"Men seem to have a gradual rather than an abrupt change in fertility and in the potential ability to produce viable healthy offspring."
In the study, Wyrobek, Eskenzai, and colleagues examined the genetic quality of sperm from 97 healthy, nonsmoking men between the ages of 22 and 80. The men were current and retired employees of the Livermore laboratory.
The study sample included at least 15 men from each ten-year period from 20 to 60 years of age and 25 men 60 to 80 years old.
In earlier research on the same sperm samples, the team found that sperm count, mobility, and the ability of sperm to move in a straight line declines with age. The new research shows that mobility has a high correlation with DNA fragmentation.
The new study also found that men face increased risk of fathering children with achondroplasia, a genetic mutation that causes a form of dwarfism.
The condition stunts bone growth; affected individuals have short arms and legs and grow to only about 4 feet (1.2 meters) tall.
As women grow older, they are more likely to give birth to a baby with Down syndrome. Not so for men, apparently.
However, the study found no general correlation among male age and chromosome changes that cause Down syndrome.
Even so, 4 of the 97 men were at increased risk for transmitting multiple genetic and chromosomal defects, according to the results. Age may have nothing to do with these subjects' condition, though; one was in his 20s and three were over 60.
Interestingly, Wyrobek added, the sperm in the Lawrence Livermore sample showed no increase with age in Apert syndrome, a disfiguring birth defect.
However, the likelihood of fathering a child with Apert increased with age in men from inner-city Baltimore, Maryland, tested by researchers at the Johns Hopkins Medical Center for a different study.
Wyrobek cautioned against reading too much into the Baltimore study group's results, since they were all from men who lived in the same area.
"There're other things going on besides age. It could be socioeconomic, or diet, or ethnicity."
Further studies, he added, will examine the Apert syndrome factor in greater detail.
Craig Niederberger is a urologist at the University of Illinois in Chicago. He said the research findings are interesting and warrant further study. But he cautioned that the technique used to test the DNA of sperm is new and controversial.
"Older men should not yet be concerned about fathering children. The evidence is still inconclusive," he said.
According to Niederberger, who is president of the Society for Male Reproduction and Urology, researchers need to examine the integrity of sperm DNA with other methods before sounding alarms about male infertility.
Nevertheless, he said, the finding that a genetic mutation that leads to dwarfism increases with age is cause for "some concern. We ought to pursue it."
Wyrobek said the research raises more questions than answers. But it suggests certain kinds of DNA damage and genetic defects go up with age.
Bottom line: "There are consequences of delaying fatherhood," he said.
Free Email News Updates
Best Online Newsletter, 2006 Codie Awards
Sign up for our Inside National Geographic newsletter. Every two weeks we'll send you our top stories and pictures (see sample).
SOURCES AND RELATED WEB SITES