Do low carb diets “boost metabolism”? New meta-analysis on energy expenditure

As we have seen frequently on this blog and as I have explained in several of my books, studies show that low-carb diets are effective for short-medium term weight loss. The mechanisms that cause this effect are still not clear and researchers continue to test the relevance of various hypotheses, such as increased satiety (study), reduced processed foods (study) or metabolic changes caused by reduced insulin levels (explanation), among others.

Its added value compared to other investigations is that in this case the analysis has been segmented with respect to the duration of the trials, dividing them into two groups, depending on whether the intervention lasted less or more than 2.5 weeks (17 days). .

In the image that incorporates the work you can clearly see the results:

As can be seen, in most of the studies of less than 2.5 weeks – the ones at the top – energy expenditure was higher in high-carbohydrate diets, which in principle is not positive for weight loss. Approximately 50 kcal per day above low carb.

However, in the six trials below, in which the duration of the intervention was greater than 17 days, the situation was reversed and the low-carbohydrate diets had a higher energy expenditure, some 135 kcal more per day.

This is how the authors tell it:

“In trials of less than 2.5 weeks, low-carbohydrate diets slightly reduced energy expenditure (…). In contrast, in trials of more than 2.5 weeks, low-carbohydrate diets substantially increased energy expenditure (by approximately 50 kcal/day for every 10% decrease in energy intake from carbohydrate) with minimal residual heterogeneity.These results suggest that shorter versus longer studies have examined different physiological states. The former consist of trials in which participants experienced varying degrees of metabolic adaptation to carbohydrate reduction; the latter consist of trials of sufficient duration to allow adequate adaptation and lead to a consistent result.

This result supports the carbohydrate-insulin model and suggests the existence of a mechanism by which the reduction of carbohydrates in the diet could help in the prevention and treatment of obesity. According to this model, the high insulin/glucagon ratio of a high glycemic load diet (…) shifts the utilization of metabolic fuels from oxidation in lean tissue to storage in adipose tissue.

If the observed effects persist in the long term, reducing dietary carbohydrate intake to half of 60% of energy intake (a typical level for low-fat diets) would increase energy expenditure by ~150 kcal daily, which would offset (if not offset by other factors) much of the increased energy intake that some think is the basis of the obesity epidemic.”

Some quite interesting results, although personally I think that more research is needed regarding whether there really are metabolic changes after those two or three weeks, the period that the defenders of this type of diet say is necessary to get the body to “adapt” . The truth is that there is no objective and rigorous definition that defines said “adaptation”, based on some type of indicator or measure, or on the metabolic and physiological conditions that occur at that moment.

In any case, this review represents a new “push” for low-carbohydrate diets, which seem to be gradually consolidating as an interesting option for some circumstances and people.

Update:

A few months after this meta-analysis, the article “Overestimated Impact of Lower-Carbohydrate Diets on Total Energy Expenditure” (2021) was published, in which the authors recalculated the results, considering the trials that, according to them, were more reliable measuring expenditure energetic. And they confirmed the existence of an increase in energy expenditure, but with more modest figures: over 70 kcal per day.