Obesity-related phenotypes and the β₃-adrenoceptor gene variant in postmenopausal women

We examined the hypothesis that postmenopausal women with the β₃- adrenoceptor gene variant (Trp64Arg) have reduced total daily energy expenditure (TEE), altered free fatty acid kinetics, and increased intra- abdominal fat. A secondary objective was to examine whether the obese state masks the effect of the variant on resting metabolic rate (RMR). There were 23 obese heterozygous women with the genetic variant (age 58 ± 6 years; BMI 36 ± 7 kg/m²) who were compared with 19 homozygous obese women with the normal allele (age 56 ± 4 years; BMI 36 ± 3 kg/m²). Daily energy expenditure was determined from doubly labeled water and indirect calorimetry, lipolysis from infusion of [1-¹³C]palmitate, and body fat distribution from computed tomography. No significant differences were found in TEE, RMR, energy expenditure of physical activity, the thermic effect of a meal, fat oxidation as estimated by fasting and postprandial respiratory quotients (RQs), or rate of lipolysis. Similarly, no difference was found in visceral adipose tissue and abdominal subcutaneous fat areas. When RMR was compared between obese (n = 23) and never-obese women with the Trp64Arg variant (n = 16), we found a 317 kcal/day lower RMR in never-obese women after controlling for fat mass, fat-free mass, and age (P < 0.0017). These results do not support the hypothesis that already obese women with the Trp64Arg polymorphism of the β₃-adrenergic receptor gene have lower daily energy expenditure, altered lipolysis, and increased abdominal obesity. On the other hand, the lower RMR in never-obese women suggests that the obese state may mask moderate effect of the Trp64Arg variant on energy expenditure. Although these results need to be confirmed in other populations, the obese state may have been a confounding factor in previous studies of the β₃- adrenoceptor Trp64Arg variant and energy expenditure.