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.
Red meat: An issue for human health and the health of the planet.
Consumption of red meat and processed meat is associated with an increase in all-cause mortality and mortality from cardiovascular diseases, diabetes, respiratory diseases, liver and kidney diseases, and certain cancers. On the contrary, consumption of white meat and fish has been associated with a decreased risk of premature death. Another troubling aspect with the production of red meat is that it is harmful to the global environment.
In traditional European agricultural societies, meat was consumed once or less than once a week, and annual meat consumption rarely exceeded 5 to 10 kg per person. In some rich countries (U.S.A., Australia, New Zealand), meat consumption now stands at 110–120 kg per person per year, > 10 times more than in traditional agricultural societies. Livestock farming occupies more than 30% of the world’s land area, and more than 33% of arable land is used to produce livestock feed. World consumption of red meat is rising sharply, especially in developing countries. This has adverse consequences for the environment and represents an unsustainable situation according to several experts.
The main harmful effects to our planet caused by meat production (Potter, BMJ 2017)
- Depletion of aquifers (producing 1 kg of meat requires more than 110,000 L of water).
- Groundwater pollution.
- Decrease of biodiversity.
- Destruction of rainforest for livestock and the production of greenhouse gases by livestock. Both combined contribute more to climate change than fossil fuels used for transport.
- Production of 37% of methane (CH4) from human activity (with 23 times the global warming
potential of CO2).
- Production of 65% of nitrous oxide (N2O) from human activity (almost 300 times the global
warming potential of CO2).
- Production of 64% of ammonia (NH3) from human activity, which contributes significantly to
acid rain and acidification of the ecosystem.
Other potential negative effects associated with red meat include accelerated sexual development, caused either by the consumption of meat and fat, or by the intake of growth hormones naturally present in meat or added to livestock feed; more extensive antibiotic resistance caused by their use to promote animal growth; a reduction in the food available for human consumption (for example, 97% of the world’s soybeans are used to feed livestock); and higher risks of infections (such as bovine spongiform encephalopathy or “mad cow disease”) due to faulty practices in intensive farming.
Experts agree that we will have to reduce our consumption of red and processed meat in order to live longer, healthier lives, but especially so that our planet is in better condition and can support human activity long term. Eating mostly cereals, fruits, vegetables, nuts, and legumes, and little or no meat is probably the ideal solution to this environmental problem, but for many, red meat is a delicious food that is hard to replace. To satisfy meat lovers who still want to reduce their consumption, companies have recently developed products made only from plants whose appearance, texture and taste are similar to meat, whereas others are trying to produce artificial meat from in vitro cell cultures.
New plant-based patties: Beyond Burger and Impossible Burger
Plant-based burgers have long been available in grocery stores, but these meatless products are intended for vegetarians and consumed mainly by them. New products made from plants, but designed to have the same appearance, texture, and taste as meat have appeared on the market recently. These meat alternatives target omnivorous consumers who want to reduce their meat consumption. Among the most popular products, there is the Beyond Burger, available at the fast food chain A&W and recently in most supermarkets in Quebec, as well as the Impossible Burger, which will soon be on the menu at fast food chain Burger King under the name “Impossible Whopper.”
The main ingredients of Beyond Burger are pea protein isolate, canola oil and refined coconut oil. This food also contains 2% or less of other ingredients used to create a meat-like texture, colour and flavour, as well as natural preservatives (see box). It is an ultra-processed food that does not contain cholesterol, but almost as much saturated fat (from coconut oil) and 5.5 times more sodium than a lean beef patty. Nutrition and public health experts have suggested avoiding coconut oil in order not to increase blood LDL cholesterol (“bad cholesterol”) and maintain good cardiovascular health (see “Saturated fats, coconut oil and cardiovascular disease”). Moreover, the nutritional contribution of these two products is similar (calories, proteins, total lipids).
Beyond Burger ingredients:
Water, pea protein isolate, canola oil, refined coconut oil, 2% or less of: cellulose from bamboo, methylcellulose, potato starch, natural flavour, maltodextrin, yeast extract, salt, sunflower oil, vegetable glycerine, dried yeast, gum arabic, citrus extract, ascorbic acid, beet juice extract, acetic acid, succinic acid, modified food starch, annatto.
