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In the study, researchers compared the effects of a high-protein, low-carbohydrate diet against a high-carbohydrate, low-protein diet combined with exercise in 48 obese women. Both diets contained 1,700 calories, 30% of calories from fat, and about 17 grams of fiber.
However, women on the high-protein diet substituted high-protein foods (e.g. meats, dairy, eggs, and nuts) for foods high in carbohydrate (e.g. breads, rice, cereal, pasta, and potatoes) to get about 30% of their total calories from protein (1.6 grams of protein per kilogram of body weight).
Women on the high-carbohydrate diet ate about half that amount of protein (0.8 grams per kilogram of body weight) and got about 60% of their daily calories from carbohydrate.
The women also followed two different exercise programs.
Exercise in group one involved voluntary "light walking activity." Women in this group averaged about 100 minutes per week of added exercise.
While group two walked a minimum of five days per week, they also did resistance exercise (30 minutes of weight training) twice weekly. The exercise was supervised and averaged 200 minutes or more each week.
After four months, both groups of dieters lost weight. Not surprisingly, body composition tests show that women who did resistance exercise lost less muscle and more fat.
• High-protein dieters who did resistance exercise group lost an average of 22 pounds and less than one pound of muscle.
• High-carbohydrate dieters who also did resistance exercise group lost an average of 15 pounds. But they also lost over 2 pounds of muscle.
So, nearly 100% of the weight lost in the high-protein exercise group was fat, while 25%-30% of the weight lost in the high-carbohydrate exercise group was muscle. This was due mainly to their low protein intake, which averaged just 0.7 grams per kilogram of bodyweight per day.
What about the group who only did the walking?
The high-protein dieters in this group lost an average of 19 pounds. However, over 4 pounds came from muscle. The high-carbohydrate group lost 17 pounds, but nearly 6 pounds came from muscle.
"Both diets work because, when you restrict calories, you lose weight. But the people on the higher-protein diet lost more weight," says lead-author Dr. Donald Layman, professor of food science and human nutrition at the University of Illinois. "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."
The main weakness with the study is that calorie intake was self reported. This is a notoriously inaccurate way to measure calorie intake, and goes a long way to explaining why the high-protein group lost more fat than the high-carbohydrate group.
There's nothing really new about the findings reported here. If you want to lose fat, set your protein intake at a level that's adequate to preserve muscle, get around 25-30% of your calories from fat, then adjust your carbohydrate intake according to how active you are.
Then, do some form of resistance and aerobic exercise on a regular basis.A minimum of two cardiovascular workouts and two bouts of resistance exercise each week. And you don't need to spend hours in the gym if you don't want to. With the right program, your workouts won't need to last more than 30-45 minutes.
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