Prostate cancer affects blacks disproportionately and its evolution may be affected by a sedentary lifestyle. The growth of prostate tumors may be modulated by androgens and insulin‐like growth factors. This study tested the hypothesis that the serum of males having a low aerobic capacity contains concentrations of substances which promote the growth of prostate cancer cells. Eight low fit (peak oxygen uptake 38< ml · kg−1· min−1) and 9 high fit (peak oxygen uptake >40 ml · kg−1· min−1) males severed as subjects. The serum concentrations of testosterone, insulin‐like growth factor‐1 (IGF‐1) and the relative cell growth of the LNCaP tumor cells were measured. The significance of intergroup differences was evaluated by a t‐test at P<0.05. IGF‐1 concentration was higher in the serum from low fitness group compared to that from the high fitness group. The relative growth of the LNCaP prostate cancer cells was significantly higher in the serum from individuals in the low fitness group than in the high fitness group. There was no significant intergroup difference in serum testosterone level. It is concluded that a lower growth rate of prostate cancer cells may be associated with a lower serum concentration of IGF‐1 in physically active males with a higher aerobic capacity than in sedentary males with a lower aerobic capacity. It is concluded that a sedentary lifestyle may promote the growth of prostate cancers.

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