Choosing the right sources of carbohydrates is essential for preventing cardiovascular disease

Choosing the right sources of carbohydrates is essential for preventing cardiovascular disease

OVERVIEW

  • Recent studies show that people who regularly consume foods containing low-quality carbohydrates (simple sugars, refined flours) have an increased risk of cardiovascular events and premature mortality.
  • Conversely, a high dietary intake of complex carbohydrates, such as resistant starches and dietary fibre, is associated with a lower risk of cardiovascular disease and improved overall health.
  • Favouring the regular consumption of foods rich in complex carbohydrates (whole grains, legumes, nuts, fruits and vegetables) while reducing that of foods containing simple carbohydrates (processed foods, sugary drinks, etc.) is therefore a simple way to improve cardiovascular health.

It is now well established that a good quality diet is essential for the prevention of cardiovascular disease and the maintenance of good health in general. This link is particularly well documented with regard to dietary fat: several epidemiological studies have indeed reported that too high a dietary intake of saturated fat increases LDL cholesterol levels, an important contributor to the development of atherosclerosis, and is associated with an increased risk of cardiovascular disease. As a result, most experts agree that we should limit the intake of foods containing significant amounts of saturated fat, such as red meat, and instead focus on sources of unsaturated fat, such as vegetable oils (especially extra virgin olive oil and those rich in omega-3s such as canola), as well as nuts, certain seeds (flax, chia, hemp) and fish (see our article on this subject). This roughly corresponds to the Mediterranean diet, a diet that has repeatedly been associated with a lower risk of several chronic diseases, especially cardiovascular disease.

On the carbohydrate side, the consensus that has emerged in recent years is to favour sources of complex carbohydrates such as whole grains, legumes and plants in general while reducing the intake of simple carbohydrates from refined flour and added sugars. Following this recommendation, however, can be much more difficult than one might think, as many available food products contain these low-quality carbohydrates, especially the entire range of ultra-processed products, which account for almost half of the calories consumed by the population. It is therefore very important to learn to distinguish between good and bad carbohydrates, especially since these nutrients are the main source of calories consumed daily by the majority of people. To achieve this, we believe it is useful to recall where carbohydrates come from and how industrial processing of foods can affect their properties and impacts on health.

Sugar polymers
All of the carbohydrates in our diet come from, in one way or another, plants. During the photosynthetic reaction, in addition to forming oxygen (O2) from carbon dioxide in the air (CO2), plants also simultaneously transform the energy contained in solar radiation into chemical energy, in the form of sugar:

6 CO2 + 12 H2O + light → C6H12O6 (glucose) + 6 O2 + 6 H2O

In the vast majority of cases, this sugar made by plants does not remain in this simple sugar form, but is rather used to make complex polymers, i.e., chains containing several hundred (and in some cases thousands) sugar molecules chemically bonded to one another. An important consequence of this arrangement is that the sugar contained in these complex carbohydrates is not immediately accessible and must be extracted by digestion before reaching the bloodstream and serving as a source of energy for the body’s cells. This prerequisite helps prevent sugar from entering the blood too rapidly, which would unbalance the control systems responsible for maintaining the concentration of this molecule at levels just sufficient enough to meet the needs of the body. And these levels are much lower than one might think; on average, the blood of a healthy person contains a maximum of 4 to 5 g of sugar in total, or barely the equivalent of a teaspoon. Dietary intake of complex carbohydrates therefore provides enough energy to support our metabolism, while avoiding excessive fluctuations in blood sugar that could lead to health problems.

Figure 1 illustrates the distribution of the two main types of sugar polymers in the plant cell: starches and fibres.

Figure 1. The physicochemical characteristics and physiological impacts of starches and dietary fibres from plant cells. Adapted from Gill et al. (2021).

Starches. Starches are glucose polymers that the plant stores as an energy reserve in granules (amyloplasts) located inside plant cells. This source of dietary carbohydrates has been part of the human diet since the dawn of time, as evidenced by the recent discovery of genes from bacteria specializing in the digestion of starches in the dental plaque of individuals of the genus Homo who lived more than 100,000 years ago. Even today, a very large number of plants commonly eaten are rich in starch, in particular tubers (potatoes, etc.), cereals (wheat, rice, barley, corn, etc.), pseudocereals (quinoa, chia, etc.), legumes, and fruits.

Digestion of the starches present in these plants releases units of glucose into the bloodstream and thus provides the energy necessary to support cell metabolism. However, several factors can influence the degree and speed of digestion of these starches (and the resulting rise in blood sugar). This is particularly the case with “resistant starches” which are not at all (or very little) digested during gastrointestinal transit and therefore remain intact until they reach the colon. Depending on the factors responsible for their resistance to digestion, three main types of these resistant starches (RS) can be identified:

  • RS-1: These starches are physically inaccessible for digestion because they are trapped inside unbroken plant cells, such as whole grains.
  • RS-2: The sensitivity of starches to digestion can also vary considerably depending on the source and the degree of organization of the glucose chains within the granules. For example, the most common form of starch in the plant kingdom is amylopectin (70–80% of total starch), a polymer made up of several branches of glucose chains. This branched structure increases the contact surface with enzymes specialized in the digestion of starches (amylases) and allows better extraction of the glucose units present in the polymer. The other constituent of starch, amylose, has a much more linear structure which reduces the efficiency of enzymes to extract the glucose present in the polymer. As a result, foods with a higher proportion of amylose are more resistant to breakdown, release less glucose, and therefore cause lower blood sugar levels. This is the case, for example, with legumes, which contain up to 50% of their starch in the form of amylose, which is much more than other commonly consumed sources of starches, such as tubers and grains.
  • RS-3: These resistant starches are formed when starch granules are heated and subsequently cooled. The resulting starch crystallization, a phenomenon called retrogradation, creates a rigid structure that protects the starch from digestive enzymes. Pasta salads, potato salads, and sushi rice are all examples of foods containing resistant starches of this type.

