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Healthy Plant-Based Diet
What Does it Really Mean? Editorial - Healthful and Unhealthful Plant-Based Diets and the Risk of Coronary Heart Disease in U.S. Adults
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Download the PDF here
Download the PDF here
Kim Allan Williams Sr., Hena Patel - JACC July 2017
Despite improvements in cardiovascular mortality rates over the past several decades, cardiovascular disease (CVD) remains a leading cause of death in the United States. Multiple studies have linked dietary patterns with incidence of CVD and found that groups consuming predominantly plant-based foods, versus animal-based, have lower rates of heart disease (1,2). Plant-based dietary patterns are becoming increasingly popular because of a variety of reported health benefits to overall health and cardiovascular risk and disease in particular (3,4).
However, the concept of "plant-based diet" varies widely in definition, ranging from exclusion of all animal products (3) to only having "high factor loadings for vegetables, fruits, fruit juice, cereal, beans" while including "fish, poultry, and yogurt" (5).
Some plant-based diets reduce or eliminate intake of highly refined plant foods such as white flours, sugars, and oils. Other publications categorize plant-based diets by actual content, (e.g., semi-vegetarian [typical American diet with smaller portions or lower frequency of animal products], pescovegetarian [seafood with or without eggs and dairy], lacto-ovovegetarian [eggs and dairy], and vegan [no animal products] diets). Randomized controlled trials and epidemiological studies indicate that plant-based diets, particularly vegan diets, are associated with significant improvement in CVD events, lowering risk factors such as diabetes and hypertension (1) and decreasing symptomatic and scintigraphic myocardial ischemia (6) and coronary artery disease (7); thus, revolutionizing our understanding about heart-healthy food patterns and the biological mechanisms linking dietary factors and CVD (1,8).
These data are strengthened by several recent landmark publications, including Song et al.'s recent large prospective cohort study of U.S. nurses and other health care professionals, describing the association between animal protein intake and cardiovascular, cancer, and all-cause mortality (9). In this large cohort study, higher intake of animal protein (including processed red meat, unprocessed red meat, dairy, poultry, and eggs) was positively associated with mortality, whereas the inverse was true for high intake of plant protein. In another recent meta-analysis, Kwok et al. (10) found similar results with vegetarians experiencing a 29% lower risk of coronary heart disease (CHD) mortality relative to nonvegetarians. These findings suggest the importance of protein source and support recommendations to increase plant protein intake, which in turn calls for education of physicians, patients, and the public about the largely unrecognized protein content of plants (e.g., peanuts and beef having the same protein content, 26 g per 100 g).
Some of these studies have defined plant-based diets dichotomously as being vegetarian or not and treat all plant foods equally-but are all plant-based diets equally beneficial for health? Further, if a predominantly plant-based diet is good, is a vegan one even better? Additionally, are the advantages of plant-based diets an all-or-nothing phenomenon? Against this background, Satija et al. (11) investigate the associations between plant-based diet indices and CHD incidence in this issue of the Journal. In a prospective series of 209,298 participants, the authors examined the inverse relationship between plant dietary index (PDI) and incidence of CHD, bifurcating a healthy PDI from an unhealthy one. They used food intake surveys to quantify dietary patterns into 3 graded plant-based diet indices-overall (PDI), healthful (hPDI), and unhealthful (uPDI)-and analyzed how gradual reductions in animal protein intake along with increases in plant food intake affect cardiovascular health. Higher adherence to PDI was independently inversely associated with CHD (hazard ratio [HR] comparing extreme deciles: 0.92; 95% confidence interval [CI]: 0.83 to 1.01; p trend = 0.003), with a stronger inverse association for hPDI (HR: 0.75; 95% CI: 0.68 to 0.83; p trend <0.001). Conversely, a positive association was found for uPDI (HR: 1.32; 95% CI: 1.20 to 1.46; p trend <0.001).
This study 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. They cannot address the benefits of a purely plant-based diet (vegan) because this was a very small population in their study.
