Men who have never had children have a lower risk of prostate cancer compared with fathers, concludes a large study, but the reasons for that are unclear.
Lead author Kristian T. Jorgensen of the Statens Serum Institut in Copenhagen and colleagues followed all men born in Denmark between 1935 and 1988. Among the group, 3,400 developed prostate cancer during up to 35 years of follow-up.
They found that, compared with fathers, childless men were 16 per cent less likely to develop prostate cancer.
But at the same time, the risk of the disease gradually declined among fathers as the number of children in their family increased. Men with more than four children were less likely to get the disease than men with fewer offspring.
The results are published in the journal Cancer.
The reasons for the seemingly contradictory links could be determined from the study data, according to the researchers.
However, they note, that this is not the first time a study has found such a link.
One theory is that men who are childless may be infertile and may have lower testosterone levels, which would then lower their prostate cancer risk. This has yet to be proven, however, Jorgensen's team notes.
Why men with several children had a lower prostate cancer risk than those with one or two children may potentially be explained by the "healthy father" phenomenon, the researchers speculate. That is, men who have many children may have healthy semen quality and healthy prostates.
As well, men with a large number of children may retain their fertility to a relatively late age, and may be more resistant to developing prostate cancer.
A recent study found that men with sons might be at a lower risk of prostate cancer than men with daughters only. But this most recent study found no association between prostate risk and child gender.
More studies will be needed to understand the "biologic, environmental, social and/or behavioral factors" that underlie these findings, the researchers conclude.
Prostate cancer is the most common cancer in Canadian men. An estimated 22,300 men in Canada were diagnosed with the disease last year.
