Physical Activity And Resting Metabolic Rate In Overweight Women During Energy Restriction Induced Weight Loss

Abstract

Obesity prevalence is disproportionately higher in African-American (AA) than Caucasian (CC) women and often is associated with metabolic and behavior differences. AA women have a lower resting metabolic rate (RMR) and a tendency for lower level of physical activity than CC women. PURPOSE The purpose of this study was to compare RMR and physical activity (PA counts) in overweight AA and CC women before and during a 4-wk energy deficit induced weight loss. METHODS Seven pre-menopausal overweight AA women (age: 31.9±6.6 y; weight: 88.1±20.6 kg; BMI: 33.9±6.6; body fat: 44.9±6.6%; lean mass: 45.0±6.9 kg) and 11 CC women (age:28.9±4.8 y; weight:85.5±14.5 kg; BMI: 31.1±4.6; body fat: 47.0±5.0%, lean mass: 43.3±4.7 kg) were recruited for this study. They received diet containing 75% of estimated individual daily energy needs for 4 weeks. Energy needs were assessed from the RMR measured using whole-room indirect calorimetry and an activity factor calculated from the activity level measured using a triaxial accelerometer worn daily on the waist during the whole study period. All meals were provided by the General Clinical Research Center (GCRC) metabolic kitchen for 2 weeks of inpatient stay at the GCRC, followed by 2 weeks of outpatient period. RMR and PA counts were assessed at baseline, mid-point (2 wk) and end of the study (4 wk). PA counts were calculated as a square root of the sum of squared activity counts in 3 dimensions every minute. Body composition was determined by dual-energy x-ray absorptiometry (DXA). RESULTS CC and AA women differed significantly in mean weight loss (3.7±1.7 vs. 1.7±1.9 kg, respectively, p =0.026). AA women had significantly higher lean body mass (LBM) and lower RMR at each time point (both p<0.05). RMR adjusted for LBM was significantly lower after weight loss in both groups (AA women:baseline; 1481±186 kcal/d, at 4th wk; 1451±170 kcal/d, p<0.01; CC women: 1603.3±152 kcal/d, at 4th wk; 1564±144 kcal/d, p<0.01). Average daily PA counts did not differ significantly during the 4-wk study period (220.0±58.7 × 103 vs. 219.2±44.5 × 10 3, p >0.05). CONCLUSIONS Difference in a magnitude of weight change during energy restriction weight loss was not associated with differences in voluntary PA between AA ad CC women but rather to differences in RMR and changes in LBM. Supported by NIH grants RR-00095 and HL 67715