An immediate consequence of this resistance of digestion-resistant starches is that these glucose polymers can be considered dietary fibre from a functional point of view. This is important because, as discussed below, the fermentation of fibre by the hundreds of billions of bacteria (microbiota) present in the colon generates several metabolites that play extremely important roles in the maintenance of good health.

Dietary fibre. Fibres are polymers of glucose present in large quantities in the wall of plant cells where they play an important role in maintaining the structure and rigidity of plants. The structure of these fibres makes them completely resistant to digestion and the sugar they contain does not contribute to energy supply. Traditionally, there are two main types of dietary fibre, soluble and insoluble, each with its own physicochemical properties and physiological effects. Everyone has heard of insoluble fibre (in wheat bran, for example), which increases stool volume and speeds up gastrointestinal transit (the famous “regularity”). This mechanical role of insoluble fibres is important, but from a physiological point of view, it is mainly soluble fibres that deserve special attention because of the many positive effects they have on health.

By capturing water, these soluble fibres increase the viscosity of the digestive contents, which helps to reduce the absorption of sugar and dietary fats and thus to avoid excessive increases in blood sugar and blood lipid levels that can contribute to atherosclerosis (LDL cholesterol, triglycerides). The presence of soluble fibre also slows down gastric emptying and can therefore decrease calorie intake by increasing feelings of satiety. Finally, the bacterial community that resides in the colon (the microbiota) loves soluble fibres (and resistant starches), and this bacterial fermentation generates several bioactive substances, in particular the short chain fatty acids (SCFA) acetate, propionate and butyrate. Several studies carried out in recent years have shown that these molecules exert a myriad of positive effects on the body, whether by reducing chronic inflammation, improving insulin resistance, lowering blood pressure and the risk of cardiovascular disease, or promoting the establishment of a diversified microbiota, optimal for colon health (Table 1).

A compilation of many studies carried out in recent years (185 observational studies and 58 randomized trials, which equates to 135 million person-years) indicates that consuming 25 to 30 g of fibre per day seems optimal to benefit from these protective effects, approximately double the current average consumption.

Table 1. Main physiological effects of dietary fibre. Adapted from Barber (2020).

Physiological effectsBeneficial health impacts
MetabolismImproved insulin sensitivity
Reduced risk of type 2 diabetes
Improved blood sugar and lipid profile
Body weight control
Gut microbiotaPromotes a diversified microbiota
Production of short-chain fatty acids
Cardiovascular systemDecrease in chronic inflammation
Reduced risk of cardiovascular events
Reduction of cardiovascular mortality
Digestive systemDecreased risk of colorectal cancer

Overall, we can therefore see that the consumption of complex carbohydrates is optimal for our metabolism, not only because it ensures an adequate supply of energy in the form of sugar, without causing large fluctuations in blood sugar, but also because it provides the intestinal microbiota with the elements necessary for the production of metabolites essential for the prevention of several chronic diseases and for the maintenance of good health in general.

Modern sugars
The situation is quite different, however, for several sources of carbohydrates in modern diets, especially those found in processed industrial foods. Three main problems are associated with processing:

Simple sugars. Simple sugars (glucose, fructose, galactose, etc.) are the molecules responsible for the sweet taste: the interaction of these sugars with receptors present in the tongue sends a signal to the brain warning it of the presence of an energy source. The brain, which alone consumes no less than 120 g of sugar per day, loves sugar and responds positively to this information, which explains our innate attraction to foods with a sweet taste. On the other hand, since the vast majority of carbohydrates produced by plants are in the form of polymers (starches and fibres), simple sugars are actually quite rare in nature, being mainly found in fruits, vegetables such as beets, or even some grasses (sugar cane). It is therefore only with the industrial production of sugar from sugar cane and beets that consumers’ “sweet tooth” could be satisfied on a large scale and that simple sugars became commonly consumed. For example, data collected in the United States shows that between 1820 and 2016, the intake of simple sugars increased from 6 lb (2.7 kg) to 95 lb (43 kg) per person per year, an increase of about 15 times in just under 200 years (Figure 2).

 

 

Figure 2. Consumption of simple sugars in the United States between 1820 and 2016.  From Guyenet (2018).

Our metabolism is obviously not adapted to this very high intake of simple sugars, far beyond what is normally found in nature. Unlike the sugars found in complex carbohydrates, these simple sugars are absorbed very quickly into the bloodstream and cause very rapid and significant increases in blood sugar. Several studies have shown that people who frequently consume foods containing these simple sugars are more likely to suffer from obesity, type 2 diabetes and cardiovascular disease. For example, studies have found that consuming 2 servings of sugary drinks daily was associated with a 35% increase in the risk of coronary heart disease. When the amount of added sugars consumed represents 25% of daily calories, the risk of heart disease nearly triples. Factors that contribute to this detrimental effect of simple sugars on cardiovascular health include increased blood pressure and triglyceride levels, lowered HDL cholesterol, and increased LDL cholesterol (specifically small, very dense LDLs, which are more harmful to the arteries), as well as an increase in inflammation and oxidative stress.

It is therefore necessary to restrict as much as possible the intake of simple sugars, which should not exceed 10% of the daily energy intake according to the World Health Organization. For the average adult who consumes 2,000 calories per day, that’s just 200 calories, or about 12 teaspoons of sugar or the equivalent of a single can of soft drink.

