Marriage and having lots of friends - even having a dog - can lower the risk of heart problems and cardiac-related deaths, research suggests. Similarly, kids might help take care of you, or give you a reason to take better care of yourself.

Also, it takes reasonably good genes to father a child. An inability to do so might be indicative of a genetic weakness that could spell heart trouble down the road.

"There is emerging evidence that male infertility is a window into a man's later health," said Dr Michael Eisenberg, a Stanford University urologist and fertility specialist who led the study. "Maybe it's telling us that something else is involved in their inability to have kids."

The study was published online Monday by medical journal Human Reproduction.

Last week, another study of 600 men in the Philippines found that testosterone, the main male hormone, drops after a man becomes a dad. Men who started out with higher levels of the hormone were more likely to become fathers, suggesting that low levels might reflect an underlying health issue that prevents reproduction, Eisenberg said.

In general, higher levels of testosterone are better, but too much or too little can cause HDL, or "good cholesterol," to fall - a key heart disease risk factor, said Dr Robert Eckel, former president of the American Heart Association and professor of medicine at the University of Colorado.