Fatherhood may be a kick in the old testosterone, but it also may help keep a man alive. New research suggests that dads are a little less likely to die of heart-related problems than childless men are.

The study, by the AARP, the government and several universities, is the largest ever on male fertility and mortality, involving nearly 138,000 men. Although a study like this can't prove that fatherhood and mortality are related, there are plenty of reasons to think they might be, several heart disease experts said.

Marriage, having lots of friends and even having a dog can lower the chance of heart problems and cardiac-related deaths, previous 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 mean a genetic weakness that can 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 the journal Human Reproduction.

Last week, a study by other researchers 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 it 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, past president of the American Heart Association and professor of medicine at the University of Colorado, Denver. "This is a hot topic," Eckel said. "I like this study because I have five children," he joked, but he said many factors such as job stress affect heart risks and the decision to have children.

For this study, researchers excluded men who had never been married so they could focus on those most likely to have the intent and opportunity to father a child. Men with cancer or heart disease also were excluded.

Of the remaining 137,903 men, 92 percent were fathers and half had three or more children. After an average of 10 years of follow-up, about 10 percent had died. Researchers calculated death rates according to the number of children, and adjusted for differences in smoking, weight, age, household income and other factors.

They saw no difference in death rates between childless men and fathers. However, dads were 17 percent less likely to have died of cardiovascular causes than childless men were.

Now for all the caveats: