Low-Fat Plant-Based Diet Is Effective for Reducing Body Weight and Increasing Insulin Sensitivity

A 16-week low-fat plant-based diet significantly improves diabetes parameters in adult patients with type 2 diabetes mellitus (T2DM). 

Obesity, particularly if one has access weight around their abdomen, accounts for 80-85% of the risk of developing T2DM. Recent studies suggest that people with obesity are up to 80 times more likely to develop type 2 diabetes than those with a BMI of less than 22 kg/m2. Studies have shown that obesity is uncommon in humans whose diets are based on plant-derived foods as these foods cause weight loss due to 1). high fiber content, and thus reduced energy intake, and 2). Increase in the thermic effect of food, which accounts for approximately 10% of total energy expenditure. In addition, there is increasing evidence that the accumulation of lipids in muscle and liver cells is associated with insulin resistance and T2DM as these accumulations decrease mitochondrial activity and postprandial metabolism. Prior studies have observed that people following a vegan diet have lower intramyocellular lipids concentrations than omnivorous ones, thus increasing mitochondrial activity and postprandial metabolism. This study tested whether a low-fat vegan diet reduces body weight, insulin resistance, postprandial metabolism, and intramyocellular and hepatocellular lipid levels in adults with overweight. 

This single-center (Washington, DC), randomized, open parallel study was conducted between January 2017 and February 2019 on 244 adults aged 25 to 75 years who have BMI between 28 and 40 kg/m2. Adults who had diabetes, pregnant or breastfeeding, waist circumference >102cm, were smoking or were using alcohol or drugs, or on a vegan diet were excluded from this study at screening. Participants were randomly assigned in a 1:1 ratio to either the low-fat vegan diet group or a control group with no diet change (n=122 in each group) and were followed for 16 weeks. The intervention diet consisted of vegetables, grains, legumes, and fruits without animal products or added fats, totaling approximately 1300 kcal/day, with approximately 75% of energy from carbohydrates, 15% from protein, and 10% from fat. In addition, vitamin B12 was supplemented (500 μg/d). The intervention group attended weekly classes for detailed instruction and cooking demonstrations and received printed materials and small food samples, but no meals were provided. All participants were continued with their regular daily habits unless specifically changed by their respective physicians. The primary outcomes were body weight, insulin resistance, postprandial metabolism, and the concentrations of intramyocellular and hepatocellular lipids. 3-member Outcome Assessment Panel was established at the beginning of the study, with all three members of this panel blinded to group assignment. Forty-four participants (23 in the intervention group and 21 in the control group) underwent proton magnetic resonance spectroscopy to quantify hepatocellular and intramyocellular lipids. ANOVA, χ2, t-test, and Fisher exact test were used for statistical analysis. Two-tailed tests were used to determine significance at the 5% level (P<0.05). 

At baseline, except for the mean age difference (53 years for the intervention group vs. 57 years for the control group), there were no significant differences between the two groups. The average weight was 93.15 kg, and the average BMI was 33.4 kg/m2 among all the participants at baseline. Two hundred twenty-two participants (91%) completed the study as 5 participants from the intervention group, and 16 participants from the control group dropped out due to non-study-related reasons. At 16-weeks, the intervention group has significantly reduced energy intake, cholesterol intake, saturated, monosaturated, and polyunsaturated fatty acid intake compared to the control group (P<0.001 for all). The intervention group had a significant weight reduction (-5.9 kg, P<0.001), BMI (-2.2 kg/m2, P<0.001), fat mass, lean mass, VAT volume (P<0.001 for all), and hepatocellular lipids (P=0.002) compared to control group. At the same time, there was a significant reduction in fasting plasma insulin level and fasting plasma glucose level (-46.2pmol/L and -0.6mmol/L, respectively) and reduction in total and LDL cholesterol (P<0.001 for both) compared to the control group. The intervention group also had a significantly increased fiber intake than the control group. Postprandial energy expenditure (the thermic effect of food) increased by 18.7% in the intervention group from baseline to 16 weeks and did not change significantly in the control group (P<0.001). In the 44 participants who underwent magnetic resonance spectroscopy, hepatocellular lipid content decreased in the intervention group by 34.4% (P =0 .03) and remained unchanged in the control group. 

This RCT shows that a 16-week ad libitum diet of approximately 1300kcal/day resulted in a moderate weight loss of approximately 6kg. This weight reduction is due to low energy intake and postprandial energy expenditure. These effects also caused normalization of liver lipid content, fasting plasma glucose concentration, and increased insulin sensitivity. With its randomized parallel design, 16-week follow-up, low attrition, and scientific measurements of various parameters, this study provided valuable evidence of the low-fat vegan diet benefits. However, a long-term follow-up study is needed to assess the beneficial effects of a low-fat vegan diet in the long term.   

Practice Pearls: 

• As per 2017-18 National Health and Nutrition Examination Survey (NHANES) data, the average American adult eats 2100 calories/day, out of which 47% comes from carbohydrates, 16% comes from protein, and 36% comes from fat. In addition, 22% of all calories come from added sugar.  
• As per a 2016 study published in the BMJ journal, Americans get 58% of all calories from ultra-processed foods, such as bread, cakes, cookies, salty snacks, soft drinks, pizza, instant and canned soups, etc.  
• As per “The State of US Health, 1990-2010” study published in JAMA in 2013, our diet is our most significant risk factor for disease, disability, and death. In other words, what we eat in America is more dangerous to our health than anything else.  

Kahleova, Hana et al. “Effect of a Low-Fat Vegan Diet on Body Weight, Insulin Sensitivity, Postprandial Metabolism, and Intramyocellular and Hepatocellular Lipid Levels in Overweight Adults: A Randomized Clinical Trial.” JAMA network open vol. 3,11 e2025454. 2 Nov. 2020, doi:10.1001/jamanetworkopen.2020.25454  

Kandarp Mehta, MS, PharmD Candidate, University of Colorado Anschutz Medical Campus, Skaggs School of Pharmacy and Pharmaceutical Sciences