Interest in plant-based eating is surging in the US. As more people lean towards meat-free lifestyles that promote both human and planetary health, two new studies link vegetarian and mostly-vegetarian diets to some unexpected outcomes.
In a recent prospective cohort study, investigators at the University of Oxford examined the occurrence of heart disease and stroke in a group of more than 48,000 meat eaters, pescetarians, and vegetarians.
Like many others reseaerchers, the Oxford team found that fish eaters and vegetarians, including vegans, were less likely than carnivores to develop ischemic heart disease––also called coronary heart disease (CHD). In pescetarians and vegetarians, rates of ischemic heart disease were 13% and 22% lower than in meat-eaters (Tong, T. et al. BMJ. 2019; 366: 14897). No big surprise there.
But the Oxford group did report a very surprising discovery: stroke risk was greatest among non-meat eaters.
The vegetarians had a 20% higher total stroke rate than the carnivores. The researchers note that of the two main types of stroke––the more common ischemic stroke and rarer hemorrhagic stroke––vegetarians were at increased risk of the latter.
An Unexpected Finding
The findings are based on analysis of data from EPIC-Oxford, a component of the larger ongoing European Prospective Investigation into Cancer and Nutrition (EPIC) study. The EPIC-Oxford study group includes 65,000 men and women living in the United Kingdom, many of whom are vegetarians.
“Vegetarian diets have become increasingly popular in recent years, partly due to the perceived health benefits, but also concerns about the environment and animal welfare,” said nutritional epidemiologist, Tammy Tong, PhD, lead author of the Oxford study. “However, the full extent of the potential health benefits and hazards of these diets is not well understood.”
“The higher risk of stroke in vegetarians is still small compared to their lower risk of coronary heart disease.”–Tammy Tong, PhD
Tong’s team based their conclusions on an extensive review of diet and health history information from 48,188 individuals recruited for EPIC-Oxford between 1993 and 2001. Participants were an average age of 45 years old and had no prior history of either CHD or stroke at enrollment.
She and colleagues grouped study participants into three categories based on dietary preference: meat eaters, or those who consumed meat regardless of whether they also consumed fish, dairy, or eggs (n=24,428); pescetarians who consumed fish but no other meat (n=7,506); and vegetarians/vegans (n=16,254).
Owing to the small number of strictly vegan participants, all lacto-vegetarians, lacto-ovo-vegetarians, and vegans were combined into a single catchall “vegetarian” group in the study’s main analyses. Most participants across groups had been following their reported current diets for more than five years at the time of recruitment.
Over an 18-year follow up period, investigators identified 2,820 new cases of ischemic heart disease and 1,072 total cases of stroke (519 ischemic stroke and 300 hemorrhagic stroke) in those included in the study.
Evaluating Relative Risk
Though the Oxford group’s observations about stroke risk do raise important questions, it is important to put them in context. First, this is an epidemiological analysis based on participant self-reporting about dietary habits. In no way should the study be dumbed down into simplistic syllogisms like “Veganism causes stroke”
Secondly, the Oxford group’s findings, published in September 2019 in the British Medical Journal, echo the vast body of evidence that overall, vegetarian diets do improve cardiovascular health.
Plant-based eaters often cite improved heart health as a top reason for eschewing meat, and there is a mountain of epidemiological data to support that assertion.
While there are many studies of the impact on vegetarianism on the heart, there are far fewer looking specifically at stroke risk. The Oxford group is one of the first to quantify stroke rates in meat versus meat-free eaters.
“There is limited prior evidence on stroke, especially subtypes of stroke, but some older studies from Japan have found that people with very low intake of animal products had higher risks of hemorrhagic and total stroke, which is consistent with our findings,” Tong told Holistic Primary Care.
But there’s no question that the protective benefits of vegetarianism against CHD largely outweigh its potential for raising stroke risk.
“It is important to note that in the current study, the absolute risk difference was equivalent to 10 fewer cases of coronary heart disease and 3 more cases of total stroke in the vegetarians than the meat eaters, in every 1,000 people consuming these diets over 10 years,” Tong said.
Put simply, “the higher risk of stroke in vegetarians is still small compared to their lower risk of coronary heart disease.”
