“Protein helps you stay on track,” say the Special K granola bar boxes. “Special K cereal, shakes, meal bars, and snacks have a winning combination of protein and fiber you need to help you outsmart hunger.”
“Protein: Your Secret Weight- Loss Weapon,” proclaimed the headline in Women’s Health magazine in 2010. “The moment it leaves your fork, protein starts winnowing your waistline.”
“Dietary protein does have a modest impact on satiety in some studies, but not in others,” says researcher Wayne Campbell, a professor of nutrition science at Purdue University.
What’s more, feeling less hungry or more full may not mean you eat less. “The majority of studies don’t show any impact of satiety on how much people consume at a later meal,” notes Heather Leidy, a nutritional physiologist at the University of Missouri.
When it comes to weight loss, it’s the calories—not the amount of carbohydrate, fat, or protein—that count.
For example, 18 of 23 weight-loss trials that lasted an average of 12 weeks found no greater weight loss with higher-protein diets than with lower-protein diets. When a meta- analysis combined the results of all 23 trials (on a total of 1,063 people), those on the higher-protein diets lost only an ex¬tra 1.7 pounds.
In the most ambitious attempt so far to see if protein is a “secret weight-loss weapon,” the Pounds Lost study advised 811 overweight or obese U.S. men and women to eat 750 fewer calories a day. Half were told to get 15 percent of their calories from protein; the other half 25 percent. (The trial also tested different levels of carbohydrate and fat.)
After two years, the higher-protein group had lost no more weight than the lower-protein group. By that time, though, both groups had drifted back to getting about 20 percent of their calories from protein.
However, extra protein may still help dieters: “A higher- protein diet does have a more consistent effect on preserving lean body mass during weight loss,” says Campbell.
In a meta-analysis of 11 studies that lasted at least 12 weeks, dieters on higher-protein diets lost 1.3 fewer pounds of lean tissue than dieters who ate less protein.
“If you eat a higher proportion of your calories as protein while you’re losing weight, you’ll lose less lean tissue,” says Campbell.
Sources: J. Am. Diet. Assoc. 111: 290, 2011; Am. J. Clin. Nutr. 89: 831, 2009; Am. J. Clin. Nutr. 96: 1281, 2012; N. Engl. J. Med. 360: 859, 2009.