The trial on almost 50 overweight women confirms previous studies showing that a high-protein diet can lead to greater fat loss than a low-calorie, high carbohydrate diet.
But the researchers from the University of Illinois have also demonstrated that when both regimes are combined with an exercise programme, the protein-rich diet is still more effective at reducing body fat.
"There's an additive, interactive effect when a protein-rich diet is combined with exercise. The two work together to correct body composition; dieters lose more weight, and they lose fat, not muscle," said author Donald Layman, professor of food science and human nutrition.
Layman's team recruited 48 women aged around 46 years old with a body mass index of 33 kg/m(2) during weight loss.
Half the women ate a protein-rich diet containing specific levels of leucine, one of the essential amino acids, for four months. The others followed a diet based on the US food guide pyramid, which contained higher amounts of carbohydrates.
Both groups consumed the same number of calories, but the first group substituted protein foods, like meat, dairy products, eggs, and nuts, for foods high in carbohydrates, such as breads, rice, cereal, pasta, and potatoes.
"Both diets work because, when you restrict calories, you lose weight. But the people on the higher-protein diet lost more weight," said Layman.
High-protein diets have been controversial as they counter the accepted weight-loss diet and there is little information on their impact on health over the long-term. But recent studies suggest that they may indeed work better than low-calorie, high-carbohydrate diets by increasing satiety and reducing fat mass.
In the current study, published in the August issue of the Journal of Nutrition (vol 135, issue 8, pp1903-10), the subjects were also required to follow one of two different exercise programmes. The first involved walking two to three times a week, while the second group included five 30-minute walking sessions and two 30-minute weightlifting sessions per week.
In both groups of dieters, the exercise helped spare lean muscle tissue and target fat loss. But, the protein-rich, high-exercise group, lost even more weight, and almost 100 per cent of the weight loss was fat, report the researchers.
In the high-carbohydrate, high-exercise group, however, as much as 25 to 30 per cent of the weight lost was muscle.
The protein-rich diet seems to be even more effective for people at higher risk of heart disease.
"The protein-rich diet dramatically lowered triglycerides and had a statistically significant effect on trunk fat, both risk factors associated with heart disease," said Layman.
"Exercise helped dieters lose an even greater percentage of body fat from the abdominal area."
The protein-rich diet is thought to work well because it contains a high level of leucine. The amino acid works with insulin to stimulate protein synthesis in muscle.
"The diet works because the extra protein reduces muscle loss while the low-carbohydrate component gives you low insulin, allowing you to burn fat," explained Layman.