Low-carbohydrate diets have been around since the 1860s, but the Atkins books are the most successful to date. Ad-libitum consumption of butter, fatty meat, and high-fat dairy products are advocated, while carbohydrate intake is restricted to under 30 g a day.

Three randomised trials have examined the long-term effects of lowcarbohydrate diets. In the first, severely obese individuals were randomised to either an energy-restricted low-fat diet or an ad-libitum low-carbohydrate diet for 6 months. At the end of this period, individuals on the low-carbohydrate diet had lost more weight, but by 12 months no difference was noted between the two groups.

The third study produced similar results, while in the second obese women on the low-carbohydrate diet lost more weight after 6 months than women on a comparative diet.

How does the diet work? Professor Astrup and colleagues state that when carbohydrate intake is severely restricted, glycogen stores and associated bound water are depleted, and so weight loss could mainly be fluid loss.

However, X-ray absorptiometry studies did not show an excessive reduction in lean body-mass, suggesting that the weight loss is attributable to fat loss. The Atkins book claims that weight is lost because of increased energy expenditure, while Professor Astrup and colleagues suggest that it is more likely to be attributable to restriction of food choices, and the fact that protein has a stronger satiating effect than fat and carbohydrate.

Is the diet safe? Some cardiovascular risk factors improved in people on the low-carbohydrate diet, but restricted intake of whole-grain bread and cereals, fruit, and vegetables does not equal a healthy diet, and absence of these food groups might increase the risk of cancer and cardiovascular disease.

The low carbohydrate content of the diet is below the minimum needed to supply the brain and muscles with sugar. Followers of the Atkins diet suffer from muscle cramp, diarrhoea, general weakness, and rashes much more frequently than those people on a recommended low-fat diet. Professor Astrup and colleagues conclude that there is a need for longer (up to 2 years) studies to assess the weight-loss efficacy of low-carbohydrate diets. They recommend that people who want to lose weight and keep it off should eat a diet reduced in calories and fat, and increase physical activity.

Contact Professor A Astrup, Department of Human Nutrition, Centre of Advanced Food Research, RVA University, 1958 Frederiksberg C, Copenhagen, Denmark; E) ast.kvl.dk

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