Men maintain a certain level of reproductive function lifelong but this function declines very gradually over time. Studies have shown that advanced paternal age is associated with changes in reproductive hormone production, sexual function, sperm production and fertility, as well as with adverse pregnancy outcomes: an increased risk of sperm mutations, birth defects and offspring diseases.
Paternal age is highly associated with reduced fertility, in couples where men are older than 40 years and women are at least 35 years (probably because after 35, maternal age is more closely linked to maternal fertility status).
Changes in the levels of male hormones, such as an increase in testosterone, are associated with a decrease in the function of cells that support sperm cell production, resulting in reduced sperm count. In addition, increased age is associated with changes in normal sperm parameters, declining semen volume, sperm motility and morphology, and in changes in the integrity of chromosomes leading to genetic abnormalities. These changes in sperm also affect the chances of fertilization using assisted reproduction technologies.
A number of very large studies have observed that paternal aging is associated with an increased risk of pregnancy loss after an established natural pregnancy. These data provide some evidence that paternal age may affect the sperm DNA integrity such that it impacts negatively on late embryo development.
It is not clear why these observed changes occur with age, however these may occur due to associated diseases (atherosclerosis, diabetes) and life-style (diet, weight, alcohol, smoking). Indeed, since production of sperm occurs throughout adulthood, as men age the testicular environment is more influenced by life-style insults, such as oxidative stress and reduction in protective micronutrients; this reduction in natural protection renders sperm production sensitive to environmental stress which affects the integrity of DNA, in addition to other sperm properties. Indeed, it was shown that increased mutations in offspring were correlated to increased paternal age, and not to increased maternal age (4% increase per year of paternal age).