The authors of a large, long term study conclude that pescatarianism and vegetarianism have an association with a reduced risk of ischemic heart disease, but they note that vegetarians have a slightly higher risk of stroke.

Over recent years, increasing numbers of people have decided to reduce the amount of meat in their diet.

Vegetarians, vegans, and pescatarians (people who eat fish but not meat) are a growing demographic.

Following any one of these meat-free diets is nothing new, but due to the spike in popularity, researchers are keen to understand the possible health implications.

A recent study, which features in BMJ, looks specifically at plant based diets and their effect on the risk of stroke and ischemic heart disease (IHD).

IHD refers to any problems that occur due to a narrowing of the arteries to the heart. Without treatment, it can lead to a heart attack.

What do we already know?

Earlier studies have concluded that vegetarians have a lower risk of obesity and IHD, but as a review of relevant research explains, there is a need for more long term studies involving larger numbers of people.

As for stroke risk, only a few studies have looked into the relationship between a plant based diet and stroke risk. According to the authors of the current study, these "found no significant differences in risk of total stroke deaths between vegetarians and nonvegetarians."

The latest study aimed to fill in some of these gaps. In all, the scientists took data from 48,188 people whom they followed for an average of 18.1 years.

The participants, who had an average age of 45 years at the start of the study, had no history of IHD or stroke.

The researchers assigned each participant to one of three groups:

• Meat eaters: people who reported eating meat
• Fish eaters: those who ate fish but no meat
• Vegetarians and vegans: people who did not eat meat or fish

The team combined vegans with vegetarians for the main analysis due to the small number of vegans in the dataset.

Using food questionnaires, the researchers could also assess overall food intake and nutrient levels. Aside from dietary information, they collected information about factors such as body mass index (BMI), height, and blood pressure.

A double edged sword

During the 18.1 years of follow-up, there were 2,820 cases of IHD and 1,072 cases of stroke.

After adjusting for sociodemographic and lifestyle factors, the analysis revealed both positive and negative relationships between cardiovascular health and reduced meat intake.

The rate of IHD among pescatarians was 13% lower than that of meat eaters, while vegetarians had a rate that was 22% lower. To put these numbers into perspective, the authors explain:

"This difference was equivalent to 10 fewer cases of ischemic heart disease [...] in vegetarians than in meat eaters per 1,000 population over 10 years."

According to the authors, this positive association appears to be, at least partly, due to lower rates of hypertension and diabetes, as well as lower BMI and cholesterol levels. However, even after the scientists had adjusted the data to account for these factors, the effect was still "marginally significant."

Conversely, vegetarians had 20% higher rates of stroke than meat eaters. This difference is equivalent to three more cases of stroke per 1,000 people over 10 years. This association was mostly due to hemorrhagic stroke rather than ischemic stroke.

No previous studies have shown this type of relationship between vegetarianism and stroke risk. The authors believe that this might be because earlier work reported stroke mortality rather than incidence. Strokes are only fatal in 10–20% of cases, so many cases would not count toward the reported total.

Why the scientists saw this increase in stroke risk is up for debate. The authors believe that it might be due to lower levels of other circulating nutrients in the blood of vegetarians. These might include essential amino acids and vitamins B-12 and D.

Strengths, limitations, and further work

The study has a number of strengths; first and foremost, the researchers used a large sample size and a long follow-up period. They also linked participants to their medical records to ensure the accurate collection of health outcomes.

In addition, the researchers checked the participants' eating habits at two time points that were years apart, finding that adherence was good overall.

However, there were certain limitations. For instance, the participants self-reported their diet, which leaves room for error and misreporting. Diet can also fluctuate over days, weeks, and years.

Also, researchers did not have access to the use of drugs, including statins, among participants.

As the study is observational, it is not possible to conclude that the effect is causal. In other words, the changes in risk could be due to other factors that the scientists did not measure.

Lastly, because the participants were predominantly European and white, the findings may not be widely applicable.

An editorial by Prof. Mark A. Lawrence and Prof. Sarah A. McNaughton from Deakin University in Australia accompanies the paper.

In it, the authors call for caution, explaining how the conclusions are "based on results from just one study, and the increase is modest relative to meat eaters."

They also explain that studies "have reported mostly protective associations between vegetarian diets and chronic disease risk factors."

These results are sure to open debate and spark more research. That vegetarianism protects against IHD is not surprising given past findings. However, the fact that giving up meat might slightly increase stroke risk is unexpected. More work is sure to follow.