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Obese men with prostate cancer tend to have lower serum concentrations of the tumour marker prostate specific antigen (PSA) than thinner men. Is it because they are less androgenic—PSA is controlled by testosterone—or is it a simple matter of haemodilution in men with a higher circulating plasma volume? To find out, researchers studied data from three large cohorts of men who had had a radical prostatectomy for cancer in the US.
As expected, increasing body mass index was associated with increasing plasma volume and decreasing serum concentration of PSA preoperatively. But the trend disappeared when the researchers analysed circulating mass (total amount) of PSA instead. In one cohort, obese men actually had a significantly greater mass of PSA than non-obese men. The results, which were extensively adjusted for confounders such as tumour severity and prostate weight, suggest that PSA concentrations are lower in obese men because the protein is diluted in their high plasma volume.
Three quarters of American men over 50 take at least one PSA test, to check for prostate cancer. Many of them are obese and risk falling below the threshold for a biopsy because their serum concentrations of PSA are spuriously low, say the researchers. Late diagnosis could be one reason why obesity is associated with a worse outcome in obese men.