As the incidence of obesity in the United States and other developed nations reaches record highs, the percentage of these populations that are dieting is also climbing. A new study finds further support for the idea that low-carb diets can be especially effective, as long as they don't lead people to eat extra fat or avoid exercise.
In nutritional parlance, carbohydrates refer to sugars and starches. Amongst dieters, carbs have come to refer to foods especially rich in these food constituents, namely potatoes, rice, grains, breads, candy, fruits, and vegetables.
Cutting carbohydrates to trim weight—often under the rubric of the Atkins Diet or the Zone Diet plan—has grown popular in recent years. Boosting these regimens' appeal have been several major studies showing that, compared with equal-calorie diets rich in carbohydrates, low-carb ones help people shed the pounds more quickly and yet experience less hunger while doing so.
However, many people who have pulled carbs from their diets have replaced sweet and starchy foods with fatty ones. The fact that many low-carb diets are, in fact, high in fat may explain some potentially detrimental cholesterol trends in a substantial subset of low-carb dieters (SN: 7/17/04, p. 40).
The new study explored what would happen if the ratio of fats in the diet were held constant and the pared carbohydrates were replaced, gram-for-gram and calorie-for-calorie, with protein. This 4-month trial, conducted in 48 obese women between the ages of 40 and 56, also assigned half of the volunteers on each diet to a low-intensity exercise regime.
The findings, reported in the August Journal of Nutrition, showed not only that the dieters lost more weight on the low-carb, protein-rich fare, but also that they lost more body fat than muscle. Moreover, women on the protein-rich diet who exercised lost 20 percent more weight than did the more-sedentary women on this diet. That's a bit surprising, notes study leader Donald K. Layman of the University of Illinois at Urbana-Champaign, because the prescribed exercise shouldn't have been enough extra activity to translate into any discernible weight loss. Indeed, the exercise regimen didn't provide any extra weight loss to the women eating the carb-rich diet.
What this means, he told Science News Online, is that the extra protein some women were eating somehow collaborated with exercise to reduce weight.
"This is really surprising, and, frankly, pretty important," Layman says, since the observation flies in the face of most nutrition guidelines, which advise dieters and everyone else in the United States to eat less protein, not more.
For their new trial, Layman and his colleagues gave their volunteers 2-week menus and directions on how to prepare the recipes. Participants were instructed to weigh portions to ensure she didn't eat more than the recommended amounts. Each woman's energy intake was expected to run about 1,700 calories per day. In fact, based on weight losses and records, it became clear that most women consumed even fewer calories, in the range of 1,400 to 1,600 per day.
Each woman ate the same foods, regardless of her diet. What differentiated the two diet groups were the allowed portions. For instance, the high-carb group was instructed to eat eight servings of starchy foods per day, which included breads, cereals, rice, and potatoes. "The high-protein group also ate bread and other starchy foods, just half as much," Layman says. Similarly, while the high-protein group was instructed to eat 9 ounces of meat and eggs per day, the high-carb eaters were restricted to just 5 ounces.
In the end, women on the high-carb diet ate about the same proportion of macronutrients as they had been downing before taking part in the study: 55 percent of their calories as sugars and starches, 30 percent as fat, and 15 percent as protein. It was the other group that made major changes in the ratio of these macronutrients. The high-protein group consumed only 40 percent carbs, 30 percent fat, and 30 percent protein.
Moreover, the proteins included in each day's menus were dominated by what Layman terms "high-quality" protein—the type especially rich in the amino acids that build muscle. Some of these amino acids, such as leucine, aren't made by the body and must be obtained from the diet—primarily from foods such as meats, dairy, eggs, and soybeans.
Ensuring that each diet provided adequate leucine was a focus of the menu planning, Layman says. He explains that this amino acid is valued for "regulating one of the first steps in turning on the machinery for protein synthesis." That's important, since muscle is almost all protein.
Adding a little exercise to the diet regime helped keep a woman's metabolism revved up longer and her muscles conditioned. The two groups that were prescribed exercise were required to take part in a supervised 30-minute walk 5 days a week and to do 30 minutes of stretching and resistance exercise twice a week, using gym machines dialed to minimal weights. Even the two more-sedentary groups were advised to walk 30 minutes a day 5 days a week, Layman says, although their compliance wasn't monitored.
The protein focus seems to have paid rich dividends, Layman says, since women on the protein-enriched diets preserved more of their muscle than the high-carb diners did. That means that protein and exercise combined to reduce the women's weight by burning body fat.
Both dieting groups trimmed more body fat when they undertook some extra exercise. Sedentary women on the protein diet dropped 15 percent of their body fat during the trial, and those who added in extra exercise lost 21.5 percent of their body fat. By contrast, those who exercised and ate the high-carb fare lost 15 percent of their body fat, while their sedentary counterparts on that diet shed only 12.3 percent of their fat.
Preserving muscle is important, Layman stresses, since—unlike fat—it burns substantial energy when the body's at rest. The higher the proportion of the body that is lean muscle, the higher its energy demand and the more likely that an individual will burn most of the calories she eats—not store them as fat.
The encouraging news, he says, is that the short-term advantages seen in the high-protein part of this trial can be maintained. Some of the women were recruited to stay on their regimens for another year, he notes, "and we find that basically the same results continued" for each group.
In a pair of papers he coauthored during the past 2 years, Layman has reported that a key feature of the protein diet's advantages may be leucine. Although a building block of protein, it may have additional metabolic activities, such as being a signaling agent that helps regulate the rate of muscle building and the body's use of blood sugar, he notes. For these functions, leucine may have to be present in concentrations higher than those needed just to build protein.
In fact, Layman says that leucine-rich diets might even help stabilize blood-sugar concentrations before and after meals—a boon to anyone with type 2 diabetes or a constellation of related heart-disease risk factors known as Syndrome X (SN: 4/8/00, p. 236). He plans to investigate leucine's potential value for such individuals in upcoming studies.
Donald K. Layman
Department of Food Science and Human Nutrition
Room 437, Bevier Hall
University of Illinois
Urbana, IL 61801
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