Deeper Dive: What are the differences between people who lose an expected amount of weight on a restricted diet and people who don't?

Individual responses to the same dietary intervention vary significantly. This variance poses a problem during weight loss because some people don’t lose the expected amount of weight for a given calorie deficit. The underlying reason(s) for the different response is not completely understood. The body does, however, experience adaptive reductions in energy expenditure meant to minimize the loss of precious energy stores. In the context of a calorie deficit, compensatory mechanisms^[1] can act to both reduce energy expenditure and increase^[2] energy intake. One of the mechanisms allowing reductions in energy expenditure appears to be a reduction in resting metabolic rate (RMR), which is the amount of energy expended for maintaining bodily functions.

It has been observed in multiple studies that RMR decreases by a larger amount than expected from the loss of body mass. Scientists refer to this phenomenon as “metabolic adaptation.” Differences in the magnitude of adaptation to reduced energy intakes might affect the weight loss response to a given calorie deficit. Even though researchers have known about this phenomenon for many years, its exact physiological mechanisms are not well characterized. Identifying metabolic markers that differentiate people who are sensitive and resistant to weight loss would help to design better interventions for specific individuals. On the other hand, it is theoretically possible that defects at any level of fat oxidation might also contribute to a reduction in the amount of fat loss during a caloric deficit.

The authors of the study under review hypothesized that participants who don’t lose the expected amount of weight after a low-calorie diet have impaired fat oxidation and increased metabolic adaptation after a low-calorie diet.

In real life and scientific studies, weight loss success on otherwise identical diets is highly heterogeneous, with some people not losing the expected amount of weight. There are compensatory physiological mechanisms meant to slow down the loss of energy stores. One of them is a reduction in metabolic rate that is greater than predicted by changes in body mass, called metabolic adaptation. Therefore, it is possible that individual differences in weight loss might be related to different degrees of metabolic adaptation. It has also been speculated that alterations in fat oxidation pathways may also contribute to the comparatively low reduction in body fat.