CVD kills more people in the US than any other condition, accounting for nearly a quarter of all lives lost annually. Because CHD is the most common type of heart disease, the need to identify interventions that reduce CHD risk remains urgent.
Limiting or eliminating meat consumption is a promising potential solution. But Tong cautioned that lack of data from large-scale intervention studies make it challenging to fully understand the impact—positive and negative—that vegetarian and vegan diets have on human health.
“Previous evidence has also shown vegetarians had lower risks of diabetes, but there is limited evidence for most other chronic conditions,” she said, underscoring the need for further research into meatless diets.
Dr. Tong and her colleagues aren’t exactly sure why stroke risk was higher in the vegetarians. They propose that nutritional deficiencies could be a contributing factor.
The vegetarians and vegans in the EPIC-Oxford study had lower circulating levels of several important nutrients, including vitamin B12, vitamin D, essential amino acids, and long chain n-3 polyunsaturated fatty acids. The authors argue that differences in those or other nutritional factors might have contributed to the elevated stroke risk.
Obtaining adequate vitamin B12 is a challenge for many vegetarians and vegans, since it comes primarily from animal products. Problematically, “low levels of vitamin B12 are associated with higher homocysteine levels, which are also linked to higher risk of stroke,” Tong hypothesized. This, too, is an emerging association warranting additional study before anyone can make conclusive clinical recommendations.
The Oxford findings “do not contradict the evidence on primarily plant-based diets and better health.” —Walter Willett, MD
Cholesterol status might also play a role in stroke risk–– but not in the way most would expect.
“Recent evidence has shown that while low cholesterol levels are protective against both heart disease and ischemic stroke, very low levels may be linked to higher risk of hemorrhagic stroke, the subtype found to be higher in the vegetarians,” Tong said.
She hopes that future studies involving different populations will help scientists better identify the connections between cholesterol, dietary factors, and stroke risk.
Filling Nutritional Gaps
According to Harvard professor Walter Willett, MD, the Oxford findings “do not contradict the evidence on primarily plant-based diets and better health.”
Willett heads the EAT-Lancet Commission, an international group of independent medicine, nutrition, agriculture, and environmental science experts studying the impacts of global food production systems on human and environmental health.
Earlier this year, the group issued a report in The Lancet entitled Food in the Anthropocene: The EAT-Lancet Commission on Healthy Diets from Sustainable Food Systems, which recommends widespread adoption of plant-based diets as a crucial strategy for preventing major worldwide health and environmental crises.
Echoing Tong’s concerns about adequate nutrition, Willett explained, “We do know that strictly vegan diets do leave some nutritional gaps,” with vitamin B12 being the most important.
“Also, just avoiding animal products does not mean that a diet is healthy,” he pointed out. A plant-based diet might eliminate meat, but it may still be high in refined starches, sugars, and unhealthy fats, which pose separate health risks of their own.
“As seen in the recent Oxford study, there have been data for some time that diets very low in cholesterol and saturated fats may increase risk of hemorrhagic stroke,” Willett said. What researchers do not yet know for certain is whether cholesterol and other lipids are to blame for the observed risk patterns, or if other dietary factors like excessive carbohydrate consumption are the real culprits.
“We need to understand this better,” he told Holistic Primary Care.
In the meantime, researchers typically agree on the fundamentals of a healthy diet.
“Most current dietary guidelines recommend a high consumption of plant-based foods, including fruits and vegetables, whole grains, legumes, and a low consumption of red and processed meat,” Tong said. “Dietary patterns such as the Mediterranean diet or the Nordic diet, which incorporate these guidelines, have been linked to lower risks of some chronic diseases.”
A second paper published this Fall examined another potential nutritional consequence of plant-based eating: choline deficiency.
In BMJ Nutrition, Prevention & Health, Emma Derbyshire, PhD, director of UK-based Nutritional Insight, a consulting firm specializing in nutrition and biomedical science, argued that vegetarians are more likely than meat eaters to suffer from health complications related to insufficient choline intake (Derbyshire, E. BMJ Nut Prev Health. 2019; Epub ahead of print. doi: 10.1136/bmjnph-2019-000037).
