- Metabolomic screening has identified a new metabolite associated with cardiovascular disease in the blood of people with type 2 diabetes.
- This metabolite, phenylacetylglutamine (PAGln), is produced by the intestinal microbiota and the liver, from the amino acid phenylalanine from dietary proteins.
- PAGln binds to adrenergic receptors expressed on the surface of blood platelets, which results in making them hyper-responsive.
- A beta blocker drug widely used in clinical practice (Carvedilol) blocks the prothrombotic effect of PAGln.
A research group from the Cleveland Clinic in the United States recently identified a new metabolite of the microbiota that is clinically and mechanistically linked to cardiovascular disease (CVD). This discovery was made possible by the use of a metabolomic approach (i.e. the study of metabolites in a given organism or tissue), a powerful and unbiased method that identified, among other things, trimethylamine oxide (TMAO) as a metabolite promoting atherosclerosis and branched-chain amino acids (BCAAs) as markers of obesity.
The new metabolomic screening has identified several compounds associated with one or more of these criteria in the blood of people with type 2 diabetes: 1) association with major adverse cardiovascular events (MACE: myocardial infarction, stroke or death) in the past 3 years; 2) heightened levels of type 2 diabetes; 3) poor correlation with indices of glycemic control. Of these compounds, five were already known: two which are derived from the intestinal microbiota (TMAO and trimethyllysine) and three others that are diacylglycerophospholipids. Among the unknown compounds, the one that was most strongly associated with MACE was identified by mass spectrometry as phenylacetylglutamine (PAGln).
In summary, here is how PAGln is generated (see the left side of Figure 1):
- The amino acid phenylalanine from dietary proteins (animal and plant origin) is mostly absorbed in the small intestine, but a portion that is not absorbed ends up in the large intestine.
- In the large intestine, phenylalanine is first transformed into phenylpyruvic acid by the intestinal microbiota, then into phenylacetic acid by certain bacteria, particularly those expressing the porA
- Phenylacetic acid is absorbed and transported to the liver via the portal vein where it is rapidly metabolized into phenylacetylglutamine or PAGln.
Figure 1. Schematic summary of the involvement of PAGln in the increase in platelet aggregation, athero-thrombosis and major adverse cardiovascular events. From Nemet et al., 2020.
Researchers have shown that PAGln increases the effects associated with platelet activation and the potential for thrombosis in whole blood, on isolated platelets and in animal models of arterial damage.
PAGln binds to cell sites in a saturable manner, suggesting specific binding to membrane receptors. The researchers then demonstrated that PAGln binds to G-protein coupled adrenergic receptors, expressed on the surface of the platelet cell membrane. The stimulation of these receptors by PAGln causes the hyperstimulation of the platelets, which then become hyper-responsive and accelerate the platelet aggregation and the thrombosis process.
Finally, in a mouse thrombus model, it has been shown that a beta blocker drug widely used in clinical practice (Carvedilol) blocks the prothrombotic effect of PAGln. This result is particularly interesting because it suggests that the beneficial effects of beta blockers may be partly caused by reversing the effects of high PAGln levels. The identification of PAGln could lead to the development of new targeted and personalized strategies for the treatment of cardiovascular diseases.
- Voluntary and regular exercise in mice decreases the number of inflammatory leukocytes (white blood cells) in the bloodstream.
- Exercise causes a decrease in leptin (a digestive hormone) secreted by fat cells, which decreases the production of leukocytes by hematopoietic stem and progenitor cells in the bone marrow.
- Cardiac patients who exercised four or more times a week had lower leptin and leukocyte blood levels.
- These results suggest that a sedentary lifestyle contributes to cardiovascular risk through increased production of inflammatory leukocytes.
It is well established that regular exercise has many benefits for cardiovascular health, but the underlying mechanisms have not yet been fully identified and understood. A recent study published in Nature Medicine shows that in mice, voluntary exercise reduces the proliferation of hematopoietic stem and progenitor cells (HSPC), which has the effect of reducing the number of inflammatory leukocytes in the bloodstream. Remember that HSPC cells have the ability to transform into different types of cells that are involved in the immune response (leukocytes, lymphocytes, macrophages, etc.).
A sedentary lifestyle, chronic inflammation and abnormally high white blood cell count (leukocytosis) promote atherosclerosis, which can potentially cause myocardial infarction, stroke or heart failure.
To test whether regular exercise can modulate hematopoiesis, the researchers put mice in cages in the presence (or not for the control group) of an exercise wheel, where they could exercise at their will. Mice use these exercise wheels readily and with great zeal and are therefore not subjected to stress as when they are forced to exercise such as, for example, forced swimming that has already been used in other studies. After six weeks, mice that exercised voluntarily (doing about 20 times more physical activity than sedentary mice) reduced their body weight and increased their food intake.
Analyses have shown that exercise reduced the proliferation of hematopoietic stem and progenitor cells by 34%. The decrease in HSPC through exercise had the effect of reducing the number of inflammatory leukocytes (white blood cells) in the bloodstream. In addition, the mononuclear cells in the bone marrow of mice that exercised were less able to differentiate into granulocytes, macrophages, and pre-B cells. The researchers showed that the mechanism involves a decrease in the production of leptin (a hormone secreted during digestion to regulate fat stores and control the feeling of satiety) in fat tissues. The decrease in leptin in the bloodstream of mice had the effect of increasing the production of factors of quiescence and retention of hematopoietic stem cells in the bone marrow, and consequently of decreasing the number of leukocytes in the bloodstream (see figure below).
