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Effect of Low-Fat Vegan Diet in Overweight Adults
Quick Takes
- A vegan diet without calorie restriction is associated with relatively lower kcal/day intake with relatively more weight loss/reduction in calories.
- The incremental weight loss with the vegan diet is at least in part related to the thermal effect of eating the meatless low glycemic index food groups, which require more energy for digestion, absorption, and disposal of the ingested nutrients.
Study Questions:
What are the effects of a low-fat vegan diet on body weight, insulin resistance, postprandial metabolism, and intramyocellular and hepatocellular lipid levels in overweight adults?
Methods:
The authors conducted a 16-week randomized clinical trial between January 2017 and February 2019. A total of 3,115 people responded to public advertisements, and 244 met the participation criteria (ages 25-75 years; body mass index [BMI] of 28-40 kg/m2) with telephone screening. Participants were randomized in a 1:1 ratio. Those in the intervention group (n = 122) were to follow a low-fat vegan diet and the control group (n = 122) to make no diet changes for 16 weeks. At weeks 0 and 16, body weight was assessed and body composition and visceral fat were measured by dual x-ray absorptiometry. Insulin resistance was assessed with the homeostasis model assessment index and the predicted insulin sensitivity index (PREDIM). Thermic effect of food was measured by indirect calorimetry over 3 hours after a standard liquid breakfast (720 kcal). In a subset of participants (n = 44), hepatocellular and intramyocellular lipids were quantified by proton magnetic resonance spectroscopy. Repeated measure analysis of variance was used for statistical analysis.
Results:
Among the 244 study participants, 211 (87%) were female, 117 (48%) White, and mean age was 54.4 (11.6) years. Mean BMI was about 33 kg/m2 and fat mass 41%. Self-reported energy intake was -355 kcal/d in the intervention group and -191 kcal in controls (p < 0.001). Over the 16 weeks, body weight treatment effect in the intervention group was a decrease of 5.9 kg; p < 0.001) largely related to decrease in fat mass and visceral fat volume. Thermic effect of food increased in the intervention group by 14.1% (p < 0.001). The homeostasis model of insulin resistance decreased (−1.3; p < 0.001) and insulin sensitivity increased (0.9; p < 0.001) in the intervention group with no change in the control group. Hepatocellular lipid levels decreased in the intervention group by 34.4% (3.2-2.4%; p = 0.002), and intramyocellular lipid levels decreased by 10.4% (1.6-1.5%; p = 0.03). None of these variables changed significantly in the control group over the 16 weeks. The change in PREDIM correlated negatively with the change in body weight (r = −0.43; p < 0.001). Changes in hepatocellular and intramyocellular lipid levels correlated with changes in insulin resistance (both r = 0.51; p = 0.01).
Conclusions:
A low-fat plant-based dietary intervention reduces body weight by reducing energy intake and increasing postprandial metabolism. The changes are associated with reductions in hepatocellular and intramyocellular fat and increased insulin sensitivity.
Perspective:
Diets limited to 1,200 kcal result in moderate weight loss and reduction in intracellular hepatic and muscle fat as well as improving insulin sensitivity, but are hard to maintain. In contrast, vegan diets are often associated with a significant decrease in energy intake and weight loss that is helped by improvement in insulin sensitivity that increases the thermic effect of food on post-prandial metabolism. While I do not prescribe it, I support patients with the metabolic syndrome trying a plant-based diet. Experience is that many cannot tolerate associated gastrointestinal symptoms.
Clinical Topics: Diabetes and Cardiometabolic Disease, Dyslipidemia, Noninvasive Imaging, Prevention, Lipid Metabolism, Diet
Keywords: Absorptiometry, Photon, Body Mass Index, Calorimetry, Indirect, Diagnostic Imaging, Diet, Fat-Restricted, Diet, Vegetarian, Insulin, Insulin Resistance, Intra-Abdominal Fat, Lipids, Magnetic Resonance Spectroscopy, Metabolic Syndrome, Overweight, Primary Prevention, Weight Loss
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