Good news about low carb diets and weight loss

Low-carb diets — like the popular Atkins and South Beach diets — have gained great popularity over recent years. Yet there is "a paucity of scientific evidence on their effectiveness," writes researcher Dr. William S. Yancy, a professor of Medicine at Duke University Medical Center.

Two trials reported in the Annals of Internal Medicine compared low carb diets with low-fat diets. Both found that people on the low-carb diets lost weight and had improved triglyceride levels and slightly improved HDL (the so-called "good" cholesterol) levels.

In both studies, at six months, the low-carb group had lost more weight than the low-fat group. But at the end of the 12-month study, both the low-carb and low-fat groups had lost about the same amount of weight.

Low carb diets

For one year, researchers at the Veterans Affairs Medical Center in Philadelphia followed 132 obese adults randomized into two groups [2]. One restricted carbohydrate intake to less than 30 grams per day (low carb diet).

The other restricted calorie intake by 500 calories, with 30 percent of calories from fat (conventional diet). Eighty-three percent of the study group had diabetes or other risk factors for heart disease.

At the six-month mark, the low-carb group had lost more weight than the low-fat group. However, by the 12-month point, both groups had lost about the same amount of weight (11 to 19 pounds for the low-carb group and 7 to 19 pounds for the low-fat group).

The low fat group continued to lose weight from 6 to 12 months, whereas the average weight in the low-carb group remained steady after 6 months.

This twelve-month study is the longest study comparing low-carb vs. low-fat diets using subjects who have a high prevalence of diabetes.

"Americans are overweight because we're eating too much food and ingesting too many calories," said Linda Stern, MD, the lead author of the study and an internist at the Philadelphia VA Medical Center.

"I think a low-carbohydrate diet is a good choice because much of our overeating has to do with consumption of too many carbohydrates," Stern said.

Stern said that she was impressed that the low-carbohydrate group lost the weight in the first six months of the twelve-month study and didn't gain it back, but she cautioned, "More research is needed to see if a low-carbohydrate diet remains safe and effective for the longer term."

The second article in the Annals of Internal Medicine involved 120 overweight people, who were followed for six months [1]. Subjects were assigned to either a low-fat or a low-carb group.

The low-carb group was instructed to eat no more than 20 grams of carbohydrate per day. They were also told to use nutritional supplements, including chromium (1,800 micrograms), D,L-phenylalanine (900 milligrams), L-carnitine (3,000 milligrams) and a green tea extract (80 milligrams).

The low-fat group was instructed to consume 30% or fewer of their daily calories from fat and 10% or fewer of their calories from saturated fats, and to keep cholesterol intake under 300 milligrams per day.

The average fat loss at the end of the study was significantly greater in the low-carbohydrate group than in the low-fat group (21 pounds versus 11 pounds).

The low-carbohydrate group had more beneficial changes in blood triglyceride levels and HDL cholesterol levels than the low-fat diet group.

Also, fewer people dropped out of the low-carb group. "A lot of people say it's hard to stick to the low-carb diet. But it's hard for people to stick to any diet," says lead author Dr. William S. Yancy. "With all the low-carb products coming out — and with restaurants making changes, too — it should be easier for people to stay on a low-carb diet."

However, those low-carb products are "a double edged sword," he admits. "If people end up eating more calories during the day, they won't lose weight. It's an important part of the formula — cutting back on calories."

A slightly shorter trial shows that both low-fat and low-carb diets lead to similar weight loss [3].

Thirty-one overweight and obese men and women took part in the ten-week study.

Subjects on the low-fat diet consumed an average of 17.8% of energy from fat, compared with their habitual intake of 36.4%, and had a resulting energy restriction of 607 calories per day.

Subjects on the low-carb diet consumed an average of 15.4% carbohydrate, compared with habitual intakes of about 50% carbohydrate, and had a resulting energy restriction of 764 calories per day.

Those on the low-fat diet lost an average of 14.96 pounds, compared with a loss of 15.4 pounds in the low-carb subjects. Preservation of lean muscle was slightly greater in the low-fat group.

One of the most recent trials, published in the Journal of Clinical Endocrinology and Metabolism, shows greater weight loss with a low-carbohydrate diet compared with a low-fat diet [4].

A group of 50 healthy, moderately obese women were randomized to four months of an ad libitum low-carbohydrate diet or an energy-restricted, low-fat diet.

