BOSTON, MA — A new study suggests that to reduce the risk of coronary heart disease it is simply not enough to eat a plant-based diet but that the foods in that diet must be high quality.
After 2 decades of follow-up in more than 200,000 adults, researchers found that adherence to a plant-based diet rich in whole grains, fruits, vegetables, nuts, and legumes was associated with a substantially lower relative risk of coronary heart disease (CHD), whereas following a plant-based diet emphasizing less healthy foods such as refined grains and sugar-sweetened beverages had an adverse effect.
Their findings were published July 17, 2017 in the Journal of the American College of Cardiology.
“I think an important contribution of this paper is about the public-health message,” lead author Dr Ambika Satija (Harvard School of Public Health, Boston, MA) told theheart.org | Medscape Cardiology. “Just because you’re vegetarian or eating more plant-based foods doesn’t necessarily mean you have a healthy diet. It’s important to think about the quality of foods you’re consuming; more whole grains rather than refined grains, more whole foods rather than juices—that’s the right direction to take.”
She noted that this approach is already reflected in the latest 2015–2020 Dietary Guidelines for Americans, which recommends focusing on nutrient-dense foods across all food groups.
Dr Alice Lichtenstein (Friedman School of Nutrition Science and Policy, Tufts University, Boston, MA) was vice-chair of the US dietary guidelines committee. “What the guidelines said is that there are a number of different dietary approaches—Mediterranean, [Dietary Approaches to Stop Hypertension] DASH, or vegetarian, which will get you to the same end, and that’s not inconsistent with what this is saying,” Dr Lichtenstein said, commenting on the current findings.
“Because it doesn’t matter what basic diet you consume; but if you consume more plant-based foods, you’re going to have a better outcome,” she added. “The new factor here is that not all plant-based diets are created equal and you have to use some judgement in choosing the plant-based foods you’re going to include in that diet, just as you use judgment in the animal foods you include in your diet.”
Previous studies have linked plant-based diets with a lower risk of CHD but have defined these diets dichotomously as being vegetarian or not and treated all plant foods equally, the investigators note in the article.
To overcome these limitations and understand how gradual reductions in animal foods affect cardiovascular health, Satija and colleagues examined data from semiquantitative food-frequency questionnaires including about 133 foods collected every 2 to 4 years from 73,710 women in the Nurses’ Health Study (NHS), 92,329 women in the NHS2, and 43,259 men in the Health Professionals Follow-Up Study.
The data were collapsed into 18 food groups within three larger categories (healthy plant foods, less healthy plant foods, and animal foods), and then ranked into quintiles.
Positive scores were assigned for healthy plant foods (whole grains, fruits/vegetables, nuts/legumes, vegetable oils, tea/coffee) and reverse scores assigned for less healthy plant-based foods (fruit juices, refined grains, potatoes, sugar-sweetened beverages, sweets/desserts) and animal foods (animal fat, ice cream, meat, miscellaneous animal-based foods). Group scores were summed to create plant-based diet indices.
The indices ranged from a median of 42–44 in the lowest decile to a median of 66–68 in the highest decile. Animal food intake ranged from three to four servings per day in the lowest decile to five to six servings per day in the highest decile.
Participants with higher scores on the plant-based diet index (PDI) and healthy PDI (hPDI) were older, more active, leaner, and less likely to smoke than those with lower scores. Concerningly, high consumers of an unhealthy PDI (uPDI) were younger, less active, and more likely to smoke.
Over 4,833,042 person-years of follow-up, 8631 participants developed CHD, defined as nonfatal MI and fatal CHD.
After full adjustment for relevant covariates, adherence to PDI was inversely associated with CHD (hazard ratio [HR] 0.92 comparing extreme declines; 95% CI 0.83–1.01). The association was modest, but Satija said this makes sense because the PDI is an aggregate, and thus participants who have a higher intake of both healthy and unhealthy plant foods may have a higher score on the PDI.
“This is kind of getting at the idea that, okay you’re a vegetarian, but if we don’t know the quality of plant foods you’re consuming, we don’t know what your risk profile for CHD is going to be,” she added.
When the hPDI and uPDI were analyzed separately, however, the inverse association was considerably stronger for hPDI, lowering the relative risk of CHD by 25% (HR 0.75 comparing extreme deciles; 95% CI 0.68–0.83, P<0.001 for trend). At the same time, uPDI was positively associated with a 32% higher relative risk of CHD (HR 1.32 comparing extreme deciles; 95% CI 1.20–1.46, P<0.001 for trend).
The associations of hPDI and uPDI with CHD risk were consistent across age, BMI, family history of CHD, and sex.
Associations of both indices were significantly stronger among more active relative to less active participants (P for interaction=0.002 for both), a finding that should be interpreted with caution, Satija said. “We don’t know exactly what’s going on and it would be good to look in an intervention at what happens if you do both a physical intervention and dietary modifications.”
To quantify the benefit of the hPDI due to lower red meat intake, the final model was individually adjusted for red meat and the results were largely unchanged (HR 0.93 for extreme PDI deciles; 95% CI 0.84–1.03).
“For those people who want to improve their diets, want to have a vegetarian or vegan diet but think that change is too extreme or that they won’t be able to make this big lifestyle change, this is good news,” Satija said. “Because even if they reduce the amount of animal foods by a couple of servings per day, they still benefit in terms of CHD risk.”
In an accompanying editorial, Drs Kim Allan Williams and Hena Patel (Rush University Medical Center, Chicago, IL) concur that it’s not an all-or-nothing proposition: “Just as physical exercise is a continuum, perhaps an emphasis on starting with smaller dietary tweaks rather than major changes would be more encouraging and sustainable.”
Although the study can’t address the benefits of a purely vegan diet, they note that it “adds to the evidence of gradations of adherence to an overall PDI with CHD incidence, such that one could propose a risk-based approach to PDI prescription: secondary prevention after cardiovascular events and patients at high risk having a stronger recommendation for a strictly hPDI.”
Finally, while a plant-based diet is more environmentally sustainable, Williams and Patel point to the potential implications of a healthier diet on downstream healthcare costs. “If, for example, widespread adoption of plant-based nutrition reduced the incidence of hypertension to 25% of the current rate, this could result in savings of nearly 30% of the Medicare budget.”
The study was supported by research grants from the National Institutes of Health. Satija reported no financial relationships. Disclosures for the coauthors are listed in the paper. Lichtenstein, Williams, and Patel reported no relevant conflicts of interest.
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