The effects of body fat distribution on metabolism at rest and during exercise in women

Many of the metabolic complications of obesity have been strongly associated with the pattern of distribution of adipose tissue on the body. Fat metabolism has been shown to vary between various sites on the body. Abdominal adipose tissue is much more labile and sensitive to hormonal (catecholamine) stimulation than adipocytes from the gluteal-femoral region. Thus, it has been speculated that abdominally obese people have a higher potential for lipolysis and as a corollary, weight loss, than lower body obese individuals. Also, differences in energy balance regulation between individuals with different body fat distribution patterns have been suggested as a basis for the inconsistent success rates of exercise-based weight loss programs in obese women. The purpose of this investigation was to examine metabolic differences at rest and in response to exercise between two groups of obese women distinctly characterized by their body fat locations. Twenty-one overfat (% fat ≥ 30%) women volunteers between 18-40 years of age with normal health and blood lipid profiles were selected. The waist to hip circumference ratio (WHR) was used to categorize an upper body (UB) group (N = 10) with a WHR ≥ 0.85 and a lower body (LB) group (N = 11) with a WHR ≤ 0.75. Subjects participated in a resting metabolic rate (RMR) test, two exercise tests (weight supported and weight bearing) to examine mechanical efficiency, and a prolonged walking test to assess fuel utilization. In addition, differences in the adrenergic regulation of metabolic rate and fuel mobilization were studied by pharmacologically blocking the beta-adrenergic receptors with propranolol at rest and during a bout of prolonged walking. The two groups of women in this study were essentially similar in all basic characteristics (i.e., age, body size, body composition, and maximal aerobic power). The results of this study indicated no differences between the groups in resting metabolic rate (i.e., UB = 1730 ± 110 vs. LB = 1640 ±.81 kcals*day^-1). The mechanical efficiency values indicated that gross, net and delta efficiencies were not different between groups for either weight carrying or weight bearing exercise modes. The net efficiency for both groups for cycle exercise ranged between 17-20% and 10-19% for the treadmill. Prolonged walking induced a significant (p < 0.0001) decline in RQ with time in both groups. However, no difference in the amount or rate of fuel utilized as fat (estimated by the steady-state RQ values) during 60 minutes of walking was noted between the two groups (i.e., range 35-65% fat). Finally, the effect of beta-blockade did not selectively alter the metabolic responses between the groups. Propranolol administration decreased heart rate but did not significantly depress RMR in either group. A significant (p < 0.006) depression of lipid utilization by propranolol during prolonged walking was noted after 30 minutes in both groups. In conclusion, the results from this study can be interpreted to mean that obese women with distinctly different body fat distribution patterns have similar physiological potentials for energy expenditure at rest and during exercise. There was no indication from the current findings that one group was more energy thrifty than the other. Both groups of women showed equal capabilities for performing prolonged exercise with the typical metabolic shift in fuel utilization. Thus, there was no indication that the obese women volunteers in this study had an impaired or differing ability for substrate mobilization or fuel utilization during exercise. The implications of this study are that despite the well-defined differences in metabolic activity shown in vitro between various adipose tissue sites, the manifestation of energy regulation mechanisms in vivo is much more obscure.