Refined flour.  Cereals are a major source of carbohydrates (and calories) in most food cultures around the world. When they are in whole form, i.e., they retain the outer shell rich in fibre and the germ containing several vitamins and minerals, cereals are a source of complex carbohydrates (starches) of high quality and beneficial to health. This positive impact of whole grains is well illustrated by the reduced risk of coronary heart disease and mortality observed in a large number of population studies. For example, recent meta-analyses have shown that the consumption of about 50 g of whole grains perday is associated with a 22–30% reduction in cardiovascular disease mortality, a 14–18% reduction in cancer-relatedmortality, and a 19–22% reduction in total mortality.

On the other hand, these positive effects are completely eliminated when the grains are refined with modern industrial metal mills to produce the flour used in the manufacture of a very large number of commonly consumed products (breads, pastries, pasta, desserts, etc.). By removing the outer shell of the grain and its germ, this process improves the texture and shelf life of the flour (the unsaturated fatty acids in the germ are sensitive to rancidity), but at the cost of the almost total elimination of fibres and a marked depletion of several nutrients (minerals, vitamins, unsaturated fatty acids, etc.). Refined flours therefore essentially only contain sugar in the form of starch, this sugar being much easier to assimilate due to the absence of fibres that normally slow down the digestion of starch and the absorption of released sugar (Figure 3).

Figure 3. Schematic representation of a whole and refined grain of wheat.

Fibre deficiency. Fortification processes partially compensate for the losses of certain nutrients (e.g., folic acid) that occur during the refining of cereal grains. On the other hand, the loss of fibre during grain refining is irreversible and is directly responsible for one of the most serious modern dietary deficiencies given the many positive effects of fibre on the prevention of several chronic diseases.

Poor-quality carbohydrates
Low-quality carbohydrate sources with a negative impact on health are therefore foods containing a high amount of simple sugars, having a higher content of refined grains than whole grains, or containing a low amount of fibre (or several of these characteristics simultaneously). A common way to describe these poor-quality carbohydrates is to compare the rise in blood sugar they produce to that of pure glucose, called the glycemic index (GI) (see box). The consumption of food with a high glycemic index causes a rapid and dramatic rise in blood sugar levels, which causes the pancreas to secrete a large amount of insulin to get glucose into the cells. This hyperinsulinemia can cause glucose to drop to too low levels, and the resulting hypoglycemia can ironically stimulate appetite, despite ingesting a large amount of sugar a few hours earlier. Conversely, a food with a low glycemic index produces lower, but sustained, blood sugar levels, which reduces the demand for insulin and helps prevent the fluctuations in blood glucose levels often seen with foods with a high glycemic index. Potatoes, breakfast cereals, white bread, and pastries are all examples of high glycemic index foods, while legumes, vegetables, and nuts are conversely foods with a low glycemic index.

Glycemic index and load
The glycemic index (GI) is calculated by comparing the increase in blood sugar levels produced by the absorption of a given food with that of pure glucose. For example, a food that has a glycemic index of 50 (lentils, for example) produces a blood sugar half as important as glucose (which has a glycemic index of 100). As a general rule, values below 50 are considered to correspond to a low GI, while those above 70 are considered high. The glycemic index, however, does not take into account the amount of carbohydrate in foods, so it is often more appropriate to use the concept of glycemic load (GL). For example, although watermelon and breakfast cereals both have high GIs (75), the low-carbohydrate content of melon (11 g per 100 g) equates to a glycemic load of 8, while 26 g of carbohydrates present in breakfast cereals result in a load of 22, which is three times more. GLs ≥ 20 are considered high, intermediate when between 11 and 19, and low when ≤ 10.

PURE study
Results from the PURE (Prospective Urban and Rural Epidemiology) epidemiological study conducted by Canadian cardiologist Salim Yusuf have confirmed the link between low-quality carbohydrates and the risk of cardiovascular disease. In the first of these studies, published in the prestigious New England Journal of Medicine, researchers examined the association between the glycemic index and the total glycemic load of the diet and the incidence of major cardiovascular events (heart attack, stroke, sudden death, heart failure) in more than 130,000 participants aged 35 to 70, spread across all five continents. The study finds that a diet with a high glycemic index is associated with a significant (25%) increase in the risk of having a major cardiovascular event in people without cardiovascular disease, an increase that reaches 51% in those with pre-existing cardiovascular disease (Figure 4). A similar trend is observed for the glycemic load, but in the latter case, the increased risk seems to affect only those with cardiovascular disease at the start of the study.

Figure 4. Comparison of the risk of cardiovascular events according to the glycemic index or the glycemic load of the diet of healthy people (blue) or with a history of cardiovascular disease (red). The median glycemic index values were 76 for quintile 1 and 91 for quintile 5. For glycemic load, the mean values were 136 g of carbohydrates per day for Q1 and 468 g per day for Q5. Note that the increased risk of cardiovascular events associated with a high glycemic index or load is primarily seen in participants with pre-existing cardiovascular disease. From Jenkins et al. (2021).

The impact of the glycemic index appears to be particularly pronounced in overweight people (Figure 5). Thus, while the increase in the risk of major cardiovascular events is 14% in thin people with a BMI less than 25, it reaches 38% in those who are overweight (BMI over 25).

 

Figure 5. Impact of overweight on the increased risk of cardiovascular events related to the glycemic index of the diet. The values shown represent the increased risk of cardiovascular events observed for each category (quintiles 2 to 5) of the glycemic index compared to the category with the lowest index (quintile 1). The median values of the glycemic indices were 76 for quintile 1; 81 for quintile 2; 86 for quintile 3; 89 for quintile 4; and 91 for quintile 5. Taken from Jenkins et al. (2021).

