- Low-fat vegan diets can help aid in weight loss, but the mechanisms behind this have been unclear.
- Adults on a low-fat vegan diet reduced dietary energy density by 30% compared with controls in a secondary analysis of a randomized trial.
- Reductions in dietary energy density were significantly associated with greater weight loss after adjusting for overall energy intake.
Adults with overweight who adopted a low-fat vegan diet ate fewer calories per gram of food without intentional calorie restriction, a secondary analysis of a randomized trial showed.
Among 244 participants, those assigned to a low-fat vegan diet for 16 weeks had an approximate 30% reduction in dietary energy density — or the amount of calories in a given weight of food — while those in the control group had no change (effect size -0.28 kcal/g/d, 95% CI -0.38 to -0.19, P<0.001).
Energy intake decreased in both groups, but more so in the group who ate a vegan diet (effect size -357.1 kcal/d, 95% CI -521.9 to -192.3, P<0.001), reported Hana Kahleova, MD, PhD, of the Physicians Committee for Responsible Medicine in Washington, D.C., and colleagues in a research letter in JAMA Network Open.
Reductions in dietary energy density were significantly associated with greater weight loss after adjusting for overall energy intake (r=0.21, 95% CI 0.08-0.34, P=0.002). A 1-kg weight loss corresponded to a 0.36-unit decrease in energy density after adjustment (P=0.002).
“The approximately 30% reduction in energy density observed in this study represents a substantial shift that is unlikely to be achieved and sustained through portion control alone,” the authors wrote. “These findings suggest that dietary composition — specifically, an emphasis on plant foods — can meaningfully influence energy intake regulation.”
“Plant-based foods are high in water and fiber and low in fat, increasing food volume without a proportional increase in energy content,” they explained. “This promotes satiety and reduces energy intake without deliberate caloric restriction.”
Kahleova told MedPage Today that participants were eating essentially the same amount of food by weight, but the low-fat vegan food — consisting of fruits, vegetables, grains, and legumes — had far fewer calories per gram.
The findings may alter the approach clinicians take when giving weight-loss advice to patients, she added.
“For years, the default advice we give patients trying to lose weight is to eat less: smaller portions, count calories, leave food on the plate. It’s advice that’s technically correct but very hard for most people to sustain, because it asks them to override hunger, meal after meal, indefinitely,” Kahleova noted.
“What this analysis points to is a different lever: instead of telling patients to eat less food, we can help them shift toward foods that are naturally less energy-dense … and let the calorie reduction happen on its own,” she said.
Kahleova emphasized that the trial participants ate until they were full and still lost a clinically meaningful amount of weight, with no daily calorie targets. “I’d like physicians to hear that as an invitation to have a different conversation with patients: not ‘eat less,’ but ‘eat differently,'” she suggested.
These findings build upon the results from the primary analysis, which found that a low-fat vegan diet resulted in weight loss and improved body composition in adults with overweight.
“We knew then that the low-fat vegan diet produced meaningful weight loss, but we hadn’t yet been able to explain the mechanism behind it,” Kahleova said. “This new analysis finally gives us a clear, measurable answer — and one that’s actionable in a clinic visit: instead of asking a patient only how much they’re eating, ask what they’re eating, and specifically how energy-dense it is.”
The randomized, open-label parallel study was conducted between January 2017 and February 2019 in Washington, D.C. Participants were assigned 1:1 to a low-fat vegan diet or asked to make no dietary changes (control group). Neither group was given limits on overall energy intake.
Most participants (87%) were women, 48% were white, and average age was 54.4. All had a body mass index of 28 to 40 at baseline.
Both groups completed 3-day diet records at baseline and week 16. Dietary energy density was calculated as a ratio of total daily energy intake (in kilocalories) to total daily food weight (in grams).
Study limitations included the use of self-reported food records and the potential lack of generalizability, since participants were research volunteers and may not fully represent the general population.
