If you’re cutting calories, it makes sense to cut carbs and fat rather than protein. But does extra protein keep you from losing muscle as you lose weight?

Researchers randomly assigned 70 middle-aged women with obesity to a:

  • RDA-protein diet that cut calories and averaged about 80 grams of protein a day, the Recommended Dietary Allowance (0.36 grams for each pound of body weight, which averaged about 215 pounds when the study started),
  • higher-protein diet that cut the same number of calories but had about 50 percent more protein (115 grams, on average), thanks to extra whey protein, or
  • control group that didn’t cut calories and had the RDA for protein.

After six months, women on the higher-protein diet had lost less lean body mass (mostly muscle) than the other dieters. But just slightly less.

“The protein supplement blunted the loss of lean mass, but the difference was trivial,” says Bettina Mittendorfer, professor of medicine and nutritional science at the Washington University School of Medicine in St. Louis.

Dieters who got the higher-protein diet lost a pound less lean body mass. But that difference was so minimal that their thigh muscles were no bigger or stronger than the muscles of dieters on the RDA-protein diet.

The surprise: the higher-protein diet had a downside.

In many people with excess weight, the body resists insulin’s attempt to admit blood sugar into cells. Losing weight should make their cells less resistant to insulin. But that didn’t happen in those who got more protein.

“Protein supplementation during weight loss completely eliminated the beneficial effect on muscle insulin resistance,” says Mittendorfer. “It wasn’t just a blunting, but a complete elimination. The extent to which it happened was remarkable.”

And insulin resistance matters. “Insulin resistance is at the core of many metabolic abnormalities,” she explains, especially type 2 diabetes.

Why might extra protein make cells more resistant to insulin?

“We know that when people consume protein, it causes an increase in insulin secretion,” says Mittendorfer. “If you chronically put out too much insulin, the body may adapt by making you more insulin resistant.”

Once someone has type 2 diabetes, replacing some carbs with protein may help, notes Mittendorfer. “In a person with diabetes, a higher-protein, low-carb diet may help control blood sugar. But that may be simply because it’s low-carb.”

But things are different in people with prediabetes. “They already have high blood insulin levels to compensate for insulin resistance, so if they pump out more, it’s a vicious cycle that could be detrimental,” says Mittendorfer.

“It’s not proven,” she cautions. “But our study shows that there is a potential concern.”

Photo: 9dreamstudio/fotolia.com.