This result is not so surprising, since it has long been known that excess fat disrupts sugar metabolism, especially by producing insulin resistance. A diet with a high glycemic index therefore exacerbates the rise in postprandial blood sugar already in place due to excess weight, which leads to a greater increase in the risk of cardiovascular disease. The message to be drawn from this study is therefore very clear: a diet containing too many easily assimilated sugars, as measured using the glycemic index, is associated with a significant increase in the risk of suffering a major cardiovascular event. The risk of these events is particularly pronounced for people with less than optimal health, either due to the presence of excess fat or pre-existing cardiovascular disease (or both). Reducing the glycemic index of the diet by consuming more foods containing complex carbohydrates (fruits, vegetables, legumes, nuts) and fewer products containing added sugars or refined flour is therefore an essential prerequisite for preventing the development of cardiovascular disease.

Refined flours
Another part of the PURE study looked more specifically at refined flours as a source of easily assimilated sugars that can abnormally increase blood sugar levels and increase the risk of cardiovascular disease. Researchers observed that a high intake (350 g per day, or 7 servings) of products containing refined flours (white bread, breakfast cereals, cookies, crackers, pastries) was associated with a 33% increase in the risk of coronary heart disease, 47% in the risk of stroke, and 27% in the risk of premature death. These observations therefore confirm the negative impact of refined flours on health and the importance of including as much as possible foods containing whole grains in the diet. The preventive potential of this simple dietary change is enormous since the consumption of whole grains remains extremely low, with the majority of the population of industrialized countries consuming less than 1 serving of whole grains daily, well below the recommended minimum (half of all grain products consumed, or about 5 servings per day).

Wholemeal breads are still a great way to boost the whole-grain intake. However, special attention must be paid to the list of ingredients. In Canada, the law allows up to 5% of the grain to be removed when making whole wheat flour, and the part removed contains most of the germ and a fraction of the bran (fibres). This type of bread is superior to white bread, but it is preferable to choose products made from whole-grain flour which contains all the parts of the grain. Note also that multigrain breads (7-14 grains) always contain 80% wheat flour and a maximum of 20% of a mixture of other grains (otherwise the bread does not rise), so the number of grains does not matter, but what does matter is whether the flour is whole wheat or ideally integral, which is not always the case.

In short, a simple way to reduce the risk of cardiovascular events and improve health in general is to replace as much as possible the intake of foods rich in simple sugars and refined flour with plant-based foods containing complex carbohydrates. In addition to carbohydrates, this simple change alone will influence the nature of the proteins and lipids ingested as well as, at the same time, all the phenomena that promote the appearance and progression of atherosclerotic plaques.

Reducing calorie intake by eating more plants

Reducing calorie intake by eating more plants

OVERVIEW

  • Twenty volunteers were fed a low-fat or low-carbohydrate diet in turn for two weeks.
  • Participants on the low-fat diet consumed an average of nearly 700 fewer calories per day than with the low-carbohydrate diet, a decrease correlated with a greater loss of body fat.
  • Compared to the low-carbohydrate diet, the low-fat diet also led to lower cholesterol levels, reduced chronic inflammation, and lowered heart rate and blood pressure.
  • Overall, these results suggest that a diet mainly composed of plants and low in fat is optimal for cardiovascular health, both for its superiority in reducing calorie intake and for its positive impact on several risk factors for cardiovascular disease.

It is estimated that there are currently around 2 billion overweight people in the world, including 600 million who are obese. These statistics are truly alarming because it is clearly established that excess fat promotes the development of several diseases that decrease healthy life expectancy, including cardiovascular disease, type 2 diabetes, and several types of cancer. Identifying the factors responsible for this high prevalence of overweight and the possible ways to reverse this trend as quickly as possible is therefore essential to improve the health of the population and avoid unsustainable pressures on public health systems in the near future.

Energy imbalance
The root cause of overweight, and obesity in particular, is a calorie intake that exceeds the body’s energy needs. To lose weight, therefore, it is essentially a matter of restoring the balance between the calories ingested and the calories expended.

It might seem simple in theory, but in practice most people find it extremely difficult to lose weight. On the one hand, it is much easier to gain weight than to lose weight. During evolution, we have had to deal with periods of prolonged food shortages (and even starvation, in some cases) and our metabolism has adapted to these deficiencies by becoming extremely efficient at accumulating and conserving energy in the form of fat. On the other hand, the environment in which we currently live strongly encourages overconsumption of food. We are literally overwhelmed by an endless variety of attractive food products, which are often inexpensive, easily accessible, and promoted by very aggressive marketing that encourages their consumption. The current epidemic of overweight and obesity thus reflects our biological predisposition to accumulate reserves in the form of fat, a predisposition that is exacerbated by the obesogenic environment that surrounds us.

Eating less to restore balance
The body’s innate tendency to keep energy stored in reserve as fat makes it extremely difficult to lose weight by “burning” those excess calories by increasing the level of physical activity. For example, a person who eats a simple piece of sugar pie (400 calories) will have to walk about 6.5 km to completely burn off those calories, which, of course, is difficult to do on a daily basis. This does not mean that exercise is completely useless for weight loss. Research in recent years shows that exercise can specifically target certain fat stores, especially in the abdominal area. Studies also show that regular physical activity is very important for long-term maintenance of the weight lost from a low-calorie diet. However, there is no doubt that it is first and foremost the calories consumed that are the determining factors in weight gain. Moreover, contrary to what one might think, levels of physical activity have hardly changed for the last thirty years in industrialized countries, and the phenomenal increase in the number of overweight people is therefore mainly a consequence of overconsumption of food. Exercise is essential for the prevention of all chronic diseases and for the maintenance of general good health, but its role in weight loss is relatively minor. For overweight people, the only realistic way to lose weight significantly, and especially to maintain these losses over prolonged periods, is thus to reduce calorie intake.

