Substrate utilization during prolonged exercise in obese women differing in body fat distribution

The present investigation compared the pattern of substrate utilization during prolonged walking in obese women with upper (UB) and lower (LB) body fat patterns and the effect of beta-adrenergic receptor blockade on that response. Twenty healthy, premenopausal, obese volunteers (% fat ≥ 30%) were characterized by waist-to-hip (WHR) circumference ratios into two distinct groups (UB: WHR ≥ 0.85, LB: WHR ≤ 0.75). The groups (n = 10 each) were similar for BMI, age, % fat, and maximal aerobic power (VO₂max). Subjects participated in two 60-min walk bouts at 55-60% VO₂max. The trials (placebo (C) and 80 mg propranolol (PR)) were performed in random order, 110 min after ingestion, with 2-4 days between tests. Open-circuit spirometry was employed to calculate the VO₂, VCO₂ and respiratory exchange ratio (RER) at 10 min intervals. A significant (P < 0.0001) decline in RER with time was evident in both trials. However, no group differences existed in RER values during either the C trial (RER range = 0.81-0.88) or the PR trial (RER range = 0.86-0.91). The PR trial induced a 23% reduction in the amount of calories utilized as fat compared to C in both WHR groups. Both groups of women showed equal capabilities for performing prolonged exercise with typical and appropriate metabolic shift in fuel mobilization from carbohydrate to fat. Despite the well-defined differences in metabolic activity of adipose cells shown in vitro, this study indicates that women with significantly different regions of adipose tissue stores have similar physiological potentials for substrate use during exercise.