Hollstein T, Heinitz S, Ando T, et al. - This study was undertaken to determine whether metabolic responses to 24-h fasting and mild cold exposure (COLD) in overweight individuals are correlated and accompanied by changes in FGF21 concentration. Since the thermogenic response to COLD may similarly characterize this human phenotype identified by acute fasting conditions, the researchers analyzed changes in 24EE and sleeping metabolic rate (SLEEP) in a whole-room indirect calorimeter during 24h fasting at thermoneutrality (24°C) and during energy balance both at thermoneutrality (24°C) and mild cold (19°C) in 20 healthy volunteers (80% male, age: 36.6 ± 11.4y, percentage body fat: 34.8 ± 10.5%). The authors conclude that a liver-derived hormone FGF21 associates and mediates the metabolic responses to short-term fasting and COLD. Thus, the 24EE response to COLD further describes the thrifty vs spendthrift phenotype, offering an additional setting for examining the physiological mechanisms underlying the human metabolic phenotype and characterizing the individual susceptibility to weight change.