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Does science support plant-based eating?

A recent study called into question the science that backs plant-based eating. Here’s why I’m not paying attention to it, and what science really says about a plant-forward lifestyle.

There’s no confusion about death rates. We will all leave this earth for one reason or another, and the numbers don’t lie: Diseases affected by unhealthy eating are the leading causes of death in the United States. In fact, poor dietary factors have now surpassed tobacco use as the leading lifestyle choice that causes death. That is, aside from unfortunate accidents and some cancers, major portions of the top risks of death are within our control.

As a registered dietitian, I know that what we eat matters. And I also know that we, as a human race, have the knowledge right now about what is more harmful and what is more helpful to eat when it comes to our physical, mental and, yes, even our emotional, health. Nutrition research is not perfect, but it’s not insignificant either. In fact, the evidence about certain eating patterns, whether beneficial or harmful, just continues to grow with consistent outcomes. So how can we know so much, yet redundantly claim to know so little about nutrition? 

Yet still, people say that they are extremely confused about what it takes to eat healthfully. My take on this? Some of it is disrespect. After all, we are in the midst of a war on science, where expertise is self-prescribed and opposing views are met with distrust rather than curiosity. And, on the other hand, I think some of the confusion is just plain ignorance. I mean, can you think of anything more personal and protected than what you choose to eat every day?

During my clinical rotations as a young dietitian-in-training, it didn’t take long before I realized that people are not very forthcoming about their eating habits. In fact, I had better luck getting intel on their bowel movements. Turns out, talking about the way one eats is an intimate dissection of their person.  

And no wonder we’re all so protective of our personal food profiles. Eating is complex. Our food and beverage choices are impacted by more factors than anything else we encounter on a daily basis. What we put in our mouths is influenced in part by taste or preference, in part by availability or price, in part by hunger or hormones, in part by beliefs or religion, and in part by whom we’re with or where we’re at—and then some!  

But yet another important reason that so many people claim to be confused about what healthy eating looks like? It is simply because the debate, history, politics and reporting of what healthy eating looks like is confusing. Despite what research shows, headlines seem to ping-pong and media outlets love clickbait and controversy. Do you love to eat bacon and chocolate, even though something (physical or metaphorical) in your heart tells you it might be best to enjoy them sparingly? Not to worry. You needn’t dig deep to find news stories (like this or this) that affirm your choices, and even give them a health halo. Phew. Change is hard. Crisis diverted. Pass the chocolate-covered bacon. 

But I’ve said it before, and I’ll say it again: No nutrition topic has been studied more than the impact on human health of vegetables, fruits, grains, nuts and other plant-based foods (and diets with stable intake of these). And consistent evidence supports, specifically, a whole food, plant-based diet as one of the most important factors associated with a long, healthy life while minimizing disease risk. 

But confusion—including mine—rose to new heights when a recent controversial study printed in the Annals of Internal Medicine undermined the plethora of research that highlights the health benefits of plant-based eating and concluded that adults should continue to eat their current consumption of processed and red meat, without any input from nutrition experts nor an explanation of how much is OK (hello, "current consumption" can mean so many different things). This was confusing advice to digest—for consumers, scientists, nutrition experts and basically everyone. Even proponents of regenerative agriculture who enjoy occasionally eating responsibly raised livestock and drinking organic milk and cooking with pasture-raised eggs (one of which is me, by the way) were confused by the outcomes. My main worry is that consumers who read this study’s conclusion will believe they can eat as much red and processed meat as they want without any ill effect on their personal health, not to mention any ill impact on planetary health. 

The root issue of this recent controversy is the claim that what we know about nutrition science and methodology for studying the impact of food on human health is weak and wrong. That is at the center of the AIM study’s claim, which cited several systematic reviews, which—interestingly—all showed decreases in diseases, such as heart disease, Type 2 diabetes and certain cancers, when people reduced their meat intake. These outcomes, the researchers said, were “weak recommendations based on low-certainty evidence” because much of it was observational research. But instead of saying that their research was inconclusive, the researchers did something preposterous in the world of science: They recommended that low (not the absence of, just low) evidence of the health benefits of reducing meat consumption meant that people should just eat their “current” levels of meat. But wasn’t that the opposite of what their research showed?

