Vegetarians and vegans: lower heart disease risk but higher stroke risk?

But vegetarians and vegans had a higher risk of stroke than meat eaters, particularly hemorrhagic stroke (when blood from an artery starts bleeding into the brain), which the researchers suggest may reflect low blood levels of total cholesterol or a low intake of certain vitamins.

More and more people have been turning to vegetarian and vegan diets, partly due to the perceived health benefits, as well as concerns about the environment and animal welfare.

Previous studies have suggested that vegetarians have a lower risk of coronary heart disease (CHD) than non-vegetarians, but data from large studies are limited and little has been reported on the difference in risk of stroke.

The new study, published in highly regarded medical journal The BMJ, was by a team of researchers from the Nuffield Department of Population Health, Oxford University and University of Auckland’s Dr Kathryn Bradbury, who was at Oxford thanks to a Girdler’s New Zealand HRC Fellowship.  

“From an environmental perspective the evidence is very clear – as a population we need to be eating less meat. It is therefore important to know what the health impacts of this might be,” says Dr Bradbury, a senior research fellow in the School of Population Health.

The team, led by Oxford University’s Dr Tammy Tong, used data from the EPIC-Oxford study to explore the risks of CHD and stroke in meat eaters, pescatarians (those who eat some fish but not meat) and vegetarians over an 18 year period.

The study included information on 48,188 people (average age 45 years) who were recruited between 1993-2001, and had no history of CHD or stroke. They were then grouped into meat eaters (24,428), pescatarians (7,506), and vegetarians, including vegans (16,254).

There were 2,820 cases of CHD and 1,072 cases of stroke during the study period, including 519 cases of ischaemic stroke (when a blood clot blocks the flow of blood and oxygen to the brain) and 300 cases of hemorrhagic stroke.

After taking account of potentially influential factors, such as medical history, smoking, use of dietary supplements and physical activity, pescatarians and vegetarians had a 13 percent and 22 percent lower risk of CHD than meat eaters, respectively.

This is equal to 10 fewer cases of CHD in vegetarians than in meat eaters per 1000 people consuming these diets over 10 years. The difference may be at least partly due to lower BMI and lower rates of high blood pressure, high blood cholesterol and diabetes linked to these diets, say the authors.

In contrast, vegetarians and vegans had a 20 percent higher risk of stroke than meat eaters, equivalent to three more cases of stroke per 1000 people over 10 years, mainly due to a higher rate of hemorrhagic stroke.

Blood tests on a subset of participants revealed that vegetarians and vegans had lower circulating cholesterol and levels of several nutrients than meat eaters (e.g. vitamin B12), which could explain these findings, the authors suggest.

“This study was observational, and we can’t be certain that the links we found are causal,” says Dr Bradbury. “A randomised controlled trial is one way to test causality, but it would be very difficult to randomly allocate large amounts of people to become vegetarian or be a meat eater for 10 years or so and compare stroke rates between the groups, so this current study design is probably the best we are likely to get on this topic.”

The study had a large sample size and long-term monitoring, but further research is needed to replicate the results in other populations and ethnicities, and it should include measurements of other nutritional factors, says lead author Dr Tammy Tong.

In a linked editorial, Professor Mark Lawrence at Deakin University, Australia, suggests that stroke risk should be kept in perspective. “It is based on results from just one study and the increase is modest relative to meat eaters,” he says.

“Relevance to vegetarians worldwide must also be considered,” he writes. “Participants were all from the UK where dietary patterns and other lifestyle behaviours are likely very different from those prevalent in low and middle-income countries where most of the world’s vegetarians live.”

Current dietary guidelines contain the most evidence based advice available for vegetarians, as well as for fish and meat eaters. They also recognise plant based diets for their environmental sustainability as well as health benefits, he adds.

He concludes, “Shifting towards plant based dietary patterns for reasons of personal or planetary health does not necessarily mean becoming a vegetarian. Indeed, populations in some low and middle income countries who consume very low amounts of animal source foods may benefit from being able to eat a little more of these foods to gain additional nutrients necessary to help combat all forms of malnutrition.”

For those who want to follow a healthy vegan diet, Dr Bradbury has this advice: “Vitamin B12 is only found in animal-source foods, so vegans should take a B12 supplement to ensure they are getting enough.”

Read the article:

The BMJ: 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

Editorial: Vegetarian diets and health