Losing about 10% of body weight improved health whether people followed a ketogenic, Mediterranean or very-low-fat plant-forward diet. But in a small clinical trial, the ketogenic diet produced a bigger drop in liver fat and stronger improvements in several measures tied to blood sugar control.
The randomized trial, published in Cell Metabolism, included 42 adults with obesity, prediabetes and excess fat in the liver. Researchers provided all of their food and adjusted calories so that each group lost about the same amount of weight over roughly five months.
That design helped answer an important question: If people lose the same amount of weight, does the type of diet still make a difference?
In this study, the answer was yes, at least for some short-term metabolic measures.
“We know weight loss can improve metabolic health in people who are obese,” said Samuel Klein, the study’s corresponding author at Washington University School of Medicine in St. Louis. “Our data suggest that if you have high liver fat content and prediabetes, reducing carbohydrate intake can have important therapeutic effects on metabolism beyond just weight loss alone.”
Participants were randomly assigned to one of three diets. The keto diet got just 4% of calories from carbohydrates and 73% from fat. The Mediterranean diet got 50% of calories from carbohydrates and 35% from fat, while the plant-forward diet got 70% of calories from carbohydrates and 15% from fat.
Researchers prepared and supplied the food, and participants reported eating more than 95% of the meals and snacks they were given.
All three groups lost almost exactly 10% of their starting body weight.
That weight loss improved how well the body responded to insulin in all three groups. But the keto group showed a much larger improvement in how the liver responded to insulin, one of the study’s main outcomes.
Liver fat also fell in every group.
It dropped by about 67% in the keto group, compared with about 45% in both the Mediterranean and plant-forward groups. Participants started the trial with liver fat averaging around 18% to 19%. By the end, it had fallen to about 6% in the keto group and about 10% in the other two groups.
Blood sugar also improved.
Fasting blood sugar and insulin levels fell in all three groups, but the decreases were generally larger with keto. A1C, a measure of average blood sugar over the past few months, also improved more in the keto group than in the other two groups.
At the end of the trial, seven of the 14 people in the keto group no longer met the study’s criteria for prediabetes. That compared with four of 14 in the Mediterranean group and one of 14 in the plant-forward group.
Those numbers sound dramatic, but the groups were very small.
The study does not show that keto “reversed” prediabetes, nor does it tell us whether those improvements lasted. Researchers measured participants soon after the controlled diet period ended, not years later.
The trial also found some reassuring short-term results for blood fats.
Despite getting much more fat and saturated fat, the keto group did not have worse LDL cholesterol than the other groups after weight loss. Fasting triglycerides fell the most in the keto group.
But that does not mean a high-fat keto diet has been proven safe for the heart over the long term.
The study lasted only a few months and included too few people to measure outcomes such as heart attacks or strokes. It also took place during active weight loss, which can itself improve many cardiovascular risk markers.
There were also similarities across the diets.
All three groups lost body fat. All three lost some lean mass. And all three improved several measures of blood sugar and insulin response.
In other words, keto was not the only diet that worked.
The biggest advantage appeared in the liver, where keto produced a larger drop in fat and a stronger improvement in how the liver handled insulin.
That may be especially relevant for people with fatty liver disease and prediabetes, which is exactly the population studied here.
The findings should not be generalized to everyone trying to lose weight.
Participants were adults with obesity, prediabetes and excess liver fat. The study does not show that people without those conditions would see the same benefits.
It also does not tell us which diet is easiest to maintain in everyday life.
During the trial, participants received prepared food, met weekly with a dietitian and were closely monitored. That is very different from following the same eating pattern independently for months or years.
The small sample size is another important limitation. Only 14 people completed each diet, so some of the differences between groups may be less stable than the percentages make them appear. Many of the additional findings beyond the study’s main outcomes will also need to be confirmed in larger trials.
“The most surprising finding was that participants in the ketogenic diet group didn’t experience an increase in blood fat or cholesterol levels despite consuming the most fat,” said first author Max Petersen of Washington University School of Medicine.
For now, the study suggests that a very-low-carbohydrate ketogenic diet may offer some additional short-term metabolic benefits during weight loss for people with obesity, prediabetes and fatty liver disease.
It does not prove that keto is the healthiest weight-loss diet overall, nor that it is the best long-term option for everyone.
The study was supported by the National Institutes of Health and the Foundation for Barnes-Jewish Hospital.
Samuel Klein reported receiving advisory board fees from AbbVie, 89Bio and Boehringer Ingelheim, an investigator-initiated research grant from Merck and support for an industry-sponsored clinical trial from Viking Therapeutics.