Rate of weight loss during underfeeding: Relation to level of physical activity

Abstract

The kinetics and bioenergetic-metabolic determinants of weight loss were examined in obese women ingesting 900 kcal/d for 5 weeks. The patients were assigned either to a sedentary group (n = 5) or to an exercise group (n = 6) in which the participants expended an additional ( X ± SD) 346 ± 61 kcal/d in aerobic physical activity. The percentage weight loss and the fractional rates (K1 = fast component; K2 = slow component) of weight loss were almost identical between the two groups. The failure of added exercise to increase the velocity of weight loss could not be explained by differences between the groups in any of the following: gastrointestinal energy and nitrogen (N) absorption; fractional rates of urinary urea N and total N loss; or the thermic effect of the formula diet. The cumulative and fractional rates of protein (ie N) loss were also similar between the groups. The exercise group lost more fat (5.3 ± 1.0 kg) than the non-exercise group (4.4 ± 1.6 kg, P < .001) as measured by underwater weighing. The maximum between-group difference in the rate of fat loss, as determined by energy-N balance, occurred during early underfeeding. With continuation of the 900 kcal/d diet, the between-group differences in the rate of fat loss diminished. The exercise subjects significantly lowered their resting heat losses relative to the non-exercise subjects ( P < .025). This in turn reduced the degree of negative energy balance in the more energy-deficient exercise group. While the rates of protein plus fat loss in the exercise group exceeded that observed in the non-exercise subjects, the rate of H 2O depletion was diminished in the exercise group. Hence, the combined effects of an equivalent rate of protein depletion, a decrease in the rate at which added exercise promotes fat loss over time, and a reduced rate of H 2O loss account for the failure of additional physical activity to accelerate weight loss over a 5-week underfeeding period.