Vegetarians had lower risk of heart disease
People who avoided meat and fish had a lower risk of cardiovascular diseases like angina and heart attacks.
In recent years, interest in plant-based diets has increased. Many people have become vegetarians and even more want to do so, with climate and animal welfare often being part of the reason for making the switch.
But what about health? Is it healthier to be a vegetarian?
Jarle Sæby Dybvik, a medical student at the University of Oslo, collaborated with researchers Mette Svendsen and Dagfinn Aune from Imperial College London to review previous studies on vegetarian diets and cardiovascular disease in order to find out more.
The researchers considered data from 13 large population studies that have followed vegetarians and non-vegetarians over many years. Vegetarians were defined as participants who did not eat seafood or meat from animals or birds. However, they could eat both eggs and dairy products.
The studies included almost 850 000 participants. Approximately 115 000 of them developed cardiovascular disease during the follow-up period. Of those who became sick, approximately 30 000 had ischemic heart disease, such as angina and heart attack. Over 14 000 had suffered strokes.
This data allowed the researchers to see if any connection existed between a vegetarian diet and the development of heart disease.
Lower risk
The results showed that the vegetarians had, on average, a 15 per cent lower risk of cardiovascular disease in general. For ischemic heart disease, such as angina and heart attacks, the risk was 21 per cent lower in vegetarians.
This reduction is significant, said Aune.
“If we assume that 30 per cent of the population gets ischemic heart disease during their lifetime and that the results we found apply to a person’s whole life, this means that the risk of heart disease drops to below 24 per cent if you eat a vegetarian diet,” he says.
However, the researchers did not find any definite link between diet and stroke.
They found a tendency towards a lower risk of stroke, but the figures were not statistically significant, meaning that the probability of the result being due only to chance is too great for the researchers to count it as a finding.
Unsure about vegans
The results concur with previous studies, which have also suggested a lower risk of cardiovascular disease, but an uncertain or in some cases opposite link with stroke.
More studies are needed to gain a better understanding of the connection between diet and stroke, according to Aune’s and his colleagues’ article in the European Journal of Nutrition.
The same applies to the connection between heart disease and a vegan diet, which is plant based and does not contain any products from animals. Here, too, the data hinted at a similar reduced risk, but they are too uncertain to give clear answers.
Several factors distinguish vegetarians from others
So what do these findings mean?
The study shows that most vegetarians have a slightly lower risk of cardiovascular disease than non-vegetarians. But is this due to the vegetarian diet?
An observational study, such as the one Aune and his colleagues have carried out, cannot answer this with certainty. Although the study shows that vegetarians have better heart health, reasons other than diet might explain this difference.
The challenge is that often the diet is not the only factor that separates vegetarians from non-vegetarians. People who choose to eat vegetarian are more health conscious than the average person in the population, for example. Perhaps they use less tobacco and alcohol and exercise more?
It is difficult to know whether the vegetarian diet itself promotes better health or if the effect is due to other aspects of lifestyle.
Smoking, alcohol and BMI can’t explain everything
Aune has extensive experience with this problem. He has carried out a number of large studies that summarize and interpret data from population surveys.
Numerous indicators suggest that the results in this study cannot be explained by lifestyle factors like smoking and physical activity, Aune believes.
“All the studies had adjusted for smoking and several had adjusted for physical activity and alcohol use,” he says.
“The difference in the results between studies that had and hadn’t adjusted for this, or within studies with and without such adjustments, was small.”
Some studies also found similar results among non-smokers, in groups with other risk factors, and among Adventists who mostly neither smoke nor drink alcohol.
Differences in BMI appeared to play a role, but could only explain a small part of the difference between vegetarians and non-vegetarians.
Aune and his colleagues found no strong evidence that such disruptive factors were behind the entire connection between a vegetarian diet and a lower risk of cardiovascular disease.
Diet is likely the cause
In addition, other types of studies suggest which mechanisms of action might be behind the beneficial effects of a vegetarian diet.
Experiments have shown that a vegetarian diet can lower LDL cholesterol and blood pressure. Calculations indicate that this can in turn lead to reducing the risk of dying from ischemic heart disease, which roughly corresponds to the figures from Aune's survey.
“We came to the conclusion that there was a probable causal connection,” he says.
The researchers therefore believe that the diet itself probably led to better heart health.
But definitive answers can only be obtained from other types of research.
Aune would like to see larger randomized controlled studies with hard outcomes, that is, experiments where the researchers divide the participants into groups that eat vegetarian or non-vegetarian, and then monitor how many actually end up with heart disease over time.
Not surprising
Helle Margrete Meltzer is a dietitian, a member of Norway’s National Nutrition Council and the former head of research at the Norwegian Institute of Public Health. She recently helped prepare an expert report for the Norwegian Directorate of Health on vegan and vegetarian diets (in Norwegian).
Meltzer, who was not involved in the new study, believes the results concur with the picture we have from before.
“The review findings aren’t new or surprising, but they confirm and support previous findings,” she says.
Meltzer also mentions that previous research suggests possible mechanisms behind the connection between a vegetarian diet and a lower risk of cardiovascular disease.
“Vegetarians usually weigh somewhat less than non-vegetarians, have lower blood pressure and a healthier blood lipid profile. These factors alone can probably explain a lot of the reduced risk of cardiovascular disease and ischemic heart disease,” she notes.
More emphasis might be placed on a vegetarian diet
Aune and his colleagues believe that the results of their study should be reflected in the dietary guidelines for the Norwegian population.
“The findings are in line with current recommendations for a plant-based diet to prevent cardiovascular disease, but they suggest that more emphasis could be placed on a vegetarian diet,” the researchers write in the European Journal of Nutrition.
Meltzer does not necessarily believe that the new results will bring about much change.
“But,” she says, “sustainability considerations dictate that we should eat less meat and more plant food, so in that sense an article like this can be reassuring. It indicates that there are additional benefits, health-wise, by going in a vegetarian direction.”
Vegetarianism doesn’t automatically mean healthy
The expert opinion from the Directorate of Health concludes that both vegetarian and vegan diets can be nutritionally complete. However, Meltzer warns against thinking that any diet without meat and fish is healthy.
“Being vegetarian or vegan doesn’t automatically mean that you have a healthy diet. Living on pasta, white bread and jam is technically vegan, but not necessarily healthy,” she writes.
Recently, a study showed that Norwegian vegetarians ate less fruit and vegetables and more sugar than recommended.
- MORE ABOUT THIS STUDY HERE: Vegetarians do not eat enough fruits and vegetables (Article produced by UiA)
And a 2017 survey concluded that people who eat unhealthy plant foods have an even greater risk of heart disease than people who eat a lot of animal foods.
“Vegetarian and vegan diets can consist of so many foods,” writes Meltzer.
“But on average, these groups probably eat healthier than the average population, and this pushes the statistics in the direction shown in this new review article,” she says.
Reference:
Jarle Sæby Dybvik, Dagfinn Aune et.al.: Vegetarian and vegan diets and the risk of cardiovascular disease, ischemic heart disease and stroke: a systematic review and meta‑analysis of prospective cohort studies. European Journal of Nutrition, 2022.
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Read the Norwegian version of this article at forskning.no
