Energy Expenditure and Maximal Oxygen Consumption
The resting metabolic rate was determined by indirect calorimetry on two mornings by methods described previously. The average of the two values was used unless the coefficient of variation was greater than 7 percent, in which case a third day of measurements was scheduled. The mean (±SD) coefficient of variation between the measurements on the two days was 3.2 ±2.5 percent.
Heart-rate monitoring was used to estimate energy expenditure, as described elsewhere13. Calibration was performed individually for each woman during each study period (at base line and at the midpoint and end of the study) by simultaneously measuring the heart rate and the oxygen consumption of each woman while she was sedentary (resting, sitting, or standing) and active (walking on a treadmill at various speeds or riding a stationary bicycle). During each study period, each woman wore a heart-rate monitor (Uniq Heart Watch, model 8799, Computer Instruments, Westbury, N.Y.) for three days (for women in the exercise group, this included two exercise days). Oxygen consumption during sleep was assumed to be equivalent to that during the measurement of the resting metabolic rate. The total daily energy expenditure was calculated by totaling oxygen consumption during periods of sleep and activities other than sleep and multiplying that value by 4.83 kcal per liter of oxygen (20.2 kJ per liter). Group means for energy expenditure based on heart-rate monitoring are very similar to those obtained with the method using doubly labeled water.