The new study was carried out by scientists at the University of Oxford in the United Kingdom, and the findings were presented at the National Cancer Research Institute (NCRI) Cancer Conference, held in Liverpool, U.K.
To the authors' knowledge, this is the first time that scientists have examined how low levels of testosterone affect the risk of prostate cancer. Their research is also the largest study of the link between hormones and prostate cancer to have ever been conducted.
Prostate cancer affects more than 170,000 men each year in the United States, of whom almost 30,000 die from the disease.
While it is not yet known what causes the condition, researchers have identified a number of risk factors, such as age, ethnicity, and genetic mutations.
Male hormones, or androgens, such as testosterone are known to promote tumor cell growth in men with prostate cancer. Lowering or blocking testosterone levels in the body is the standard treatment for prostate cancer because androgens help the prostate cancer cells to grow.
The so-called androgen saturation model proposes that in order to grow, prostate tissue — whether malignant or benign — needs just enough testosterone for the androgen receptors to be saturated.
However, after the saturation point has been reached, any further increase in testosterone does not lead to an additional increase in prostate tissue or cancer tumor growth.
As the authors of the new study explain, "[B]ecause the saturation point is thought to be low, until now there have been insufficient prospective data available to test this theory."
So, the team set out to fill this research gap by testing the theory in more than 19,000 men.
Low testosterone reduced risk by 20 percent
The researchers examined 20 prospective studies including more than 19,000 men, 6,933 of whom had prostate cancer and 12,088 of whom were examined as healthy controls.
Participants were aged between 34 and 76 years old, and their blood was collected for analysis between 1959 and 2004.
The researchers grouped the men into 10 categories according to their testosterone level. These ranged from those with the lowest blood levels of the androgen to those with the highest.
Then, prostate cancer risk was calculated using conditional logistic regression methods.
As predicted, men in the lowest testosterone group were considerably less likely to develop prostate cancer. In fact, they had a 20 percent reduced likelihood of developing the disease.
Surprisingly, however, the study also found that if these men do end up developing the disease, they are 65 percent more likely to have an aggressive form of the condition.
In the other nine groups of varying testosterone levels, the researchers found no association with prostate cancer risk.
Findings may help to prevent prostate cancer
Prof. Malcolm Mason — a prostate cancer expert for the nonprofit organization Cancer Research U.K. — comments on the findings, saying, "Testosterone's role in prostate cancer's development has been a hotly debated area of research, so it's great to see some strong evidence."
"This," he adds, "puts another piece of the jigsaw into place in terms of understanding the biology of what causes prostate cancer."
Study co-author Prof. Tim Key, of the University of Oxford, also weighs in. He says, "This is an interesting biological finding that could help us understand how prostate cancer develops and progresses."
"Until now, we didn't have a clear idea of the role testosterone played in prostate cancer risk. This is the first population study to support the theory that risk is lowered below a certain threshold of the hormone."
Prof. Tim Key
Prof. Matt Seymour, the NCRI's clinical research director, says, "In future, these results could be important in helping to devise an approach to reducing men's risk of developing the disease."
"[It's] possible that this could help unravel ways to diagnose and treat fatal prostate cancers before they can do any harm," concurs Prof. Mason, "but that's very far down the line."