Impossible Burger ingredients: Water, soy protein concentrate, coconut oil, sunflower oil, natural flavours, 2% or less of: potato protein, methylcellulose, yeast extract, dextrose, food starch modified, soy leghemoglobin, salt, soy protein isolate, mixed tocopherols (Vitamin E), zinc gluconate, thiamine hydrochloride (vitamin B1), sodium ascorbate (vitamin C), niacin (vitamin B3), pyridoxine hydrochloride (vitamin B6), riboflavin (vitamin B2), vitamin B12.
The Impossible Burger is made from soy protein, coconut oil and sunflower oil. It also contains ingredients that are used to create a meat-like texture, colour and flavour, as well as vitamins and natural preservatives. Among the ingredients added to mimic the colour and flavour of meat is soy leghemoglobin, a hemoprotein found in the nodules on the roots of legumes that has a similar structure to animal myoglobin. Rather than extracting this protein from the roots of soybean plants, the manufacturer uses leghemoglobin produced by yeast (Pichia pastoris) in which the DNA encoding for this protein has been introduced. The use of P. pastoris soybean leghemoglobin was approved by the US Food and Drug Administration in 2018. The fact that the leghemoglobin used is a product of biotechnology rather than from a natural source does not appear to pose a particular problem, but some researchers suspect that the heme it contains could have the same negative health effects as those associated with the consumption of red meat, i.e., an increased risk of cardiovascular disease and certain types of cancer. A causal link between heme and these diseases has not been established, but population studies (see here and here) indicate that there is a significant association between heme consumption and a rise (19%) in mortality risk from all causes. In contrast, non-heme iron from food (vegetables and dairy products) is not associated with an increased risk of mortality.
Beyond Burger and Impossible Whopper, served with mayonnaise and white bread, are not suitable for vegans (eggs in mayonnaise) or a particularly healthy option because of the saturated fat and salt they contain. However, the manufacture of these products requires much less energy and has a much smaller environmental footprint than real red meat, which is their strong selling point. According to one study, the production of a Beyond Burger patty generates 90% less greenhouse gas emissions and requires 46% less energy, 99% less water and 93% less arable land than a beef patty.
We believe that it is preferable, as much as possible, to obtain unprocessed fresh plant products and to do the cooking yourself, in order to control all the ingredients and thus avoid ingesting sodium or saturated fat in excessive amounts, as is the case with most ultra-processed products, including these new meatless patties. Fatty and salty foods taste good to a large majority of human beings, and the food industry takes this into account when designing the ultra-processed food products it offers on the market. If you want to eat a “burger” without meat, why not try to prepare it yourself with black beans (recipes here and here), oats, lentils or quinoa?
Production of “meat” in the laboratory
In vitro “meat” production involves culturing animal muscle cells (from undifferentiated cells or “stem cells”) in a controlled or laboratory environment. The first beef patty produced in a laboratory in 2013 cost 215,000 pounds (Can$363,000), but the price has dropped considerably since then. However, this product is not yet ready to be commercialized, as there are still several technological problems to solve before it can be produced on a large scale. Moreover, if the current experimental product can be used to successfully mimic ground meat, we are still far from being able to grow cells in a three-dimensional form that looks like a steak, for example.
The technology could be used to produce, for example, “Fugu” (puffer fish) meat, a delicacy prized by the Japanese, but which can be deadly if the chef or specialized companies do not prepare the fish properly. Indeed, tetradoxine contained in the liver, ovaries and skin of the fugu is a powerful paralyzing poison for which there is no antidote. Laboratory-made fugu meat would not contain any poison and would be safe for consumers.
Another example of an advantageous application would be the production of duck foie gras. A majority of the French (67%) are against the traditional method of production by gavage, which makes the animals suffer. One company (Supreme) is developing a method to obtain fatty liver from isolated duck egg cells.
Other companies are developing methods to produce egg white and milk proteins by fermentation rather than using animals. Although this “cellular agriculture” still seems a little “futuristic”, it could become increasingly important in the food industry and help reduce the production of meat that is harmful to our planet.
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.
We often hear that all plants are created equal in terms of positive impact on health, and that the important thing is simply to eat them as often as possible, without worrying about the nature of the fruits, vegetables or seeds consumed. In other words, essentially it would be the quantity that counts, and there would be no difference between eating iceberg lettuce or broccoli, or blueberries rather than a banana. This reductionist view is somewhat outdated, because we now know that there are enormous differences in the biochemical composition of plants, and some of them are in a class of their own for their content in molecules known to have positive effects on health. The inclusion of these foods in dietary habits could thus enhance the benefits associated with a diet rich in plants, especially in terms of prevention of cardiovascular disease.
