April 15, 2005 -- Diets that lower carbohydrates may not get any extra advantage from boosting protein.
When diets substitute proteins for carbohydrates, studies show greater fat loss in women. But it's not known whether the effect is due to the increased protein content of the diets or the reduction in carbohydrates, write the researchers.
The study appears in the April 1 edition of The American Journal of Clinical Nutrition.
"In previous studies, we have shown differences between high-protein diets and lower-protein diets when we substituted protein for carbohydrate and kept fat constant," says Peter M. Clifton, FRACP, PhD, of the University of Adelaide's medicine department.
"The question: Was it protein or the carbohydrates? This study suggested that perhaps it was the decrease in carbohydrates, rather than the increase in protein that made the difference we saw previously," Clifton tells WebMD in an email.
Do Carbs Count More Than Protein?
Both diets cut carbs to the same level: no more than 30% of total daily calories. One diet featured low-fat (29% total calories), high-protein (24% total calories) items. It was based on lean meat, poultry, and low-fat dairy foods, says Clifton's study.
The other low-carb diet had a standard amount of protein (8% total calories) and a higher amount of monounsaturated fat (45% total calories). Those menus included lean meat, poultry, higher-fat milk, and oil and nuts high in monounsaturated fat.
After 12 weeks of the calorie-restricted diet -- followed by four weeks of a maintenance diet -- the two groups had no differences in weight loss, fat or lean-mass loss, insulin resistance (a risk factor signaling heart disease and type 2 diabetes), or fasting cholesterol and triglyceride levels.
On average, the low-fat, high-protein group lost about 21 pounds. The average weight lost by the high-fat, standard-protein group was 22.5 pounds.
Both diets were well tolerated; no negative side effects were seen. Neither plan hurt bone mass or kidney function, says the study.
Participants said they were less hungry after the low-fat, high-protein meal, both at the beginning and end of the study. However, "having a lower desire to eat did not translate into a lower intake," Clifton tells WebMD.
The diets' long-term effects aren't known. Neither are the results for people who aren't obese (body mass index of 30 or higher).
Which Diet Is Best?
Calories count on any diet, and sticking to the diet is important. "Most studies have shown that energy intake and not macronutrient composition is the key determinant of total weight loss," says Clifton's study. Macronutrients include fats, carbohydrates, and proteins.
"I don't think low-carbohydrate diets are quite as popular here [in Australia] as in the U.S.A.," says Clifton.
When asked what he would tell obese people considering either diet, he says, "For a short-term -- say, 12-week weight loss -- I would not say anything. But long term, I would recommend at least 100 grams of carbohydrates from bran cereals, fruit, and vegetables to keep micronutrient intake normal and bowel function normal."
Other studies have recommended favoring complex carbohydrates (in fruits, vegetables, whole grains, and legumes) over simple carbohydrates, which include sugary foods and refined grains.