Vegans and the risk of stroke

Stroke is a sudden and life-threatening event in which an artery supplying the brain is blocked (ischaemic stroke) or a blood vessel in the brain bursts (haemorrhagic stroke). Both result in a part of the brain suddenly losing some or all of its blood supply which leads to a rapid loss of brain function and requires immediate medical care. Stroke symptoms may include loss of the ability to move and feel, usually on one side of the body, loss of speech, vision, inability to understand or react and dizziness. How much damage a stroke causes depends on how long brain cells remain without blood supply, the size of the affected area and the extent of the blood vessel blockage or damage. Some people recover fully, some partially, whilst for others a stroke is fatal. There are also mini-strokes where the damage isn’t so big and they can cause only mild symptoms. However, they are dangerous too because if the person doesn’t seek medical care, their health may further deteriorate.

Ischaemic stroke is much more common than haemorrhagic – about 87 per cent of strokes are ischaemic (Mozzafarian et al., 2016). In the UK, there are over 100,000 strokes each year and data suggests that the average age for someone having a stroke is decreasing, with over a third of strokes in adults aged between 40 and 69 (NICE, 2019).

There are many risk factors for stroke, including advanced age, high blood pressure, diabetes, high blood fats and cholesterol, atrial fibrillation, smoking, physical inactivity, poor nutrition, family history, chronic kidney disease, obesity, coronary heart disease, sleep apnea and depression (Campbell, 2017; Chiu et al., 2020). Some of these are out of our control, such as age or genetics, yet most of them are linked to lifestyle. As Spence (2018) puts it, “When ranked in order of importance, among the interventions available to prevent stroke, the three most important are probably diet, smoking cessation and blood pressure control”.

High blood pressure

Of all the risk factors, hypertension (high blood pressure) is probably the most significant in that people who eventually suffer a stroke also have raised blood pressure (Campbell, 2017; Chiu et al., 2020).

Across scientific studies, vegans consistently have lower blood pressure than other diet groups (Alexander et al., 2017; Chiu et al., 2020) and when compared with meat-eaters, vegans have a 63 per cent lower risk of high blood pressure (Pettersen et al., 2012).

In people who suffer from high blood pressure, a switch to a wholefood plant-based diet usually achieves a significant blood pressure drop (Alexander et al., 2017). In fact, a healthy vegan diet is more effective at blood pressure lowering than a vegetarian one (Lee et al., 2020).

As one study summarised, each 20 mmHg increase in systolic blood pressure or each 10 mmHg increase in diastolic blood pressure more than doubles the risk of death from stroke, while a reduction of 5 mmHg in systolic blood pressure leads to a seven per cent reduced risk of all-cause mortality, a nine per cent reduced risk of heart disease, and a 14 per cent reduced risk of stroke (Kahleova et al., 2017).

According to an analysis of several major studies, vegetarian and vegan diets are associated with lower blood pressure – on average a 6.9 mmHg lower systolic and 4.7 mmHg lower diastolic blood pressure compared to meat-eaters – and clinical trials of vegetarian or vegan diets that lasted at least six weeks resulted in mean decreases of 4.8 mmHg systolic and 2.2 mmHg diastolic blood pressure (Yokoyama et al., 2014). Campbell (2017) suggested that this effect could be seen partly due to increased potassium intake and lower sodium intake. And another study agrees that higher potassium intake lowers blood pressure among people with hypertension (Kahleova et al., 2017). Plant foods are good sources of potassium but that’s not the only reason why plant-based diets lower our blood pressure – high protein intake, especially from meat, increases blood pressure so cutting it out has a noticeable effect (Kahleova et al., 2017). Another major factor is salt and several studies highlight the importance of salt intake reduction for the prevention and treatment of high blood pressure (Spence, 2018).