Figure. Schematic summary of the effects of exercise on leukocyte levels and the risk of cardiovascular disease. LepR+: expressing the leptin receptor. Adapted from Frodermann et al., 2019.
Leptin supplementation in mice that exercised (using subcutaneous micropumps) reversed the exercise-induced effects on hematopoiesis, proving that this digestive hormone is involved in this phenomenon.
The exercise wheel was removed from the mouse cage after six weeks. Three weeks later, the effect on leptin production faded, but the effects of exercise on hematopoiesis persisted, i.e. the leukocyte levels of exercise mice were still lower than that of sedentary mice. There is therefore a “memory” of the exercise, which was related to epigenetic changes, i.e. to a difference in the expression of certain genes without alteration of their DNA sequence.
A reduction in leukocyte levels in the blood can lead to an increased risk of infection, as has already been observed for high-intensity exercise. The researchers wanted to see if this was the case with the mice in their study. A component of the cell wall of bacteria (lipopolysaccharide) was injected into the stomachs of mice to induce an inflammatory response. The mice responded quickly by increasing the number of HSPC and defence cells (neutrophils, monocytes, B lymphocytes, T-cells) in the blood and at the site of infection. Mice who exercised reacted more than sedentary mice to lipopolysaccharide injection and had a lower mortality rate when real sepsis was provoked. It is therefore clear that regular voluntary exercise in mice does not decrease the emergency immune response to infection.
The researchers then wanted to find out if the decrease in leukocytes caused by exercise could reduce atherosclerosis and inflammation of atherosclerotic plaques. To do this they used a “knockout” mouse line in which the gene encoding apolipoprotein E was inactivated (Apoe–/–). This protein carries lipids into the blood and is essential for their elimination. Inactivation of the Apoe gene causes hypercholesterolemia and atherosclerosis in mice. Apoe–/– mice that developed atherosclerosis were placed in cages containing an exercise wheel, which led to a decrease in leptin levels, a decrease in leukocytes, and a decrease in plaque size. The same beneficial effects of exercise on atherosclerosis were observed in a mouse line in which the gene encoding the leptin receptor was inactivated specifically at the level of stromal cells.
The researchers finally wanted to know if exercise could have beneficial effects on hematopoiesis in patients with cardiovascular disease. To do this, they checked whether there was an association between the amount of exercise and the blood levels of leptin or the number of leukocytes in 4,892 participants of the CANTOS study, who were all recruited after having a heart attack. Participants who exercised four or more times a week had significantly lower leptin blood levels. Another study (Athero-Express Study) also showed a favourable relationship between the amount of exercise and levels of leptin and leukocytes. The results of these two clinical studies, combined with those obtained in mice, indicate that physical activity has beneficial effects on leptin levels and leukocytosis in patients with cardiovascular disease.
This new study suggests that a sedentary lifestyle contributes to cardiovascular risk through an increased production of inflammatory leukocytes, and confirms the idea that physical activity reduces chronic inflammation. Let us recall the main recommendations of the World Health Organization’s (WHO) regarding physical activity for health:
“In order to improve cardiorespiratory and muscular fitness, bone health, reduce the risk of noncommunicable diseases and depression,
- Adults aged 18–64 should do at least 150 minutes of moderate-intensity aerobic physical activity throughout the week or do at least 75 minutes of vigorous-intensity aerobic physical activity throughout the week or an equivalent combination of moderate- and vigorous-intensity activity.
- Aerobic activity should be performed in bouts of at least 10 minutes duration.
- For additional health benefits, adults should increase their moderate-intensity aerobic physical activity to 300 minutes per week, or engage in 150 minutes of vigorous-intensity aerobic physical activity per week, or an equivalent combination of moderate- and vigorous-intensity activity.
- Muscle-strengthening activities should be done involving major muscle groups on 2 or more days a week.”
To learn more about the benefits, quantity and types of exercise, check out these articles:
How much exercise to live longer?
Exercise on an empty stomach to burn more fat
Can regular exercise compensate for long periods spent sitting?
Exercise benefits in cardiovascular disease: beyond attenuation of traditional risk factors
- A study of 400,000 middle-aged people (average age 51) shows that above-normal cholesterol levels are associated with a significant increase in the risk of cardiovascular disease in the decades that follow.
- This risk is particularly high in people who were under the age of 45 at the start of the study, suggesting that prolonged exposure to excess cholesterol plays a major role in increasing the risk of cardiovascular disease.
- Reducing cholesterol levels as early as possible, from early adulthood, through lifestyle changes (diet, exercise) can therefore limit the long-term exposure of blood vessels to atherogenic particles and thus reduce the cardiovascular events during aging.
It is now well established that high levels of cholesterol in the bloodstream promote the development of atherosclerosis and thereby increase the risk of cardiovascular events such as myocardial infarction and stroke. It is for this reason that the measurement of cholesterol has been part of the basic blood test for more than 30 years and that a deviation from normal values is generally considered a risk factor for cardiovascular disease.
Remember that cholesterol is insoluble in water and must be combined with lipoproteins to circulate in the blood. Routinely, the way to determine cholesterol levels is to measure all of these lipoproteins (what is called total cholesterol) and then distinguish two main types:
- HDL cholesterol, colloquially known as “good cholesterol” because it is involved in the elimination of cholesterol and therefore has a positive effect on cardiovascular health;
- LDL cholesterol, the “bad” cholesterol because of its involvement in the formation of atherosclerotic plaques that increase the risk of heart attack and stroke.
