This study compared the bowel function, fecal hemoglobin (Hb) loss, iron status and diets of 36 women of varying activity levels. In addition, we examined the effect of a marathon race on bowel function and fecal Hb loss in 15 of these women. The subjects were divided into 4 groups based on estimated daily exercise energy expenditure: Group A) less than 400 kcal/d (less than 1674 kJ/d); Group B) 400-599 kcal/d (1674-2507 kJ/d); Group C) 600-800 kcal/d (2511-3348 kJ/d); and Group D) greater than 800 kcal/d (greater than 3348 kJ/d). There were no differences in hematological and iron indexes among the 4 groups. Mean daily stool wet weights and individual stool weights were greater for groups B and D as compared to group A. However, transit time and frequency of defecation were not different. Daily fecal Hb losses (mg/day) were greater in groups B, C, and D as compared to A, but there was no difference in fecal Hb concentrations (mg/g feces) among the groups. Dietary intakes of energy, protein, fat and fiber were similar for all groups. Ascorbic acid and iron intakes from food significantly increased with increased estimated daily energy expenditure. Total intakes of ascorbic acid and iron, including oral supplements, were not different among the groups. Marathon racing resulted in a 21% increase in mean transit time, a 27% decrease in daily stool weight and 21% lower frequency of defecation. Fecal Hb concentrations and daily Hb losses before and after the race were not significantly different, despite an increased daily Hb loss in 10 of the 15 runners after the race.(ABSTRACT TRUNCATED AT 250 WORDS)