Yet, there are other mechanisms through which plant-based nutrition leads to lower blood pressure – they include improved vasodilation (the ability of blood vessels to relax and widen their diameter), greater antioxidant content and anti-inflammatory effects resulting in healthier blood vessel walls, and modification of gut bacteria – reducing those producing TMAO and supporting bacteria producing beneficial by-products (Alexander et al., 2017). Each of these is described in more detail below.

Campbell (2017) points out that there’s one inconsistency in population data, diet and the risk of stroke – haemorrhagic stroke used to be more common in some Asian countries, notably in Japan, when they still had a more traditional diet. With increasing popularity of Western foods, Japan’s rates of obesity, diabetes and heart disease have also increased but haemorrhagic stroke decreased. It’s been suggested (Campbell, 2017) that this may be a coincidence since traditional Japanese diet is very high in salt and over the past several decades, there have been many initiatives to reduce salt intake as it’s linked to high blood pressure – a major risk factor for stroke. At the same time, the healthcare system in Japan has also improved hypertension treatment and monitoring so that would have played a role in reducing stroke risk as well. High salt intake causes water retention and so increases blood pressure through different mechanisms than diets high in animal protein and fat, which may lead to atherosclerosis (described below).

Atherosclerosis

High blood cholesterol means many small, sticky cholesterol articles circulating in the blood – these particles stick to the walls of blood vessels (particularly arteries, the blood vessels carrying blood away from the heart) and can form thick layers called cholesterol plaques. This leads to the narrowing and hardening of the arteries – atherosclerosis – and is a big risk factor for stroke because it increases blood pressure and there’s always the risk that one of the plaques will tear off and block a blood vessel in the brain, causing a stroke. Atherosclerosis in the brain is one of the major causes of stroke worldwide (Banerjee and Chimowitz, 2017).

As Campbell (2017) summarised; animal protein, excessive added sugars and saturated fat, which are all characteristic of a Western diet, increase cholesterol levels in the blood. High cholesterol levels are one of the major risk factors for stroke (Spence, 2018; Chiu et al., 2020).

Metabolic syndrome, elevated blood lipids and cholesterol have all been identified as risk factors for atherosclerosis in the brain in particular, posing a serious threat of ischaemic stroke (Chiu et al., 2020).

Very low-fat, plant-based diets have been shown to reduce and even eliminate cholesterol plaques in clinical trials with patients suffering from ischaemic heart disease (Campbell, 2017).

For atherosclerosis, and therefore cardiovascular disease, to progress, inflammation also plays a major role (Li et al., 2020). Vegetarians have been shown to have lower C-reactive protein levels (protein measuring the level of inflammation in the body) and dietary interventions based around plant foods have been shown to reduce C-reactive protein (Campbell, 2017).

Another study found that the more pro-inflammatory foods people ate, the higher their risk of stroke, up to a 28 per cent increase (Li et al., 2020). Foods such as processed, red and organ meats, refined carbohydrates and sweetened beverages were labelled as pro-inflammatory. Foods such as green leafy vegetables, yellow vegetables, wholegrains, fruit, tea, coffee and moderate amounts of wine were labelled as anti-inflammatory.

TMAO

The body normally transports cholesterol back into the liver where it is either broken down or turned into bile salts. A compound called trimethylamine-N-oxide (TMAO) prevents this from happening and increases the stickiness of cholesterol particles, contributing to atherosclerosis, which is why it’s a risk factor for heart disease and stroke (Chiu et al., 2020). Human gut bacteria produce TMAO parent molecule from foods rich in L-carnitine (meat) and choline (eggs) and the liver then finishes the last step in TMAO creation (Campbell, 2017; Spence, 2018).

Spence (2018) recommends that people at risk of stroke avoid egg yolk and red meat and consume a mostly plant-based diet. Red meat has about four times as much carnitine as chicken or fish and egg yolks are the richest source of choline (Spence, 2018).

In general, vegetarians are about 90 per cent less likely to have high TMAO levels than meat-eaters (Chiu et al., 2020).