LDL cholesterol is difficult to measure directly and its concentration is rather calculated from the values determined for total cholesterol, HDL cholesterol and triglycerides using a mathematical formula:
[LDL cholesterol] = [Total cholesterol] – [HDL cholesterol] – [Triglycerides] / 2.2
However, this method has its limits, among other things because a large proportion of cholesterol can be transported by other types of lipoproteins and therefore does not appear in the calculation. However, it is very easy to measure all of these lipoproteins by simply subtracting HDL cholesterol from total cholesterol:
[Total cholesterol] – [HDL cholesterol] = [Non-HDL cholesterol]
This calculation makes it possible to obtain the concentration of what is called “non-HDL” cholesterol, i.e. all of the atherogenic lipoproteins [VLDL, IDL, LDL and Lp(a)] that are deposited at the level of the wall of the arteries and form atheromatous plaques that significantly increase the risk of cardiovascular problems. Although clinicians are more familiar with LDL cholesterol measurement, cardiology associations, including the Canadian Cardiovascular Society, now recommend that non-HDL cholesterol also be used as an alternative marker for risk assessment in adults.
The decision to initiate cholesterol-lowering therapy depends on the patient’s risk of experiencing a cardiovascular event in the next 10 years. To estimate this risk, clinicians use what is called a “risk score” (the Framingham risk score, for example), a calculation based primarily on the patient’s age, history of cardiovascular disease, family history and certain clinical values (blood pressure, blood sugar, cholesterol). For people who are at high risk of cardiovascular disease, especially those who have suffered a coronary event, there is no hesitation: all patients must be taken care of quickly, regardless of LDL or non-HDL cholesterol levels. Several clinical studies have shown that in this population, the main class of cholesterol-lowering drugs (statins) helps prevent recurrences and mortality, with an absolute risk reduction of around 4%. As a result, these drugs are now part of the standard therapeutic arsenal to treat anyone who has survived a coronary event or who has stable coronary heart disease.
The same goes for people with familial hypercholesterolemia (HF), a genetic disorder that exposes individuals to high levels of LDL cholesterol from birth and to a high risk for cardiovascular events before they even turn 40. A study has just recently shown that HF children who were treated with statins at an early age had a much lower incidence of cardiovascular events in adulthood (1% vs. 26%) than their parents who had not been treated early with statins.
However, the decision to treat high cholesterol is much more difficult for people who do not have these risk factors. Indeed, when the risk of cardiovascular events over the next 10 years is low or moderate, the guidelines tolerate much higher LDL and non-HDL cholesterol levels than in people at risk: for example, when we usually try to keep LDL cholesterol below 2 mmol/L for people at high risk, a threshold twice as high (5 mmol/L) is proposed before treating people at low risk (Table 1). In this population, there is therefore a great deal of room for maneuver in deciding whether or not to start pharmacological treatment or to fundamentally change lifestyle habits (diet, exercise) to normalize these cholesterol levels.
Table 1. Canadian Cardiovascular Society guidelines for dyslipidemia treatment thresholds. *FRS = Framingham Risk Score. Adapted from Anderson et al. (2016).
This decision is particularly difficult for young adults, who are generally considered to be at low risk of cardiovascular events over the next 10 years (age is one of the main factors used for risk assessment and therefore the younger you are, the lower the risk). On the one hand, a young person, say in their early forties, who has above-normal LDL or non-HDL cholesterol, but without exceeding the recommended thresholds and without presenting other risk factors, probably does not have a major risk of being affected by a short-term cardiovascular event. But given their young age, they may be exposed to this excess cholesterol for many years and their risk of cardiovascular disease may become higher than average once they turn 70 or 80.
Recent studies indicate that it would be wrong to overlook this long-term negative impact of higher-than-normal non-HDL cholesterol. For example, it has been shown that an increase in non-HDL cholesterol at a young age (before age 40) remains above normal for the following decades and increases the risk of cardiovascular disease by almost 4 times. Another study that followed for 25 years a young population (average age of 42 years) who presented a low risk of cardiovascular disease at 10 years (1.3%) obtained similar results: compared to people with normal non-HDL cholesterol (3.3 mmol/L), those with non-HDL cholesterol above 4 mmol/L had an 80% increased risk of cardiovascular mortality. As shown in Table 1, these non-HDL cholesterol values are below the thresholds considered to initiate treatment in people at low risk, suggesting that hypercholesterolemia that develops at a young age, even if it is mild and not threatening in the short term, may nevertheless have longer-term adverse effects.
This concept has just been confirmed by a very large study involving nearly 400,000 middle-aged people (average age 51) who were followed for a median period of 14 years (maximum 43 years). The results show a significant increase as a function of time in the risk of cardiovascular disease based on non-HDL cholesterol levels: compared to the low category (<2.6 mmol/L), the risk increases by almost 4 times for non-HDL cholesterol ≥ 5.7 mmol/L, as much in women (increase from 8% to 34%) as in men (increase from 13% to 44%) (Figure 1).
Figure 1. Increased incidence of cardiovascular disease based on non-HDL cholesterol levels. From Brunner et al. (2019).
The largest increase in risk associated with higher non-HDL cholesterol levels was observed in people who were under 45 years of age at the beginning of the study (risk ratio of 4.3 in women and 4.6 in men for non-HDL cholesterol ≥5.7 mmol/L vs. the reference value of 2.6 mmol/L) (Figure 2). In older people (60 years or more), these risk ratios are much lower (1.4 in women and 1.8 in men), confirming that it is prolonged exposure (for several decades) to high levels of non-HDL cholesterol that plays a major role in increasing the risk of cardiovascular disease.
