Researchers at the University of Texas Health Science Center in San Antonio studied 2,779 men without prostate cancer between 2001-04. In the study released online Monday in the journal Cancer, they reported finding that the more obese the men were, the lower their levels of prostate-specific antigen or PSA. A PSA of 4.0 or lower usually means no cancer.
Previous studies have shown that prostate cancer is more aggressive in obese men than in men of average weight. The researchers wanted to see if the cancer's detection was somehow being delayed in obese men.
The Texas study found that the most morbidly obese men had about 30 percent lower PSA levels than men of normal weight.
"That tells us it's likely or it's possible that prostate cancer detection may be delayed in overweight or obese men," said Jacques Baillargeon, associate professor of epidemiology at the health science center.
The research may encourage many doctors to take a closer look at the tests of obese men.
"For sure, I will be more vigilant in my patients who are obese in evaluating their PSA," said Dr. Nelson Stone of Mount Sinai School of Medicine, who was not involved in the study. "We may be losing some of the sensitivity of the test in the obese patient in our ability to detect prostate cancer. We may have to set our sights lower."
The antigen used in the screening test is made by normal prostate cells and is measured in blood. The higher the antigen level, the more likely the chance of prostate cancer, as the cells multiply uncontrollably, according to the American Cancer Society.
But having high PSA levels is not a definitive diagnosis of cancer, which is why the Atlanta-based society recommends men with high PSA levels have a biopsy.
The latest study builds on previous research released in May in the New England Journal of Medicine that found that men with a "normal" PSA actually had cancer 15 percent of the time and that two-thirds of those men with cancer had aggressive cases.
The Texas study did not explain why obese men have lower PSA levels. But doctors believe obese men produce more estrogen, which drives down testosterone levels and could affect cells that produce the antigen used in the test.