Endothelial function

Healthy blood vessels have a strong wall that contracts and relaxes based on what you’re doing and how much blood is needed in your tissues. High blood pressure and atherosclerosis both prevent blood vessels from relaxing and allowing more blood through them (vasodilation). Studies that tested how plant-based diets affect this ability of blood vessels found that they invariably improve endothelial function (Campbell, 2017).

Plant-based dietary patterns in general

According to several scientific studies, plant-based diets lower blood pressure and cholesterol levels, reduce the risk of heart disease and stroke, and halt progress of existing cardiovascular issues (Freeman et al., 2017; Chiu et al., 2020).

A large study conducted in Taiwan, comparing diets and stroke risk of vegetarians and non-vegetarians, found a stark division (Chiu et al., 2020) – vegetarians had half the risk of stroke in general, 60-74 percent lower risk of ischaemic stroke and 65 per cent lower risk of haemorrhagic stroke. It is worth noting that vegetarians in this study had a relatively low intake of eggs – 0.3 servings a day, and dairy – 0.2-0.3 servings a day, making their diet predominantly plant-based. Their diet provided the stroke-risk-lowering benefit despite the fact that 64 per cent of these vegetarians had inadequate vitamin B12 intake.

In the Adventist study in the US, vegetarians had a 29 per cent lower risk of stroke and this risk reduction was predominantly seen in men (Kwok et al., 2014).

Data from China shows how a dietary shift changes population health – over the past few decades, traditional Chinese diets have been in decline due to increasing popularity of Western foods resulting in a higher intake of meat and eggs and decreased intake of fruit, vegetables and wholegrains. This shift has brought about a marked rise in cardiovascular disease. Between 2003 and 2013, heart disease/attack mortality in China increased by 213 per cent, while stroke mortality increased by 26.6 per cent (Spence, 2018).

Vitamin B12

Lack of vitamin B12 can lead to hyperhomocysteinemia – too much homocysteine in the blood. Homocysteine is an amino acid which your body processes further so it doesn’t stay in the blood for long but in order for that to happen, vitamin B12 is needed. High homocysteine levels in the blood can damage the lining of the arteries and may also make the blood clot more easily than it should, which can increase the risk of blood vessel blockages and contribute to cholesterol plaque build-up, increasing the risk of stroke (Chiu et al., 2020).

It is difficult to get enough of vitamin B12 from foods alone so it’s best to take a supplement on a regular basis. It should provide at least 50 µg daily or a higher dose a couple of times a week.

Controversial study results

A comprehensive study of diets and health of meat-eaters, pescaterians and vegetarians – the EPIC-Oxford study – brought somewhat unexpected results that are in stark contrast with other studies (Tong et al., 2019). The study found that although vegetarians had 22 per cent lower rates of ischaemic heart disease than meat-eaters, they had 20 per cent higher rates of total stroke – haemorrhagic stroke in particular.

When the scientists divided the group into vegetarians and vegans, the latter had an 18 per cent lower risk of ischaemic heart disease but a 35 per cent higher for total stroke than meat-eaters. However, these results were not statistically significant because the number of vegans in the study and stroke cases among them were low. It is therefore possible that an odd case could have resulted in such controversial numbers.

Some experts suggested that aside from the small number of cases, low vitamin B12 levels could have played a role, increasing the level of homocysteine in the vegetarians’ blood, or possibly low omega-3 intakes and high salt consumption. These are all factors that people weren’t very well educated about when the study started but nutrition education has advanced and vegans are advised to take vitamin B12 supplements, make sure they have enough omega-3 fats and avoid excessive salt.

