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Vegetarian diet linked to lower risk of heart disease but higher risk of stroke

Thursday 5 September 2019

"Being vegetarian 'lowers heart disease risk but increases chance of stroke'," reports The Guardian in what would appear to be a case of 'good news, bad news', for vegetarians.

The headline is prompted by a study of more than 48,000 adults in the UK. The study found that people who said they followed a vegetarian or vegan diet were less likely to develop heart disease during 18 years of follow-up, but more likely to have a stroke.

The researchers calculated that for every 1,000 vegetarians in the study, there were 10 fewer cases of heart disease, but 3 more strokes, compared with meat eaters over a 10-year period. People who ate fish but not meat (pescatarians) had a reduced risk of heart disease, but no difference in risk of stroke.

The heart disease results may be partly down to vegetarians having lower body mass index (BMI), lower blood pressure and lower cholesterol levels. The higher number of strokes were mostly due to bleeding inside or around the brain (haemorrhagic), rather than a blood clot blocking the supply of blood to the brain (ischaemic), which is the more common type of stroke.

High blood pressure is a cause of haemorrhagic stroke, so it's difficult to know the reason for these results. We do not know of any plausible mechanism by which eating a vegetarian diet would increase your blood pressure.

It's also very hard to say from this type of study whether diet directly caused the difference in risk of disease. Diets are complex and other factors may be involved. There's no need to be alarmed by the findings, if you follow a healthy diet of any type.

And any increase in risk of stroke for vegetarians is 'outweighed' by a potentially greater decrease in heart disease risk.

Find out more about a healthy diet.

Where did the story come from?

The researchers who did the study were from the University of Oxford. The study was funded by the UK Medical Research Council and the Wellcome Trust. It was published in the peer-reviewed British Medical Journal on an open-access basis, so is free to read online.

Most of the UK media headlines focused on the raised stroke risk for vegetarians, rather than the reduced heart disease risk. Mail Online stated: "Vegetarians have a 20% higher risk of suffering a stroke than meat eaters 'because they miss out on key vitamins'," although the reason for the higher stroke risk is not clearly understood.

In most media, the lower risk of heart disease for vegetarians is not mentioned until several paragraphs into the story. A casual reader could be misled into thinking that a vegetarian diet posed more of a risk to health than a meat-based diet, which would be untrue.

The clear decrease in risk of heart disease may outweigh any potential increased risk of haemorrhagic stroke, although this cause-and-effect link is not certain. Only The Guardian reported both findings of the study in its headline.

What kind of research was this?

This was a cohort study. Cohort studies are useful for investigating links between risk factors (such as different types of diets) and outcomes (such as heart disease and stroke). But they cannot tell us whether those risk factors directly cause certain outcomes. Also, the study was based on self reporting, and the differences in health outcomes may have more to do with other lifestyle factors that were not considered in the study.

What did the research involve?

Researchers recruited 48,188 volunteers from across the UK via GP surgeries and advertisements between 1993 and 2001. They targeted vegetarians through adverts in health food stores, magazines, and newsletters sent out by the Vegetarian Society and Vegan Society, to increase the numbers of non meat eaters in the study.

Volunteers filled in a questionnaire about their diet, which also included questions about their lifestyle, health, education, age, height and weight, and where they lived. They were categorised based on their answers into:

  • people who ate meat (24,428)
  • people who ate fish but not meat (7,506)
  • people who ate no meat, but may eat either eggs or dairy products (vegetarians and vegans combined, 16,254)

The volunteers were sent a follow-up questionnaire in 2010 to see if their diet had changed. They were followed up for an average 18.1 years, with NHS registers used to find out if they had ischaemic heart disease (caused by a blockage or narrowing of the blood vessels supplying the heart) or any type of stroke. The 2 types of stroke are ischaemic (caused by clots in the blood vessels of the brain) and haemorrhagic (caused by bleeding from the blood vessels of the brain).

The study did not include people with heart disease, stroke, angina or any type of cardiovascular disease at the start.

