Metabolic Determinants of Weight Gain in Humans

One of the fundamental challenges in obesity research is to identify subjects prone to gain weight so that obesity and its comorbidities can be promptly prevented or treated. The principles of thermodynamics as applied to human body energetics demonstrate that susceptibility to weight gain varies among individuals as a result of inter-individual differences in energy expenditure and energy intake, two factors that counterbalance one another and that together determine daily energy balance and, ultimately, bodyweight change. This review focuses on the variability among individuals in human metabolism that determines weight change. Conflicting results have been reported about the role of inter-individual differences in energy metabolism during energy balance in relation to future weight change. However, recent studies show that metabolic responses to acute, short-term dietary interventions that create energy imbalance, such as low-protein overfeeding or fasting for 24 hours, may reveal the underlying metabolic phenotype that determines the degree of resistance to diet-induced weight loss or the propensity to spontaneous weight gain over time. Metabolically “thrifty” individuals, characterized by a predilection for saving energy in settings of undernutrition and dietary protein restriction, display a minimal increase in plasma Fibroblast Growth Factor 21 (FGF21) concentrations in response to a low-protein overfeeding diet and tend to gain more weight over time as compared to metabolically “spendthrift” individuals. Similarly, inter-individual variability in the causal relationship between energy expenditure and energy intake (“energy sensing”) and in the metabolic response to cold exposure (e.g., brown adipose tissue activation) seems to some extent to be indicative of individual propensity to weight gain. Thus, an increased understanding and the clinical characterization of phenotypic differences in energy metabolism among individuals (metabolic profile) may lead to new strategies to prevent weight gain or improve weight loss interventions by targeted therapies on the basis of metabolic phenotype and susceptibility to obesity in individual persons.