While this study found that vegetarians had a higher risk of stroke (Tong et al., 2019), the Taiwanese study showed the exact opposite (Chiu et al., 2020). The authors of the latter offered an explanation – nearly 80 percent of vegetarians in the EPIC-Oxford study drank alcohol but most vegetarian participants in Taiwan did not. It’s been suggested that alcohol consumption may negate the protective effect of low cholesterol levels (typical of vegetarians) on haemorrhagic stroke risk (Chiu et al., 2020). Spence (2018) also highlighted that high alcohol consumption increases the risk of haemorrhagic stroke – the risk increases from about nine standard drinks per week for women and 14 for men. The Epic-Oxford vegetarians also had a higher cheese consumption than meat-eaters – this fact is important because cheese is high in saturated fat and salt, both of which contribute to high blood pressure and cholesterol. As mentioned above, the Taiwanese vegetarians had minimal intakes of dairy so this difference may help to explain the different study results.

Another study from the UK found that vegetarians (including vegans) had lower risk of heart disease, stroke and heart-attacks than meat-eaters (Petermann-Rocha et al., 2020). However, when the authors compared the non-obese with obese participants, the data revealed that only the non-obese vegetarians had significantly lower risk of the above – obesity decreased and almost cancelled this advantage. It’s worth noting that vegetarians in this study had a higher intake of crisps than meat-eaters and this highlights that a plant-based diet isn’t necessarily a healthy one.

Different foods and stroke risk

Several studies have investigated the effect of different foods on the risk of stroke. Even though it is difficult to assess separate food groups because our diets are varied, these studies gathered data that offers clear indicators for stroke prevention – wholesome plant-based foods.

Fruit and vegetable consumption has been found to be protective against stroke – people who eat the most fruit and vegetables have a 21 per cent lower risk compared to people who eat very little, with several studies revealing similar results (Campbell, 2017). According to one study, the strongest risk reduction was seen at an intake of 800 grams a day of fruit and vegetables (Aune, 2019).

Whole grains have been found to be protective against a variety of cardiovascular issues, including stroke (Campbell, 2017). The amount that seems to confer the biggest benefit is 225 grams a day (Aune, 2019).

Berries are a great source of anthocyanins (powerful antioxidants) and a regular intake from around three servings per week seems to have a protective effect on blood vessels, reducing the risk of cardiovascular events, including stroke (Freeman et al., 2017).

Nuts are a healthy food group offering many essential nutrients and a moderate consumption – a small handful a day (20 grams) – seems to help protect against cardiovascular diseases, including stroke, and helps to reduce blood cholesterol as well (Freeman et al., 2017; Aune, 2019).

Green leafy vegetables, such as kale, cabbage, rocket, broccoli, watercress or Brussels sprouts, reduce arterial stiffness and blood pressure, and have an anti-inflammatory effect (Freeman et al., 2017). They should be on our daily menu.

Some research shows a mild protective effect of pulses against stroke, other data doesn’t show a clear association (Freeman et al., 2018). Either way, pulses are an excellent source of protein, fibre, antioxidants, vitamins and minerals.

It’s interesting that data on coffee and population health shows that daily coffee consumption may lower the risk of stroke – according to several studies, three to four cups daily is a safe amount and may reduce the risk (Freeman et al., 2018).

Saturated fats (from animal foods but also coconut and palm oil), and cholesterol (mainly from eggs) may increase the risk of stroke, while vegetable oils (olive oil in particular) seem to reduce the risk (Freeman et al., 2017).

A growing number of studies have linked increased consumption of added sugars to heart disease, stroke and fatal cardiovascular events (Freeman et al., 2018).

And, predictably, higher-than-low meat consumption has been shown to correlate with increased stroke risk – study results range from 14 to 24 per cent increased risk (Campbell, 2017).

Bottom line

A vegan or vegetarian diet may not reduce the risk of stroke if it’s based around fatty, processed foods high in sugar and salt, and if the person is obese. However, if your diet consists of fruit and vegetables, wholegrains, pulses and nuts and seeds, with the addition of vitamin B12, research suggests that it will reduce the risk of developing cardiovascular disease, including stroke, and that it can even reverse atherosclerosis if you already have it.

References

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