In the first multicenter trial to look at the high-fat, low-carbohydrate Atkins diet, researchers have found that at three and six months, the Atkins diet produces significantly greater weight loss than a conventional low-fat, high-carbohydrate diet.
Because of the high amounts of fat that people consume on the Atkins diet, many have worried that over the long term, it might have serious side effects. The researchers found no differences in side effects during this study and even found benefits in blood lipid profiles.
All study subjects had medically significant obesity, weighing at least 20 percent more than their ideal body weight and began the study an average of 50 pounds overweight.
Atkins dieters lost twice as much weight during the first six months of the study. However, over the next six months, dieters on both plans tended to regain weight, and there was no statistical weight difference between the groups at one year.
The study also found that people on the Atkins diet had greater improvements in blood lipids than those on the conventional diet. High-density lipoprotein (HDL), or “good” cholesterol, increased more in the Atkins group, and their serum triglycerides decreased more than conventional dieters. Low HDL and high triglyceride levels increase risk of cardiovascular disease.
The study, which appears in the May 22 issue of The New England Journal of Medicine, was conducted by researchers at Washington University, the University of Pennsylvania School of Medicine and the University of Colorado Health Sciences Center.
“This study demonstrates that a low-carbohydrate diet can have beneficial effects in treating obesity,” said senior investigator Samuel Klein, M.D., the Danforth Professor of Medicine and Nutritional Science and director of the Division of Geriatrics and Nutrition. Klein added that additional research is needed to understand why low-carbohydrate dieters lost more weight than conventional dieters and to evaluate the long-term efficacy and safety of low-carbohydrate diets.
The researchers wanted to mimic what happens when most people diet, so they did not offer dieters behavior modification or extensive clinical supervision.
The participants were randomly selected to follow either the low-carbohydrate or low-calorie diet. They met a registered dietitian at the start of the study and again at three, six and 12 months.
Conventional female dieters ate 1,200-1,500 calories per day, and males followed a daily 1,500-1,800-calorie diet that consisted of 60 percent carbohydrate, 25 percent fat and 15 percent protein, based on the Food Guide Pyramid. The other group was instructed to read and follow the diet prescribed in Dr. Atkins’ New Diet Revolution.
At three months, Atkins dieters lost an average of 17.6 pounds while the conventional dieters lost about 8.3 pounds. After six months, Atkins dieters lost an average of 21.2 pounds; conventional dieters lost an average of 11.5 pounds. At 12 months, the Atkins group was down an average of 15.9 pounds versus 9.7 pounds for conventional dieters. The difference is not considered statistically significant, partly because almost half of the participants dropped out of the study before the one-year mark.
HDL cholesterol levels were up 11 percent at one year in the Atkins diet group, compared to an increase of only 1.6 percent of those on the conventional diet. Serum triglycerides declined an average of 17 percent after one year on the Atkins diet versus no significant change among conventional dieters. Part of that improvement may result from greater weight loss, but the changes in HDL cholesterol and triglycerides were greater than expected from moderate weight loss alone.
The multicenter research team is recruiting larger numbers of participants in a five-year, NIH-funded study of low- and high-carbohydrate diets to more fully assess the benefits and risks of the diets on bone mass, kidney function, blood vessel health and exercise tolerance. It will also look at whether behavior intervention and modification might help people stick to these diets.
“A calorie is still a calorie, whether the calorie comes from fat, carbohydrates or protein,” Klein said. “But it might be that certain types of calories are more filling than others and result in an overall decrease in total calorie intake.”