Substantial evidence indicates that a predominantly plant-based diet is associated with improved cardiovascular risk factors, reduced incidence, and progression of CHD. Not all plant-based foods are equally healthy; rather, plant-based diets including whole grains as the main form of carbohydrate, unsaturated fats as the predominate form of dietary fat, an abundance of fruit and vegetables, and adequate n-3 fatty acids can play an important role in preventing CVD. Such diets, which have many other health benefits including the prevention of several chronic diseases, deserve more emphasis in dietary recommendations.
Getting to the "meat" of the issue, does this mean that healthful eating is an all-or-none phenomenon? Public health recommendations on exercise, for instance, now report that some activity is better than none when it comes to improving health and well-being. Reaching the weekly recommended goal of physical activity may initially seem unattainable to many, and thus discourage them from any exercise. Similarly, this "all-or-none" mindset is seen with eating habits. Just as physical activity is a continuum, perhaps an emphasis on starting with smaller dietary tweaks rather than major changes would be more encouraging and sustainable for those finding it difficult to make a complete and precipitous change in dietary habits. A simple approach was recently coined by Michael Pollan, "Eat food. Not too much. Mostly plants" (12).
Potential implications of healthier diet include a reduction in disease-related expenditures, such as drug costs and hospitalizations for stroke, heart failure, chronic kidney disease, and myocardial infarction associated with systemic hypertension, which is present in 58% of Medicare beneficiaries (13). If, for example, widespread adoption of plant-based nutrition reduced the incidence of hypertension to 25% of the current rate (1), this could result in savings of nearly 30% of the Medicare budget (14).
What, then, does hPDI really mean? It means both a challenge and an opportunity for cardiology. Until recently, as a group, cardiologists have not delved deeply into nutrition, treating CVD's downstream effects rather than obliterating its roots, leaving primary and secondary prevention opportunities on the table. It is time that we educate ourselves on dietary patterns, risk, and outcomes, and focus more on "turning off the faucet" instead of "mopping up the floor" (15).
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Healthful and Unhealthful Plant-Based Diets and the Risk of Coronary Heart Disease in U.S. Adults
Ambika Satija, Shilpa N. Bhupathiraju, Donna Spiegelman, Stephanie E. Chiuve, JoAnn E. Manson, Walter Willett, Kathryn M. Rexrode, Eric B. Rimm, Frank B. Hu
Abstract
Background Plant-based diets are recommended for coronary heart disease (CHD) prevention. However, not all plant foods are necessarily beneficial for health.
Objectives This study sought to examine associations between plant-based diet indices and CHD incidence.
Methods We included 73,710 women in NHS (Nurses' Health Study) (1984 to 2012), 92,329 women in NHS2 (1991 to 2013), and 43,259 men in Health Professionals Follow-up Study (1986 to 2012), free of chronic diseases at baseline. We created an overall plant-based diet index (PDI) from repeated semiquantitative food-frequency questionnaire data, by assigning positive scores to plant foods and reverse scores to animal foods. We also created a healthful plant-based diet index (hPDI) where healthy plant foods (whole grains, fruits/vegetables, nuts/legumes, oils, tea/coffee) received positive scores, whereas less-healthy plant foods (juices/sweetened beverages, refined grains, potatoes/fries, sweets) and animal foods received reverse scores. To create an unhealthful PDI (uPDI), we gave positive scores to less-healthy plant foods and reverse scores to animal and healthy plant foods.
Results Over 4,833,042 person-years of follow-up, we documented 8,631 incident CHD cases. In pooled multivariable analysis, higher adherence to PDI was independently inversely associated with CHD (hazard ratio [HR] comparing extreme deciles: 0.92; 95% confidence interval [CI]: 0.83 to 1.01; p trend = 0.003). This inverse association was stronger for hDPI (HR: 0.75; 95% CI: 0.68 to 0.83; p trend <0.001). Conversely, uPDI was positively associated with CHD (HR: 1.32; 95% CI: 1.20 to 1.46; p trend <0.001).
