Healthier sperm may mean longer life, according to a study that followed more than 40,000 Danish men for up to 40 years.

In an interview with Reuters Health, Dr. Tina Kold Jensen, who was involved in the study, said: "No matter what you look at, the risk of dying is decreased if you have a good semen quality compared to low; the poorer the semen quality, the higher the risk of dying."

While the findings shouldn't scare men whose semen quality isn't tip-top, they do suggest that these men should be checked out for other illnesses, especially testicular cancer, said Jensen, of the University of Southern Denmark in Odense.

Male infertility has become increasingly common over the past 50 years, Jensen and her team point out in the American Journal of Epidemiology, and some investigators have suggested that abnormal development of male reproductive organs in the womb could be responsible. This "fetal origins hypothesis" has also been tied to widespread illnesses in later life like heart disease and diabetes, they add.

To test the hypothesis that semen quality might therefore be related to illness and death, the researchers looked at men who had been referred to the Copenhagen Sperm Analysis Laboratory between 1963 and 2001, following them through the end of 2001 or until they died. They restricted their analysis to 43,277 men with viable sperm in their semen.

As the concentration of sperm in the men's semen increased, so did their lifespan, the researchers found. Men whose sperm concentration was 40 million per milliliter were 40 percent less likely to die during the course of the study than men with sperm concentrations below 10 million per milliliter.

Longevity also rose steadily with the percentage of a man's sperm that were "motile," meaning they moved around normally; and the percentage of normally formed sperm. For example, men with 75 percent or more normal sperm had a 54 percent lower risk of dying than men who had less than 25 percent normal sperm.

The researchers also found that men in the study who had fathered children lived longer than childless men, in line with previous research showing that fertile men, and women, live longer.

Childless men are known to be less healthy, poorer, and more likely to have chronic illnesses, the researchers note. But the increase in longevity with semen quality was seen in both men who had kids and those who didn't, suggesting that the relationship between healthy sperm and longer life was independent of these factors.

Men with higher sperm quality were at lower risk of a wide variety of diseases, the researchers found, including illnesses not traditionally associated with the fetal environment hypothesis, like cancer, respiratory problems, and digestive disease.

Jensen and her colleagues hypothesize that "good semen quality may be a biomarker of general health associated with better survival."