Figure 2. Age-specific and sex-specific association of non-HDL cholesterol and cardiovascular disease. From Brunner et al. (2019).
According to the authors, there would therefore be great benefits in reducing non-HDL cholesterol levels as soon as possible to limit the long-term exposure of blood vessels to atherogenic particles and thus reduce the risk of cardiovascular events. An estimate based on the results obtained indicates that in people 45 years of age and under who have above-normal non-HDL cholesterol levels (3.7–4.8 mmol/L) and other risk factors (e.g. hypertension), a 50% reduction in this type of cholesterol would reduce the risk of cardiovascular disease at age 75 from 16% to 4% in women and from 29% to 6% in men. These significant reductions in long-term risk therefore add a new dimension to the prevention of cardiovascular disease: it is no longer only the presence of high cholesterol levels which must be considered, but also the duration of exposure to excess cholesterol.
What to do if your cholesterol is high
If your short-term risk of cardiovascular accident is high, for example, because you suffer from familial hypercholesterolemia or you combine several risk factors (heredity of early coronary artery disease, hypertension, diabetes, abdominal obesity), it is certain that your doctor will insist on prescribing a statin if your cholesterol is above normal.
For people who do not have these risk factors, the approach that is generally recommended is to modify lifestyle habits, particularly in terms of diet and physical activity. Several of these modifications have rapidly measurable impacts on non-HDL cholesterol levels: weight loss for obese or overweight people, replacing saturated fat with sources of monounsaturated fat (olive oil, for example) and omega-3 polyunsaturated fats (fatty fish, nuts and seeds), an increase in the consumption of soluble fibres, and the adoption of a regular physical activity program. This roughly corresponds to the Mediterranean diet, a diet that has repeatedly been associated with a decreased risk of several chronic diseases, particularly cardiovascular disease.
The advantage of adopting these lifestyle habits is that not only do they help normalize cholesterol levels, but they also have several other beneficial effects on cardiovascular health and health in general. Despite their well-documented clinical utility, randomized clinical studies indicate that statins fail to completely reduce the risk of cardiovascular events, both in primary and secondary prevention. This is not surprising, since atherosclerosis is a multifactorial disease, which involves several phenomena other than cholesterol (chronic inflammation in particular). This complexity means that no single drug can prevent cardiovascular disease alone. And it is only by adopting a comprehensive approach based on a healthy lifestyle that we can make significant progress in preventing these diseases.
The old debate over whether egg consumption is detrimental to cardiovascular health has been revived since the recent publication of a study that finds a significant, albeit modest, association between egg or dietary cholesterol consumption and the incidence of cardiovascular disease (CVD) and all-cause mortality. Eggs are an important food source of cholesterol: a large egg (≈50 g) contains approximately 186 mg of cholesterol. The effect of eggs and dietary cholesterol on health has been the subject of much research over the last five decades, but recently it has been assumed that this effect is less important than previously thought. For example, the guidelines of medical and public health organizations have in recent years minimized the association between dietary cholesterol and CVD (see the 2013 AHA/ACC Lifestyle Guidelines and the 2015–2020 Dietary Guidelines for Americans). In 2010, the American guidelines recommended consuming less than 300 mg of cholesterol per day; however, the most recent recommendations (2014–2015) do not specify a daily limit. This change stems from the fact that cholesterol intake from eggs or other foods has not been shown to increase blood levels of LDL-cholesterol or the risk of CVD, as opposed to the dietary intake of saturated fat that significantly increases LDL cholesterol levels, a significant risk of CVD.
Some studies have reported that dietary cholesterol increases the risk of CVD, while others reported a decrease in risk or no effect with high cholesterol consumption. In 2015, a systematic review and meta-analysis of prospective studies was unable to draw conclusions about the risk of CVD associated with dietary cholesterol, mainly because of heterogeneity and lack of methodological rigour in the studies. The authors suggested that new carefully adjusted and rigorously conducted cohort studies would be useful in assessing the relative effects of dietary cholesterol on the risk of CVD.
What distinguishes the study recently published in JAMA from those published previously is its great methodological rigour, in particular a more rigorous categorization of the components of the diet, which makes it possible to isolateindependent relationships between the consumption of eggs or cholesterol from other sources and the incidence of CVD. The cohorts were also carefully harmonized, and several fine analyses were performed. The data came from six U.S. cohorts with a total of 29,615 participants who were followed for an average of 17.5 years.
The main finding of the study is that greater consumption of eggs or dietary cholesterol (including eggs and meat) is significantly associated with a higher risk of CVD and premature mortality. This association has a dose-response relationship: for every additional 300 mg of cholesterol consumed daily, the risk of CVD increases by 17% and that of all-cause mortality increases by 18%. Each serving of ½ egg consumed daily is associated with an increased risk of CVD of 6% and an increased risk of all-cause mortality of 8%. On average, an American consumes 295 mg of cholesterol every day, including 3 to 4 eggs per week. The model used to achieve these results took into account the following factors: age, gender, race/ethnicity, educational attainment, daily energy intake, smoking, alcohol consumption, level of physical activity, use of hormone therapy. These adjustments are very important when you consider that egg consumption is commonly associated with unhealthy behaviours such as smoking, physical inactivity and unhealthy eating. These associations remain significant after additional adjustments to account for CVD risk factors (e.g. body mass index, diabetes, blood pressure, lipidemia), consumption of fat, animal protein, fibre and sodium.
