Obesity may affect the accuracy of a screening test for prostate cancer, research suggests.
Prostate screening is not always accurate
The test looks for raised levels of a chemical called prostate specific antigen (PSA), which can be sign of cancer.
However, the test is far from infallible, and now University of Texas researchers have found obesity is associated with lower PSA levels.
Details of the study are published in the journal Cancer.
Every year 27,000 men in the UK are diagnosed with prostate cancer and 10,000 men die from it.
Obesity is thought to increase the risk of the disease.
Obese men with prostate cancer also tend to have a poorer prognosis than others.
This might be linked to the fact that the disease is often not picked up in obese men until it has reached an advanced stage.
However, some experts believe that elevated levels of hormones associated with obesity may make the cancer more difficult to treat.
The latest study measured PSA levels in 2,779 men without prostate cancer.
The researchers found that as body mass index (BMI) increased, so PSA levels fell.
Body Mass Index
Underweight: less than 20
Normal weight: 20-25
Overweight: 25 - 29.9
Obese: over 30
Morbidly obese: over 40
The finding was consistent regardless of age and race.
The researchers say their findings suggest that low levels of PSA among obese men may make it more difficult for the screening test to detect any problem.
They say that doctors may need to consider adjusting PSA values when screening obese men for prostate cancer.
Chris Hiley, of the Prostate Cancer Charity, said: "We know that the PSA level can be affected by factors such as the size of the prostate gland and exercise.
"However, this is the first study to show that obesity may mask an obese man's true PSA level.
"Contradictory results came out from the same research centre in 2004, which showed no relationship between obesity and PSA levels.
"More research is needed to look at the effect of obesity on PSA to ensure that all men receive a prompt diagnosis."
Dr Emma Knight, of the charity Cancer Research UK, said studies were underway to weigh up the pros and cons of the PSA test in different settings.
"It may be most reliable, for example, in men with a family history of prostate cancer.
"Meanwhile, it is important to remain aware of its limitations and to interpret PSA test results with caution.
"This study suggests the PSA test, as it stands at present, might fall short of accurate diagnosis in obese men." Dr Knight said high PSA levels were not always a sign of prostate cancer.
For example, they may simply reflect a non-cancerous enlargement of the prostate gland.
"Furthermore, the test does not distinguish between aggressive and more slow-growing tumours that are unlikely to cause problems.
"This can lead to unnecessary treatment for some men."
African Caribbean men are three times more likely to be diagnosed with prostate cancer than their white counterparts.