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Did you know...
... that according to the World Health Organization (WHO), 347 million people worldwide have diabetes?
Diabetes is known to adversely affect some organs and systems in the body. These include ill-effects on your blood vessels and nerves, leading to major health issues. These health complications caused due to long-term diabetes are known by the inclusive term 'diabetic complications'.

However, the effects of diabetes on fertility have not been studied as extensively as the others. It was thought to have an adverse effect for years, but no study had made any irrefutable claim either way.

The situation remains largely unchanged even today. Diabetes is known to affect the female reproductive system, but its effects on male fertility are still not conclusively established. No large-scale studies have been done on diabetic men; one major study concluded that diabetes can indeed affect male fertility (it is explained further in the article), but its sample size was too small to make an all-encompassing conclusion.

However, diabetic complications can affect the male reproductive system, albeit indirectly. Here's how.
Nerve Damage
Diabetes (both type I and type II diabetes) primarily damages blood vessels by decreasing their internal circumference and thus constricting blood flow. This is mainly linked to high blood pressure and cardiac issues (because the heart has to pump blood more forcefully, so that it can travel through the narrowed blood vessels), but it also affects nerve endings in various areas of the body, due to the reduced and abnormal blood supply.
Among others, it affects the nerves present in the penile region. This itself can cause erectile dysfunction problems, and can lead to infertility. Furthermore, this can cause an ejaculatory condition known as retrograde ejaculation. In this condition, semen is produced in a normal way and the man experiences orgasm, but due to nerve damage, the semen can't be projected outside the body, and instead travels in the opposite direction - into the urinary bladder. This is not dangerous in any way, but due to obvious reasons, the man still won't be able to impregnate a woman.
Sperm DNA Damage
The aforementioned study about diabetic male patients was conducted with 27 men who were suffering from Type I diabetes (which occurs naturally, as an autoimmune disorder), and 29 male patients who were undergoing infertility treatment. No patient was known to be both diabetic and infertile.

The study, conducted by researchers based in Belfast, found that both groups had similar levels of sperm count and motility. The diabetic men had slightly lower semen volumes, but they were still within the healthy-to-acceptable limits accepted by the WHO.

The disparity was revealed when the researchers examined the semen for signs of sperm DNA damage. They found that the semen samples from the diabetic group had much more sperm DNA damage than the group undergoing infertility treatments. The female egg is capable of repairing some amount of damage to the sperm, but the DNA damage seen in the diabetic group was beyond that.

Though not all sperms were damaged in the samples from diabetic men, it was a clear sign of a damaged reproductive system, and can lead to infertility. Semen from men who are neither diabetic nor undergoing infertility treatment is likely to contain drastically healthier sperms.
This test showed that diabetes could be linked to male infertility indirectly, but more wide-ranging research is necessary before the results can be proclaimed as a fact. Whatever the outcome of the research, it is clear that diabetes does indeed have an adverse effect on male fertility. The extent of that, though, is still unclear.