Weight

Vegans have the lowest risk of obesity

By WhyStopHarmingAnimals.com

Vegans are consistently the leanest group in studies of diet and body weight, with the lowest obesity risk of the patterns studied[1]. Randomized trials show the same thing from the other end: assign people a vegan diet, often with no instruction to eat less[5][8], and they lose weight[4][5][6][7][8][9].

What the cohorts show

A review in Obesity Reviews found vegan diets associated with the lowest obesity risk[1]. The reasons are ordinary. Vegan meals carry more fiber, the foods are less calorie-dense, and they leave out the heavy fat that comes with animal products. A large cohort in JAMA Network Open found the same pattern with body fat: people who ate more plant foods had less of it[2]. Losing weight doesn't require animal products[4][5][7][8], and it often means getting fewer calories from them.

The meat side of the ledger points the same way[3]. A meta-analysis covering 1,135,661 people (Obesity Reviews, 2014) found that eating more red and processed meat came with 37% higher odds of obesity[3], along with higher BMI and larger waist circumference at the top of the intake range[3].

What happens when people are assigned the diet

Observational studies can't rule out that leaner people are simply more likely to choose a vegan diet. Trials can. A 2016 meta-analysis of 12 randomized trials (Journal of General Internal Medicine) found that people assigned a vegetarian diet lost 2.0 kg more[4] than those assigned non-vegetarian diets over a median of 18 weeks[4]. On vegan diets the difference was 2.5 kg[4]. A 2015 meta-analysis in the Journal of the Academy of Nutrition and Dietetics found that prescribing vegetarian diets, vegan ones included, lowered average body weight[5], and it did so even in trials where weight loss wasn't the stated goal[5].

A 2022 pool of 11 trials lasting 12 weeks or longer, in people with overweight or type 2 diabetes, found vegan diets cut body weight by 4.1 kg and BMI by 1.4[6]. HbA1c fell 0.18 points[6], total cholesterol 0.30 mmol/L[6] and LDL 0.24 mmol/L[6]. Blood pressure and triglycerides did not move[6].

Individual trials show larger effects[7][8]. In a 16-week randomized trial of overweight adults (Nutrition & Diabetes, 2018), the low-fat vegan group lost 6.5 kg[7], 4.3 kg of it fat mass[7], while the control group showed no significant change[7]. Visceral fat, the fat packed around the organs, fell only in the vegan group[7], and so did HOMA-IR[7], a measure of insulin resistance. In the BROAD community trial, people with obesity, ischaemic heart disease or type 2 diabetes ate a whole-food plant-based diet with no calorie counting[8] and were about 12 kg down at six months[8], with better BMI and cholesterol[8].

More calories burned in digestion

Part of the explanation is what goes onto the plate: fiber fills people up on fewer calories. A 16-week randomized trial of 244 people (JAMA Network Open, 2020) found a second mechanism[9]. On a low-fat vegan diet, body weight fell 5.9 kg[9], and the thermic effect of food, the calories the body burns digesting a meal, rose 14%[9]. The same diet cut liver fat by 34%[9] and fat inside muscle by 10%[9], and insulin sensitivity improved[9]. None of these changed in the control group[9].

The 14% rise means the body spent more energy handling the same meals after the switch. It is a modest effect on any single day, but it runs every day, and it helps explain why people on vegan diets lose weight without counting calories. The drop in liver and muscle fat matters beyond the scale, since fat stored in those tissues is closely tied to insulin resistance and type 2 diabetes.

The limits

The cohort evidence is observational. Most trials run for weeks or months, with a median of 18 weeks in the 12-trial pool[4], and the pooled differences of 2 to 4 kg are smaller than the results of the strongest single trials[4][6][7][8]. In the 2022 pool, blood pressure and triglycerides didn't change[6]. What the trials do show is consistent: taking animal products off the plate lowers body weight[4][5][6][7][8][9], whether or not weight was the aim[5].