Forty women completed the trial, with an equal number of dropouts from each diet group. The low-carbohydrate group lost more weight (9.8 vs. 6.1 kilograms) and more body fat (6.2 vs. 3.2 kilograms) than the low-fat group.

There were no significant differences in energy intake between the diet groups as reported on three-day food records at the conclusion of the study (1422 vs. 1530 calories).

Interestingly, the low-fat (69% carbohydrate, 11% protein, 20% fat) meal caused a greater increase in metabolic rate than did the low-carbohydrate (5% carbohydrate, 26% protein, 69% fat) meal (53 versus 31 calories).

The patterns of energy expenditure after the meal suggest that the low-carbohydrate meal was absorbed more slowly than the low-fat meal, a reasonable assumption given the known effects of the nutrient fat to slow gastric emptying.

It's possible that the five-hour period used to measure the metabolic rate may have underestimated the full effect of the low-carbohydrate meal to a greater extent than the low-fat meal.

However, given the differences in weight loss between the groups, the researchers don't think this is why the low-carbohydrate group lost the extra weight.

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.

However, based on the subjects' food records, there was a reduction in energy of approximately 850 calories per day at two months and 700 calories per day at four months in both the low-fat and low-carbohydrate groups.

To account for the ~ 3.6 kilogram difference in weight loss between the groups over four months, with similar energy intake, the low-carbohydrate group would have to expend ~ 225 calories per day more than the low-fat group.

However, the researchers could not account for differences of this magnitude in measurements of resting energy expenditure, thermic effect of food, or estimates of physical activity.

Instead, they think the best explanation for the difference in weight loss between the groups is a difference in energy intake that was not apparent in their self-reported three-day food records. This has been shown in other studies [5, 6].

One reason for underreporting by the low-fat dieters may have been that they were following a prescribed energy restriction and so were faced with a limit in daily energy intake. Since energy intake was not restricted in the low-carbohydrate group, it is plausible that they felt less pressure to meet any goals for energy intake.

In addition, since the low-carbohydrate group followed a diet that differed dramatically from their usual intake, with more limited food choices that were likely easier to catalogue and record, it's possible that their reporting was more accurate.

"We can no longer dismiss very-low-carbohydrate diets," says Walter C. Willett, MD, of the Harvard School of Public Health, in an editorial accompanying the two articles.

We can encourage overweight patients to experiment with various methods for weight control, including reduced carbohydrate diets, as long as they emphasize healthy sources of fat and protein and incorporate regular physical activity, " Willet adds. "Patients should focus on finding ways to eat that they can maintain indefinitely rather than seeking diets that promote rapid weight loss."

References
1. Yancy, W.S. Jr., Olsen, M.K., Guyton, J.R., Bakst, R.P., & Westman, E.C. (2004). A low-carbohydrate, ketogenic diet versus a low-fat diet to treat obesity and hyperlipidemia: a randomized, controlled trial. Annals of Internal Medicine, 140, 769-777
2. Stern, L., Iqbal, N., Seshadri, P., Chicano, K.L., Daily, D.A., McGrory, J., Williams, M., Gracely, E.J., & Samaha, F.F. (2004). The effects of low-carbohydrate versus conventional weight loss diets in severely obese adults: one-year follow-up of a randomized trial. Annals of Internal Medicine, 140, 778-785
3. Meckling, K.A., O'Sullivan, C., & Saari, D. (2004). Comparison of a low-fat diet to a low-carbohydrate diet on weight loss, body composition, and risk factors for diabetes and cardiovascular disease in free-living, overweight men and women. Journal of Clinical of Endocrinology and Metabolism, 89, 2717-2723
4. Brehm BJ, Spang SE, Lattin BL, Seeley RJ, Daniels SR, D'Alessio DA. (2004). The role of energy expenditure in the differential weight loss in obese women on low-fat and low-carbohydrate diets. Journal of Clinical Endocrinology and Metabolism
5. Lichtman SW, Pisarska K, Berman ER, Pestone M, Dowling H, Offenbacher E, Weisel H, Heshka S, Matthews DE, Heymsfield SB. (1992). Discrepancy between self-reported and actual caloric intake and exercise in obese subjects. New England Journal of Medicine, 327, 1893-1898
6. Forbes GB. (1993). Diet and exercise in obese subjects: self-report versus controlled measurements. Nutrition Reviews, 51, 296-300