With a diet devoid of animal fats, vegans seem unlikely candidates for heart disease.
But new research suggests that people who eat only plant foods may lack nutrients vital for cardiovascular health.
Vegan diets tend to be low in iron, zinc, vitamin B12 and omega-3 fatty acids, according to a review of dozens of published articles on the biochemistry of vegetarianism, published in the Journal of Agricultural and Food Chemistry.
As a result, vegans tend to have high blood levels of homocysteine and low levels of HDL, or "good," cholesterol. Both are risk factors for heart disease, notes the study's lead author, Duo Li, a professor of nutrition at Zhejiang University in Hangzhou, China.
Dr. Li found that vegans also have a high platelet volume, which is associated with heart attacks, and increased platelet aggregation - a clumping of platelets that can lead to blood clots.
"From our study, I do not encourage people to be vegan," he says.
The research compared data on vegan populations worldwide. Dr. Li found that many vegans, especially in Asian countries, are against taking supplements, often for religious reasons.
"We're trying to encourage vegans to balance their diets," Dr. Li says, "and for vegan families, especially with infants, you definitely should take supplements" to compensate for nutrient deficiencies.
Other experts dispute the link between veganism and cardiovascular disease.
Markers for heart disease do not necessarily indicate higher rates of actual disease among vegans, says David Jenkins, a professor of nutritional sciences at the University of Toronto.
Dr. Jenkins points out that the role of "good" cholesterol is to carry "bad" cholesterol away from the arterial wall to the liver. For vegans whose diet is low in artery-clogging fats, he says, "the good cholesterol doesn't have to be high."
The jury is out regarding homocysteine levels as well, he says. In studies, folic acid and B12 supplements given to patients to reduce homocysteine levels "have not necessarily changed the heart disease or stroke risk."
Dr. Jenkins, a vegan for many years, says he monitors various indicators in his blood "and when it dips low, I start taking supplements again."
Other vegans eschew pills of any kind, including Aaron Ash, owner of Gorilla Food, a raw-food restaurant in Vancouver that has gained a cult following.
Mr. Ash says he has read enough books on plant-based nutrition to understand the importance of a balanced diet. Processed foods such as vegan chips and tofu dogs are low in nutrition, including essential fatty acids, he says. "You can eat a pretty junkie diet as a vegan."
Mr. Ash says he isn't concerned about being deficient in nutrients, even though he hasn't had his blood checked. His diet includes nuts and hemp seeds, rich in omega-3 fatty acids, and blue-green algae, a source of B12 that he eats about six days a week. "It's so easy to put in a smoothie," he says.
But plant foods, including seaweeds, do not provide adequate B12, according to Vesanto Melina, a registered dietitian in Langley, B.C., and co-author of Becoming Vegan. In fact, blue-green algae contains an analogue of B12 that interferes with B12 absorption, she says. "It can fool the body."
Ms. Melina urges vegans to eat B12-fortified foods or take B12 supplements. As well, she recommends plant sources of omega-3s such as flax, hemp and chia seeds, walnuts and canola oil.
A vegan diet is generally a positive step towards lowering the risk for heart disease, Ms. Melina says, adding that she disagrees with Dr. Li's conclusions. "It just depends what studies they looked at."
But when vegans don't include sufficient omega-3s and B12 in their diets, she warns, "you undermine the whole thing."