The present findings represent the first study examining if metabolic adaptation, at the level of RMR, was associated with time to reach weight loss goals. We found that the larger the metabolic adaptation (RMRm-RMRp) after weight loss, the longer was the time needed to reach weight loss goal (BMI = 25 kg/m2), even after adjusting for baseline body weight, baseline TEE and adherence to the diet, suggesting strategies to decrease metabolic adaptation and adherence to diet may expedite weight loss.
In the present analysis, women with overweight who experienced an average weight loss of 13 kg (16.1%) over 155 ± 49 days, had a metabolic adaptation of approximately − 50 kcal/day. It needs to be emphasized that this metabolic adaptation was seen after 4 weeks of weight stabilization following the active weight loss phase and, as such, is probably much lower that what would be expected during the active weight loss phase. In line with this, we have recently shown a significant reduction, in fact more than halving, in metabolic adaptation when measurement were done after 4 weeks of weight stabilization, in comparison to when measurements were performed immediately after weight loss (19). More specifically, a significant metabolic adaptation, at the level of RMR, of -92 ± 110 kcal/day was seen immediately after a 14 kg (13%) weight loss in individuals with obesity, which was significantly reduced, despite still significant (-38 ± 124 kcal/day), after 4 weeks of weight stabilization (19). Moreover, a significant positive moderate correlation was found between metabolic adaptation immediately after weight loss and metabolic adaptation after 4 weeks of weight stabilization (r = 0.663, P < 0.001, n = 71). Therefore, it is reasonable to expect that metabolic adaptation during active weight loss in the present study may have been closer to -120 kcal/day than − 46 kcal/day.
Our regression model showed that even after adjusting for baseline body weight, baseline TEE and adherence to the diet, metabolic adaptation was still a significant predictor of time to reach weight loss goals. For each 10 kcal/day increase in metabolic adaptation, time to reach weight loss goal increased by 1 day. This might not seem like much, since the average metabolic adaptation was only approximately − 50kcal/day. However, as discussed in detail in the previous paragraph, this value is likely to be underestimated and a more reasonable estimation would be -120 kcal/day during active weight loss, with a large inter-individual variation, ranging from − 700 to + 750 kcal/day. That means that those with the largest magnitude of metabolic adaptation would need to stay on the diet for 70 additional days (compared with a person with 0 metabolic adaptation) in order to reach their weight loss goal, even after adjusting for adherence to the diet. This probably helps to explain some of the variation in time needed to reach weight loss goals (range 66–252 days). Importantly, this is assuming that metabolic adaptation during active weight loss would only occur at the level of resting energy expenditure, which has been shown not to be the case, and metabolic adaptation might in fact be of a larger magnitude at the level of non-resting energy expenditure (8, 9).
We and others have shown that metabolic adaptation, either during active weight loss (11, 19), or after a period of weight stabilization (19, 20), is not a risk factor for weight regain. However, metabolic adaptation might lead to resistance to weight loss, as shown in the present study. Even though adherence to the diet is clearly the most important determinant of time to reach weight loss goals, the present findings are of great clinical relevance as they mean that individuals who are struggling to achieve weight loss goals, despite assuring compliance with the diet, may indeed be “suffering” from metabolic adaptation during active weight loss.
The present findings are in line with recent data from our group showing that metabolic adaptation at the level of RMR is associated with less weight and fat mass loss following low-energy diets, in individuals with obesity (30). Moreover, Goele and colleagues (31) reported that in women who experience metabolic adaptation after a low-energy diet, 38% of the difference between measured and predicted weight loss was due to metabolic adaptation at the level of RMR. These two studies and the present analysis suggest that a lower than expected weight loss, or a delay in reaching weight loss goals, may not necessarily result from lack of compliance to the intervention. Metabolic adaptation can modify the outcome of a weight loss intervention, albeit to varying degrees (due to the very large inter-individual differences in metabolic adaptation). The success in the clinical management of individuals with obesity needs, therefore, to be tailored according to individual variations for any relevant phenotype, including the presence or absence of metabolic adaptation in response to weight loss.
Our study has both strengths and limitations. Gold standard procedures were used for the measurements of RMR (after a 4-day GCRC in-patient stay and an overnight sleep, under controlled condition of feeding and physical activity), body composition (4CM) and estimation of diet adherence (TEE by DLW). However, this study also suffers from some limitations. First, it includes a very homogenous sample of premenopausal (20–41 years) women with overweight. This prevents the generalisation of our results to men, other BMI groups and older subjects. Moreover, this also explains why our regression model had an R2 of only 32%, i.e. a truncated range for both BMI and age and only women. Second, metabolic adaptation was measured after 4 weeks of weight stabilization, and as such, is likely underestimated, which might have weakened the association between metabolic adaptation and time to reach weight loss goals.