The low-fat, low-carb debate and the theory of relativity.

The 2013 American College of Cardiology/American Heart Association Task Force on Practice Guidelines and The Obesity Society (1) reviewed the available evidence-based literature with regard to the macronutrient content of diets and their health benefits and concluded that weight loss was similar with all types of diets as long as the diets achieved similar calorie restriction. There was moderate evidence to conclude that lower-fat/highercarbohydrate diets, compared with higher-fat/lower-carbohydrate diets, resulted in a greater reduction in LDL cholesterol, lesser reductions in triglycerides, and lesser increases in HDL cholesterol. Despite the differences in lipids, one point goes to those who claim that “a calorie is a calorie.” However, in this issue of the Journal, the low-fat/low-carbohydrate debate continues to rage on beyond the conclusion that a calorie is a calorie, especially in those with cardiovascular disease risk (2). We still consider that a calorie is a calorie, but evidence continues to accumulate since the publication of the guidelines that a calorie is metabolized differently depending on the underlying physiologic and genetic status of the human consuming that calorie (3). In fact, the weight loss seems to be similar because we are talking about the same number of calories restricted; however, depending on the metabolic status of the patient, cardiovascular disease risk markers are affected differently. Perhaps we should use the theory of relativity in physics and apply it to human physiology: i.e., a calorie’s worth is relative to the physiologic status of the person ingesting that calorie. That calorie exerts its influence relative to the host’s genetic makeup. In this issue of the Journal, Tay et al. (4) report the results of a 52-wk randomized trial of lowvs. high-carbohydrate diets in 115 obese adults with type 2 diabetes. Completion rates were similar in the 2 groups as were weight loss, blood pressure, glycated hemoglobin (HbA1c), and reduction in fasting glucose. The low-carbohydrate-diet group showed greater reductions in diabetes medications and glycemic variability, as well as triglycerides, and greater increases in HDL cholesterol. Several aspects of the design of this study allow it to stand out and add to the knowledge obtained from previous studies. Other studies lasting for up to 1 y that compared low-fat with low-carbohydrate approaches in those with or without type 2 diabetes concluded that there were negligible differences in weight loss but reported changes in HDL cholesterol and triglycerides; this would favor a low-carbohydrate approach at least in this time period (5). Shorter studies were able to document reduced inflammatory markers in low-carbohydrate compared with low-fat interventions (6). The studies came to these similar conclusions despite variability in terms of subject characteristics and health risks, duration of intervention, primary outcomes, and the monitoring of compliance to diet, as well as definitions of low-carbohydrate and low-fat diet. In addition, a confounder for the health effects of macronutrient content is always weight loss (7). What does the study by Tay et al. add to the literature? Several aspects of this study relating to methodology increase the confidence one has in its conclusions, as follows: