A plant-based diet boosts after-meal burn, leads to weight loss and improves cardiometabolic risk factors in overweight individuals, according to the outcomes from a randomised control trial by researchers with the Physicians Committee for Responsible Medicine.
For the study, ‘Effect of a Low-Fat Vegan Diet on Body Weight, Insulin Sensitivity, Postprandial Metabolism, and Intramyocellular and Hepatocellular Lipid Levels in Overweight Adults’, published in JAMA Network Open, the researchers randomly assigned 244 participants (n=122 in each group) - who were overweight and had no history of diabetes - to an intervention or control group in a 1:1 ratio. For 16 weeks, participants in the intervention group followed a low-fat, plant-based diet based on fruits, vegetables, whole grains, and legumes with no calorie limit. The control group made no diet changes. Neither group changed exercise or medication routines, unless directed by their personal doctors.
Body composition and visceral fat were measured by dual x-ray absorptiometry and 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 three hours after a standard liquid breakfast (720 kcal).
Of the 244 participants, 211 (87%) were female, 117 (48%) were white and the mean (SD) age was 54.4 (11.6) years. Over the 16 weeks, body weight decreased in the intervention group by 5.9 kg (p<0.001). Thermic effect of food increased in the intervention group by 14.1% (p<0.001). The homeostasis model assessment index decreased (p<0.001) and PREDIM increased (p<0.001) in the intervention group. Hepatocellular lipid levels decreased in the intervention group by 34.4%, from a mean (SD) of 3.2% (2.9%) to 2.4% (2.2%) (p=0.002) and intramyocellular lipid levels decreased by 10.4%, from a mean (SD) of 1.6 (1.1) to 1.5 (1.0) (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<.001). Changes in hepatocellular and intramyocellular lipid levels correlated with changes in insulin resistance (both r = 0.51; p=0.01).
"These findings are groundbreaking for the 160 million Americans struggling with overweight and obesity," explained study author, Dr Hana Kahleova, director of clinical research for the Physicians Committee. "Over the course of years and decades, burning more calories after every meal can make a significant difference in weight management."
The researchers also teamed up with Yale University researcher, Drs Kitt Petersen and Gerald Shulman to track intramyocellular lipid and hepatocellular lipid - the accumulating fat in muscle and liver cells - in a subset of 44 participants using proton magnetic resonance spectroscopy. Those in the plant-based group reduced the fat inside the liver and muscle cells by 34% and 10%, respectively, while the control group did not experience significant changes. Fat stored in these cells has been linked to insulin resistance and type 2 diabetes.
"When fat builds up in liver and muscle cells, it interferes with insulin's ability to move glucose out from the bloodstream and into the cells," said Kahleova. "After just 16 weeks on a low-fat, plant-based diet, study participants reduced the fat in their cells and lowered their chances for developing type 2 diabetes."
The study also offered new insight into the link between fat within the cells and insulin resistance. The plant-based group decreased their fasting plasma insulin concentration by 21.6 pmol/L, decreased insulin resistance and increased insulin sensitivity, while the control group saw no significant changes. The plant-based group also reduced total and LDL cholesterol by 19.3 mg/dL and 15.5 mg/dL, respectively, with no significant changes in the control group.
"Not only did the plant-based group lose weight, but they experienced cardiometabolic improvements that will reduce their risk for type 2 diabetes, heart disease, and other health problems,” added Kahleova.
“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,” the paper’s authors concluded. “This randomised clinical trial found that a low-fat plant-based dietary intervention reduces body weight by reducing energy intake and increasing postprandial metabolism, apparently owing to increased insulin sensitivity resulting from reduced hepatocellular and intramyocellular fat. This intervention may be an effective treatment for overweight adults.”
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