The Protein Solution
To reach a healthier weight with fewer hunger pangs, consider eating more lean protein.
A new study adds to a growing body of research that points to protein's power to satisfy hunger better than either fat or carbohydrates.
The findings could also help explain the recent but short-lived enthusiasm for low-carb diets, which happen to be high in protein.
"It's telling us that one of the reasons why the low-carbohydrate diets seemed to work is not because of low carbohydrates, but because of high protein," said Arne Astrup, head of the department of human nutrition at the Royal Veterinary and Agricultural University in Copenhagen. "Look at Atkins, South Beach and the Zone," said Astrup, who wrote an editorial accompanying the protein study, which appears in this month's American Journal of Clinical Nutrition. "They're all characterized as having 30 to 40 percent of calories from protein."
Thirty percent of daily calories as protein -- about twice what most Americans eat, and the upper limit recommended by the Institute of Medicine -- is the amount that University of Washington researchers gave to 19 participants in the latest study. They had an average body mass index of 26, equal to being about 10 pounds overweight, and had maintained their current weight for at least three months.
Throughout the four-month study, food was prepared and supplied by the research team. Although the participants lived at home, they met with researchers twice a week to check their weight and receive more food. Their blood was tested regularly to measure insulin, leptin and ghrelin, hormones that regulate appetite.
During the first two weeks of the trial, researchers supplied enough food for participants to maintain their weight on a standard American diet composed of 15 percent of calories from protein, 35 percent from fat and 50 percent from carbohydrates. Study subjects were instructed to eat all the food supplied. If any remained, they were told to return it so that researchers could accurately calculate their daily calories.
For the third and fourth weeks, participants still received enough food to maintain their weight but shifted to a diet with twice the amount of protein, about half the fat and with the remaining calories coming from healthy carbohydrates, including plenty of fruit, vegetables and whole grains. As before, participants were instructed to eat all the food supplied daily. Any remaining food was returned for measurement.
"They complain about it, because they feel very full on the high protein diet," said David Scott Weigle, professor of medicine at the University of Washington and the study's lead author.
For the last 12 weeks of the study, which was designed to mimic ordinary living conditions, participants received the same high protein fare, plus extra food containing up to 15 percent more calories per day. They also got the freedom to eat as much or as little as they wanted. To avoid nutritional boredom, participants could also eat one meal of their choosing per week and drink up to three alcoholic beverages weekly. They recorded their "free meal" in food records, kept logs rating their appetites and received instructions on how to choose healthy, high-protein fare.
Left to their own devices on the high-protein diet, participants spontaneously cut their intake by 441 calories daily -- roughly a quarter of their daily calories. They lost an average of 11 pounds, including about eight of fat, while reporting feeling full and satisfied. "I was surprised by the magnitude of the weight loss," Weigle said, noting that participants' weight "just cruised down."
Blood tests showed that appetite-controlling hormones reached levels that normally boost hunger. "It means that the effect of the high-protein diet was stronger than the biological stimulus to eat more," Weigle said. One way to explain the results, he said, is that a high protein intake affects the brain, a finding already confirmed in animal studies. If the brain perceives that lower levels of appetite-stimulating hormones are normal, Weigle said, "then it would continue to turn down hunger."