In an unintendedly comical way, science occasionally supplies data for exactly those claims that it regards as “not scientifically proven”. One of them is that a vegan diet results in a higher metabolic rate and a lower body weight.
First clues from animal experiments
Already C.T. Campbell reported from his animal experiments with animal and plant protein that the animals, who received only plant protein, were much more active and used their exercise wheel about twice as much as the other animals. Many people who change to a vegan diet report that they feel much fresher, more active and more energetic. Evidence, however, that vegan diets really induce a higher calorie-burning rate in the body, was lacking.
Same number of calories – different weight?
Now, I am currently looking at the data of the largest “Adventist Health Study 2” again, which included over 73.000 Adventists. About 35.000 of these participants were omnivores, 4.000 half-vegetarians, 7.000 were fish-consuming, 21.000 were lacto-ovo-vegetarians (no meat, but milk and eggs) and about 5.500 were vegans. The average body mass index (BMI) was 28.3 for omnivores, 27.3 for half-vegetarians, 26 for fish consumers, 26.1 for lacto-ovo-vegetarians and 24.1 for the vegans. The different groups, however, on average consumed (in the same order): 1884, 1720, 1939, 1912 and 1897 kcal a day! There was consequently barely a difference between the different groups in terms of calorie intake and vegans nearly had exactly the same amount as the omnivores. And still, they had a lower weight by 4.2 BMI levels! (Which corresponds to a weight difference of 12.1 kg for a person of 1.70 cm height!)
More exercise – voluntarily!
A part of the solution to the riddle lies in the increased physical activity of vegans, because out of this group, the amount of people, who exercised for more than an hour, was double the amount of people, who exercised for less than an hour. For the omnivores, however, there were more people who exercised less than an hour in comparison to people who exercised more than an hour each week. But even under the assumption that this time was spent doing high intensity sports (MET10), this difference could not explain a weight difference of 4.2 BMI levels. Vegans who exercise more would have to burn 84.700 kcal more than their counterparts. I thus calculated the average calorie intake for both groups and carefully considered the different percentages of people who exercised for 0 min, up to 20 min, 20-60 min, 60-150 min and over 150 min a week. The result was a calorie expense of 293.870 kcal for vegans and 235.182 kcal for omnivores over the course of 5.79 years. This means a difference of 58.688 kcal. Subtracting this from the original 84.700 kcal difference, 26.011 kcal remain as unexplained burned calories.
If we convert this number into fat, this would mean a weight difference of approximately 3.7 kg, which results solely from the vegan diet, because the both the caloric intake and the exercise load remain the same. How nice!
Photo: istockphoto.com, “Happy young housewife in kitchen pointing in camera”
1: Poehlman ET, Arciero PJ, Melby CL, Badylak SF. Resting metabolic rate and postprandial thermogenesis in vegetarians and nonvegetarians. Am J Clin Nutr. 1988 Aug;48(2):209-13.
2: Campbell TC, Chen J. Energy balance: interpretation of data from rural China. Toxicol Sci. 1999 Dec;52(2 Suppl):87-94. Review.
3: Orlich MJ, Singh PN, Sabaté J, Jaceldo-Siegl K, Fan J, Knutsen S, Beeson WL, Fraser GE. Vegetarian dietary patterns and mortality in Adventist Health Study 2. JAMA Intern Med. 2013 Jul 8;173(13):1230-8. doi: 10.1001/jamainternmed.2013.6473.