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Am Fam Physician. 2007;76(1):32-35

to the editor: In the article “Low-Carbohydrate Diets,” Drs. Last and Wilson reviewed the existing state of knowledge on this subject.1 They seemingly suggest that weight loss is not sustainable with a low-carbohydrate diet and that caloric count, rather than nutrient restriction, is the key to effective weight loss. Indeed, persons need to have a lower caloric intake than output to lose weight, but consideration of macronutrients cannot be ignored.

One study examined the long-term stability of weight loss in 16 patients who were maintained on a low-carbohydrate diet.2 At six months, there was a decrease in mean body weight from 221.8 ± 32.4 lb (100.6 ± 14.7 kg) to 196.7 ± 31.5 lb (89.2 ± 14.3 kg); most of this weight loss persisted after 22 months.2 Although this is a small sample, it highlights the need to further explore the long-term use of low-carbohydrate diets.

There is ample evidence from three decades of literature demonstrating the potential benefits of low-carbohydrate diets over and above caloric restrictions.35 A meta-analysis of 87 studies of various weight-loss diets found that after controlling for energy intake (comparing diets with equal caloric content), the diets that obtained less than 41.4 percent of calories from carbohydrates were associated with more weight loss than diets that obtained a higher percentage of calories from carbohydrates. The conclusion was that low-carbohydrate, high-protein diets affect weight loss independent of energy intake.6

Pending further studies, physicians should continue to have an open mind about whether low-carbohydrate diets lead to sustainable weight loss and whether this weight loss exceeds that expected solely from caloric restriction.

in reply: We agree with Dr. Standard-Goldson's contention that physicians should have an open mind with regard to the long-term benefits of low-carbohydrate diets on weight loss. Low-carbohydrate diets appear to be safe and to result in weight loss and an improved metabolic profile. In persons with high insulin levels, low-glycemic index diets are more effective than low-fat diets for attaining weight loss and fat loss and, regardless of insulin status, more effectively increase high-density lipoprotein levels and decrease triglycerides.1

Still, low-glycemic index diets are not strictly the same as low-carbohydrate diets. Even though there has been no conclusive evidence that long-term use of low-carbohydrate diets leads to a greater sustained weight loss than other, similarly low-calorie diets, an extension of the hormonal findings of the 2007 study discussed above could readily be made.1

The study Dr. Standard-Goldson cites to support the effectiveness of low-carbohydrate diets over 22 months is not a comparison study; it is a report of a cohort of 16 patients on a low-carbohydrate diet and cannot establish supremacy of one diet over any other.2 This lack of evidence for a long-term advantage of low-carbohydrate diets over other diets neither supports nor refutes the possibility of long-term benefit. It is simply unproved either way.

We disagree with Dr. Standard-Goldson's proposition that low-carbohydrate diets have been demonstrated to induce a metabolic advantage. The meta-analysis cited does not prove that low-carbohydrate diets induce any real metabolic advantage.3 Patients consuming a low-carbohydrate diet did have a greater decrease in fat-free mass than those consuming more carbohydrates. Unfortunately, no studies continuing beyond 26 weeks were included. Additionally, studies were excluded if the patients were not hypocaloric, the very group that would be most helpful in proving a metabolic advantage. If weight loss was induced in patients consuming a low-carbohydrate and high- or normal-caloric diet, then the proposed metabolic benefits might actually be demonstrated. The discussion section of that article addresses the possibility that increased water loss from ketosis could account for any differences in weight loss. As such, it is still unproven conjecture that low-carbohydrate or low-glycemic index diets can in some way induce a greater weight loss than that expected from caloric restriction and/or water loss.

Until more is known about the physiologic effects of various diets and the physiology of the dieter, diets should be primarily hypocaloric and linked to the patient's medical condition (e.g., insulin resistance) and dietary preferences (e.g., low-carbohydrate, lower-carbohydrate, low-fat, balanced). Ultimately, the most successful weight-reducing diet will be the one that patients can adhere to for the longest duration.

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This series is coordinated by Kenny Lin, MD, MPH, deputy editor.

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