Because of the substantial controversy regarding low-carbohydrate diets and even disagreements in interpreting the results of specific studies it is difficult to objectively summarize the research in a way that reflects scientific consensus. Although there has been some research done throughout the twentieth century,[67][68] most directly relevant scientific studies have occurred in the 1990s and early 2000s and, as such, are relatively new. Researchers and other experts have published articles and studies that run the gamut from promoting the safety and efficacy of these diets[69][70] to questioning their long-term validity to outright condemning them as dangerous. Until recently a significant criticism of the diet trend was that there were no studies that evaluated the effects of the diets beyond a few months. However, studies are emerging which evaluate these diets over much longer periods, controlled studies as long as two years and survey studies as long as two decade
A meta-analysis of randomized controlled trials by the Cochrane Collaboration in 2002 concluded that fat-restricted diets are no better than calorie restricted diets in achieving long term weight loss in overweight or obese people. A 2003 meta-analysis that included a randomized controlled trials published after the Cochrane review found that "low-carbohydrate, non-energy-restricted diets appear to be at least as effective as low-fat, energy-restricted diets in inducing weight loss for up to 1 year.
Potential favorable changes in triglyceride and high-density lipoprotein cholesterol values should be weighed against potential unfavorable changes in low-density lipoprotein cholesterol values when low-carbohydrate diets to induce weight loss are considered. A 2008 systematic review of randomized controlled studies that compared low-carbohydrate diets to low-fat/low-calorie diets and found that measurements of weight, HDL cholesterol, triglyceride levels and systolic blood pressure were significantly better in groups that followed low-carbohydrate diets. The authors of this review also found a higher rate of attrition in groups with low-fat diets, and concluded that "evidence from this systematic review demonstrates that low-carbohydrate/high-protein diets are more effective at 6 months and are as effective, if not more, as low-fat diets in reducing weight and cardiovascular disease risk up to 1 year," but they also called for more long-term studies.
In addition to research on the efficacy of the diets some research has directly addressed other areas of health affected by low-carbohydrate diets. For example, contrary to popular belief that low-carbohydrate diets damage the heart, one study found that women eating low-carbohydrate, high-fat/protein diets had the same or slightly less risk of coronary heart disease, compared to women eating high-carbohydrate, low-fat diets. Other studies have found possible benefits to individuals with diabetes, cancer, and autism. The ketogenic diet, with 80% of energy from fat and much of the remaining from protein, has been used since the 1920s to treat epilepsy. Modern effective anticonvulsant drugs mean that it is now used only for children with difficult-to-control epilepsy, and there may be cause for concern over issues such as stunted growth for these children.
A recent study has come out that shows how carbohydrate intake and excess weight is related among healthy adults. The study found out that the risk of excess weight and obesity decreased with a healthy intake of carbohydrates. Consuming a low-carbohydrate (less than 47% of ones energy intake) diet is correlated with a greater risk of being overweight among adults. The lowest risk of gaining weight is an intake of between 47% to 64% energy from carbohydrates.