Each day a huge number of men (something in the region of 13,000 in the US alone) call to see their doctor with a variety of problems associated with urination and come away having been told that they are suffering from an enlarged prostate.

Benign prostatic hyperplasia, benign prostatic hypertrophy or BPH, to give it its proper medical names, is an extremely common and treatable problem seen in many men over the age of 65 and is suffered by as many as half of all men in their sixties and ninety percent of men in their seventies and eighties.

But what causes this condition which is a simple part of the natural aging process and is as common as your hair turning grey with age?

The simple answer is that, despite all the advances in modern medicine, we don't know. We do, however, have enough evidence to start to unravel the mystery.

Men are more likely to suffer from BPH if their father, or a brother, has suffered from an enlarged prostate. This suggests that there is, at least in part, a genetic component to the problem.

One theory here is that genetic instructions given to the cells of the prostate to control its growth during puberty are again activated in later life and either cause further growth in their own right or make the cells of the prostate more sensitive to hormones which promote growth.

We also know that men who have their testes removed at an early age (before puberty) do not suffer from BHP. There is therefore a link between the aging of the testes and the development of an enlarged prostate.

Although the exact role played by the testes is not clear we know that men produce both the male hormone testosterone and small quantities of the female hormone estrogen throughout life. With increasing age however the quantity of testosterone present in the blood decreases although the level of estrogen does not fall in the same proportion. The theory, supported to some extent by research carried out on animals, is that the higher proportion of estrogen present in the prostate promotes the activity of other hormones which are responsible for cell growth.

Although this theory has been around for some time now researchers have found it somewhat difficult to explain satisfactorily the link between the theory and the fact that the removal of the testes before puberty results in a failure to develop the problem of an enlarged prostate.

The final, and perhaps at present most credible, theory is that an enlarged prostate is caused by the presence of a hormone known as dihydrotestosterone (DHT).

We know that DHT plays a crucial role in the development of the sex organs during pregnancy and that it is also largely responsible for the development of facial hair, the deepening of the voice and the development of the prostate during puberty.

DHT is derived from testosterone and is found in the prostate where it is believed to play a role in the growth of the prostate gland. However, as testosterone levels drop with age the body's ability to produce DHT does not appear to diminish and indeed DHT accumulates in the prostate and continues to promote growth. It is also interesting to note that men who lose their ability to produce this hormone derived from testosterone do not develop enlarged prostate glands.

Although it may be some years yet before researchers uncover the exact cause of BPH. While it is almost certainly the result of hormonal changes resulting from the aging process, the precise nature of these changes and the effects which they trigger remain something of a mystery today.