Researchers in the U.S. say that obese men have lower levels of a protein used to screen for prostate cancer risk, and doctors should take weight into account when determining a man’s risk.

Jay H. Fowke of Vanderbilt University Medical Center in Nashville, Tennessee, says this may explain why overweight men are more likely to be diagnosed when their disease is advanced.

The test for prostate specific antigen, or PSA is for a protein produced only by prostate cells which circulates in the blood.

When a prostate is growing the levels rise which could be an indication of cancer, or a usually harmless but annoying condition called benign prostatic hypertrophy.

A PSA test can tell a doctor whether to carry out further tests for cancer, but evaluating PSA is not an exact science, and there is no clearly safe level.

The researchers say obese men should possibly be more concerned about a lower PSA level than if they were slender.

The researchers tested blood from 150 black men and 149 white men and found that the more overweight the men were, the lower their PSA levels while race did not appear to be a factor.

The researchers say that whether equal PSA levels in an obese versus thin man convey the same biologic relevance is unclear but their findings suggest that a marginally elevated PSA level in an obese person, could be suspicious.

Prostate cancer is the most common cancer among men after lung cancer, and will affect 234,460 men in the United States alone this year, killing more than 27,000, according to the American Cancer Society.

The findings will be published in the Nov. 15 issue of Cancer.