Vegan Diet Brings Slew of Metabolic Benefits in Randomized Trial

A plant-based diet low in fat yielded several metabolic benefits, according to a new trial.

Over a 16-week period, adults with overweight or obesity who were randomized to a low-fat vegan diet lost an average 13 pounds (95% CI 11-14.8 lbs, P<0.001), reported Hana Kahleova, MD, PhD, of the Physicians Committee for Responsible Medicine in Washington, D.C., and colleagues in JAMA Network Open.

On the other hand, overweight adults in the control group who made no dietary changes at all maintained their weight throughout the 4-month trial.

Participants on the low-fat vegan diet also reaped several other benefits as well. As measured by indirect calorimetry over the course of 3 hours following a standard liquid breakfast of 720 calories, those on a vegan diet saw more than a 14% (95% CI 6.5-20.4, P<0.001) increase in postprandial energy expenditure (the thermic effect of food).

In addition, this dietary change increased the predicted insulin sensitivity index (0.9, 95% CI 0.5-1.2, P<0.001) and simultaneously decreased the homeostasis model assessment index (-1.3, 95% CI -2.2 to -0.3, P<0.001). This increase in insulin sensitivity was negatively correlated with a decrease in body weight (r=-0.43, P<0.001).

Using proton magnetic resonance spectroscopy, the researchers also found a significant decrease in lipid levels among those on the low-fat vegan diet. Specifically, this diet helped overweight adults achieve a 34.4% decrease in hepatocellular lipid levels and a 10.4% decrease in intramyocellular lipid levels. Both of these changes were positively correlated with changes in insulin resistance (both r=0.51, P=0.01).

Changes in these specific lipid levels are "clinically important," Kahleova's group pointed out, explaining how "[h]epatocellular and intramyocellular lipids play central roles in hepatic and muscle insulin resistance, respectively, and in type 2 diabetes."

The trial included a total of 224 adults ages 25 to 75 from Washington, D.C., with a BMI of 28 to 40. Since no meals were provided, the half of participants who were randomized to the low-fat vegan diet were instructed to follow a diet of about 75% carbohydrates, 15% protein, and 10% fat from vegetables, grains, legumes, and fruits, without animal products or added fats. These individuals also took 500 μg/d of vitamin B12.

Naturally, this group saw an increase in average carbohydrate and fiber intake, paired with a decrease in fat, protein, and cholesterol intake. Although these participants were allowed to eat as much as they desired, they reduced their overall energy intake -- a finding consistent with previous studies on vegan diets.

The control group participants were instructed to follow their usual diet. Both groups had a limit on alcoholic beverages of one or two drinks per day for women and men, respectively.

Interestingly, levels of physical activity dropped slightly in both groups, although this wasn't statistically significant.

During the 16-week trial, only five participants out of the 122 who were randomized to the vegan diet dropped out. However, Kahleova's group pointed out that these participants were "health-conscious" and therefore "may not be representative of the general population but may be representative of a clinical population seeking help for weight problems or type 2 diabetes."

Another limitation of the study was the self-reporting of dietary intake.

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