How do plant-based diets prevent cardiovascular disease
In is now recognised that atherosclerosis appears to be the result of oxidative damage to the endothelial cells that line the blood vessels. The damage to the endothelial layer of arteries begins with inflammation caused by oxidative stress, which is a result of oxidised low-density lipoproteins penetrating the endothelial layer. This process leads on to the development of atherosclerotic plaques.
Cardiovascular risk factors are influenced by diet in a number of ways.
1) Hypertension (persistent elevation in blood pressure) is the second most important known global risk factor for chronic disease after poor diet. In the UK, high blood pressure is the third most important risk factor for chronic disease after tobacco smoking and poor diet. High blood pressure is, however, the single most important risk factor for cardiovascular disease and related disability and increases the risk of stroke, ischaemic heart disease and heart failure. Almost 1 in 3 adults will be diagnosed with hypertension. Plant-based diets (both vegetarian and vegan) have been shown to be effective at maintaining a normal blood, especially if the diet is also low / avoids added salt. In the Adventist Health Study-2 and the EPIC-Oxford, vegetarians and vegans have the lowest risk of hypertension (up to 60% reduced risk compared with omnivores). In fact the DASH diet specifically designed for treating hypertension was inspired by the blood pressure lowering effects of a vegetarian diet. However a vegetarian diet was not deemed to be universally acceptable, animal-derived foods are permitted in the DASH diet. Plant-based diet patterns are able to lower blood pressure in individuals with hypertension to a greater extent than most blood pressure lowering medications.
A healthy plant-based diet contains foods that are known to maintain a blood pressure in the normal range. These include whole grains, which in randomised studies have been shown to significantly lower blood pressure. Nitrate-rich vegetables can be converted to nitric oxide in the body, which is required to maintain the health of arteries and results in dilatation of blood vessels thus reducing blood pressure and the risk of cardiovascular disease. Leafy green vegetables but also beetroot have a high nitrate content. In fact, a randomised study of beetroot juice, showed that its consumption could lead to a reduction in blood pressure to the same magnitude as medication.
High salt intake is a major risk factor for hypertension. Salt in the diet mainly comes from processed and prepared foods, meat and restaurant foods. Salt is made up of around 40% sodium and 60% chloride. Sodium is an essential nutrient but adequate amounts can be obtained from whole plant foods. The overconsumption of sodium from the diet leads to water retention and a rise in blood pressure. It also damages the endothelial lining of blood vessels. Thus a plant-based diet helps to maintain sodium consumption at a healthful level, which should not exceed 1500mg (3/4 teaspoon of salt) if hypertension is an issue and really this is applicable to those without hypertension, regardless of current guidelines. An easy to follow rule when checking food labels is that if the sodium content in milligrams is greater than the calorie content in kcal, then that’s too much. Aim to keep sodium intake in milligrams less than or at least no more that the total number of calories (kcal) eaten in a day. In fact a healthy plant-based diet is beneficial for blood pressure because of the abundant potassium content, a nutrient that the majority of individuals on a Western-style diet are low in. Potassium is necessary for maintaining muscle function but also relaxing blood vessels.
Regular consumption of flaxseeds (linseed) has also been shown to significantly lower pressure in randomised studies. The beneficial amount is about 30g per day.
2) High blood cholesterol is a significant risk factor for cardiovascular disease, especially if LDL-cholesterol is high. The main driver for increased cholesterol levels is the consumption of saturated fat and to some extent dietary cholesterol (both found in animal-derived foods). Mechanisms by which saturated fat elevates LDL cholesterol is by stimulating its production and reducing turnover by reducing LDL receptor activity and protein abundance, thus elevating blood levels. Unsaturated fatty acids do the opposite. Plant-based diets have been shown to reduce blood cholesterol levels as effectively as first line statin medications. In the Adventist Health Studies and EPIC-Oxford study, vegans have the lowest blood cholesterol of all diet patterns.
It is also interesting to note that conventional guidelines have recommended the consumption of ‘white’ meat rather than ‘red’ meat for lower cholesterol levels. However, a recent study tested the effects of different sources of protein on blood cholesterol level. Participants were randomly assigned to a diet high in protein from red meat, white meat and plants and with high or low amounts of saturated fat. Each participant had 4 weeks on each diet with a washout period in-between. The results showed that both red and white meat consumption elevated blood cholesterol levels to a similar degree, independently of saturated fat consumption, whereas the diet composed of plant protein did not elevate blood cholesterol levels. The authors concluded ‘The findings are in keeping with recommendations promoting diets with a high proportion of plant-based food but, based on lipid and lipoprotein effects, do not provide evidence for choosing white over red meat for reducing CVD risk’.
A key step in the process of developing atherosclerosis and heart disease is penetration of cholesterol into the arterial wall, which sets up an inflammatory reaction. Cholesterol is carried around the body by lipoproteins such as low density lipoprotein (LDL) and high density lipoprotein (HDL). However, there are subtypes of these lipoproteins. For example LDL can be small, dense LDL or large LDL and it is the small/dense LDL that is most associated with the risk of heart disease. In addition, the number of LDL particles is also important in predicting heart disease. For example, the same amount of cholesterol in the blood can be carried by a high number of small LDL particles or a small number of large particles. The number of LDL particles in the blood can be estimated by measuring apolipoprotein B (ApoB) levels. Each LDL particle has one ApoB protein, so the higher the number, the higher the LDL particle number. Atherosclerosis results from the infiltration of apoB-containing lipoprotein particles into the coronary artery wall and hence it seems that measuring apoB in the blood is a better predictor of heart disease than measuring LDL-cholesterol, although this testing is not routinely performed. This argument goes some way to explaining why there are doctors who state that the blood cholesterol and LDL-cholesterol levels are not important and not a good predictor of heart disease. We can all think of examples of people that have a high blood cholesterol and do not have heart disease and those with a normal cholesterol that have had a heart attack. It is likely that this is down to differences in the level of small/dense LDL and apoB levels. A plant-based/vegan diet has also been shown to be effective at lowering apoB levels.
