Vegetarians and pescetarians (people who eat fish but not meat) have a lower risk of developing coronary heart disease than meat-eaters, according to a study published late last week in The BMJ. But vegetarians — including vegans — are at a higher risk of having a stroke, the study also reports.
These findings are based on data collected during an 18-year period from more than 48,000 British adults, of whom about a third were vegetarians.
Vegetarians should not be alarmed, however. Although large and long running, the study is observational, which means it can’t prove that diet was the direct cause of the differences in disease risk. Other factors in the lives of the study’s participants — factors not addressed in the study — may also explain the results.
Also, the decreased risk of coronary heart disease observed in this study outweighs the increased risk of stroke, particularly given that heart disease is much more common than stroke.
Some experts have expressed skepticism about the stroke finding. Dr. Malcolm Finlay, a consulting cardiologist at Queen Mary University of London who was not involved in study, told the Science Media Centre that the study’s authors put “too much weight on a complex statistical method to try and correct for the fact that the vegetarians were very much healthier than meat eaters.”
“So, while this method can say the risk of stroke isn’t as low as one might expect it to be in vegetarians considering how much healthier they are in general compared to meat-eaters, their overall risk of a major life-changing cardiovascular event happening still appears much lower,” he added.
Unanswered questions
As background information in the study points out, vegetarian diets, including veganism, have become increasingly popular in recent years, not only because of their perceived health benefits but also for reasons having to do with global warming and growing concerns about animal welfare.
The potential benefits and risks of vegetarian diets are not fully understood, however. Previous research has suggested that vegetarians are at lower risk for heart disease, for example, but little is known about how vegetarian diets affect the risk of death from stroke.
The authors of the current study, a team led by epidemiologist Tammy Tong of the Nuffield Department to Population health at the University of Oxford, set out to address that research gap.
How the study was done
For the study, the researchers analyzed data collected from 48,188 adults living in the United Kingdom (average age: 45) who had been recruited between 1993 and 2001 into the ongoing EPIC-Oxford study, which is investigating the long-term effects of diet on health. None of the participants had a history of coronary heart disease or stroke when they entered the study.
At the start of the study, the participants filled out lengthy questionnaires, which included detailed questions about their diet. Based on their answers, 24,428 of them were categorized as meat eaters, 7,506 as pescetarians and 16,254 as vegetarians (a group that included vegans). All were sent a second questionnaire in 2010 to see if their eating habits had changed.
The study followed the participants for an average of 18 years. During that period, 2,820 of the participants developed coronary heart disease (caused by plaque buildup in the walls of the arteries that feed blood to the heart) and 1,072 had a stroke.
The stroke cases involved 519 ischemic strokes (ones in which a blood clot blocks the flow of blood and oxygen to the brain) and 300 hemorrhagic strokes (ones in which blood from an artery begins bleeding into the brain).
Key findings
Tong and her colleagues analyzed all this data to determine if there were any associations between type of diet and the risk of coronary heart disease and stroke. First, however, they adjusted the results to take into account other factors known to increase the risk of those two conditions, such as smoking, alcohol use, physical activity, use of dietary supplements and (in women) use of oral contraceptives and menopausal hormone therapy.
Here’s what they found:
- The vegetarians in the study had a 22 percent lower risk of coronary heart disease than the meat eaters. Over a 10-year period, that drop in risk means there would be 10 fewer cases of coronary heart disease among vegetarians than among meat eaters for every 1,000 people.
- The pescetarians had a 13 percent lower risk of coronary heart disease than the meat eaters. That equals 5.8 fewer cases of the disease for every 1,000 people over 10 years.
- Vegetarians had a 20 percent increased risk of stroke compared to meat eaters. That’s the equivalent of three more cases of stroke per 1,000 people over 10 years. Most of the increase in stroke risk among the vegetarians was for hemorrhagic stroke
- Pescetarians had no increased risk of stroke.
“Overall,” Tong and her colleagues conclude, “the present study has shown that UK adults who were fish eaters or vegetarians had lower risk of [coronary] heart disease than meat eaters but that vegetarians had higher risk of stroke.”
Possible explanations
The reason for the lower risk among vegetarians for coronary heart disease is most likely due to the fact that they tend to have a lower body mass index (BMI) as well as lower blood pressure and cholesterol levels, the researchers point out. Vegetarians are also less likely to develop type 2 diabetes, a disease known to raise the risk of heart disease.
What might be behind the observed link between vegetarianism and a higher risk of stroke is more perplexing, particularly given that the vegetarians in this study had, on average, lower blood pressure than the meat eaters. High blood pressure is a major risk factor for stroke as well as for coronary heart disease.
The vegetarians in the study did tend to have lower levels of certain nutrients (such as vitamin B12), as well as lower levels of circulating cholesterol. Those differences might explain the higher risk of stroke, the researchers say. In other studies, low levels of LDL cholesterol (the so-called bad cholesterol) have been connected to a higher risk of hemorrhagic stroke.
That explanation is somewhat problematic, however. In other studies, low LDL levels raised the risk of stroke primarily when it occurred alongside high blood pressure. That wasn’t the case in this study.
Not the final word
Tong and her colleagues call for more research to see if the findings in this study can be replicated, particularly in more diverse populations.
In the meantime, as an editorial that accompanies the study stresses, no one should be altering their diet based on this (or any other) single study. Furthermore, the increased stroke risk identified in this study needs to be kept in perspective. “It is based on results from just one study and the increase is modest relative to meat eaters,” the editorial states.
FMI: You can read the study in full on The BMJ’s website. The BMJ was formerly known as the British Medical Journal.
