This Diet Helped People Lose Weight Without Eating Less Food

The study points to food volume, not willpower, as a weight-loss lever.

12 Aug 2026, 15:27 by Tudor Tarita · ZME Science

Credit: Pexels

Weight-loss advice comes in many different forms, but it usually boils down to one blunt instruction: reduce calorie intake.

But according to a new study, there’s another way to get there. Instead of deliberately shrinking portions, people can change the kinds of foods they eat—and end up consuming fewer calories almost automatically.

That’s exactly what happened in a 16-week trial of a low-fat vegan diet. People lost weight without calorie counting and without eating less food by weight.

The Weight of Food Stayed Similar

The trial included 244 adults between the ages of 25 and 75 with BMIs ranging from 28 to 40. A normal or healthy weight body mass index (BMI) for adults ranges from 18.5 to 24.9.

Half of the participants were assigned to a low-fat vegan diet, while the other half were asked to make no changes to their usual diet.

After 16 weeks, the vegan group lost an average of 5.9 kilograms (about 13 pounds). Their daily energy intake fell by roughly 355 calories more than the control group, according to the study. The researchers also reported improved insulin sensitivity, lower fat stored in the liver and muscle, and an increase in calories burned after meals.

Researchers say this is linked to a concept called energy density.

“This is really the heart of what energy density explains,” Dr. Hana Kahleova, the study’s lead author and director of clinical research at the Physicians Committee for Responsible Medicine, told Newsweek. “There’s a well-documented tendency for people to eat a fairly consistent weight or volume of food at a meal, almost regardless of how many calories that food contains.”

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If you want to have a healthier diet, having compact, high-calories foods is usually a bad way to go about it. Foods that are rich in water and fiber tend to be better for you. Illustration: ZME Science..

Basically, a bowl of beans, greens and grains can weigh as much as a meal built around cheese, meat, and added oils, but it often carries fewer calories. Water and fiber add bulk. Fat, by contrast, packs more than twice as many calories per gram as carbohydrates or protein.

“In our study, participants who shifted toward a diet built from vegetables, fruits, whole grains, and legumes ate the same total weight of food each day as before,” Dr. Kahleova added. “The amount of food didn’t change, but the energy density of that food fell by nearly 30%.”

A Plate Strategy

Credit: Pexels.

So what changed in the participants’ diet?

Consumption of fruit, vegetables, legumes, whole grains and meat alternatives increased, while intake of meat, dairy, eggs, nuts, seeds and added fats fell. Increased legume consumption was particularly strongly associated with weight loss. Fiber intake also increased substantially. In the original trial, people in the vegan group went from about 24 grams of fiber per day to nearly 35 grams.

The findings arrive as injectable weight-loss drugs have reshaped obesity treatment. The effects of this vegan diet were comparable to weight-loss drugs, though the comparison isn’t exactly perfect.

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For people who do not want to go fully vegan, the study still offers a practical lesson. You don’t have to go all the way. Put more high-volume, lower-calorie foods at the center of the meal, and you’ll generally do better.

“Fill half your plate with vegetables before adding anything else,” Dr. Kahleova concluded. “Swap meat for beans, lentils or tofu in some meals each week. Choose whole fruit over juice or dried fruit… and go easy on added oils, butter and cheese.”