All of the research they reviewed consistently—and with statistical significance—echoed that there are harms of eating more rather than less meat. And, by the way, don’t some consumers eat two servings of meat in a week while others eat 20? Does that make a difference? Of course it does! Does how that meat is raised, fed, processed and cooked make a difference? Of course it does! Also confusing were the omission of some extremely large studies that showed disease decline with plant-based eating that the researchers chose not to include in their systematic review. The reason? They were too large. 

The researchers of this “eat meat” study used systematic review and meta-analysis plus an assessment called GRADE (Grading of Recommendations, Assessment, Development and Evaluations) as the gold standard of nutrition research, a methodology that summarizes the results of available literature and well-designed health care studies on a topic to find consistent conclusions among the large pool of data. Through GRADE, they prioritized evidence from randomized controlled trials. The problem here is that there are no randomized controlled trials examining eating red and processed meat and its link to diseases specifically because that’s impossible.

There is simply no way to get someone to eat only one food or type of food over a significant portion of time and to measure the results, nor is it humane. All nutrition experts, including me, would agree that systematic reviews, however, are the types of complete, exhaustive summaries of current evidence that are the gold standard, and we do have those. Systematic review helps us avoid weighting the conclusions of a 2-day, poorly controlled animal study versus the outcomes of a 5,000-person, multiyear, double-blind clinical trial. Unfortunately, many health media outlets don’t make methodology differences clear in their reporting on health and nutrition research, so everything gets a headline, and it seems especially the shocking, contrary-to-gut-instincts, should-never-be-covered-in-the-first-place kind of stuff takes precedence. If it bleeds, it leads (here, quite literally).

But again, I want to reiterate that the group of researcher-statisticians involved in the AIM study—through their systematic review and meta-analysis—found entirely consistent, clinically meaningful, statistically significant adverse effects of eating more versus less red and processed meat on all-cause mortality, cardiovascular disease, cancer and Type 2 diabetes. Yet, instead of recommending people eat less, they recommended that people eat their “current” intake of red and processed meat. Confused? Me, too. 

The one thing I can agree with from this controversy is that nutrition science is imperfect. All science is, yet science is the best tool available to allow us to pursue objective truths. 

So where does this leave you when you’re making decisions about what to eat in your quest for health? You have to rely of the biggest pools of data and consistency in the evidence, as well as do the work to become a more intuitive eater (that is, truly listen to and watch your body). As a registered dietitian, I’m here to tell you that the vast weight of data and consistent evidence still points to a plethora of health benefits when your eating pattern includes a lot of plant-based foods (vegetables, fruit, nuts, seeds, beans, and plant-based oils), plus fish, chicken and quite limited amounts of red meat, ultra-processed foods and added sugars in order to reduce your risk of the leading causes of diet-related premature death, such as heart disease, hypertension, Type 2 diabetes, dementia and some cancers. 

So what is the most compelling, evidenced-based research that supports plant-based eating? Here are a few standouts: 

Diabetes Prevention Program (The DPP Study)
This randomized, controlled clinical trial was conducted at 27 clinical centers around the U.S. from 1996 to 2001. The trial enrolled 3,234 participants; 55% were Caucasian, and 45% were from minority groups at high risk for the disease, including African American, Alaska Native, American Indian, Asian American, Hispanic/Latino or Pacific Islander. Participants were assigned to either a Lifestyle Change Group (with intensive training on diet and physical activity), a Metformin Group (with medication and standard advice about diet and physical activity) or a Placebo Group (with placebo medication and standard advice about diet and physical activity). 

