Healthcare providers usually recommend either a low-carbohydrate and high-fat, Mediterranean-style or a low-fat and more plant-based diet for weight loss. These popular diets differ in the nutrients they include.

The low-carb, high-fat diet, sometimes called the keto or Atkins diet, gets most of its nutrients from fat and restricts carbohydrate consumption. It can be high in saturated fats and include animal products.

The Mediterranean diet contains mostly plant-based foods, such as fruit, vegetables, nuts and olive oil, along with some fish and low-fat dairy products. It includes little or no red meat. It is high in monosaturated fats: it

A low-fat, plant-forward diet can derive as much as 70 percent of its calories from carbohydrates.

But what, if any, metabolic benefits do these diets offer in addition to weight loss? The findings of a new study led by researchers at the Washington University School of Medicine in St. Louis suggest that for people with obesity the keto diet delivers the greatest metabolic benefits.

Reducing carbohydrate intake has the most important metabolic benefits beyond weight loss alone.

Forty-two people with obesity, prediabetes and fatty liver disease were enrolled in this study. They were randomly assigned to either the keto diet, the Mediterranean diet or a low-fat, plant-forward diet.

All three diets helped with weight loss. Participants lost about 10 percent of their body weight by the end of the study period, and a significant percentage of body fat. Their bodies also became more sensitive to insulin, which improved their ability to regulate their blood sugar levels.

It was in measures of liver metabolism and fat loss that the keto diet excelled. Of the study participants who followed the keto diet for four or five months, about half no longer met the criteria for prediabetes, compared to 29 percent of those who followed the Mediterranean diet and seven percent who followed a low-fat, plant-forward diet.

The findings suggest reducing carbohydrate intake has the most important metabolic benefits beyond weight loss alone, Samuel Klein, corresponding author on the study and a professor of nutritional science and medical director of the Washington University Weight Management Center, explained.

The keto diet produced the greatest improvements in several markers of metabolic health. Liver fat, for example, dropped by about 67 percent in the keto diet group, compared to 45 percent in the Mediterranean and low-fat, plant-forward groups.

“The most surprising finding was that participants in the keto group did not experience an increase in fat or cholesterol levels in the blood, despite consuming the most fat.”

Participants in the keto group had the greatest increase in glucagon, a hormone that promotes fat loss in the liver, over a 24-hour period, compared to those in the other groups. The keto group also had greater reductions in insulin levels, which reduces fat content in the liver.

“The most surprising finding was that participants in the keto group did not experience an increase in fat or cholesterol levels in the blood, despite consuming the most fat,” said Max Petersen, first author on the study and an instructor in the department of medicine at Washington University.

The study is published in Cell Metabolism.