“Satisfy your hunger,” say Kellogg’s Special K Shakes. Does protein keep you satiated—that is, feeling full—for longer?

“The results of human studies are mixed,” says Barbara Rolls, professor of nutritional sciences at Penn State.

One reason for the mixed results: many studies don’t account for calorie density. Calorie density is how many calories you’re getting in, say, a bite of food.

The effect of protein and calorie density

“If you take some fat out and put some protein into foods, people are going to consume fewer calories because you’re lowering the calorie density,” explains Rolls. That’s because, ounce for ounce, fat has more calories than protein.

And calorie density has a bigger effect than macronutrients (protein, fat, or carbs) on satiety, Rolls points out.

In a study that did take calorie density into account, women ate as much as they wanted of lunch and dinner entrées with 10, 15, 20, 25, or 30 percent of calories from protein, with starches making up the difference. (Fat levels and calorie density didn’t vary.)

“We used real food like chicken and shrimp, but everything was finely chopped in casseroles so people couldn’t tell that there were differences in protein content,” says Rolls.

The results: protein had no impact on how much people ate—or on how hungry or full they felt—throughout the day.

Do expectations matter?

Of course, extra protein may curb appetite after a meal in some studies simply because people expect it to.

“Many people were raised to believe that protein is the center of a meal, and that a meal is more satisfying if it has meat or eggs or fish or legumes or another protein source,” says Rolls.

But that’s a matter of expectations, not protein per se.

“If we’re going to develop a high-satiety diet, it’s not going to depend on one magic ingredient that you add to a food,” says Rolls. “The hope that one simple tweak to your diet will automatically help you manage your food intake is way too simplistic.”

Photo: anaumenko/stock.adobe.com.