Body fat increases in all who overeat, regardless of the level of protein eaten, the researchers found. But those who overate with low protein levels in their diet stored a higher percent of calories as fat. They also lost lean body mass, while those on the higher-protein diets gained lean body mass.
The messages are clear, says researcher George Bray, MD, Boyd professor and professor of medicine at the Pennington Biomedical Research Center in Baton Rouge, La.
Calories count, and so does protein. "Very low protein diets are clearly detrimental," Bray says. "You lose lean body mass."
The study is published in the Journal of the American Medical Association.
Overeating, Protein, & Weight Gain: Study Results
Other experts have suggested that overeating either on a low- or high-protein diet would produce less weight gain than overeating with normal protein intake.
Bray and his team set out to assess how the level of protein affects not only weight gain when you overeat, but also body composition (what percent of you is lean vs. fat) and resting energy expenditure (the amount of calories your body burns at rest).
Bray's team studied 25 healthy men and women, ages 18 to 35, between 2005 and 2007. Their average body mass index or BMI ranged from nearly 20 to nearly 30. A BMI of 18.5 to 24.9 is considered normal weight.
Each participant first ate a weight-stabilizing diet for 13 to 25 days. Next, they were assigned to one of three groups: a 5%, 15%, or 25% protein diet. During the next eight weeks, they were fed one of the three protein levels in a diet that had about 1,000 extra calories a day.
The researchers evaluated their weight, body composition, and resting energy expenditure before and after the study.
The findings could help experts advise people on how to eat to avoid weight gain and obesity and to maintain lean body mass.
The results: All groups gained weight, as expected. The low-protein group gained the least, about 7 pounds. The normal-protein group gained 13.3 pounds. And the high-protein group gained 14.4 pounds.
But "the low-protein group stored a higher percentage of calories as fat than the other groups," Bray tells WebMD.
In addition, the low-protein group lost lean body mass, about 1.5 pounds. The normal-protein group gained 6.3 pounds of lean body mass, and the high-protein group gained 7 pounds of lean body mass. Also, the calories burned while at rest increased in the normal- and high-protein groups, but not in the low-protein group.
While excess body fat is linked with obesity, increased lean muscle mass has a positive effect on metabolism.
Overeating & Protein Intake: Lessons
"The low-protein diet was clearly not good in terms of preserving lean body mass," Bray says.
The researchers did not find much difference in terms of body composition changes between the 15% or the 25% protein diet, Bray says.
"Based on the study, healthy adults should consider getting 12% to 15% protein [from their diet]," Bray says. (That recommendation does not apply to the elderly or athletes, he says, who may need more.)
For a 2,000-calorie diet, that level would mean taking in about 300 calories or 75 grams of protein daily. (A gram of protein has 4 calories.) A 3-ounce skinless chicken breast has about 27 grams of protein. Six ounces of Greek yogurt has about 14 grams.
Bray serves as a consultant for Abbott Laboratories and Takeda Global Research Institute. He is an advisor to Medifast, Herbalife, and Global Direction in Medicine. He has received royalties for the Handbook of Obesity.
Overeating, Protein, & Weight Gain: 'The Danger of Eating Low Protein'
The study results suggest the downsides of eating too little protein, says David Heber, MD, professor of medicine at the UCLA David Geffen School of Medicine and founding director of UCLA's Center for Human Nutrition. He co-authored an editorial to accompany the study.
"The danger of eating low protein is you eat too many refined carbohydrates," he tells WebMD. White bread, for instance, is a refined carbohydrate. "That puts on weight. And that weight tends to be fat."
"Protein is a great thing to control your appetite and to maintain lean body mass," Heber says. "I have been a proponent of increasing protein without increasing fat. Right now, most Americans are at the lower end of protein intake [recommendations]. You have to have protein to maintain lean body mass."
Heber reports serving as counselor for the Obesity Society. He is an advisor for POM Wonderful, Herbalife, and McCormick Spices. He has received book royalties for What Color Is Your Diet? and The L.A. Shape Diet.