Stroke risk higher but heart disease lower in vegetarians

Just as people have started convincing themselves to eat veggies rather than meat, a new study says that vegetarian and vegan diets just might put them at a 20% higher risk of stroke – though the risk of heart disease is correspondingly lower.

This study is the earliest to look specifically at the risk of stroke in vegetarians.

How was the study done?

The researchers analyzed health data from over 48,000 people in the UK, with an average age of 45 years. They were classified as meat eaters, fish eaters and vegetarians/vegans. The corresponding numbers were about 24,000, 7,500 and 16,000 respectively. the follow up was carried out for 18 years, on average. They looked at smoking and exercise habits as well as other potential confounding factors that could affect the chances of stroke or heart disease.

What did the study show?

During the study period, there were over 2,800 heart events and almost 1,100 stroke events. Of these, 500 were ischemic and 300 hemorrhagic strokes, caused by cut-off of blood flow to a part of the brain, and bleeding into the brain, respectively. There was no significant difference in the rates of heart attack or ischemic stroke among diet groups.

The stroke risk was least among the fish eaters, who had slightly higher cholesterol levels in the blood compared to vegetarians, as well as adequate levels of vitamin B12. They also had a 13% lower risk of coronary heart disease (CHD).

On the other hand, vegetarians had a 20% increased risk of stroke, which means about 3 cases more per 1,000 vegetarians over 10 years, and a 43% increase in hemorrhagic stroke risk. However, they also had a 22% lower risk of coronary heart disease translating into 10 less heart-related incidents/ 1,000 vegetarians over 10 years.

The lower incidence of heart disease in these groups could be linked to the generally lower rates of obesity and overweight, as well as lower average blood pressure, cholesterol and diabetes mellitus, all of which are known risk factors for both heart disease and stroke. This confirms the results of the European Prospective Investigation into Cancer (EPIC)-Oxford) study which reported a higher risk of ischemic heart disease.

This study had too short a follow up to be able to determine risks in fish eaters and vegans separately. Nonetheless, it failed to show any difference in the number of stroke deaths between those on a vegetarian or non-vegetarian diet.

Heart disease occurs much more commonly than stroke, which means that vegetarians still have the edge when it comes to overall cardiovascular disease risks. Secondly, the absolute change in risk is small. The study only means, therefore, that a vegetarian diet is not guaranteed to bring about improvements in all areas of health. Instead, it should be part of a healthy overall lifestyle.

Why would stroke risk increase in vegetarians?

Recent studies show a higher chance of hemorrhagic stroke when the cholesterol levels are extremely low. Low B12 levels are also more likely in vegans and vegetarians, because it is naturally found only in animal foods. This might also be a risk factor, though not supported by strong evidence so far.
The researchers pointed out that the effect of a vegetarian diet on other health conditions must also be examined to provide a well-rounded picture.

In an accompanying editorial, Mark Lawrence and Sarah McNaughton said, “Participants were all from the United Kingdom where dietary patterns and other lifestyle behaviors are likely to differ from those prevalent in low and middle income countries, where most of the world's vegetarians live.” They also recommended sticking to dietary guidelines, irrespective of the type of diet one chooses, as these are designed to provide an optimal health outcome relative to multiple health conditions.

Other dietitians commented on the need for further research to provide solid evidence for this increased risk. British Heart Foundation’s senior dietitian Tracy Parker says, “Whilst this is an interesting finding, this study is observational and doesn't provide us with enough evidence, so more research in this area would be needed.”

Researcher Tammy Tong said, “Additional studies in other large scale cohorts with a high proportion of non-meat eaters are needed to confirm the generalizability of these results and assess their relevance for clinical practice and public health.”

The report was published on September 4, 2019, in the British Medical Journal.

Journal reference:

Risks of ischaemic heart disease and stroke in meat eaters, fish eaters, and vegetarians over 18 years of follow-up: results from the prospective EPIC-Oxford study. Tammy Y. N. Tong, Paul N. Appleby, Kathryn E. Bradbury, Aurora Perez-Cornago, Ruth C. Travis, Robert Clarke, & Timothy J Key. BMJ 2019;366:l4897. https://doi.org/10.1136/bmj.l4897. https://www.bmj.com/content/366/bmj.l4897