Conclusions Higher intake of a plant-based diet index rich in healthier plant foods is associated with substantially lower CHD risk, whereas a plant-based diet index that emphasizes less-healthy plant foods is associated with higher CHD risk.
Plant-based diets have been associated with a lower risk of various diseases (1-3), including coronary heart disease (CHD) (4-9), the leading global cause of death (10). However, these studies suffer from key limitations. With the exception of a recent investigation (3), prior studies (4-9) have defined plant-based diets as "vegetarian" diets, which constitute a family of dietary patterns that exclude some or all animal foods. As recommendations based on incremental dietary changes are easier to adopt, it is important to understand how gradual reductions in animal food intake with concomitant increases in consumption of plant foods affect cardiovascular health. Additionally, in studies of vegetarian diets, all plant foods are treated equally, even though certain plant foods, such as refined grains and sugar-sweetened beverages (SSB) are associated with higher cardiometabolic risk (11-13).
To overcome these limitations, we have created 3 versions of plant-based diet indices using a graded approach: an overall plant-based diet index (PDI), which emphasizes consumption of all plant food while reducing animal food intake; a healthful plant-based diet index (hPDI), which emphasizes intake of healthy plant foods associated with improved health outcomes such as whole grains, fruits, and vegetables; and an unhealthful plant-based diet index (uPDI), which emphasizes consumption of less healthy plant foods known to be associated with a higher risk of several diseases (14). In 3 U.S. cohorts, we previously documented that the PDI was inversely associated with type 2 diabetes risk with a stronger inverse association for hPDI and a positive association for uPDI (14). In the present study, we examined the associations of these plant-based diet indices with CHD incidence in more than 200,000 male and female health professionals in the United States.
Discussion
In 3 ongoing prospective cohort studies, higher adherence to PDI was modestly associated with lower CHD incidence (HR comparing extreme deciles: 0.92; 95% CI: 0.83 to 1.01). This inverse association was considerably stronger for adherence to a healthier version (hPDI) (HR: 0.75; 95% CI: 0.68 to 0.83), but positive for adherence to a less healthy version (uPDI) (HR: 1.32; 95% CI: 1.20 to 1.46) of a plant-based diet index. These associations remained robust to adjustment for multiple confounders and were consistently observed in various subgroups.
In a previous analysis (14), we found similar associations of these 3 indices with type 2 diabetes. Our current analysis extends the potentially protective association with hPDI to CHD. The mechanisms through which hPDI could reduce CHD risk are likely shared with the mechanisms for type 2 diabetes risk reduction (2,24-32). Specifically, greater adherence to hPDI would lead to diets high in dietary fiber, antioxidants, unsaturated fat, and micronutrient content, and low in saturated fat and heme iron content (Online Table 1), all of which could aid in weight loss/maintenance, enhance glycemic control and insulin regulation, improve lipid profile, reduce blood pressure, improve vascular health, decrease inflammation, and foster more favorable diet-gut microbiome interactions (e.g., through lowered levels of trimethylamine N-oxide), thereby lowering CHD risk. Greater adherence to uPDI, on the other hand, leads to diets with higher glycemic load and index; added sugar; and lower levels of dietary fiber, unsaturated fats, micronutrients, and antioxidants, which could result in higher CHD risk through the above-mentioned pathways. This is also illustrated in the fact that the associations of hPDI and uPDI with CHD incidence were slightly attenuated on adjustment for some of these pathways, specifically hypercholesterolemia, hypertension, and diabetes.
Prospective cohort studies examining the association of plant-based diets with CHD have focused on CHD mortality. Most of these studies have been carried out in Europe, with only 3 studies in the United States (Adventist Health Studies [7]). A pooled analysis of 5 of the above-mentioned cohorts found a 24% lower risk of CHD mortality (95% CI: 6% to 38%) comparing vegetarians with nonvegetarians (5). A recent meta-analysis found similar results with vegetarians experiencing a 29% lower risk of CHD mortality (95% CI: 13% to 43%) relative to nonvegetarians (6). The EPIC (European Prospective Investigation into Cancer and Nutrition)-Oxford study, 1 of the few studies to examine the association of a vegetarian diet with CHD incidence in addition to mortality, found a 32% lower 11-year CHD incidence (95% CI: 19% to 42%) among vegetarians relative to nonvegetarians (8).
