Eating a diet that’s high in protein is often recommended for people trying to lose weight, since high-protein foods make people feel more full, preventing overeating. However, a new study suggests that while the diet may help people slim down, it doesn’t necessarily improve other health problems under the hood. (For more on that, see: How Much Protein Should I Eat Every Day.)
In a small study, researchers at Washington University School of Medicine in St. Louis followed 34 postmenopausal women with obesity for about six months. The women were split into three groups: One group kept their diet the same, one group went on a calorie-restricted weight loss diet (with the daily recommended amount of protein), and another group went on the same diet but also increased their protein intake by about 150-250 calories. The researchers provided all the meals for the women, and besides the increased protein, the diets were virtually the same.
The study authors found that while both groups of women were able to lose about 10% of their body weight, the women who ate more protein experienced no changes in their insulin sensitivity—which is important for overall health.
Improved insulin sensitivity is important to cut down on a person’s risk for type 2 diabetes—which is common in people with obesity. It’s one of the reasons weight loss is recommended for better health in the first place. The women who lost weight without increasing the amount of protein they ate experienced a 25 to 30% improvement in insulin sensitivity. But the women who ate more protein experienced no change in their insulin sensitivity at all. “We definitely expected a blunting of the effect, but to completely eliminate it was a little bit surprising,” says lead study author Bettina Mittendorfer, a professor of medicine.
The number of people in the study, which was published Tuesday in the journal Cell Reports, is very small, but Mittendorfer says the report is not the first to raise skepticism about high-protein diets. “There is a reported association from epidemiological studies between protein intake and an increased risk for type 2 diabetes,” she says.
Mittendorfer and her fellow researchers plan to continue studying the issue to better understand why people who eat more protein did not experience the same metabolic benefits, and whether the type of protein a person consumes matters. For instance, does plant protein have different effects compared to animal protein? “It’s a tremendous effort to go through a 10% loss in body weight. To not see an improvement in one of these key factors is significant I think,” says Mittendorfer.
Though the study is small and more research is needed to better understand the findings, Mittendorfer says she would advise people to be “cautious” about adopting a high-protein diet to lose weight. “I think there is no reason to go for high protein intake during weight loss, based on our results,” she says. “There’s no reason to do it, and potentially there is harm or lack of a benefit.”