Friday, October 2, 2009
No medical condition has generated as many dietary remedies as obesity. All diets have their followers, but hard data on the efficacy of the diets are scarce. In this issue of the Journal, Sacks et al.1 report the results of a large, long-term trial that tested the efficacy of weight-loss diets that were high or low in carbohydrates, protein, or fat. Highcarbohydrate, low-fat diets became popular approximately 20 years ago, when it was thought that calories from carbohydrates were less fattening than the same number of calories from fat.
A high-fat, low-carbohydrate diet was popularized by Dr. Robert Atkins in the 1970s2 and recently enjoyed a revival. The appeal of high-protein diets is that protein is thought to provide more satiation per calorie than fat or carbohydrates. The trial by Sacks et al. lasted longer than most, the dropout rate was low, treatment was intensive, and compliance was assessed with objective biomarkers.1 Unfortunately, the dietary goals were only partly achieved. Protein intake was intended to differ by 10% of energy between the high-protein-diet group and the average-proteindiet group, but the actual difference, as assessed by the measurement of urinary nitrogen excretion, was 1 to 2% of energy (according to my calculations, which were based on a diet that provided 1700 kcal per day).
Extreme carbohydrate intakes also proved hard to achieve. When fat is replaced isocalorically by carbohydrate, high-density lipoprotein (HDL) cholesterol decreases in a predictable fashion.3 The authors used the difference in the change in HDL cholesterol levels between the lowest- and highest-carbohydrate groups to calculate the difference in carbohydrate content between those diets. That difference turned out to be 6% of energy instead of the planned 30%.