A review of this study suggests that the association between cholesterol and the incidence of CVD and mortality may be due in part to residual confounding factors. The authors of this review believe that health-conscious people reported eating fewer eggs and cholesterol-containing foods than they actually did. Future studies should include “falsification tests” to determine whether a “health consciousness” factor is the cause of the apparent association between dietary cholesterol and CVD risk.
Eggs, TMAO and atherosclerosis
A few years ago, a metabolomic approach identified a compound in the blood, trimethylamine-N-oxide (TMAO), which is associated with increased cardiovascular risks. TMAO is formed from molecules from the diet: choline, phosphatidylcholine (lecithin) and carnitine. Bacteria present in the intestinal flora convert these molecules into trimethylamine (TMA), then the TMA is oxidized to TMAO by liver enzymes called flavin monooxygenases. The main dietary sources of choline and carnitine are red meat, poultry, fish, dairy products and eggs (yolks). Eggs are an important source of choline (147 mg/large egg), an essential nutrient for the liver, muscles and normal foetal development, among others.
A prospective study indicated that elevated plasma concentrations of TMAO were associated with a risk of major cardiac events (myocardial infarction, stroke, death), independent of traditional risk factors for cardiovascular disease, markers of inflammation, and renal function. It has been proposed that TMAO promotes atherosclerosis by increasing the number of macrophage scavenger receptors, which carry oxidized LDL (LDLox) to be degraded within the cell, and by stimulating macrophage foam cells (i.e. filled with LDLox fat droplets), which would lead to increased inflammation and oxidation of cholesterol that is deposited on the atheroma plaques. A randomized controlled study indicates that the consumption of 2 or more eggs significantly increases the TMAO in blood and urine, with a choline conversion rate to TMAO of approximately 14%. However, this study found no difference in the blood levels of two markers of inflammation, LDLox and C-reactive protein (hsCRP).
Not all experts are convinced that TMAO contributes to the development of CVD. A major criticism is focused on fish and seafood, foods that may contain significant amounts of TMAO, but are associated with better cardiovascular health. For example, muscle tissue in cod contains 45–50 mmol TMAO/kg. For comparison, the levels of choline, a precursor of TMAO, are 24 mmol/kg in eggs and 10 mmol/kg in red meat. The only sources of choline that are equivalent to that in TMAO in marine species are beef and chicken liver. TMAO contained in fish and seafood is therefore significantly more important quantitatively than TMAO that can be generated by the intestinal flora from choline and carnitine from red meat and eggs. This was also measured: plasma levels of TMAO are much higher in people who have a fish-based diet (> 5000 μmol / L) than in people who eat mostly meat and eggs (139 μmol / L). In their response to this criticism, the authors of the article point out that not all fish contain the same amounts of TMAO and that many (e.g. sea bass, trout, catfish, walleye) do not contain any. Fish that contain a lot of TMAO are mainly deep-sea varieties (cod, haddock, halibut). The TMAO content of other fish, including salmon, depends on the environment and when they are caught.
Other experts believe this could be a case of reverse causality: the reduction in renal function associated with atherosclerosis could lead to an accumulation of TMAO, which would mean that this metabolite is a marker and not the cause of atherosclerosis. To which the authors of the hypothesis counter that the high concentration of TMAO is associated with a higher risk of cardiovascular events even when people have completely normal kidney function.
Diabetes and insulin resistance
People who are overweight (BMI> 25) and obese (BMI> 50) are at higher risk of becoming insulin resistant and having type 2 diabetes and metabolic syndrome, conditions that can, independently or in combination, lead to the development of cardiovascular disease. There is evidence that dietary cholesterol may be more harmful to diabetics. Intestinal absorption of cholesterol is impaired in diabetics, i.e. it is increased. However, in a randomized controlled trial, when diabetic patients consumed 2 eggs per day, 6 times per week, their lipid profile was not altered when their diet contained mono- and polyunsaturated fatty acids. Other studies (mostly subsidized by the egg industry) suggest that eggs are safe for diabetics.
Dr. J. David Spence of the Stroke Prevention & Atherosclerosis Research Center believes that people at risk for CVD, including diabetics, should avoid eating eggs (see also this more detailed article). This expert in prevention argues that it is the effects of lipids after a meal that matter, not fasting lipid levels. Four hours after a meal high in fat and cholesterol, harmful phenomena such as endothelial dysfunction, vascular inflammation and oxidative stress are observed. While egg whites are unquestionably a source of high-quality protein, egg yolks should not be eaten by people with cardiovascular risks or genetic predispositions to heart disease.
The association between the consumption of eggs or foods containing cholesterol and the risk of CVD is modest. But since this risk increases with the amount consumed, people who eat a lot of eggs or foods containing cholesterol have a significant risk of harming their cardiovascular health. For example, according to the study published in JAMA, people who consume two eggs per day instead of 3 or 4 per week have a 27% higher risk of CVD and a 34% higher risk of premature mortality. It is therefore prudent to minimize the consumption of eggs (less than 3 or 4 eggs per week) and meat in order to limit the high intake of cholesterol and choline and avoid promoting atherosclerosis.
The role of dietary fat in the development of obesity, cardiovascular disease and type 2 diabetes has been the subject of vigorous scientific debate for several years. In an article recently published in the prestigious Science, four experts on dietary fat and carbohydrate with very different perspectives on the issue (David Ludwig, Jeff Volek, Walter Willett, and Marian Neuhouser) identified 5 basic principles widely accepted in the scientific community and that can be of great help for non-specialists trying to navigate this issue.
This summary is important as the public is constantly bombarded with contradictory claims about the benefits and harmful effects of dietary fat. Two great, but diametrically opposed currents have emerged over the last few decades:
- The classic low-fat position, i.e., reducing fat intake, adopted since the 1980s by most governments and medical organizations. This approach is based on the fact that fats are twice as caloric as carbohydrates (and therefore more obesigenic) and that saturated fats increase LDL cholesterol levels, a major risk factor for cardiovascular disease. As a result, the main goal of healthy eating should be to reduce the total fat intake (especially saturated fat) and replace it with carbohydrate sources (vegetables, bread, cereals, rice and pasta). An argument in favour of this type of diet is that many cultures that have a low-fat diet (Okinawa’s inhabitants, for example) have exceptional longevity.
- The low-carb position, currently very popular as evidenced by the ketogenic diet, advocates exactly the opposite, i.e., reducing carbohydrate intake and increasing fat intake. This approach is based on several observations showing that increased carbohydrate consumption in recent years coincides with a phenomenal increase in the incidence of obesity in North America, suggesting that it is sugars and not fats that are responsible for excess weight and the resulting chronic diseases (cardiovascular disease, type 2 diabetes, some cancers). One argument in favour of this position is that an increase in insulin in response to carbohydrate consumption can actually promote fat accumulation and that low-carb diets are generally more effective at promoting weight loss, at least in the short term.
Reaching a consensus from two such extreme positions is not easy! Nevertheless, when we look at different forms of carbohydrates and fat in our diet, the reality is much more nuanced, and it becomes possible to see that a number of points are common to both approaches. By critically analyzing the data currently available, the authors have managed to identify at least five major principles they all agree on:
1) Eating unprocessed foods of good nutritional quality helps to stay healthy without having to worry about the amount of fat or carbohydrate consumed.
A common point of the low-fat and low-carb approaches is that each one is convinced it represents the optimal diet for health. In fact, a simple observation of food traditions around the world shows that there are several food combinations that allow you to live longer and be healthy. For example, Japan, France and Israel are the industrialized countries with the two lowest mortality rates from cardiovascular disease (110, 126 and 132 deaths per 100,000, respectively) despite considerable differences in the proportion of carbohydrates and fat from their diet.
It is the massive influx of ultra-processed industrial foods high in fat, sugar and salt that is the major cause of the obesity epidemic currently affecting the world’s population. All countries, without exception, that have shifted their traditional consumption of natural foods to processed foods have seen the incidence of obesity, type 2 diabetes, and cardiovascular disease affecting their population increase dramatically. The first step in combating diet-related chronic diseases is therefore not so much to count the amount of carbohydrate or fat consumed, but rather to eat “real” unprocessed foods. The best way to do this is simply to focus on plant-based foods such as fruits, vegetables, legumes and whole-grain cereals, while reducing those of animal origin and minimizing processed industrial foods such as deli meats, sugary drinks, and other junk food products.
2) Replace saturated fat with unsaturated fat.
The Seven Countries Study showed that the incidence of cardiovascular disease was closely correlated with saturated fat intake (mainly found in foods of animal origin such as meats and dairy products). A large number of studies have shown that replacing these saturated fats with unsaturated fats (e.g., vegetable oils) is associated with a significant reduction in the risk of cardiovascular events and premature mortality. A reduction in saturated fat intake, combined with an increased intake of high quality unsaturated fat (particularly monounsaturated and omega-3 polyunsaturated), is the optimal combination to prevent cardiovascular disease and reduce the risk of premature mortality.
These benefits can be explained by the many negative effects of an excess of saturated fat on health. In addition to increasing LDL cholesterol levels, an important risk factor for cardiovascular disease, a high intake of saturated fat causes an increase in the production of inflammatory molecules, an alteration of the function of the mitochondria (the power plants of the cell), and a disturbance of the normal composition of the intestinal microbiome. Not to mention that the organoleptic properties of a diet rich in saturated fats reduce the feeling of satiety and encourage overconsumption of food and accumulation of excess fat, a major risk factor for cardiovascular disease, type 2 diabetes and some cancers.
3) Replace refined carbohydrates with complex carbohydrates.
The big mistake of the “anti-fat crusade” of the ’80s and ’90s was to believe that any carbohydrate source, even the sugars found in processed industrial foods (refined flours, added sugars), was preferable to saturated fats. This belief was unjustified, as subsequent studies have demonstrated beyond a doubt that these refined sugars promote atherosclerosis and can even triple the risk of cardiovascular mortality when consumed in large quantities. In other words, any benefit that can come from reducing saturated fat intake is immediately countered by the negative effect of refined sugars on the cardiovascular system. On the other hand, when saturated fats are replaced by complex carbohydrates (whole grains, for example), there is actually a significant decrease in the risk of cardiovascular events.
Another reason to avoid foods containing refined or added sugars is that they have low nutritional value and cause significant variations in blood glucose and insulin secretion. These metabolic disturbances promote excess weight and the development of insulin resistance and dyslipidemia, conditions that significantly increase the risk of cardiovascular events. Conversely, increased intake of complex carbohydrates in whole-grain cereals, legumes, and other vegetables helps keep blood glucose and insulin levels stable. In addition, unrefined plant foods represent an exceptional source of vitamins, minerals and antioxidant phytochemicals essential for maintaining health. Their high fibre content also allows the establishment of a diverse intestinal microbiome, whose fermentation activity generates short-chain fatty acids with anti-inflammatory and anticancer properties.
