Plant-based eating was not linked to stroke in 14,776 US adults, and results for heart disease were mixed

Eating more plants and fewer animal foods is widely thought to protect the heart and blood vessels. A long-running American study designed to include the people at highest risk of stroke has tested that idea. For stroke it found no link, however plant-based eating was scored.

The paper, published in August in the American Journal of Clinical Nutrition, did find that a healthy plant-based diet went with fewer cases of coronary artery disease. But a broader score that counts all plant foods alike pointed in two directions at once, with fewer coronary deaths and more new coronary cases. The authors’ own word for that pattern is heterogeneous, meaning mixed.

Why plant-based diets were tested in this stroke study

Most of what is known about plant-based diets and cardiovascular disease comes from a few large cohorts, several of them made up of health professionals. The new paper starts from who has been missing. Black and rural Americans carry a heavy burden of cardiovascular illness and death, and the authors write that both groups have had limited representation in plant-based diet research. They add that the level of protection may differ by dietary composition and individual characteristics, so results from one population may not transfer to another.

Unlike the cohorts in which plant-based scores were previously tested, the study they used was set up with exactly these groups in mind. It is called REasons for Geographic and Racial Differences in Stroke, or REGARDS. Its founding paper describes a national study of African-American and white adults aged 45 and older and states the purpose plainly: the objective is to determine the causes for the excess stroke mortality in the Southeastern US and among African-Americans. Mortality means deaths. The southeastern states with the highest stroke death rates are often called the stroke belt, and an earlier REGARDS analysis of 16,068 people reported that 33.6% of participants were Black and that 34.0% were residents of the stroke belt.

REGARDS is a cohort study. A cohort study follows a group of people over time and records who develops disease. Under the original design, participants are followed via telephone at 6-month intervals for identification of stroke events.

How the study scored plant-based diets

The analysis, led by Emily Belarmino with Suzanne Judd as senior author, included 14,776 adults. Their diets were recorded at the start, between 2003 and 2007. The three scores were, in the paper’s words, calculated from food frequency questionnaires. A food frequency questionnaire is a long checklist that asks how often a person ate each of many foods over the past year.

The scoring system was set out in a 2017 paper based on the Nurses’ Health Study and the Health Professionals Follow-up Study, two long-running US cohorts. Its authors began from the observation that not all plant foods are necessarily beneficial for health. Each score treats food groups differently:

• Overall plant-based diet index (PDI). Every plant food raises the score, whether it is lentils or white bread. Animal foods lower it.
• Healthy plant-based diet index (hPDI). In that paper’s definition, healthy plant foods (whole grains, fruits/vegetables, nuts/legumes, oils, tea/coffee) received positive scores. What they called less-healthy plant foods (juices/sweetened beverages, refined grains, potatoes/fries, sweets) and animal foods received reverse scores.
• Unhealthy plant-based diet index (uPDI). The less healthy plant foods raise the score. Healthy plant foods and animal foods lower it.

None of these scores identifies vegetarians or vegans. A person can score high on any of them while eating meat. The scores rank people by the balance of their diet.

Participants were divided into fifths on each score, and the top fifth was compared with the bottom fifth. The comparisons were made after adjusting for sociodemographic characteristics, health risk factors, and caloric intake. Sociodemographic characteristics are things such as age, sex, race, income and education.

Three outcomes were counted through 2019: stroke, new cases of coronary artery disease (CAD), and deaths from it. Coronary artery disease is narrowing of the arteries that supply the heart, the usual cause of heart attacks. Over the follow-up there were 700 strokes, 1,047 new cases of coronary artery disease and 944 coronary deaths.

What the study found about plant-based diets and stroke

On stroke the result was the same for all three scores. The paper’s summary is a single sentence: plant-based diets were not associated with stroke.

That held for the overall score, the healthy score and the unhealthy score. People whose diets were richest in whole plant foods had strokes at about the same rate as those whose diets were poorest in them, once other characteristics were taken into account.

Nor was the answer different for Black and white participants. Across all the outcomes, the paper reports, associations did not differ by race.

What the plant-based scores showed for heart disease

For the heart, the three scores gave three different answers.

The healthy score. People in the top fifth for healthy plant-based eating had a 28% lower risk of developing coronary artery disease than those in the bottom fifth, with a range from 10% to 42%. A range here is the margin of uncertainty around an estimate. For coronary deaths the picture depended on sex. A statistical test for interaction, which checks whether an association differs between subgroups, revealed an inverse association between hPDI and mortality among females only. Inverse means that as the score went up, deaths went down.