Since endogenous choline synthesis cannot sufficiently meet our health needs, we must obtain it either from our food or via dietary supplements––and therein lies the challenge for vegetarians–Emma Derbyshire, PhD
Similar to omega-3 fatty acids, choline is an essential nutrient that the body requires but cannot produce enough of endogenously. Choline fulfills many critical functions; it is involved in neurocognition, lipid metabolism, liver function, and homocysteine regulation. Low choline status is linked to conditions ranging from liver disease to potential neurological disorders and cognitive impairment.
Deficiency may result from low dietary intake, but other factors like genetic polymorphisms and hormone deficiencies can also impact choline status.
The health risks associated with inadequate choline are greater in certain groups and at crucial stages of the life cycle, particularly during pregnancy and lactation, when adequate choline is vital for infant development.
Since endogenous choline synthesis cannot sufficiently meet our health needs, we must obtain it either from our food or via dietary supplements––and therein lies the challenge for vegetarians, Derbyshire says.
But her claim that vegetarians are at increased risk of deficiency is based largely on conjecture, not new epidemiological data. She posits that since choline comes largely from animal-derived foods, and more people are eschewing these in favor of plant-based alternatives, they could be putting themselves at risk.
Animal foods do contain more choline per unit weight than plants. Beef, chicken, fish, eggs, and milk are among the richest sources, although plant foods, including some grains, nuts, and cruciferous vegetables, do provide smaller amounts of the important nutrient.
So, Derbyshire’s supposition about vegetarians is certainly plausible, though not definitively proven.
In 1998, the US Institute of Medicine (IOM) was the first health organization to establish choline intake recommendations. Nearly two decades later, a 2016 analysis of National Health and Nutrition Examination Survey (NHANES) data showed that only 11% of Americans age two and older met the recommended intake for choline. Intake was significantly higher in males than females across all age groups, suggesting that women are likelier than men to be choline-deficient (Wallace, T & Fulgoni, V. J Am Coll Nutr. 2016; 35(2): 108-12).
Derbyshire cautions that the growing popularity of plant-based eating could lead to even lower average choline intake.
“Accelerated food trends towards plant-based diets/veganism could have further ramifications on choline intake/status. Eggs, milk and meat appear to be major dietary providers and further movements away from the consumption of these could have unintended consequences.”
She argues that if people don’t get enough choline from their diets, “supplementation strategies will be required,” especially for pregnant and nursing mothers.
Nutritional Safety Nets
Other nutrition experts are less concerned about widespread choline deficiency among plant eaters.
Reviewing Derbyshire’s analysis, Dr. Willett says he “would not recommend choline supplements at this time.” He does not buy the argument that people need animal products in order to get enough choline.
“Choline is essential, but we can get a fairly good amount from a healthy plant-based diet, and we don’t have evidence that more is beneficial,” Willett explained. He pointed out that some studies even show correlations between higher choline intake and poor health outcomes.
“Of course, pregnancy and early childhood are times of increased nutritional demand, and we need more data on choline intake during these periods of life.”
By and large, the weight of evidence still supports the position that vegetarianism and mostly-vegetarian whole food diets promote good health. A widespread shift to plant-based eating will definitely reduce the suffering of animals and the environmental impact of human food production.
According to Willett, nothing about these two recent studies on stroke risk and choline deficiency changes these general conclusions.
For most people, the benefits of going plant-based far outweigh the small risks.
But following a plant-based diet does not necessarily have to mean giving up meat or animal products altogether. “A small amount of animal-sourced foods––for example, two servings per day with one being dairy foods––is also healthy and can minimize chances of being short on beneficial dietary factors,” Willett said.
Reducing meat products is an important step toward healthful eating––but what’s even more important is what replaces the meat. When patients cut animal products from their diets, Willett encourages them to swap in “a mix of nuts, legumes, soy foods, fruits, vegetables, whole grains, and healthy fats, as these foods have positive health effects.”
Fish and a modest amount of dairy are other good options for patients who are comfortable eating some animal foods.
For vegans, Willett recommends “a multiple vitamin and/or multi-mineral supplement, most importantly to ensure adequate B12 intake.” Supplementing with multivitamins and minerals “provides a nutritional safety net for other essential nutrients” in patients who entirely avoid all animal-sourced products.