Obese women who follow low-carbohydrate diets, such as the Atkins diet, may lose more weight in a four-month period than those who go on low-fat diets, new study findings show. The reason for the greater weight loss, however, is not clear.

"The differential weight loss is not explained by differences in resting energy expenditure, thermic effect of food or physical activity," write study author Dr. Bonnie J. Brehm, of the University of Cincinnati, Ohio and her team.

In a previously published study, Brehm and her colleagues compared the effects of a low-carbohydrate diet versus a low-fat diet among obese women. They found that the women on the low-carbohydrate diet lost more than twice as much weight as those in the comparison group during a six-month study period.

The researchers hypothesized that the greater weight loss among those on the low-carbohydrate diet was due to the women's greater energy expenditure. "If it's not calories in, it must be calories out," Brehm stated.

Some advocates of low-carbohydrate diets say that such diets promote increased energy expenditure, but this claim has not been formally tested, until now.

To investigate, Brehm and her team randomly assigned 50 moderately obese women to a low-carbohydrate diet group or a low-fat diet group. Only the low-fat group was told to restrict their caloric intake. Forty women completed the study.

By the end of the four-month study, women in both groups had lost weight and body fat, the researchers report in this month's issue of the Journal of Clinical Endocrinology and Metabolism. However, the low-carbohydrate group lost more than 10 percent of their body weight, while the low-fat group lost about 7 percent.

Specifically, the low-carbohydrate group lost 9.8 kilograms (21.6 pounds) of weight and 6.2 kilograms (13.7 pounds) of body fat, while the low-fat group lost about 6.1 kilograms (13 pounds) of weight and 3.2 kilograms (7 pounds) of body fat, the report indicates.

To estimate their level of physical activity, women in both groups were fitted with pedometers, which recorded the number of steps they took daily. At the start of the study, both groups of women had similar pedometer readings, and by the end of the study, there were no significant changes, according to Brehm and her team.

Resting energy expenditure was also similar between the two groups at the start of the study and remained comparable four months later.

The thermic effect of food (TEF), which comprises up to 10 percent of the amount of energy consumed daily, includes the energy expended during digestion. When the investigators obtained TEF measurements after the women ate breakfasts containing a similar number of calories, they found that those on the low-fat diet expended more energy in a five-hour period.

This suggests that the low-fat meal was absorbed more quickly than the low-carbohydrate meal, the report indicates. Yet, even if the TEF of the low-carbohydrate meal had been underestimated, the researchers "would not have approached the amount of energy needed to account for the greater weight loss in this group," they write.

"These results confirm that short-term weight loss is greater in obese women on a low-carbohydrate diet than in those on a low-fat diet even when reported food intake is similar," according to Brehm and her team.

SOURCE: Journal of Clinical Endocrinology and Metabolism, March 2005