What it revealed about plant-based eating: Participants in the Lifestyle Change Group consistently had the best outcomes, including a 58% overall reduction in developing diabetes (71% reduction in those over age 60). With this intervention, they avoided or had lower intake of processed meats, fatty red meat and poultry with skin and displayed lower blood pressure and improved cholesterol levels compared to the other groups. Additionally, participants with diabetes and prediabetes who had eating patterns that included more plant-based proteins or fish and less red meat were shown to have to lower insulin resistance and lower blood glucose (A1C). 

Primary Prevention of Cardiovascular Disease with a Mediterranean Diet (The PREDIMED and Lyon Heart Study)
This parallel-group, multicenter, randomized trial involved a total 7,447 people ages 55 to 80 years from 2003 to 2010 and studied variations on the typical Mediterranean diet versus a standard low-fat diet as it related to heart disease. Participants had no cardiovascular disease at the start of the study but were identified as high-risk, and were assigned to one of three diets: a Mediterranean diet supplemented with extra-virgin olive oil, a Mediterranean diet supplemented with mixed nuts, or a control diet (advice to reduce dietary fat).

What it revealed about plant-based eating: The traditional Mediterranean diet is characterized by a high intake of olive oil, vegetables, fruits, nuts and cereals; a moderate intake of fish and poultry; and a low intake of dairy products, red meat, processed meats and sweets. Study participants who followed the highly plant-based Mediterranean diet, whether supplemented with extra-virgin olive oil or mixed nuts, had a lower risk of cardiovascular events during the study, with a relative difference of 30%. After the study was stopped, all participants, including those in the low-fat control group, were advised to follow a plant-based Mediterranean diet.

Diet Patterns and Mortality (Journal of Nutrition, 2014)
This issue evaluated the top dietary patterns associated with outcomes of healthier, longer lives. The review included the association of four defined diet scores reflecting commonly examined patterns and risk of mortality from cardiovascular disease, cancer or all causes combined in older U.S. men and women in the NIH-AARP Diet and Health Study prospective cohort. By using information from food frequency questionnaires completed by 424,662 participants at baseline, the authors calculated how closely participants' diets matched the following diet quality scores: 1) the Healthy Eating Index–2010, 2) the Alternate Healthy Eating Index–2010, 3) the Dietary Approaches to Stop Hypertension (DASH), and 4) a modified Mediterranean Diet (aMED). 

What it revealed about plant-based eating: The three dietary scores most consistently associated with lower mortality all included high consumption of vegetables, fruits, whole-grains and legumes and lower consumption of red and processed meat. The review concluded that the diet patterns that are associated with lower risk of death from heart disease, cancer or any cause are “built on a common core of a diet rich in plant foods (whole grains, a variety of fruit and vegetables, nuts and legumes), which is supported by extensive scientific evidence.”

These large-scale reviews only scratch the surface of research that links eating patterns that emphasize more plant-based foods and fewer animal-based foods with reduced risk of poor health outcomes. It’s worth noting that the AIM study did include one review in its research that evaluated people’s attitudes about eating meat. And this research (despite the exclusion of others) is what helped influence their recommendations.

Predictably, this review found that omnivores are reluctant to give up animal foods even if they know high intake of those foods could put their health at risk. A statement about the inclusion of this particular research from the Harvard T.H. Chan School of Public Health stated: “Although taste preference is important for personalized dietary advice, it is questionable whether it should be considered as a major factor in developing dietary guidelines.” Similarly, I’d point out, many people don’t want to quit smoking, stop drinking or exercise more, but that doesn’t—and shouldn’t—change the recommendations that they should do so for improved health.

But perhaps the most egregious omission from this most recent nutrition clickbait is the science that points to our dietary and agricultural habits impact on the environment. Had any of that research been included, the researchers' outcome would have been entirely different (and yes, not so newsworthy). No one can look at that evidence and responsibly say that humans should continue eating their current intake of animal foods. Problem solvers here can look like a smattering of the most positive trends and promising innovators in the industry. They include those who support organic and regenerative agriculture, locavores and plant-based eaters on the spectrum of flexitarian to vegan—and those who form the habits, respect the research and have the shopping carts to prove it.  

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