The totality of the data points to blood cholesterol being the only direct risk factor for atherosclerosis; that is a high blood cholesterol level MUST be present for atherosclerosis to develop. If the serum total cholesterol is 2.3–3.6mmol/L and LDL-cholesterol 1.3–1.8mmol/L it seems that atherosclerotic disease does not develop. In addition, atherosclerosis does not progress when total cholesterol is 3.9mmol/L and LDL-cholesterol is <1.8mmol/L. A whole food plant-based diet is much more likely to achieve these cholesterol levels. It is also worth noting that plant-based diets often lead to a fall in HDL-cholesterol, but this does not seem to be of detriment given the significant fall in total and LDL-cholesterol that occurs.
There are various mechanisms by which plant-based diets maintain a healthy blood cholesterol level. This includes; the avoidance of cholesterol/saturated fat in the diet; the high fibre consumption, which reduces the absorption of fat from the diet; the consumption of plant sterols (equivalent of plant cholesterol as emphasised in the Portfolio diet) binds dietary cholesterol and prevents absorption. The effects of a plant-based or vegan diet are less significant on triglyceride levels.
3) Reduction of inflammation and oxidative stress. Plant-based diets reduce the risk of cardiovascular disease by reducing inflammation and oxidative stress. In general, meat-based diets result in higher levels of inflammation and oxidative stress for a number of reasons. This includes the consumption of haem iron found mainly in animal foods, which can generate reactive oxygen species and result in oxidative stress. Nitrates and nitrites in processed meat generate N-nitroso compounds, including nitrosamine, which create reactive oxygen species and oxidative stress and impair the function of blood vessels.
One measure of inflammation is high sensitivity CRP (hsCRP). There is a direct correlation between the level of hsCRP and the risk of cardiovascular disease in men and women. So, researchers have tried to combat atherosclerosis with drugs that reduce inflammation. One such drug is canakinumab, which blocks interleukin-1 beta. In patients with prior myocardial infarction the drug was able to significantly reduce the level of hsCRP and the risk of further cardiovascular events. This was at the cost of an increased risk of fatal infection. A vegan diet is also effective at reducing inflammation as determined by hsCRP levels. The open‐label, blinded end‐point, EVADE CAD (Effects of a Vegan Versus the American Heart Association‐Recommended Diet in Coronary Artery Disease) trial randomised 100 participants with coronary artery disease to 8 weeks of a vegan or American Heart Association–recommended diet with provision of groceries, tools to measure dietary intake, and dietary counselling. After 8 weeks, those on a vegan diet had a 32% greater reduction in the level of hsCRP compared to those on the AHA diet. This is not dissimilar to the reduction seen with canakinumab! However, the study was not powered to look at effects on cardiovascular events or survival.
4) A novel mechanism is the generation of trimethylamine N- oxide (TMAO), when meat-based diets are consumed. Choline and carnitine, compounds derived mainly from animal foods (red meat, poultry, fish and eggs) are converted by gut microbes to trimethylamine (TMA). TMA is then converted to TMAO by the liver. TMAO is thought to increase the risk of cardiovascular disease (both heart disease and stroke) by its effects on cholesterol and sterol metabolism, promoting inflammation and by making platelets more reactive thus increasing blood clotting. People eating a vegan diet don’t have the bacteria necessary to produce TMA even if they are fed meat!
5) Saturated fat from animal-derived foods damages the epithelial lining of the gut and allows endotoxin, including lipopolysaccharides (from the bacterial contamination of meat) into the blood, resulting in inflammation, including within arteries.
6) High blood triglyceride (TG) levels increase the risk of cardiovascular disease. When you eat, your body converts any excess calories into triglycerides. The triglycerides are stored in fat cells. Later, hormones release triglycerides for energy between meals. When the fat cells get very full, there is spill over of TGs into the blood. There are a number factors that affect TGs levels and cause an elevation. Lifestyle-related factors include diet, alcohol consumption and physical activity. Being overweight and over-consuming calories will increase TG levels. Other causes of elevated TGs include the presence of chronic disease (e.g. thyroid and kidney problems) and a number of prescribed medications.
Check out our factsheet on lower TG levels
When it comes to diet, a plant-based or vegan diet has not consistently resulted in a reduction in TG levels, although in general, those consuming a plant-based diet are more likely to have TG levels in the desired range. In fact, a Mediterranean diet may be better at reducing TG levels as shown in the CARDIVEG study. In addition, avoiding refined carbohydrates, including refined sugars and grains, are equally important for maintaining normal TG levels.
7) Gene expression has been shown to change with lifestyle interventions. A prospective study of 63 participants who either had established coronary artery disease or at least 2 risk factors for coronary artery disease and undertook intensive lifestyle interventions, including a low fat vegetarian diet (similar to that used by Dr Dean Ornish), physical activity, stress management and regular support groups were compared to 63 matched control participants. After one year the results showed that intensive lifestyle interventions could also modulate gene expression in peripheral blood, favourably altering the expression of genes involved in the immune function, lipid metabolism, blood pressure regulation, inflammation and oxidative stress. This study demonstrates, that despite the inheritability of genes that may increase the risk of cardiovascular disease, it is possible to alter their expression thus modifying the risk of developing the cardiovascular disease.
There are a multitude of reasons why a plant-based diet is best for prevention and treatment of cardiovascular disease. It addresses the root cause of the disease without the side-effects of medication.
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