A blood test could be used to detect dangerous hidden belly fat, - even if it is buried so deep within the abdomen that a person outwardly appears lean. Scientists say new research indicates that a diagnostic test could be developed to help identify these individuals, who have a higher risk of heart disease and diabetes.

For most people, a simple tape measure can indicate if they are winning the battle of the bulge. But while doctors acknowledge that waist circumference generally correlates with fitness, some seek more sophisticated methods of assessing patients' health.

"Waist circumference correlates to some extent, but it's really a crude measure," explains Barbara Kahn at the Beth Israel Deaconess Medical Center in Boston, Massachusetts, US.

Kahn notes that people who look healthy sometimes have hidden fat surrounding their abdominal organs: "It's especially possible in certain groups, such as Asians." And she stresses that such intra-abdominal fat is closely linked to a host of cardiovascular illnesses.

Costly scans

It is possible, but not easy, to get a detailed picture of much abdominal fat you have. Computed tomography (CT), a body-imaging technique that uses X-ray technology, can provide a stomach scan revealing the quantity and distribution of fat around the organs.

Kahn believes that a blood test could serve as a suitable substitute for this cumbersome method of body fat imaging. "It's very costly to get people in for a CT scan, whereas you could possibly get a blood test done at a screening desk in a shopping centre," she says.

Her team recruited 196 volunteers to compare blood protein levels in people with various amounts of abdominal fat.

Sixty-six of the subjects in the trial were classified as lean, which according to World Health Organization standards means they had a body-mass index (BMI) of 25 or less. The remaining 130 subjects had a BMI above 30, meaning they were clinically obese.

The participants gave blood samples and also underwent CT scanning, which helped researchers calculate how much fat each subject had within their abdominal area and under the skin.

Kahn and her colleagues noticed that blood levels of a molecule called "retinol-binding" protein (RBP4) correlated with the amount of belly fat.

Lean test possible

Obese subjects whose fat was mostly distributed throughout the abdominal cavity had an average of 75 micrograms of RBP4 per millilitre of blood. By contrast, obese individuals with a preponderance of subcutaneous fat (below the skin) had significantly less, with 50 micrograms per millilitre. Lean subjects had only about 25 micrograms of RBP4 per millilitre of blood.

This study did not identify lean people with risky levels of hidden fat, but the researchers now believe they can develop a cheap and accurate test based on RBP4 levels to do this.

Previous research has linked RBP4 to a higher risk of pre-diabetes. The protein is known to bind to vitamin A, but exactly how it plays a role in obesity and diabetes remains unclear.