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Senior Citizen Health & Medicine

Men Who are Too Fat Run Risk of Undetected Prostate Cancer

Men with a BMI of 35+ had 11 to 21 percent lower PSA relative to normal-weight men

Nov. 20, 2007 – Men who are too fat may put themselves in danger of having prostate cancer that goes undetected by the standard test for prostate-specific antigen (PSA). Higher body mass index (BMI) is associated with higher plasma volume, which may be related to lower prostate-specific antigen (PSA) levels among obese men, according to a study in the November 21 issue of the Journal of the American Medical Association.

The predictive value of the PSA test depends on accurate readings of a protein, (P)rostate (S)pecific (A)ntigen continually pumped out by the prostate.

When the prostate is enlarged - due to cancer or other disorders - the concentration of PSA in the bloodstream can increase, signaling the possible presence of a tumor. Physicians thus commonly regard increased PSA values as a first marker to diagnose prostate cancer, to be followed by other diagnostic tests such as physical exams and ultrasound.

Complicating the picture further, the researchers note, that both physical exams and imaging studies are more difficult in obese men.

Although recent studies have shown that PSA concentrations can be lower than expected in obese men with prostate cancer, the current research was designed to determine which of two dueling hypotheses explained this, notes Alan Partin, M.D., chief of the Department of Urology at Johns Hopkins’ Brady Urological Institute.

One idea was based on the possibility that obese men make less PSA because they tend to have less testosterone, the sex hormone that prompts PSA production. The other attributed the phenomenon to the increased amount of blood that obese men produce to support their size, which has the effect of thinning out the concentration of PSA.

Lionel L. Baρez, M.D., Duke University Medical Center, and colleagues including Partin and Stephen Freedland, M.D., Partin’s former postdoctoral fellow who is now an assistant professor at Duke, investigated both ideas by assessing how much total PSA obese and normal-weight men have.

The study consisted of men who underwent radical prostatectomy for prostate cancer from 1988 to 2006 and who were included in the databases of the Shared Equal Access Regional Cancer Hospital (1,373 cases), Duke Prostate Center (1,974 cases), and Johns Hopkins Hospital (10,287 cases).

As expected, PSA concentrations were typically lower in the obese patients than in the normal-weight ones, although the total amount of PSA was about the same in both groups of patients.

The researchers found that higher BMI was significantly associated with greater plasma volume in all study populations. Men with a BMI of 35 or greater had 21 percent to 23 percent larger plasma volumes relative to normal-weight men. After adjusting for multiple clinicopathological variables, higher BMI was associated with lower pre-operative PSA concentrations in the groups.

Men with a BMI of 35 or greater had 11 percent to 21 percent lower PSA concentrations relative to normal-weight men.

“In 3 distinct prostate cancer cohorts, all treated by radical prostatectomy, hemodilution from increased plasma volume may be responsible for the observed decreased PSA concentration in men with higher BMI. This association needs to be confirmed prospectively in screened populations that include men without prostate cancer,” the authors conclude.

 “It’s clear to us that excess blood had diluted PSA concentrations in that group,” says Partin.

Freedland says a variety of new tests currently in development for cancer and other diseases rely on the concentrations of disease markers similar to PSA circulating in the blood. “For these other tests just starting down the development pipeline,” he says, “we need to think about the actual total amount of a biological marker rather than concentration.”

This study was supported by the Department of Veterans Affairs, the Duke University Department of Surgery and Division of Urology, Department of Defense Prostate Cancer Research Program, the American Urological Association Foundation/Astellas Rising Star in Urology Award, National Institutes of Health Specialized Programs of Research Excellence Grant P50 CA58236, the Georgia Cancer Coalition, National Institutes of Health R01CA100938, National Institutes of Health Specialized Programs of Research Excellence Grant P50 CA92131-01A1 and the American Cancer Society.

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