Do low-carb eaters burn more calories?

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A new study shows people can burn more calories on low-carb diets but only after the body adequately adapts, which takes about two and a half weeks.

We have covered the numerous studies from Harvard physician and researcher Dr. David Ludwig showing that people eating a low-carb diet have higher average energy expenditure than those on a diet where the majority of calories are from carbs.

We have also covered studies by NIH researcher Kevin Hall that find no significant difference in energy expenditure between lower-carb and higher-carb diets.

How do we make sense of the conflicting evidence? It may all come down to the length of the study.

Dr. Ludwig and his colleagues published a new report in the Journal of Nutrition that focuses on correlating energy expenditure with the length of time someone is on a low-carb diet.

The researchers evaluated 29 different studies of varying duration. They concluded that energy expenditure might initially go down on low-carb diets, but after two and a half weeks, energy expenditure increases significantly.

We wrote about the differences between short-term metabolic ward trials and longer, less rigorously controlled trials and weighed their respective benefits. The current meta-analysis reveals the shortcomings of studies that last only a few days to a few weeks, no matter how well controlled they are.

There appears to be a metabolic shift that occurs roughly two and a half weeks after switching to low carb. While shorter studies may be used to investigate other scientific questions about nutrition and metabolism, these new findings suggest that longer studies are needed to provide clinicians with a more accurate picture of how different diets affect obesity and weight loss.

According to senior author Dr. Ludwig:

We updated and reanalyzed a prior, high visibility meta-analysis by Kevin Hall, and found that – contrary to the original meta-analysis – total energy expenditure was significantly higher on low-carbohydrate vs. high-carbohydrate diets, after allowing a few weeks for metabolic adaptation to the change in macronutrients (a well-documented phenomenon).

We believe this finding makes 3 contributions to the science, in that the data:

  1. Provide the best available evidence to date that all calories are not metabolically alike
  2. Support a key prediction of the Carbohydrate-Insulin Model
  3. Demonstrate the pitfalls of short diet studies (comprising the majority of published trials), a design issue of broad significance to the fields of obesity and nutrition.

This new meta-analysis is an essential contribution to the science of carbohydrate metabolism and should alter the way we interpret shorter low-carb diet studies.

Thanks for reading,
Bret Scher MD FACC


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