In other words, the group eating the low-carb diet burned more calories just sitting around, and the effect was substantial-- about 250 Calories per day. This is basically the equivalent of an hour of moderate-intensity exercise per day, as Dr. Ludwig noted in interviews (2). The observation is consistent with the claims of certain low-carbohydrate diet advocates that this dietary pattern confers a "metabolic advantage", allowing people to lose weight without cutting calorie intake-- although the study didn't actually show differences in body fatness.
In Dr. Ludwig's study, calorie intake was the same for all groups. However, the study had an important catch that many people missed: the low-carbohydrate group ate 50 percent more protein than the other two groups (30% of calories vs. 20% of calories). We know that protein can influence calorie expenditure, but can it account for such a large difference between groups?
A new study gets us part of the way to answering that question (3). This is a follow-up to a study that was published by the same authors in 2013 (4). For 8 weeks under strict metabolic ward conditions, researchers overfed 25 volunteers by 40 percent of their normal calorie needs. They compared three groups at different levels of protein intake:
- Overfeeding with 5% protein.
- Overfeeding with 15% protein.
- Overfeeding with 25% protein.
Protein intake was increased at the expense of fat, so carbohydrate intake was similar in all groups.
After eight weeks, the calorie expenditure of the groups differed substantially. It increased in all groups, as expected due to the overfeeding, but it increased much more in the high-protein group (group 3). Within one day, total calorie expenditure in the high-protein group had increased by 130 Calories. By the end of the 8-week period, the high-protein group was burning about 260 Calories more per day than the low-protein group, and about 180 Calories more than the medium-protein group!
This demonstrates that protein intake can have a large effect on calorie expenditure-- equaling the magnitude of moderate daily exercise. Although it was a different type of study than what Dr. Ludwig published, when this study is considered along with the rest of the evidence, it suggests that there probably is a low-carb "metabolic advantage", but that this advantage is probably due to increased protein intake, not reduced carbohydrate intake.
The companion study also found, strangely, that all three groups gained the same amount of fat during overfeeding, despite the differences in calorie expenditure. This is the same thing Dr. Ludwig observed, although his study was only four weeks long. It's possible that 4-8 weeks isn't long enough for these differences in calorie expenditure to manifest in fat mass-- or perhaps something more mysterious is going on that will require further investigation**. However, the higher protein groups did end up with more muscle mass than the low-protein group.
* They found that a moderate-CHO low-glycemic diet led to an intermediate calorie expenditure, although it wasn't statistically different from the other two conditions.
* For example, perhaps indirect calorimetry isn't the best tool for measuring calorie expenditure in this type of intervention. The use of indirect calorimetry to measure calorie expenditure rests on certain assumptions, which to my knowledge have not been well validated in the context of overfeeding and weight-reduced people. My friend and colleague Karl Kaiyala published a paper in 2011 discussing some of these issues (5). It's possible that indirect calorimetry gives erroneous measurements in some contexts.
Thanks to Erik Arnesen for tweeting the study, and Pedro Bastos for sending me the full text.