Flaxseed is a good example of a food that is different from other plants because of certain unique characteristics. On the one hand, these seeds contain very high amounts of polyunsaturated fatty acids (72% of all fats), with three quarters of these fats being the omega-3 type. On the other hand, flaxseed is an exceptional source of lignans, a group of polyphenols with antioxidant and anti-inflammatory properties. In recent years, several studies have suggested that these properties may have several positive effects on cardiovascular health.
Linolenic acid: A fat like no other
Flax seeds are one of only two foods of plant origin (the other being chia seeds) that contain more omega-3 fatty acids (linolenic acid) than omega-6 (linoleic acid). For example, while the omega-3/omega-6 ratio is less than 1 for all commonly consumed vegetable oils, this ratio is 4 for flaxseed, up to 500 times higher than some vegetable sources (sunflower, for example) (Table 1).
Table 1. Proportion (%) of the different types of fatty acids present in various plant sources. Adapted from Dubois (2007).
This predominance of linolenic acid in flaxseed is very interesting because this omega-3 fatty acid has positive effects on several cardiovascular disease risk factors (lowering of LDL cholesterol, lowering of blood pressure), and also has anti-inflammatory and anti-arrhythmic properties. All of these properties may contribute to the decreased risk of cardiovascular events associated with linolenic acid consumption observed in several studies (see Table 2).
Table 2. Examples of studies reporting a positive effect of linolenic acid on cardiovascular health. From Rodriguez-Leyva (2010).
|Study||Number of participants||Main results
|Hu et al. (1999)||76,283 (women)||A higher intake of linolenic acid is associated with a decrease in fatal myocardial infarction.
|Albert et al. (2005)||76,283 (women)||40% reduction in risk of sudden cardiac death in people consuming the most linolenic acid
|Baylin et al. (2003)||964||The content of linolenic acid in adipose tissue is inversely associated with a decrease in the risk of infarction.
|Djoussé et al. (2001)||2,004||A diet rich in linolenic acid is associated with a lower incidence of coronary calcified atherosclerotic plaques.
|Dilecek et al. (1992)||12,866 (men)||A high intake of linolenic acid is associated with a reduced risk of cardiovascular mortality and all-cause mortality.
|Djoussé et al. (2005)||4,594||A high intake of linolenic acid is associated with a decrease in systolic pressure and the incidence of hypertension.
Linolenic acid can also be converted into DHA and EPA, two long-chain omega-3 fatty acids that have repeatedly been associated with a reduced risk of cardiovascular mortality (this conversion to DHA and EPA is quite low, in the vicinity of 5%, but is generally higher in women). Therefore, while it is now known that simply replacing saturated fats (from animal products) in our diet with unsaturated vegetable fats significantly reduces the risk of developing cardiovascular disease, this protection could be even more important when these unsaturated fats are omega-3.
The Lyon Diet Heart Study is one of the most important demonstrations of the potential of these plant-based omega-3 fatty acids in preventing cardiovascular disease. In this study, 605 myocardial infarction survivors were randomly separated into two groups, one placed on a low-fat diet as recommended by the American Heart Association, and the other on a Mediterranean diet including margarine enriched in linolenic acid (1.1 g/day). After a two-year follow-up, the incidence of cardiovascular disease, including cardiac mortality, decreased dramatically (73%) in the intervention group, raising the interesting possibility that the inclusion of linolenic acid in the diet can significantly improve cardiovascular health.
In primary prevention, most subsequent epidemiological studies have shown that people who have a high intake of linolenic acid are less likely to be affected by cardiovascular disease, a protective effect that has been observed as much in the United States (see here and here), as in Europe (Holland) and Central America (Costa Rica). A meta-analysis of 13 prospective studies indicates that an increased linolenic acid intake of 1 g per day is associated with a 10% reduction in the risk of cardiovascular disease, a protection confirmed by data analysis from 8 American and European studies comprising a total of 148,675 women and 80,368 men. The risk reduction offered by linolenic acid could even be much higher (60%) for people whose intake of long-chain omega-3 (DHA and EPA, found mainly in oily fish) is low (<0.1 g/day). In sum, most of the data collected to date suggests that increased linolenic acid intake is associated with reduced risk of cardiovascular events.
