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

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).

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.