Low-carb diets - where is the evidence?

Researchers have expressed the need for long-term studies on the effects of low-carbohydrate diets. A review of this currently popular regime suggests that although successful, and with no obvious short-term adverse effects, it is not clear how the diet impacts people in middle age.

People who go on low-carbohydrate diets typically lose weight, but restricted caloric intake and longer diet duration are the biggest reasons why, according to the study from Stanford University Medical Center and collaborators at Yale University.

"Low-carbohydrate diets have been extremely popular as of late, and the lay press has suggested they're a safe and effective means of weight loss,"​ said lead author Dr Dena Bravata, social science research associate at Stanford's Center for Primary Care and Outcomes Research. "While these diets are effective in the short term, weight loss results from reduced calories, not carbohydrate restriction."​

The study appears in the 9 April issue of the Journal of the American Medical Association​.

Despite the popularity of low-carbohydrate/ high-protein diets, and the concern of some in the medical community that these diets are too high in fat and can lead to kidney and liver problems and other health risks, Bravata said little evidence exists on the efficacy and safety of low-carbohydrate diets.

Bravata and her colleagues collected literature on low-carbohydrate diets published between 1966 and 2003. They reviewed a total of 107 diet studies, which involved 3,268 people from around the world. The studies were small and heterogeneous, with carbohydrate and caloric intake, diet duration and participant characteristics varying greatly.

However all of the studies had two things in common: none had participants with a mean age over 53 and none of the randomised and controlled studies lasted longer than 90 days.

"Information on older adults and long-term results are scarce at best, and this should be kept in mind when looking at our findings,"​ noted Bravata.

The researchers' meta-analysis found that people on diets of 60 or fewer grams of carbohydrates a day (a threshold used in some of the popular low-carbohydrate diets) did lose weight. But the weight loss was associated with restriction of caloric intake and longer diet duration, not with reduced carbohydrate intake. It also found that the greatest weight loss occurred among those participants on diets with the highest baseline weight and lowest caloric content.

"The greatest predictors of weight loss appear to be caloric intake and diet duration,"​ she said. "The findings suggest that if you want to lose weight, you should eat fewer calories and do so over a long time period."​

The researchers found no significant adverse effects on cholesterol, glucose, insulin and blood-pressure levels among participants on the diets. But, Bravata stressed, the adverse effects may not have shown up within the short period of the studies. She also said losing weight typically leads to an improvement in some of these levels, so this could have had an impact on the numbers.

The researchers concluded that there is insufficient evidence overall to make recommendations for or against using the diets. Bravata said studies are now needed on the role of exercise in weight loss (as exercise information was excluded from this analysis), the long-term effects of these diets and the effectiveness and safety of these diets for people over the age of 53.

Co-author Christopher Gardner agreed that more studies on low-carbohydrate diets are needed. "There wasn't a lot of information from well-designed, randomised controlled trials...The obesity epidemic involves people having weight problems for years or decades, and we need long-term data on these diets' effectiveness and safety."​

A statement from the Atkins Health and Medical Information Services said: "The most significant finding of the Journal of the American Medical Association (JAMA) meta-analysis of low carbohydrate diets is that there is no scientific evidence to support the criticisms of the Atkins Nutritional Approach."​

They added that the meta-analysis "likely did not include the results from several recent studies specific to the Atkins Nutritional Approach and several others examining the impact of controlled carbohydrate nutrition on weight loss and cardiovascular health."​

These studies are either very recently published or are in the process of publication, according to the group, and are likely not to be included in the review.