grouped by distance run per week. The HDL-cholesterol concentrations increased significantly in association with each 15-mile increment in miles run per week. The graph indicates that HDL-cholesterol concentrations increase in both the older and younger men for comparable distances run weekly. Body mass index (a measure of being overweight), waist circumference, blood pressure, resting heart rate, plasma triglyceride concentrations, and ratio of total cholesterol to HDL-cholesterol were also lower in older runners who ran greater distances in comparison to those who ran less (these results are not shown). For most heart disease risk factors, their relations to distance ran were the same in older and younger runners. However, men sixty years and above showed significantly smaller reductions in LDL-cholesterol per mile ran (the so-called 'bad cholesterol') than their younger male counterparts (see graph). The different LDL responses to exercise may be due to age related changes in the metabolism of LDL. Other researchers have shown that in older men, apolipoprotein B molecules (the primary LDL protein) are produced more rapidly and the molecules reside longer in the blood than in younger men. Older men also have decreased bile acid production and decreased activity of the enzyme alpha-hydroxylase. Bile acid production is the primary mechanism for eliminating cholesterol from the body. The activity of the enzyme alpha-hydroxylase regulates the rate that cholesterol is converted to bile acid.