Why Did Energy Expenditure Differ Between Diets in the Recent Study by Dr. Ludwig's Group?

As discussed in the previous post, a recent study by Dr. David Ludwig's group suggested that during weight maintenance following fat loss, eating a very low carbohydrate (VLC) diet led to a higher metabolic rate (energy expenditure) than eating a low-fat (LF) diet, with a low glycemic index (LGI) diet falling in between the two (1).  The VLC diet was 30 percent protein, while the other two were 20 percent.  It's important to note that these were three dietary patterns that differed in many ways, and contrary to claims that are being made in the popular media, the study was not designed to isolate the specific influence of protein, carbohydrate or fat on energy expenditure in this context. 

Not only did the VLC diet lead to a higher total energy expenditure than the LF and LGI diets, the most remarkable finding is that it led to a higher resting energy expenditure.  Basically, people on the VLC diet woke up in the morning burning more energy than people on the LGI diet, and people on the LGI diet woke up burning more than people on the LF diet.  The VLC dieters burned 326 more calories than the LF dieters, and 200 more than the LGI dieters.

It's always tempting to view each new study in isolation, without considering the numerous studies that came before it, but in this case it's essential to see this study through a skeptical lens that places it into the proper scientific context.  Previous studies have suggested that:

1. The carbohydrate:fat ratio of the diet has little or no detectable impact on energy expenditure in people who are not trying to lose weight (2, 3).
2. The carbohydrate:fat ratio of the diet has little or no detectable impact on energy expenditure in people who are being experimentally overfed, and if anything carbohydrate increases energy expenditure more than fat (4, 5).
3. The carbohydrate:fat ratio of the diet has little or no detectable impact on energy expenditure during weight loss (6, 7, 8), and does not influence the rate of fat loss when calories are precisely controlled. 

This new study does not erase or invalidate any of these previous findings.  It fills a knowledge gap about the effect of diet composition on energy expenditure specifically in people who have lost weight and are trying to maintain the reduced weight.

With that, let's see what could have accounted for the differences observed in Dr. Ludwig's study.
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