Eighteen male marathon runners (mean marathon performance: 2 h 36 min, SD = 7.0 min; VO2 max = 64.1 +/- 15.1 ml/kg . min-1) were submitted to a needle biopsy in the suprailiac region and to various measurements of body fatness: percent body fat (% fat), seven skinfold thicknesses, and mean fat cell diameter. Basal and insulin-stimulated glucose conversion into triglycerides were measured in collagenase-isolated fat cells, while heparin-releasable lipoprotein lipase activity (LPLa) was determined in intact adipose tissue. All body fatness indicators were significantly smaller in marathon runners in comparison to a sedentary control group (P less than 0.001). Fat cell basal and insulin-stimulated glucose conversion into triglycerides as well as LPLa were significantly higher for the runners group (P less than 0.01), differences being particularly important when comparisons were performed between subjects paired for mean fat cell diameter. Pearson interclass correlations between body fatness and fat cell glucose incorporation into triglycerides were low and positive for the sedentary group (0.04 less than or equal to r less than or equal to 0.41), while they were negative for the marathon runners groups (-0.28 less than or equal to r less than or equal to -0.40) with the exception of % fat. Moreover, correlations between LPLa and indicators of body fatness were positive in the sedentary group (0.47 less than or equal to r less than or equal to 0.79), while they were negative in the marathon runners group (-0.03 less than or equal to r less than or equal to -0.63).(ABSTRACT TRUNCATED AT 250 WORDS)