Less sugar or less fat?
How do we get there? First, it’s important to realize that the surge in the number of overweight people has coincided with a greater availability of foods high in sugar or fat (and sometimes both). All countries in the world, without exception, that have adopted this type of diet have seen their overweight rates skyrocket, so it is likely that this change in eating habits plays a major role in the current obesity epidemic.

However, the respective contributions of sugar and fat to this increase in caloric intake and overweight are still the subjectof vigorous debate:

1) On the one hand, it has been proposed that foods high in fat are particularly obesogenic, since fats are twice as high in calories as carbohydrates, are less effective in causing a feeling of satiety, and improve the organoleptic properties of foods, which generally encourages (often unconscious) overconsumption of food. Therefore, the best way to avoid overeating and becoming overweight would be to reduce the total fat intake (especially saturated fat due to its negative impact on LDL-cholesterol levels) and replace it with complex carbohydrates (vegetables, legumes, whole-grain cereals). This is colloquially called the low-fat approach, advocated for example by the Ornish diet.

2) On the other hand, the exact opposite is proposed, i.e. that it would be mainly carbohydrates that would contribute to overconsumption of food and to the increase in the incidence of obesity. According to this model, carbohydrates in foods in the form of free sugars or refined flours cause insulin levels to rise markedly, causing massive energy storage in adipose tissue. As a result, fewer calories remain available in the circulation for use by the rest of the body, causing increased appetite and overeating to compensate for this lack. In other words, it wouldn’t be because we eat too much that we get fat, but rather because we are too fat we eat too much.

3) By preventing excessive fluctuations in insulin levels, a diet low in carbohydrates would thus limit the anabolic effect of this hormone and, therefore, prevent overeating and the accumulation of excess fat.

Less fat on the menu, fewer calories ingested
To compare the impact of low-carb and low-fat diets on calorie intake, Dr. Kevin Hall’s group (NIH) recruited 20 volunteers who were fed each of these diets in turn for two weeks. The strength of this type of cross-study is that each participant consumes both types of diets and that their effects can therefore be compared directly on the same person.

As shown in Figure 1, the two diets studied were completely opposite of each other, with 75% of the calories in the low-fat (LF) diet coming from carbohydrates versus only 10% from fat, while in the low-carb (LC) diet, 75% of calories were in the form of fat, compared to only 10% from carbohydrates. The LF diet under study consisted exclusively of foods of plant origin (fruits, vegetables, legumes, root vegetables, soy products, whole grains, etc.), while the LC diet contained mainly (82%) animal foods (meat, poultry, fish, eggs, dairy products).

Figure 1. Comparison of the amounts of carbohydrates, fats and proteins present in the low-carbohydrate (LC) and low-fat (LF) diets consumed by study participants. Adapted from Hall et al. (2021).

The study shows that there is indeed a big difference between the two types of diets in the number of calories consumed by participants (Figure 2). Over a two-week period, participants who ate an LF (low-fat) diet consumed an average of nearly 700 calories (kcal) per day less than an LC (low-carbohydrate) diet. This difference in calorie intake is observed for all meals, both at breakfast (240 calories less for the LF diet), at lunch (143 calories less), at dinner (195 calories less), and during snacks taken between meals (128 calories less). This decrease is not caused by a difference in the appreciation of the two diets by the participants, as parallel analyses did not find any difference in the level of appetite of the participants, nor in the degree of satiety and satisfaction generated by the consumption of either diet. However, the LF diet was composed exclusively of plant-based foods and therefore much richer in non-digestible fibres (60 g per day compared to only 20 g for the LC diet), which greatly reduce the energy density of meals (quantity of calories per g of food) compared to the high-fat LC diet. It is therefore very likely that this difference in energy density contributes to the lower calorie intake observed for the low-fat diet.

Overall, these results indicate that a diet consisting of plants, and thus low in fat and high in complex carbohydrates, is more effective than a diet consisting mainly of animal products, high in fat and low in carbohydrates, to limit calorie intake.

Figure 2. Comparison of the daily calorie intake of participants on a low-carbohydrate (LC) or low-fat (LF) diet. From Hall et al. (2021).

Weight loss
Despite the significant difference in calorie intake observed between the two diets, their respective impact on short-term weight loss is more nuanced. At first glance, the LC diet appeared to be more effective than the LF diet in causing rapid weight loss, with about 1 kg lost on average in the first week and almost 2 kg after two weeks, compared to only 1 kg after two weeks of the LF diet (Figure 3). However, further analysis revealed that the weight loss caused by the LC diet was mainly in the form of lean mass (protein, water, glycogen), while this diet had no significant impact on fat loss during this period. Conversely, the LF diet had no effect on this lean body mass, but did cause a significant decrease in body fat, to around 1 kg after two weeks. In other words, only the LF diet caused a loss of body fat during the study period, which strongly suggests that the decrease in calorie intake made possible by this type of diet may facilitate the maintenance of astable body weight and could even promote weight loss in overweight people.

Figure 3. Comparison of changes in body weight (top), lean mass (middle), and body fat (bottom) caused by low-carbohydrate and low-fat diets. From Hall et al. (2021).