4) A high-fat low-carb diet may be beneficial for people who have disorders of carbohydrate metabolism.
In recent years, research has shown that people who have normal sugar metabolism may tolerate a higher proportion of carbohydrates, while those with glucose intolerance or insulin resistance may benefit from adopting a low-carb diet richer in fat. This seems particularly true for people with diabetes and prediabetes. For example, an Italian study of people with type 2 diabetes showed that a diet high in monounsaturated fat (42% of total calories) was more effective in reducing the accumulation of fat in the liver (a major contributor to the development of type 2 diabetes) than a diet low in fat (28% of total calories).
These benefits seem even more pronounced for the ketogenic diet, in which the consumption of carbohydrates is reduced to a minimum (<50 g per day). Studies show that in people with a metabolic syndrome, this type of diet can generate a fat loss (total and abdominal) greater than a hypocaloric diet low in fat, as well as a higher reduction of blood triglycerides and several markers of inflammation. In people with type 2 diabetes, a recent study shows that in the majority of patients, the ketogenic diet is able to reduce the levels of glycated haemoglobin (a marker of chronic hyperglycaemia) to a normal level, and this without drugs other than metformin. Even people with type 1 diabetes can benefit considerably from a ketogenic diet: a study of 316 children and adults with this disease shows that the adoption of a ketogenic diet allows an exceptional control of glycemia and the maintenance of excellent metabolic health over a 2-year period.
5) A low-carb or ketogenic diet does not require a high intake of proteins and fats of animal origin.
Several forms of low carbohydrate or ketogenic diets recommend a high intake of animal foods (butter, meat, charcuteries, etc.) high in saturated fats. As mentioned above, these saturated fats have several negative effects (increase of LDL, inflammation, etc.), and one can therefore question the long-term impact of this type of low-carb diet on the risk of cardiovascular disease. Moreover, a study recently published in The Lancet indicates that people who consume little carbohydrates (<40% of calories), but a lot of fat and protein of animal origin, have a significantly increased risk of premature death. For those wishing to adopt a ketogenic diet, it is therefore important to realize that it is quite possible to reduce the proportion of carbohydrates in the diet by substituting cereals and other carbohydrate sources with foods rich in unsaturated fats like vegetable oils, vegetables rich in fat (nuts, seeds, avocado, olives) as well as fatty fish.
In short, the current debate about the merits of low-fat and low-carb diets is not really relevant: for the vast majority of the population, several combinations of fat and carbohydrate make it possible to remain in good health and at low risk of chronic diseases, provided that these fats and carbohydrates come from foods of good nutritional quality. It is the overconsumption of ultra-processed foods, high in fat and refined sugars, which is responsible for the dramatic rise in food-related diseases, particularly obesity and type 2 diabetes. Restricting the consumption of these industrial foods and replacing them with “natural” foods, especially those of plant origin, remains the best way to reduce the risk of developing these diseases. On the other hand, for overweight individuals with metabolic syndrome or type 2 diabetes, currently available scientific evidence suggests that a reduction in carbohydrate intake by adopting low-carb and ketogenic diets could be beneficial.
Berries are becoming increasingly popular in our diet, whether consumed fresh, frozen, dried or canned, and in related products such as jams, jellies, yogurts, juices and wines. Berries provide significant health benefits because of their high content of phenolic compounds, antioxidants, vitamins, minerals and fibres. Recognizing these health benefits has recently led to a 21% increase in world berry production.
The generic term “berries” is sometimes used to refer to small fruits, but from a botanical point of view, if some berries are genuineberries (blueberries, bilberries, cranberries, currants, lingonberries, elderberries), others are polydrupes (raspberries, blackberries), and the strawberry is a “false fruit” since the achenes (the small seeds on the outer surface of the strawberry) are the actual fruits of the strawberry. Berry fruits are rich in phenolic compounds such as phenolic acids, stilbenes, flavonoids, lignans and tannins (see the classification and structure of these compounds in Figure 1). Berries are particularly rich in anthocyanidins, pigments that give the skin and flesh of these fruits their distinctive red, blue or purple colour (Table 1).
Figure 1. Classification and chemical structure of phenolic compounds contained in berries. Adapted from Parades-López et al., 2010 and Nile & Park, 2014.
Like most flavonoids, anthocyanidins are found in nature as glycosides (compounds made of a sugar and another molecule) called anthocyanins. These anthocyanins can be absorbed in their whole form (linked to different sugars) both in the stomach and in the intestine. Anthocyanins that reach the large intestine can be metabolized by the microbiota (intestinal flora). The maximum concentration of anthocyanins in the bloodstream is reached from 30 minutes to 2 hours after eating berries. However, the maximum plasma concentration (1–100 nmol/L) of anthocyanins is much lower than what is measured in intestinal tissues, indicating that these compounds are metabolized extensively before entering the systemic circulation as metabolites. After administering a radiolabelled anthocyanin to humans, 35 metabolites were identified, 17 in blood, 31 in urine and 28 in feces. Thus, it is likely that these metabolites, rather than the intact molecule, are responsible for the health benefits associated with anthocyanins.
