CURETON, KIRK J. and PHILLIP B. SPARLING. Distance running performance and metabolic reponses to running in men and women with excess weight experimentally equated.Med. Sci. Sports Exercise, Vol. 12, No. 4, pp. 288–294, 1980. Metabolic responses to submaximal and maximal treadmill (TM) running and 12-min run performance were measured in 10 female and 10 male adults who regularly engaged in distance running to experimentally investigate the extent to which differences between men and women in metabolic responses to exercise and distance running performance are due to the sex difference in % body fat. The males were studied under two conditions: (1) with normal body weight and (2) with external weight added to the trunk so that the total percent excess weight (% EW) was equal to the % fat of a matched female. Under the added-weight condition, % EW of the males was increased by an average of 7.5%, the mean sex difference in % fat. Equating % EW reduced the mean sex difference in TM run time by 1.3 min (32%) and in 12-min run performance by 173 m (30%). These changes in running performance were the result of an average 3.0 ml/min·kg (38%) reduction in the sex difference for the oxygen required per unit fat-free weight (FFW) to run at various submaximal speeds and a 3.9 ml·min-1, kg-1(65%) reduction in the sex difference for Vo2maxexpressed relative to the total weight carried. It was concluded that the greater sex-specific, essential body fat of women is one determinant of the sex difference in metabolic responses to running and distance running performance. Because of her greater body fatness, the average woman will utilize more oxygen per unit FFW to run at any given submaximal speed, will have a lower Vo2maxexpressed relative to body weight and, as a result, will maintain a speed on the 12-min run or other similar distance running event which is slower than her male counterpart. Since the sex-specific, essential fat of women cannot be eliminated by diet or training, it provides part of a biological justification for separate distance running performance standards and expectations for men and women.