Cardiovascular risk factors
In addition to promoting lower calorie intake and fat loss, the LF diet also appears to be superior to the LC diet in terms of its impact on several cardiovascular risk factors (Table 1):

Cholesterol. It is well established that LDL cholesterol levels increase in response to a high intake of saturated fat (see our article on the issue). It is therefore not surprising that the LF diet, which contains only 2% of all calories as saturated fat, causes a significant decrease in cholesterol, both in terms of total cholesterol and LDL cholesterol. At first glance, the high-fat LC diet (containing 30% of the daily calorie intake as saturated fat) does not appear to have a major effect on LDL cholesterol; however, it should be noted that this diet significantly modifies the distribution of LDL cholesterol particles, in particular with a significant increase in small and dense LDL particles. Several studies have reported that these small, dense LDL particles infiltrate artery walls more easily and also appear to oxidize more easily, two key events in the development and progression of atherosclerosis. In sum, just two weeks of a high-fat LC diet was enough to significantly (and negatively) alter the atherogenic profile of participants, which may raise doubts about the long-term effects of this type of diet on cardiovascular health.

Table 1. Variations in certain risk factors for cardiovascular disease following a diet low in carbohydrates or low in fat. From Hall et al. (2021).

Branched-chain amino acids. Several recent studies have shown a very clear association between blood levels of branched-chain amino acids (leucine, isoleucine and valine) and an increased risk of metabolic syndrome and type 2 diabetes, two very important risk factors for cardiovascular diseases. In this sense, it is very interesting to note that the levels of these amino acids are almost twice as high after two weeks of the LC diet compared to the LF diet, suggesting a positive effect of a diet rich in plants and poor in fats in the prevention of these disorders.

Inflammation. Chronic inflammation is actively involved in the formation and progression of plaques that form on the lining of the arteries and can lead to the development of cardiovascular events such as myocardial infarction and stroke. Clinically, this level of inflammation is often determined by measuring levels of high-sensitivity C-reactive protein (hsCRP), a protein made by the liver and released into the blood in response to inflammatory conditions. As shown in Table 1, the LF diet significantly decreases the levels of this inflammatory marker, another positive effect that argues in favour of a plant-rich diet for the prevention of cardiovascular disease.

In addition to these laboratory data, the researchers noted that participants who were fed the LF diet had a slower heart rate (73 vs. 77 beats/min) as well as lower blood pressure (112/67 vs. 116/69 mm Hg) than observed following the LC diet. In the latter case, this difference could be related, at least in part, to the much higher sodium consumption in the LC diet compared to the LF diet (5938 vs. 3725 mg/day).

All of these results confirm the superiority of a diet mainly composed of plants on all the factors involved in cardiovascular health, whether in terms of lipid profile, chronic inflammation, or adequate control of calorie intake necessary to maintain body weight.

Control of inflammation through diet

Control of inflammation through diet

OVERVIEW

  • Chronic inflammation is actively involved in the formation and progression of plaques that form on the lining of the arteries, which can lead to the development of cardiovascular events such as myocardial infarction and stroke.
  • Two studies show that people whose diet is anti-inflammatory due to a high intake of plants (vegetables, fruits, whole grains), beverages rich in antioxidants (tea, coffee, red wine) or nuts have a significantly lower risk of being affected by cardiovascular disease.
  • This type of anti-inflammatory diet can be easily replicated by adopting the Mediterranean diet, rich in fruits, vegetables, legumes, nuts and whole grains and which has repeatedly been associated with a lower risk of cardiovascular events.

Clinically, the risk of having a coronary event is usually estimated based on age, family history, smoking and physical inactivity as well as a series of measures such as cholesterol levels, blood sugar level and blood pressure. The combination of these factors helps to establish a cardiovascular disease risk “score”, i.e. the likelihood that the patient will develop heart disease over the next ten years. When this score is moderate (10 to 20%) or high (20% and more), one or more specific drugs are generally prescribed in addition to recommending lifestyle changes in order to reduce the risk of cardiovascular events.

These estimates are useful, but they do not take into account other factors known to play an important role in the development of cardiovascular disease. This is especially true for chronic inflammation, a process that actively participates in the formation and progression of plaques that form on the lining of the arteries and can lead to cardiovascular events such as myocardial infarction and stroke.

The clinical significance of this chronic inflammation is well illustrated by studies of patients who have had a heart attack and are treated with a statin to lower their LDL cholesterol levels. Studies show that a high proportion (about 40%) of these people have excessively high blood levels of inflammatory proteins, and it is likely that this residual inflammatory risk contributes to the high rate of cardiovascular mortality (nearly 30%) that affects these patients within two years of starting treatment, despite a significant reduction in LDL cholesterol. In this sense, it is interesting to note that the canakinumab antibody, which neutralizes an inflammatory protein (interleukin-1 β), causes a slight but significant decrease in major cardiovascular events in coronary patients. Statins, used to lower LDL cholesterol levels, are also believed to have an anti-inflammatory effect (reduction in C-reactive protein levels) that would contribute to reducing the risk of cardiovascular events. One of the roles of inflammation is also demonstrated by the work of Dr. Jean-Claude Tardif of the Montreal Heart Institute, which shows that the anti-inflammatory drug colchicine significantly reduces the risk of recurrence of cardiovascular events.

Reducing chronic inflammation is therefore a very promising approach for decreasing the risk of cardiovascular disease, both in people who have already had a heart attack and are at a very high risk of recurrence and in healthy people who are at high risk of cardiovascular disease.

