Super Baby Food Book

The Barnard Low-Fat vs. the Usual Diet Study

Original Study

Kahleova H, Petersen KF, Shulman GI, et al. Effect of a Low-Fat Vegan Diet on Body Weight, Insulin Sensitivity, Postprandial Metabolism, and Intramyocellular and Hepatocellular Lipid Levels in Overweight Adults: A Randomized Clinical Trial. JAMA Network Open. 2020;3(11):e2025454.

Links to This Study

What Did the Researchers Study?

Researchers conducted a 16-week randomized clinical trial involving 244 adults with overweight. Participants were randomly assigned in equal numbers either to follow a low-fat vegan diet or to continue their usual diet without making dietary changes.

The researchers measured body weight, body composition, insulin sensitivity, post-meal metabolism, and other metabolic outcomes before and after the intervention.

What Was the Vegan Diet?

Participants in the intervention group were asked to follow a low-fat vegan diet based on vegetables, fruits, whole grains, and legumes. They avoided animal products and added fats.

The diet did not require calorie counting or portion restriction. Participants were instructed to eat freely from the permitted foods.

What Did They Find?

Over the 16-week study, participants in the vegan group lost an average of about 5.9 kilograms, or about 13 pounds. Body weight did not significantly change in the control group.

The vegan group also showed improved insulin sensitivity and increased post-meal energy expenditure.

What Happened to Fat Stored in the Liver and Muscles?

In a subset of participants, researchers measured fat stored inside liver and muscle cells. Liver fat decreased substantially in the vegan group, and fat stored within muscle cells also decreased.

These reductions were associated with improvements in insulin resistance.

Why Is This Interesting?

This randomized trial demonstrates that changing an overall dietary pattern can produce measurable changes in body weight and metabolic health within only 16 weeks.

The participants were not instructed to deliberately restrict calories, yet those following the low-fat vegan diet reduced their energy intake and lost weight.

Why Is This Study Included Under “What Is the Healthiest Overall Diet?”

The study tested an entire dietary pattern rather than a single food or nutrient. Participants changed many parts of their diets at the same time, including the types of foods eaten, dietary fat, fiber intake, and energy density.

For that reason, the study cannot tell us which single component produced each benefit. Instead, it shows what happened when participants adopted the overall low-fat vegan dietary pattern used in the trial.

An Important Limitation

The study lasted only 16 weeks, so it cannot tell us whether the changes would persist over many years. Participants also knew which diet they were following, as is generally unavoidable in studies comparing visibly different dietary patterns.

In addition, several dietary characteristics changed simultaneously. The trial therefore tested the combined effect of the dietary pattern rather than isolating the effect of any one food or nutrient.

Funding and Author Information

The study was funded by the Physicians Committee for Responsible Medicine (PCRM) and grants from the Yale Diabetes Center. Several authors, including Hana Kahleova and Neal Barnard, were affiliated with PCRM.

The published article states that the funders had no role in the design or conduct of the study, collection or analysis of the data, preparation or approval of the manuscript, or the decision to submit it for publication.

Published Corrections

JAMA Network Open later published corrections to this article concerning the Role of the Funder/Sponsor section and several errors in the reported results and supplementary material. When reviewing the study, use the current corrected version on the official journal website or PubMed Central.