Research has shown that obese men have lower rates of prostate cancer than thinner men, but a new study suggests that this does not reflect an actual lower risk, but a lower rate of early detection through prostate cancer screening.

The findings, researchers say, may help explain two seemingly “inconsistent” patterns: while obese men have a lower rate of prostate cancer than thinner men, they are more likely to have an aggressive form of the disease when they are diagnosed.

Using data from three U.S. government health surveys, researchers found that among men who underwent prostate cancer screening with a prostate-specific antigen (PSA) blood test, obese men generally had lower PSA levels than thinner men.

They were also less likely to have their PSA test followed up with a biopsy to rule out or confirm prostate cancer. Of obese men who had ever had PSA screening, 4.6 percent had undergone a biopsy, versus 5.8 percent of normal-weight men.

The findings, published in the American Journal of Medicine, support the theory that obese men may be diagnosed with prostate cancer less often because they are less likely than thinner men to have PSA-prompted biopsies.

“The lower incidence is not a matter of a truly lower risk, but of lower detection,” said senior researcher Dr. Grace Lu-Yao, of the Cancer Institute of New Jersey, Robert Wood Johnson Medical School in New Brunswick.

Lower rates of early detection, in turn, could help explain the higher rates of prostate cancer progression and death that have been seen among obese men.

A large study of more than 900,000 U.S. adults, for example, found that the death rate from prostate cancer was 67 per 100,000 among normal-weight men, versus 83 and 87 per 100,000 among obese and severely obese men, respectively.

However, while the current study suggests that obese men may have lower rates of PSA-driven biopsy, it is not clear that this is an entirely bad thing, Lu-Yao pointed out.

That’s because although doctors commonly perform PSA screening in men age 50 and older, such routine screening is controversial.

Much of the concern revolves around the fact that prostate tumors are often slow-growing, and screening may result in many men undergoing surgery, radiation or other treatments for cancers that would never have threatened their lives. So those treatments - with their risks of side effects like erectile dysfunction and urinary incontinence - can potentially do more harm than good for many men.

In addition, elevated PSA levels are not necessarily caused by tumors. Non-cancerous conditions, like prostate enlargement, can boost PSA levels in the blood; and factors unrelated to medical conditions, like a man’s age or weight, influence PSA concentrations.

So PSA screening can lead to unnecessary biopsies, which themselves can have side effects such as temporary erectile dysfunction and urinary incontinence.

The decisions over whether to undergo PSA screening, or to have potentially concerning findings followed up by biopsy, are complex ones and need to be made based on one-on-one discussions between men and their doctors, Lu-Yao pointed out.

These latest findings do not offer obese men specific guidance on those issues, she noted. “The main contribution of this,” Lu-Yao said, “is that it helps clarify the issue of why obesity is associated with a lower incidence of prostate cancer but a higher progression and mortality.”

The study does raise the possibility that doctors should consider men’s body mass index (BMI) in decisions about biopsy follow-up to PSA testing, according to Lu-Yao. But further research is needed to show whether that is a useful step.

Lu-Yao and her colleagues based their findings on three different nationally representative government health studies conducted between 1988 and 2004. The studies involved a total of nearly 8,100 men age 40 and older.

The researchers used one of the studies to look at the relationship between BMI and average PSA levels. Overall, they found, the average level was 17 percent lower among severely obese men compared with normal-weight men.

In another one of the studies, about two-thirds of the men had ever had a PSA test, with no significant differences according to body weight. Of obese men, 11 percent had a PSA level of 4 nanograms per milliliter (ng/mL) or higher - which, in the past, was used as the threshold for an “abnormal” result that might prompt a biopsy. That compared with 16 percent of normal-weight men.

Similarly, as mentioned, the biopsy rate was also lower among obese men, at 4.6 percent of obese men overall, versus 5.8 percent among normal-weight men.

Doctors no longer use the 4 ng/mL cutoff, or any specific PSA level, to define an abnormal or normal result. Instead, Lu-Yao pointed out, they base biopsy recommendations on a “risk calculator” method that considers not only a man’s PSA level, but also other factors - such as age, race and family history of the disease.

BMI is not included in that risk assessment. Future studies, Lu-Yao said, should investigate whether adding BMI to the calculation can help refine decisions on biopsy.

If it does, the big remaining question would be whether it ultimately affects obese men’s survival of prostate cancer.

SOURCE:  American Journal of Medicine, September 2010.