Anti-inflammatory diet
Two studies published in the Journal of the American College of Cardiology suggest that the nature of the diet can greatly influence the degree of chronic inflammation and, in turn, the risk of cardiovascular disease. In the first of these two articles, researchers analyzed the link between diet-induced inflammation and the risk of cardiovascular disease in 166,000 women and 44,000 men followed for 24 to 30 years. The inflammatory potential of the participants’ diet was estimated using an index based on the known effect of various foods on the blood levels of 3 inflammatory markers (interleukin-6, TNFα-R2, and C-reactive protein or CRP). For example, consumption of red meat, deli meats and ultra-processed industrial products is associated with an increase in these markers, while that of vegetables, fruits, whole grains and beverages rich in antioxidants (tea, coffee, red wine) is on the contrary associated with a decrease in their blood levels. People who regularly eat pro-inflammatory foods therefore have a higher inflammatory food index, while those whose diet is rich in anti-inflammatory foods have a lower index.

Using this approach, the researchers observed that a higher dietary inflammatory index was associated with an increased risk of cardiovascular disease, with a 40% increase in risk in those with the highest index (Figure 1). This increased risk associated with inflammation is particularly pronounced for coronary heart disease (acute coronary syndromes including myocardial infarction) with an increased risk of 46%, but seems less pronounced for cerebrovascular accidents (stroke) (28% increase in risk). The study shows that a higher dietary inflammation index was also associated with two risk markers for cardiovascular disease, higher circulating triglyceride levels as well as lower HDL cholesterol. These results therefore indicate that there is a link between the degree of chronic inflammation generated by diet and the risk of long-term cardiovascular disease, in agreement with data from a recent meta-analysis of 14 epidemiological studies that have explored this association.

Figure 1. Change in the risk of cardiovascular disease depending on the inflammatory potential of the diet. From Li et al. (2020). The dotted lines indicate the 95% confidence interval.

Anti-inflammatory nuts
A second study by a group of Spanish researchers investigated the anti-inflammatory potential of walnuts. Several epidemiological studies have reported that regular consumption of nuts is associated with a marked decrease in the risk of cardiovascular disease. For example, a recent meta-analysis of 19 prospective studies shows that people who consume the most nuts (28 g per day) have a lower risk of developing coronary artery disease (18%) or of dying from these diseases (23%). These reductions in the risk of cardiovascular disease may be explained in part by the decrease in LDL cholesterol (4%) and triglyceride (5%) levels observed following the consumption of nuts in intervention studies. However, this decrease remains relatively modest and cannot alone explain the marked reduction in the risk of cardiovascular disease observed in the studies.

The results of the Spanish study strongly suggest that a reduction in inflammation could greatly contribute to the preventative effect of nuts. In this study, 708 people aged 63 to 79 were divided into two groups, a control group whose diet was completely nut free and an intervention group, in which participants consumed about 15% of their calories daily in the form of walnuts (30–60 g/day). After a period of 2 years, the researchers observed large variations in the blood levels of several inflammatory markers between the two groups (Figure 2), in particular for GM-CSF (a cytokine that promotes the production of inflammatory cells) and interleukin-1 β (a highly inflammatory cytokine whose blood levels are correlated with an increased risk of death during a heart attack). This reduction in IL-1 β levels is particularly interesting because, as mentioned earlier, a randomized clinical study (CANTOS) has shown that an antibody neutralizing this cytokine leads to a reduction in the risk of myocardial infarction in coronary heart patients.

Figure 2. Reduction in blood levels of several inflammatory markers by a diet enriched with nuts. From Cofán et al. (2020). GM-CSF: granulocyte-monocyte colony stimulating factor; hs-CRP: high-sensitivity C-reactive protein; IFN: interferon; IL: interleukin; SAA: serum amyloid A; sE-sel: soluble E-selectin; sVCAM: soluble vascular cell adhesion molecule; TNF: tumour necrosis factor.

Taken together, these studies therefore confirm that an anti-inflammatory diet provides concrete benefits in terms of preventing cardiovascular disease. This preventative potential remains largely unexploited, as Canadians consume about half of all their calories in the form of ultra-processed pro-inflammatory foods, while less than a third of the population eats the recommended minimum of five daily servings of fruits and vegetables and less than 5% of the recommended three servings of whole grains. This imbalance causes most people’s diets to be pro-inflammatory, contributes to the development of cardiovascular diseases as well as other chronic diseases, including certain common cancers such as colon cancer, and reduces the life expectancy.

The easiest way to restore this balance and reduce inflammation is to eat a diet rich in plants while reducing the intake of industrial products. The Mediterranean diet, for example, is an exemplary anti-inflammatory diet due to its abundance of fruits, vegetables, legumes, nuts and whole grains, and its positive impact will be all the greater if regular consumption of these foods reduces that of pro-inflammatory foods such as red meat, deli meats and ultra-processed products. Not to mention that this diet is also associated with a high intake of fibre, which allows the production of anti-inflammatory short-chain fatty acids by the intestinal microbiota, and of phytochemicals such as polyphenols, which have antioxidant and anti-inflammatory properties.

In summary, these recent studies demonstrate once again the important role of diet in preventing chronic disease and improving healthy life expectancy.

Insufficient dietary fibre intake harms the gut microbiota and the immune system’s balance

Insufficient dietary fibre intake harms the gut microbiota and the immune system’s balance

OVERVIEW

  • The typical diet in Western countries does not contain enough fibre.
  • This insufficient fibre intake adversely affects the bacteria in the gut microbiota and therefore the immunity and health of the host.
  • An abundant and varied consumption of dietary fibre helps maintain a diverse and healthy microbiota, which produces metabolites that contribute to human physiology and health.