These studies have defined plant-based diets dichotomously as being vegetarian or not. Our study adds to the evidence base by examining the association of gradations of adherence to PDI with CHD incidence. For instance, those in the lowest decile of PDI consumed 5 to 6 servings of animal foods per day, whereas those in the highest decile consumed 3 servings of animal foods per day. This approach has the advantage of being easily translatable, as we found that even a slightly lower intake of animal foods combined with higher intake of healthy plant foods is associated with lower CHD risk. One other study adopted this approach with respect to cardiovascular disease mortality and found similar results (3). However, these studies have examined plant-based diets at a single time point, making it difficult to fully capture the association of a time-varying exposure such as diet on the development of CHD, which has a long etiologic period. Our study adds to the existing reports by demonstrating the associations of long-term cumulative intake of a plant-based diet index with more than 20-year CHD incidence.
We also found that a healthier version of a plant-based diet index, which emphasizes plant foods known to be associated with improved health outcomes, is associated with substantially lower CHD risk. Contrarily, when intake of less healthy plant foods is emphasized, the opposite association was observed. When we examined associations of the 3 food categories with CHD risk, less healthy plant foods and animal foods were both associated with increased risk, with a potentially stronger association for less healthy plant foods. This highlights the wide variation in nutritional quality of plant foods, making it crucial to consider the quality of plant foods consumed in plant-rich diets.
When we examined a diet that emphasized both healthy plant and healthy animal foods, the association with CHD was only slightly attenuated relative to that with hPDI. Thus, the moderate reductions in animal foods suggested here can be largely achieved by lowering intake of less healthy animal foods such as red and processed meats. The results of this study are in line with the recently released 2015 Dietary Guidelines for Americans (33), which recommends higher consumption of high-quality plant foods. Dietary recommendations based on the hPDI would also be environmentally sustainable, as plant-based food systems use fewer resources than food systems that are heavily reliant on animal foods (34).
Study limitations
This is one of the largest prospective investigations of plant-based diet indices and incident CHD in the world, with periodic data on diet, lifestyle, and medical history collected over more than 2 decades. However, measurement error in diet assessment is likely, although evaluating cumulatively averaged intake reduces random errors (17) while allowing for the examination of long-term dietary intake. Given the observational nature of the study, residual and unmeasured confounding are possible; thus, we should interpret modest effect sizes such as those we observed for PDI with caution. However, the results were largely unchanged when we adjusted for additional covariates, including markers of socioeconomic status. Additionally, randomized controlled trial evidence showing the protective effect of plant-based diets on intermediate outcomes, including weight change, lipid profile, glycemic control, and blood pressure lends further support to our findings (35-38).
Conclusions
We found a modest inverse association of higher adherence to PDI with CHD incidence in 3 prospective cohort studies in the United States. While this inverse association was stronger for a plant-based diet index that emphasized healthy plant foods, CHD risk was significantly elevated for a plant-based diet index that emphasized less healthy plant foods. Dietary guidelines and lifestyle interventions could recommend increasing intake of healthy plant foods, while reducing intake of less healthy plant foods and certain animal foods for improved cardiometabolic health.
Perspectives
COMPETENCY IN PATIENT CARE AND PROCEDURAL SKILLS: Medical and health professionals should guide patients to increase intake of healthy plant foods, such as whole grains, fruits, vegetables, and nuts, and reduce intake of animal foods and less healthy plant foods such as SSB for CHD prevention.
TRANSLATIONAL OUTLOOK: Future research should replicate these findings in other racial/ethnic, occupational, and socioeconomic groups and explore biological mechanisms involved in the potentially cardioprotective effects of hPDI to identify personalized clinical interventions and therapies for CHD prevention.
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