by Avik Chatterjee, MD
The first meta-analysis of trials of above-named diets combined data from 48 randomized, controlled trials (RCT’s) for a total of 7286 patients. Patients lost weight on all of the diets, but the amount of weight lost was no different between the different types (ex. low-carb, low-fat, mixed).
The second meta-analysis looked at 26 RCT’s for a total of 1016 patients comparing Weight Watchers, Atkins, South Beach, Zone and control diets. In this meta-analysis, too, patients lost a moderate amount of weight on any diet, but there was no clear winner.
Before I heard about these first two studies, however, I was drawn in by the buzz about another study. What study got more press than two large, high-quality meta-analyses of all the available RCT’s examining named diets? A single RCT with 148 patients. Why? Because in this year-long study by Bazzano et al, patients on a low-carb diet lost 8 more pounds than patients on a low-fat diet. From NPR, to the Huffington Post, to Reuters, this study was all over the news.
This concerned me. First of all, that the smaller RCT got so much more popular press than the larger meta-analyses was a problem. Second, there are several reasons to think that this single study is not applicable for the average American. Participants in the smaller RCT received intensive nutrition and cooking education, frequent dietitian appointments, and a daily meal supplement, all for free. Participants hit daily calorie counts as low as 1258 in the first 3 months in the low-carb group. This was a very motivated, well-supported group, and it’s unlikely that the average American could follow the diets in this study without intensive outside support.
In its defense, this RCT did have findings that indicate that weight loss in low-carb diets might be qualitatively different (better?) than weight loss in low-fat diets—at 12 months, low-carb participants lost more fat mass and gained more lean mass, and serum HDL (good cholesterol) increased more in the low-carb group. These findings definitely deserve further scrutiny, because it could be that while weight loss is similar, the actual impact on health might differ between different types of diets.
But the mainstream media picked up on the Bazzano study not because of the HDL improvements or improvements in lean body mass, but because of the power of the headline: lose 8 more pounds on a low-carb diet! Even I, someone who works on nutrition and obesity for a living, was taken in.
The scientific community can’t depend on the popular press to communicate the science behind diets, so the onus is on scientists. As Szilvia Szegedi pointed out in a previous post, we need to keep up-to-date on the literature, but also engage the popular press and social media (including blogs) to communicate the bottom line as we know it: find a reasonable diet, named or not, that you can stick to, and you can lose weight.