Dietary fibre is made up of complex sugars that cannot be digested by human digestive enzymes, but is an important source of energy for gut bacteria, which have the ability to break it down. This fibre comes mainly from plants, but is also found in animal tissues (meat, offal), fungi (mushrooms, yeasts, moulds), and foodborne microorganisms. The main fibres are cellulose, lignins, pectin, inulin, starches and dextrins resistant to amylases, chitins, beta-glucans and other oligosaccharides. However, not all dietary fibre can be used by the intestinal microbiota (cellulose for example), so researchers are more particularly interested in “microbiota-accessible carbohydrates” or MAC, which are found in legumes, wheat and oats, for example.

Resurgence of allergies and inflammatory and autoimmune diseases
Non-communicable diseases, such as allergies and inflammatory and autoimmune diseases have been on the rise in Western countries over the past century. Although we do not know all the causes of these increases, it is quite plausible that they have an environmental component. The transition from the traditional diet to the Western diet that occurred after the Industrial Revolution is often called into question. The typical Western diet consists primarily of processed foods high in sugar and fat, but low in minerals, vitamins, and fibre. The recommended daily intake of dietary fibre is at least 30 grams (1 ounce), while followers of the Western diet consume only 15 grams on average. In addition, people living in traditional societies consume up to 50–120 g/day of fibre and have a much more diverse gut microbiota than Westerners. A diverse microbiota is associated with good health in general, while a poorly diversified microbiota has been associated with chronic diseases common in Western countries, such as type 2 diabetes, obesity, inflammatory bowel disease (ulcerative colitis, Crohn’s disease), colorectal cancer, rheumatoid arthritis and asthma.

Metabolites of the gut microbiota
The gut microbiota contributes to human physiology by producing a multitude of metabolites. The most studied are short-chain fatty acids (SCFAs), which are organic compounds such as acetate, propionate and butyrate that together constitute ≥95% of SCFAs. These metabolites are absorbed and find their way into the bloodstream via the portal vein and act on the liver and then, via the peripheral blood circulation, on other organs of the human body. SCFAs play key roles in the regulation of human metabolism, the immune system, and cell proliferation. SCFAs are metabolites produced by microorganisms in the intestinal microbiota from dietary fibres, which are complex sugars. The microbiota produces other metabolites from amino acids derived from dietary protein, including indole and its derivatives, tryptamine, serotonin, histamine, dopamine, p-cresol, phenylacetylglutamine, and phenylacetylglycine.

A lack of dietary fibre leads to the generation of toxic metabolites by the microbiota
Insufficient fibre intake not only leads to reduced microbiota diversity and a reduction in the amount of SCFAs produced, but also causes a shift in the metabolism of microorganisms towards the use of substrates less favourable for human health. Among these alternative substrates, amino acids from food proteins are fermented by the microbiota into branched-chain fatty acids, ammonia, amines, N-nitroso compounds, phenolic compounds such as p-cresol, sulphides, and indole compounds. These metabolites are either cytotoxic and/or pro-inflammatory and they contribute to the development of chronic diseases, particularly colorectal cancer.

Effects on mucus production that protects the intestinal lining
The main substrates used by the microbiota when fibre intake is low are mucins, glycoproteins contained in the mucus that cover and protect the epithelium of the intestinal lining. Maintaining this layer of mucus is very important to prevent infections; however, a diet low in fibre alters the composition of the gut microbiota and leads to a significant deterioration of the mucus layer, which can increase the susceptibility to infections and chronic inflammatory diseases (see figure, below). Transcriptomic analyses have revealed that when there is a lack of MAC-type fibres, the enzymes that break down the mucus are expressed in greater quantities in the microorganisms of the microbiota. The consequences of the deterioration and thinning of the mucus layer are a dysfunction of the intestinal barrier, i.e. increased permeability, which increases susceptibility to infection by pathogenic bacteria. A diet rich in fibre has the opposite effect: the microbiota is diverse and the abundant production of SCFA metabolites stimulates the production and secretion of mucus by specialized epithelial cells, known as goblet cells.

Figure. Effect of a high- or low-fibre diet on the composition and diversity of the gut microbiota and the impact on human physiology. MAC: microbiota-accessible carbohydrates. From Makki et al., 2018.

Immune system
A healthy gut microbiota contributes to the maturation and development of the immune system (see this review article). For example, short-chain fatty acids (SCFAs) produced by the microbiota stimulate the production of regulatory T-cells. SCFAs have many effects on the function and hematopoiesis of dendritic cells as well as on neutrophils, which are the first leukocytes to be mobilized by the immune system in the presence of a pathogen.

Inflammation and colon cancer
The incidence of inflammatory bowel disease has increased dramatically in the West over the past few decades. A diet low in fibre has been correlated with an increased incidence of Crohn’s disease. On the contrary, a sufficient intake of dietary fibre seems to protect against the development of ulcerative colitis, an effect which has been associated with a decrease in SCFAs produced by the microbiota, butyrate in particular, which has anti-inflammatory properties. Inflammatory bowel disease can lead to the development of colon cancer. Additionally, reduced dietary fibre intake has been linked to an increased incidence of colorectal cancer.

Dietary fibre plays a much more complex role than was believed a short time ago, when it was thought that it had a purely mechanical role in intestinal transit, by an increase in the volume of the alimentary bolus and by its emollient properties. Adequate dietary fibre intake helps maintain a diverse and healthy gut microbiota, which can prevent the development of allergies as well as inflammatory and autoimmune diseases. The gut microbiota is the subject of intense research efforts, as evidenced by the numerous scientific articles published each month, and it certainly has not revealed all of its secrets!