Many cross-sectional studies have demonstrated the influences of fitness level or the regional distribution of fat on lipid and carbohydrate metabolism; however, the relative contribution of these two variables in the same subjects has not been extensively examined. The purpose of this study was to determine the impact of regional adiposity on plasma lipids and carbohydrate metabolism in middle- to older-aged men with a wide range of fitness levels. Forty-six sedentary and exercise-trained men (age [mean +/- SE], 52.8 +/- 0.88 years) were included in this study. Fitness level was assessed by (a) time to exhaustion, and (2) maximal oxygen uptake achieved during an incremental treadmill test. Plasma lipid levels were determined in the basal, fasting state. Carbohydrate metabolism was evaluated by the glucose and insulin responses (total glucose and insulin areas under the curve, insulin sensitivity index [ISI]) to a 75-g, 2-hour oral glucose tolerance test (OGTT). Abdomen to hip ratio (AHR) was used as the index of regional adiposity. Multiple regression analysis indicated that fitness level and the percentage of body fat were significant predictors (approximately 56% of total variance) for total insulin area under the curve and the ISI. A comparison between the sedentary and trained subjects showed that training resulted in an improved ISI at an equal AHR. Fitness level was also the only significant multiple regression predictor for high-density lipoprotein (HDL)-cholesterol (25% of total variance) and accounted for the greatest amount of variance in triglyceride levels (34%), although AHR was also a significant predictor (6%).(ABSTRACT TRUNCATED AT 250 WORDS)