Lignans: Protective phytoestrogens
Another unique feature of flaxseed is its exceptional content of lignans, a group of complex phenolic compounds that have antioxidant and anti-inflammatory properties. While the majority of plants contain relatively low levels of lignans, these molecules are present in much higher quantities in flaxseed (300 mg/100 g), over 1,000 times more than in some commonly consumed foods (Table 3).
Table 3. Main dietary sources of lignans. From Peterson, 2010.
|Food||Lignans (mg/100 g)
|Rye bread (whole grain)||1.2
These lignans, mainly secoisolariciresinol and matairesinol, are metabolized by the intestinal microbiota to enterolactone and enterodiol, two molecules that have a weak estrogenic action (phytoestrogens). Since oestrogens exert a cardioprotective action (and would be responsible for the lower incidence of cardiovascular disease in women compared to men), it has been suggested that the estrogenic properties of lignans could help reduce the risk of cardiovascular events. Some epidemiological studies that focused on this issue found that this is indeed the case, i.e., that a higher intake of lignans or an increase in the blood level of enterolactone (produced by the metabolism of lignans) is associated with a reduced risk of cardiovascular events.
Flaxseed and cardiovascular disease
One of the main limitations of studies on the cardioprotective role of linolenic acid and lignans is the low content of these molecules in the traditional western diet. For example, salad dressings are the main source of linolenic acid in several epidemiological studies, while the restricted distribution of lignans in plants means that their intake may be below the threshold required to generate important cardiovascular effects. Therefore, the simultaneous presence of significant amounts of linolenic acid and lignans in flaxseed suggests that the addition of these seeds to dietary habits represents a simple (and economical) way to overcome these deficiencies and improve the beneficial impact of these two classes of molecules on cardiovascular health.
So far, the best-documented effect of flaxseed supplementation is on lowering blood pressure. For example, a randomized, double-blind, placebo-controlled study in Puerto Rico (FLAX-PAD) found that adding 30 grams of ground flaxseed to the diet resulted in a significant decrease in systolic (10 mmHg) and diastolic (7 mmHg) blood pressure. This reduction is even more pronounced in people who were hypertensive at the start of the study (>140 mmHg), with a 15 mmHg reduction in systolic pressure, a decrease even more pronounced than that obtained with the help of some antihypertensive drugs. A meta-analysis of 11 studies on the impact of flaxseed supplementation on blood pressure, however, suggests a more modest antihypertensive effect, with a decrease of approximately 2 mmHg for systolic and 1.2 mmHg for diastolic pressure. This may not seem like much, but studies show that a reduction in blood pressure of this order could decrease stroke mortality by 10% and coronary heart disease by 7%.
It also appears that flaxseed supplementation may lower LDL cholesterol levels, another important risk factor for cardiovascular disease. A meta-analysis of 28 studies showed that flaxseed caused an average decrease of 0.10 mmol/L and 0.08 mmol/L in total cholesterol and LDL cholesterol, respectively, this effect being particularly pronounced in women (- 0.24 mmol/L) and in people who had high cholesterol levels at the beginning of the procedure. A 15% reduction in LDL cholesterol was also observed in patients with lower extremity osteoarthritis, this decrease being added to that caused by statin.
In recent years, much emphasis has been placed on the importance of regularly consuming long-chain omega-3 fatty acids (DHA and EPA), mainly found in oily fish such as salmon, to reduce the risk of cardiovascular diseases. It should not be forgotten, however, that omega-3 of plant origin also have a protective role and that a high intake of foods rich in linolenic acid, such as flax seeds, can also help reduce the risk of cardiovascular events. In fact, a number of studies (here and here, for example) have reported that both types of omega-3 have complementary roles, and a combined increase in linolenic and long-chain omega-3 intake may be desirable for maximum protective effect.
In practical terms, an average daily intake of 2.2 g of linolenic acid is recommended, which corresponds to one tablespoon (15 ml) of flaxseed. It is essential to grind the seeds to increase the absorption of omega-3 fatty acids and allow the transformation of lignans into active phytoestrogens by intestinal bacteria. However, omega-3 fatty acids being very fragile and sensitive to degradation, one should buy whole seeds that can be ground when needed in a simple coffee grinder, and store the ground seeds for a maximum of two weeks in the refrigerator in an airtight container. The ground seeds have a slightly nutty flavour that goes well with cereals, yogurts, smoothies, and can even be used as a salad topping.