Table 1. Content of phenolic compounds, flavonoids, and anthocyanins of different berries. Adapted from Parades-López et al., 2010 and Nile & Park, 2014.
|Berries (genus and species)||Phenolic compounds||Flavonoids||Anthocyanins
|(mg/100 g fresh fruit)||(mg/100 g fresh fruit)||(mg/100 g fresh fruit)
|Raspberry (Rubus ideaous)||121||6||99
|Blackberry (Rubus fruticosus)||486||276||82–326
|Strawberry (Fragaria x. ananassa)||313||–||54
|Blueberry (Vaccinium corymbosum)||261–585||50||25–495
|Bilberry (Vaccinium myrtillus )||525||44||300
|Cranberry (Vaccinium macrocarpon)||315||157||67–140
|Redcurrant (Ribes rubrum)||1400||9||22
|Blackcurrant (Ribes nigrum)||29-60||46||44
|Elderberry (Sambucus nigra)||104||42||45-791
|Red cranberry (Vitis vitis-idea)||652||74||77
Biological activities of berries
Data from in vitro and animal experimental models indicate that the phenolic compounds in berries may produce their beneficial effects through their antioxidant, anti-inflammatory, antihypertensive, and lipid-lowering activities, which could prevent or mitigate atherosclerosis. Perhaps the best-known of the biological activities of phenolic compounds is their antioxidant activity, which helps protect the body’s cells from damage caused by free radicals and counteract certain chronic diseases associated with aging. According to several studies using in vitro and animal models, berries also have anti-cancer properties involving several complementary mechanisms such as induction of metabolic enzymes, modulation of the expression of specific genes and their effects on cell proliferation, apoptosis (programmed cell death, an unsettled process in cancer cells), and signalling pathways inside the cell.
In a prospective study conducted in China with 512,891 participants, daily consumption of fruit (all types of fruit) was associated with an average decrease in systolic blood pressure of 4.0 mmHg on average, a decrease of 0.5 mmol/L of blood glucose concentration, a 34% reduction in the risk of major coronary events and a 40% reduction in the risk of cardiovascular mortality. These results were obtained by comparing participants who ate fruits daily to those who did not consume them at all or very rarely. In this study, there was a strong dose-response correlation between the incidence of cardiovascular events or cardiovascular mortality and the amount of fruit consumed. Studies suggest that among the constituents of fruit, it is the flavonoids, and especially the anthocyanins, that are responsible for these protective effects.
A number of prospective and cross-sectional studies have examined the association between the consumption of anthocyanins and cardiovascular risk factors (see this review). In four out of five studies that examined the risks of coronary heart disease or nonfatal myocardial infarction, anthocyanin consumption was associatedwith a reduction in coronary artery disease risk from 12% to 32%. The impact of anthocyanins on the risk of stroke was investigated in 5 studies, but no evidence of a protective effect was found in this case.
With respect to cardiovascular risk factors, studies indicate that higher consumption of anthocyanins is associated with decreased arterial stiffness, arterial pressure, and insulinemia. The decrease in blood pressure associated with the consumption of anthocyanins, -4 mmHg, is similar to that seen in a person after quitting smoking. The effect of anthocyanins on insulin concentration, an average reduction of 0.7 mIU/L, is similar to the effects of a low-fat diet or a one-hour walk per day. A decrease in inflammation has been associated with the consumption of anthocyanins and flavonols, a mechanism that may underlie the reduction of cardiovascular risk and other chronic diseases.
Randomized controlled trials
A systematic review and meta-analysis of 22 randomized controlled trials, representing 1,251 people, report that berry consumption significantly reduces several cardiovascular risk factors, such as blood LDL cholesterol [-0.21 mmol/L on average], systolic blood pressure [-2.72 mmHg on average], fasting glucose concentration [-0.10 mmol/L on average], body mass index [-0.36 kg/m2on average], glycated haemoglobin [HbA1c, -0.20% on average], and tumour necrosis factor alpha [TNF-alpha, 0.99 pg/mL on average], a cytokine involved in systemic inflammation. In contrast, no significant changes were observed for the other markers of cardiovascular disease that were tested: total cholesterol, HDL cholesterol, triglycerides, diastolic blood pressure, ApoAI, ApoB, Ox-LDL, IL-6, CRP, sICAM-1,and sICAM-2.
Another systematic review published in 2018 evaluated randomized controlled trials [RCTs] on the effects of berry consumption on cardiovascular health. Among the 17 high-quality RCTs, 12 reported a beneficial effect of berry consumption on cardiovascular and metabolic health markers. Four out of eleven RCTs reported a reduction in systolic and/or diastolic blood pressure; 3/7 studies reported a favourable effect on endothelial function; 2/3 studies reported an improvement in arterial stiffness; 7/17 studies reported beneficial effects for the lipid balance; and 3/6 studies reported an improvement in the glycemic profile.
Berries and cognitive decline
Greater consumption of blueberries and strawberries was associated with a slowdown in cognitive decline in a prospective study of 16,010 participants in the Nurses’ Health Study aged 70 or older. Consumption of berries was associated with delayed cognitive decline of approximately 2.5 years. In addition, nurses who had consumed more anthocyanidins and total flavonoids had a slower cognitive decline than participants who consumed less.
The exceptional content of phenolic compounds in berries and their positive effects on health remind us that the quality of food is not just about nutrients: proteins, carbohydrates, lipids, vitamins and minerals; a wide variety of other molecules found in plants are absorbed from the intestines and routed through the bloodstream to all cells in the body. While not essential nutrients, phytochemicals such as flavonoids can contribute to better cardiovascular health and healthier aging.