“Satisfies hunger longer,” promise Special K Protein Shakes, which are mostly blends of water, nonfat milk, whey protein concentrate, soy protein isolate, and sugar. “With every tasty shake, you’ll get the nutritional benefits of 10g protein and 5g fiber that can help satisfy your hunger so you can lose weight.”
What the research shows
Some studies—many of them funded by the food industry—report that higher-protein foods make people feel more full than lower-protein foods.1 But the best studies find no difference.2
Barbara Rolls is director of the Laboratory for the Study of Human Ingestive Behavior at Penn State University, where she studies what affects appetite and how much people eat.
“Our study gave people real foods, like chicken casserole or shrimp stir-fry, but with 10, 15, 20, 25, or 30 percent of their calories from protein,” she says. “The entrées looked and tasted the same and had the same fat and calories.”
The protein level in the food didn’t matter. “Protein had no impact on hunger or how many calories people ate at other meals.”
Liquid vs solid food
What’s more, a drink—with or without protein—may be less satiating than a solid food.
For example, researchers fed 120 lean and 60 obese adults a solid or liquid version of a high-carb food, a high-fat food, or a high-protein food. All the foods had the same number of calories. In each case, the participants ate more calories on the days they got the liquids.3 Other studies find the same.4
Effect on weight
And longer-term studies find little difference—a pound or two—or no difference in weight loss when dieters eat higher-protein versus normal-protein diets.5,6
“To count on a little more protein to satisfy hunger and then translate that to weight loss, that’s really a leap,” notes Rolls. “Even if people say they’re less hungry, that doesn’t mean they’re going to eat less.”
Still, if you’re cutting calories, it makes sense to cut carbs or fat rather than protein because protein will help prevent muscle loss when dieting.
But why bother with a Special K Protein Shake that costs about $1.50? A 10 oz. bottle has 10 grams of protein and 180 calories (thanks, in part, to its added sugar). You can get that much protein in 10 oz. of fat-free milk for only 110 calories and less money.
And the fiber in Special K’s shake comes largely from maltodextrin and polydextrose, processed fibers that may have little or no effect on appetite or regularity.
Want a better meal of protein and fiber? Try a bowl of fat-free greek yogurt, fruit, and nuts.
1 Am. J. Clin. Nutr. 87(suppl):1558S, 2008.
2 J. Am. Diet. Assoc. 111: 290, 2011.
3 Int. J. Obesity 31: 1688, 2007.
4 Brit. J. Nutr. 106: 37, 2011.
5 J. Nutr. 143: 591, 2013.
6 Nutr. Metab. Cardiovasc. Dis. 24: 224, 2014.
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