The American Heart Association (AHA) recently published a review of current knowledge on the association between dietary fats and cardiovascular disease. By taking into consideration the entirety of the available scientific evidence, the committee concluded that a lower intake in saturated fat and replacing it with unsaturated fat represents the optimal combination to reduce the incidence of heart disease in the population, especially if it is accompanied by a transition toward an overall healthy diet, such as the Mediterranean diet.
The positive impact of substituting saturated fat with unsaturated fat can mainly by explained by the opposite effects of these two types of fat on the LDL cholesterol level, a well-established risk factor for cardiovascular disease. Whereas saturated fat is associated with an increase of this cholesterol, and thus an increase in the risk of cardiovascular events, unsaturated fat leads to a decrease in LDL cholesterol levels in the blood and is associated with a significant decrease in mortality. Since animal-based protein sources (meat, dairy products, eggs) are the main sources of saturated fat in diets, whereas plant-based fats are mainly unsaturated, the mere act of reducing consumption of animal products while simultaneously increasing the consumption of plant-based foods is a very easy way to improve the quality of dietary fats, and thus reduce the risk of heart disease. In fact, it is interesting to note that several regions of the world known for their low incidence of cardiovascular disease (Okinawa, Japan; Ikaria, Greece; Sardinia; and the Tsimané people of the Amazon) all share one commonality: a diet high in plant-based foods with a low intake of animal protein and saturated fat.
Plant-based saturated fat
Whereas almost all plant-based fats mainly contain unsaturated fat, there is nevertheless one notable exception: tropical palm and coconut oils (see the Table). Indeed, palm oil (extracted from the fruit pulp) and palm kernel oil (derived from the kernels) contain very high levels of saturated fat (50% for palm oil and 82% for palm kernel oil), which gives them a semi-solid texture at room temperature. This property is used in the food industry to improve the texture of cookies, cakes and other products, and the high level of saturated fat also ensures that these oils are much more resistant to oxidation and considerably improves the shelf life of these foods. However, like all sources of saturated fat, these oils increase blood cholesterol levels and thus are not recommended for cardiovascular health. Not to mention the devastating environmental impact of the intensive cultivation of oil palm, in particular in Indonesia: almost two million hectares of tropical forest are destroyed every year for this crop, a deforestation that has disastrous environmental consequences and threatens animals such as tigers and orangutans from Sumatra and Borneo with extinction.
Table. Proportion of saturated, monounsaturated and polyunsaturated fat in different animal and plant-based fats.
|Source of fat||Saturated fat|
|Palm kernel oil||82||11||2
Coconut oil: saturated fat with positive effects?
Coconut oil is another plant-based source that contains a very high proportion of saturated fat (82%), but that, curiously, has gained a good reputation over the years. In fact, a recent survey reported that 72% of Americans consider coconut oil a “healthy” food! Two main characteristics of coconut oil, frequently mentioned in mainstream press, explain this popularity:
1) Population studies. Epidemiological studies conducted among populations that consume large quantities of coconut, such as the inhabitants of Polynesian islands like Tokelau or those of the Melanesian island Kitava, revealed a low incidence of heart disease, despite a high intake of saturated fat from this fruit. It should be noted, however, that it is the coconut meat, very high in fibre, that is consumed by these populations, so the lack of effect on cardiovascular disease cannot be extrapolated to that associated with adding purified coconut oil to Western diets, which contain a large proportion of processed foods.
2) Impact on cholesterol. The saturated fat in coconut oil has shorter chains than that found in palm oil or in butter, and, in theory, has less harmful effects on cholesterol levels. About half of saturated fat contained in coconut oil is in the form of lauric acid (12 carbon atoms), and studies show that the effect of this fatty acid on LDL cholesterol is half that of palmitic acid (16 carbon atoms). In practice, however, a systematic review of the studies conducted to date indicates that coconut oil increases LDL cholesterol levels in a similar way to other sources of saturated fat (butter, palm oil) and in a more significant way than unsaturated fat, such as olive oil for example.
Overall, these observations suggest that coconut oil is a source of saturated fat like any other, and that it should be used sparingly to prevent an increase in the risk of heart disease. For everyday cooking, virgin olive oil is a much better choice because of its very well documented positive effects on cardiovascular health. Among other plant-based oils, the use of canola oil is recommended, as it contains the highest proportion of omega-3 polyunsaturated fats, known for their anti-inflammatory effects.