'More Plants, Less Meat, Less Diabetes,' New Analysis Indicates

Marlene Busko

July 22, 2019

Middle-aged people who ate more plant-based foods — mainly semi-vegetarians but also vegetarians and vegans — were less likely to develop type 2 diabetes than their peers who ate more meat, fish, eggs, and dairy, in a large meta-analysis.

Overall, people with the highest versus lowest intake of any plant-based foods had a 23% lower risk of developing type 2 diabetes, independent of body mass index (BMI), in a 2- to 28-year follow-up, the new data show.

And those with the highest versus lowest intake of healthy plant-based foods — that is, fruits, vegetables, whole grains, legumes, and nuts — had a 30% lower risk of incident diabetes.

"To our knowledge, the present study provides the most comprehensive evidence on the association between plant-based dietary patterns and incidence of type 2 diabetes," Frank Qian, MPH, Department of Nutrition, Harvard T.H. Chan School of Public Health, Boston, Massachusetts, and colleagues summarize, in their article published online July 22 in JAMA Internal Medicine.

They acknowledge that this meta-analysis of nine observational studies in high-income countries cannot show cause and effect, and intake was based on (mainly one-time) self-reported replies to a food frequency questionnaire.

Nevertheless, "owing to the overall low feasibility of randomized clinical trials directly testing plant-based dietary patterns for the prevention of type 2 diabetes," the study supports "a possible protective role of these dietary patterns against the development of type 2 diabetes," they write.

Participants in the category of greatest adherence to a plant-based diet still consumed roughly 1.7 to 3.9 servings/day of dairy, eggs, fish, or meat, they note, so additional studies are needed to see if reducing this further would increase health benefits.  

More Plants, Less Meat Still Achievable Even in 'Food Deserts'

The takeaway message is simple, Kim Allan Williams, Sr, MD, cardiology division chief, Rush University Medical Center, Chicago, Illinois, told Medscape Medical News in an email: "More plants. Less meat. Less diabetes."

"Meat and especially processed meat," he stressed, "is associated with more diabetes, heart failure, cancer, hypertension, stroke, hyperlipidemia, heart attacks, and death."

Also invited to comment, Michelle L. O'Donoghue, MD, MPH, associate professor of medicine, Harvard Medical School, and a cardiologist at Brigham and Women’s Hospital, Boston, Massachusetts, agrees that the findings "contribute to a growing body of research that a diet rich in plants and low in animal intake may help to reduce the risk of developing many chronic Western diseases including type 2 diabetes."

Intriguingly, some studies have even suggested that a plant-based diet may actually help reverse type 2 diabetes, says O'Donoghue, who writes a blog for theheart.org | Medscape Cardiology.

And there are simple steps people can take to improve their diet, she said, to shift it more towards healthy plant-based sources, even if they live in a so-called "food desert."

Similarly, Michelle McMacken, MD, assistant professor, NYU School of Medicine, and director, Bellevue Hospital Weight Management Clinic, New York City, told Medscape Medical News in an email that the study findings "should inspire clinicians to consider recommending a more plant-based eating pattern to their patients."

Can a Plant-Based Diet Prevent Diabetes?

As background, Qian and colleagues explain that it is not clear if eating a more vegetarian-type diet might ward off type 2 diabetes.

To investigate this, they performed a meta-analysis of nine studies in five US cohorts (Nurses' Health Study I and II, Adventists Health Study I and II, and Health Professionals Follow-up Study) and four other cohorts from around the world in Greece (ATTICA), Singapore (Singapore Chinese Health Study), the Netherlands (Rotterdam Study), and Taiwan (Tzu Chi Health Study).

The analysis included 307,099 participants with a mean age of 36 to 65 years and a mean BMI of 23 to 36.7 kg/m2. Of these, 23,544 participants were diagnosed with diabetes during follow-up.

Overall, participants with a higher versus lower adherence to a plant-based diet had a reduced risk of incident type 2 diabetes (relative risk [RR], 0.77; 95% CI 0.71 - 0.84), after adjusting for risk factors such as BMI, age, smoking, and family history of diabetes.

In four cohorts with more detailed information, participants with a higher versus lower adherence to a "healthy plant-based diet" had an even greater reduced risk of the outcome (RR, 0.70; 95% CI, 0.62 - 0.79).

Nutrient Content of Plant-Based Foods Matters

O'Donoghue says emerging evidence is contributing to new dietary theories.

"We are only now beginning to understand that carbs shouldn't be shouldering all the blame" for risk of type 2 diabetes, she explains.

"Certainly, processed or refined carbohydrates and sugars may contribute to inflammation and disease development," she continued.

"However, we are finding that a plant-based diet that reduces or eliminates animal intake while increasing intake of healthy whole grain carbohydrates may actually have very favorable effects."

At the same time, "the [nutrient] quality of the food we eat is crucial, and that was reflected in the findings of the current study," where nutrient-rich plant-based foods provided greater benefit than nutrient-poor plant-based foods.

Moreover, diets such as the Mediterranean diet emphasize the intake of fruits and vegetables and may also offer clinical benefit for patients unwilling to fully commit to a plant-based diet.

And although a ketogenic diet may lead to weight loss, the data are very mixed regarding its effects on inflammation, lipid parameters, and disease progression, so this should not be recommended as a first-line strategy, she advises.

Sustained efforts are needed to inform people about the importance of good nutrition for good health, according to O'Donoghue, and it is imperative that efforts are increased to provide healthier food choices in schools.

Unfortunately, many people living in "food deserts" in low-income communities still have limited access to fresh, wholesome fruits and vegetables, or these foods are too expensive.  

However, "even in the absence of fresh produce," she said, "patients can still make healthier choices at the grocery store with canned vegetables and healthier grains, [and] more affordable or free grocery delivery is also becoming a reality in many parts of the country."

McMacken agrees: "Patients can be counseled to look in their local grocery stores for healthy and inexpensive plant-based foods, such as dried or canned legumes (beans, chickpeas, and lentils), whole grains (brown rice, oats, barley), frozen vegetables, and fresh produce that is in season," while reducing consumption of highly processed foods.

Possible Mechanisms

Several mechanisms may explain the study findings, Qian and colleagues suggest.

Plant-based foods "contain fiber, vitamins and minerals, antioxidants, phenolic compounds, and unsaturated fatty acids," they note, which "improve insulin sensitivity and blood pressure, reduce long-term weight gain, and ameliorate systemic inflammation — pathways involved in the cause of type 2 diabetes."

On the other hand, "these diets also deemphasize or avoid red and processed meats, which have been shown to adversely affect risk of type 2 diabetes, possibly owing to their high heme iron or dietary cholesterol contents." 

"Further experimental evidence," the researchers conclude, "could help provide insights into other novel pathways that could mediate the beneficial association between plant-based dietary patterns and type 2 diabetes." 

The study was supported by grants from the National Institutes of Health. Qian and Williams have reported no relevant financial disclosures. Disclosures for the other authors are listed in the article. O'Donoghue has reported receiving research grants from Merck, GlaxoSmithKline, Eisai, AstraZeneca, and Janssen. McMacken currently serves on the advisory board for Nutrinic and as faculty for Sustainable Diet.

JAMA Int Med. Published online July 22, 2019. Abstract.

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