In overweight adults with no history of diabetes, a low-fat, plant-based vegan diet can reduce visceral fat and significantly improve both pancreatic beta-cell function and insulin resistance, potentially decreasing the risk of type 2 diabetes, according to researchers.
The 16-week randomized controlled trial in 73 adults showed that participants who ate a diet of vegetables, grains, legumes, and fruits significantly improved their overall metabolic condition, say Hana Kahleova, MD, PhD, of the Physicians Committee for Responsible Medicine in Washington, DC, and colleagues.
“Our study suggests the potential of a low-fat plant-based diet in diabetes prevention, addressing both core pathophysiologic mechanisms — insulin resistance and diminished beta-cell function — at the same time,” they write in their article, published online February 9 in Nutrients.
In a statement by the Physicians Committee for Responsible Medicine, Kahleova said the study “has important implications for diabetes prevention.” An estimated 30 million Americans have type 2 diabetes and it is projected that a third of the population will develop diabetes, she pointed out.
“Fortunately, this study adds to the growing evidence that food really is medicine and that eating a healthful plant-based diet can go a long way in preventing diabetes.”
Vegan vs Normal Diet
Previous studies have shown that the prevalence of diabetes is 46% to 74% lower in people who eat a plant-based diet compared with meat lovers in the general population, according to background information in the article.
A vegan diet has also been shown to improve glycemic control in type 2 diabetes better than calorie-restricted, low-carbohydrate diets, the researchers note.
Insulin resistance leading to impaired pancreatic beta-cell function is a key factor in type 2 diabetes, even though current treatment isn’t usually focused on improving beta-cell function, they add.
The study, conducted between October 2016 and June 2017, enrolled eight men and 67 women, age 25 to 75 years, with a body mass index (BMI) of 28 to 40 kg/m2. Participants on the vegan diet were told to avoid animal products and added fats, and they took a daily vitamin B12 supplement (500 μg).
The vegan diet provided 75% of caloric energy from carbohydrates, 15% from protein, and 10% from fats (20–30 grams/day). There was no calorie restriction in the vegan diet. The control group was asked to make no changes to their diet. However, alcohol intake was restricted in both groups: one drink a day for women and two drinks a day for men.
The vegan diet elicited marked increases in meal-stimulated insulin secretion and beta-cell glucose sensitivity, along with decreased fasting insulin resistance and decreased fasting and postprandial plasma glucose concentrations in individuals with no history of diabetes.
Specifically, the homeostasis model assessment – insulin resistance (HOMA-IR) index, used to assess fasting insulin resistance, fell significantly in the intervention group (P < .001), but not in controls (treatment effect −1.0). No significant change in oral glucose insulin sensitivity was observed in either group.
Notably, changes in the HOMA-IR index correlated positively with changes in BMI (r = 0.34; P = .009) and visceral fat volume (r = 0.42; P = .001), and the latter remained significant after adjusting for changes in BMI. Changes in glucose-induced insulin secretion correlated negatively with changes in BMI (r = −0.25; P = .04), but not visceral fat.
In the control group, beta-cell glucose sensitivity did not improve.
As HOMA-IR primarily reflects hepatic insulin resistance, the results “suggest a marked improvement in hepatic, rather than peripheral, insulin sensitivity,” the researchers note. Also, the decrease in insulin resistance was related to loss of visceral fat, independent of changes in BMI, while changes in glucose-induced insulin secretion were related to changes in BMI only.
“In this context, it seems plausible that a low-fat vegan diet in our study decreased hepatic insulin resistance and led to a subsequent improvement in beta-cell function,” the researchers observe.
There was also improvement in plasma lipid concentrations in response to a low-fat vegan diet, which is consistent with previous studies.
Food intake was based on participants’ own dietary records, which may limit the generalizability of the findings, the study authors acknowledge.
Jury Still Out on Benefits of Vegan Diet
As previously reported by Medscape Medical News, the Academy of Nutrition and Dietetics said in its 2016 position statement that plant-based diets are appropriate for people from infancy to old age, and during pregnancy.
For others, however, the jury’s still out on the disease-prevention merits of a vegan diet.
The definition of a plant-based diet can vary widely, the authors of one report point out. Another warns that vegetarian and vegan diets might be associated with nutrient deficiencies that could be harmful during pregnancy.
And recently, a National Institutes of Health study linked a vegetarian diet to higher risk of depression in men.
The study was funded by the Physicians Committee for Responsible Medicine, of which Kahleova and coauthor Neal D Barnard, MD, are employees.