The overall score. Here the results pulled apart. People in the top fifth had a 27% lower risk of dying of coronary artery disease, with a range from 10% to 41%. Yet the same top fifth had a 60% higher risk of being newly diagnosed with it, with a range from 9% to 133%. That second result was confined to one group. The interaction test found the pattern differed by where people lived, with higher PDI associated with higher CAD among nonrural adults only.

The unhealthy score. It was not associated with CAD incidence or mortality in either direction. Incidence means the rate of new cases.

The authors do not claim to have resolved the contradiction in the overall score. Their conclusion is that greater adherence to healthy plant-based diets went with lower coronary risk, but overall plant-based diets had heterogeneous associations with CAD incidence and mortality. The last line of their summary is modest: healthy plant-based diets appear to support heart health.

One feature of the overall score may be relevant. Because it rewards any plant food, a diet heavy in refined grains, fries and sweetened drinks can score as high as one built on beans and vegetables. Two people in the top fifth may be eating very differently.

How this fits with other studies of plant-based diets, stroke and the heart

The heart result for the healthy score matches the study that introduced these scores. In more than 200,000 US health professionals with 8,631 cases of coronary heart disease, the highest healthy scores went with a 25% lower risk, with a range from 17% to 32%. The overall score showed a weaker link of 8%, with a range that reached to no difference.

On stroke the same cohorts did find something. With 6,241 strokes to analyze, people with the highest healthy scores had a 10% lower risk, with a range from 2% to 17%. The authors’ conclusion was that people who adhered to a healthful plant-based diet had a lower risk of stroke. The overall score showed no clear link. A study with 700 strokes is much less able to detect a difference of that size than one with 6,241, so the two results do not necessarily conflict.

Stroke has been the awkward outcome for plant-based eating before. EPIC-Oxford is a cohort in the United Kingdom with a large proportion of non-meat eaters. Over 18 years, vegetarians had 22% lower rates of heart disease than meat eaters, and 20% higher rates of stroke, with a range from 2% to 40%. That amounted to about three more strokes per 1,000 people over 10 years, mostly strokes caused by bleeding. The associations for stroke did not attenuate after further adjustment of disease risk factors. Attenuate means weaken.

Another US cohort has reported a split result of its own. In about 12,000 middle-aged adults followed for nearly three decades, the scoring worked the same way: in all indexes, higher intakes of animal foods received lower scores. The overall plant score went with 16% less cardiovascular disease. The healthy score went with 19% fewer cardiovascular deaths but showed no link to new cases, which is the reverse of the pattern in REGARDS.

REGARDS itself has looked at diet in a different way before. An analysis from 2015 let the data sort foods into patterns. One of them, the Southern pattern, was characterized by added fats, fried food, eggs, organ and processed meats, and sugar-sweetened beverages. Its heaviest consumers had a 56% higher risk of acute coronary heart disease. A later REGARDS paper found that people eating most in line with a plant-based pattern had a 41% lower risk of being hospitalized for heart failure.

What the plant-based diet and stroke study cannot show

Association only. People who eat more whole plant foods differ from those who do not in income, education, exercise and much else. The analysis adjusted for many such characteristics, and others may remain.

Diet was measured once, by recall. A questionnaire filled in between 2003 and 2007 stood for what people ate over the following decade or more. Diets change, and people misremember.

Scores, not diets. The indexes rank the balance of foods. They do not pick out vegetarians, vegans or any named diet, and the same score can describe very different plates.

Fewer strokes than the largest studies. With 700 strokes spread across five groups, a modest link could have gone undetected. A null result here does not rule one out.

Subgroup results are fragile. The link in women only, and the link in nonrural adults only, each came from testing subgroups. Such results are more likely than main results to arise by chance, and they need confirming in other studies.

A contradiction left standing. A score that goes with fewer coronary deaths and more coronary diagnoses in the same people is hard to interpret. It could reflect chance, differences in who gets diagnosed, or the mixed quality of diets that score high. The study cannot say which.

Who was studied. Participants were Black and white adults aged 45 and over when they joined. The results may not apply to younger people or to other groups.

Whether a plant-based way of eating suits a particular person, and what it means for their own risk of stroke or heart disease, is assessed by their doctor.

In a study built around the Americans most at risk of stroke, the healthy plant-based score tracked fewer coronary cases and no score tracked stroke.