Researchers adjusted their results to account for people's age, gender, year of recruitment to the study, education, socioeconomic status, smoking and alcohol use, physical activity, use of dietary supplements, use of oral contraceptives or hormone replacement therapy (HRT).

They tested the figures to see whether a history of high blood pressure, high cholesterol or BMI could account for the results. They also looked for a potential influence on the results of people's dietary intake of fruit and vegetables, dietary fibre and total energy intake.

What were the basic results?

During the 18-year study, 2,820 people had heart disease and 1,072 people had stroke, of which 519 were ischaemic and 300 haemorrhagic stroke.

Compared to meat eaters:

  • fish eaters had 13% lower risk of heart disease (hazard ratio (HR) 0.87, 95% confidence interval (CI) 0.77 to 0.99)
  • vegetarians and vegans had 22% lower risk of heart disease (HR 0.78, 95% CI 0.70 to 0.87)
  • fish eaters had no difference in risk of stroke
  • vegetarians and vegans had 20% higher risk of stroke (HR 1.20, 95% CI 1.02 to 1.40)

Analysing by type of stroke, they found no difference between meat eaters and vegetarians in the risk of ischaemic stroke. However, vegetarians had 43% raised risk of haemorrhagic stroke (HR 1.43, 95% CI 1.08 to 1.90).

For every 1000 people, over 10 years, there was:

  • 5.8 fewer cases of heart disease among fish eaters (95% CI -10 to -1)
  • 10 fewer cases of heart disease among vegetarians (95% CI -6.7 to 13.1)
  • 3 additional cases of stroke among vegetarians (95% CI 0.8 to 5.4)

Adjusting figures to account for diabetes, high blood pressure, high blood cholesterol and high BMI weakened the results for heart disease, suggesting that vegetarians' lower levels of these markers might at least partly account for their reduced risk of heart disease.

How did the researchers interpret the results?

The researchers said: "Overall, the present study has shown that UK adults who were fish eaters or vegetarians had lower risk of ischaemic heart disease than meat eaters but that vegetarians had higher risk of stroke."

They said further studies should look at levels of cholesterol, vitamins and fatty acids to search for possible explanations for their findings.

Conclusion

Headlines warning vegetarians of a 20% rise in stroke risk are understandably alarming. However, the risk is balanced by a clearly reduced risk of heart disease, which is more common than haemorrhagic stroke. And looking at the absolute figures, the raised risk translates into 3 additional strokes per 1,000 people over 10 years, which might sound less alarming.

It's also important to be aware of the limitations of the study. Observational studies cannot show that one factor (such as a vegetarian diet) directly causes an outcome( such as increased risk of stroke). Even though the researchers tried to account for some potentially confounding factors – for example that vegetarians tended to be younger than meat eaters, were more physically active and less likely to smoke – they cannot account for everything.

Vegetarians and fish eaters tended to have a lower BMI, blood pressure and cholesterol levels. The likelihood is that these factors lower the risk of heart disease, so may explain part of the lower risk for vegetarians and fish eaters.

The question of why vegetarians had a raised risk of haemorrhagic stroke remains unclear. High blood pressure is the most common cause for this type of stroke, so that does not appear to answer the question. The researchers and experts who have commented on the study say that lower levels of LDL cholesterol ("bad" cholesterol) have been linked to an increased risk of haemorrhagic stroke in other studies – particularly when in combination with high blood pressure – so this could have raised risk. However, this remains a theory that needs further study.

Overall, the study does not provide any evidence that people should switch to or from a vegetarian diet. It's possible to have a balanced, healthy diet whether you eat meat or not. And of course, many people chose to eat a vegetarian diet for primarily ethical rather than health reasons.

Regardless of the diet you eat, the study also emphasises the importance of identifying and managing high blood pressure and high cholesterol.

All people aged 40 to 74 are invited to attend an NHS Health Check to check BMI, blood pressure, cholesterol, blood sugar and risk of cardiovascular disease, and for people over 65, dementia.

Find out more with the NHS Eatwell guide.

Analysis by Bazian
Edited by NHS Website