Why Obese Men Post Lower PSA Levels

(HealthDay News) -- In recent years, doctors have learned that they need to adjust the results of blood tests to properly diagnose prostate cancer in obese men, but now researchers think they know why.

It turns out that larger men have more blood, which dilutes the levels of the protein called PSA -- a key indicator of prostate trouble.

The new study doesn't definitively prove why overweight and obese men with prostate cancer tend to score lower on the PSA (prostate specific antigen) test, but it does give doctors an idea about what may be going on, said study co-author Dr. Stephen Freedland, an assistant professor of urology and pathology at Duke University.

"PSA is not a bad test for obese men. We just need to know how to use it," he said. "If we use it correctly, it will be just as good as in normal-weight men."

Prostate cancer strikes one in six men, mostly those over the age of 65, according to the Prostate Cancer Foundation.

Older men often routinely undergo a PSA blood test that looks for an antigen made by the prostate that helps it function. The antigen leaks into the bloodstream at a steady rate, and the amount is higher in men with prostate cancer, Freedland explained.

Recent research has revealed that PSA levels are 20 percent to 25 percent lower in overweight and obese men than in men of normal weight. This can translate into diagnostic problems with bigger men, Freedland said.

"You may call him normal and not worry about him and wait another year or two or longer, and that gives time for the cancer to grow," he noted.

According to Freedland, it's not clear if there's any impact on PSA levels from being underweight.

In the new study, researchers tested a theory that the reason PSA levels are lower in larger men is because the antigen gets diluted in larger volumes of blood. "It's like taking a little bit of a drug and putting it in a cup of water versus a bowl of water," Freedland said.

The study authors looked at the records of about 14,000 men with prostate cancer who underwent removal of their prostates between 1988 and 2006. The findings are published in the Nov. 21 issue of the Journal of the American Medical Association.

The researchers found that men who were fatter had larger blood volumes, which the study authors said supports their theory that dilution caused their PSA levels to lower.

The same thing could hold true for blood markers of other cancers, Freedland said. "As we develop blood tests for other cancers, we should be keeping this in mind," he added.

However, Dr. Nelson Stone, a clinical professor of urology and radiation oncology at Mount Sinai School of Medicine, is skeptical of the results, because the research just focused on men with advanced prostate cancer. That and other factors may have skewed the results, he said.

"I don't think they've proved their point," he said. But he added, "We need to be a little bit more careful when we evaluate a patient who is larger, and we look at his PSA and compare to a patient who is thinner. The PSAs are not the same."

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For advice on losing weight, visit the U.S. Food and Drug Administration.