WASHINGTON, D.C. — A low-fat vegan diet lowers the energy density of the foods people eat by roughly 30%, allowing them to eat satisfying amounts of food while taking in fewer calories and losing weight—without deliberately restricting calories— according to new research published in JAMA Network Open .
In a randomized clinical trial of adults with overweight, those who adopted an ad libitum low-fat vegan diet substantially reduced how many calories were packed into each gram of food they ate. That drop in energy density was linked to lower calorie intake and greater weight loss, even though participants were never told to eat less.
"This helps answer a question people often have about plant-based eating: How can you lose weight without counting calories or going hungry?" said Hana Kahleova, MD, PhD, director of clinical research at the Physicians Committee for Responsible Medicine and lead author. "The answer is energy density. Plant foods are rich in water and fiber, so you can fill your plate, feel full, and still take in fewer calories."
A Mechanism Behind the Weight Loss
The analysis drew on a 16-week randomized trial in which adults with overweight were assigned to either an ad libitum low-fat vegan diet of fruits, vegetables, grains, and legumes, or a control group that made no dietary changes. Neither group was given calorie limits. Energy density was calculated for each participant as the ratio of total energy (kcal per day) to total food weight (grams per day).
Key findings include:
- Total food weight did not change significantly in either group—participants on the vegan diet ate just as much food by weight.
- Energy intake fell in both groups but more in the vegan group, by about 357 kcal/day.
- Dietary energy density did not change in the control group but dropped about 30% in the vegan group.
- Greater reductions in energy density were associated with greater weight loss, and the link held after accounting for changes in calorie intake.
Eat More Food, Take in Fewer Calories
The shift was driven by what landed on the plate. On the vegan diet, calorie-dense animal foods such as meat, dairy, and eggs were eliminated, while intake of high-volume, low-energy-density plant foods—especially vegetables and legumes—rose sharply. Because these foods carry few calories per gram, participants could eat generous portions while consuming less energy overall.
"This isn't about willpower or smaller portions," Dr. Kahleova said. "It's about choosing foods that naturally deliver fewer calories in every bite. A 30% reduction in energy density is a substantial shift that would be very hard to achieve and sustain through portion control alone."
Why Energy Density Matters
Energy density—the number of calories per gram of food—has long been studied as a driver of how much people eat. Controlled feeding studies show that lowering the energy density of meals helps people feel full and reduces how much they eat afterward, all while keeping the volume of food the same.
"When the foods you eat are lower in energy density, your body's natural appetite signals work in your favor," Dr. Kahleova said. "You eat until you're satisfied, and you simply end up with fewer calories. That's a sustainable way to manage weight."
Implications for Weight Management
With dietary guidance often focused on cutting calories or controlling portions, the study points to a different and potentially more practical lever: the composition of the diet itself. Emphasizing water-rich, high-fiber plant foods may make it easier for people to reduce calorie intake without feeling deprived.
"From a clinical standpoint, targeting energy density offers a realistic strategy for weight loss," Dr. Kahleova said. "Instead of telling people to eat less, we can help them eat differently—and the results follow."
Bottom Line
A low-fat vegan diet substantially lowers dietary energy density, helping people take in fewer calories and lose weight without intentional calorie restriction.
"You don't have to eat less," Dr. Kahleova said. "You can eat more food, feel full, and still lose weight—by choosing foods that are naturally lower in energy density."
Founded in 1985, the Physicians Committee for Responsible Medicine is a nonprofit organization that promotes preventive medicine, conducts clinical research, and encourages higher standards for ethics and effectiveness in education and research
