Raised levels of a protein in the blood may provide an early warning sign of the development of type 2 diabetes, research suggests.
Type 2 diabetes tends to be diagnosed later in life
Scientists believe testing for levels of the protein - RBP4 - may help identify those at risk of diabetes before symptoms become apparent.
Drugs to cut RBP4 levels may also help reduce the risk of full-blown disease.
The study, led by Beth Israel Deaconess Medical Center, appears in the New England Journal of Medicine.
Raised RBP4 levels were found in people with a condition called insulin resistance, which develops when the body's tissues lose the ability to respond to the hormone insulin.
Because insulin is necessary to enable the body to take up sugar from blood and convert it into energy, this impairment results in a build-up of glucose in the bloodstream.
The condition can lead not only to type 2 diabetes, but is also a major risk factor for cardiovascular disease.
However, it can be difficult to diagnose.
Last year, the same team of researchers discovered that RBP4, which is secreted from fat cells, could cause insulin resistance in mice.
Now they have shown that levels of the protein provide a reliable measure of whether or not a person has developed, or is at risk of the condition.
The finding held good whether no matter who the researchers tested, including people with normal body weight and normal blood glucose, but a strong family history of diabetes.
Next, the researchers showed that people who improved their insulin sensitivity by taking exercise also lowered their levels of RBP4.
Similarly, RBP4 levels did not go down among people whose insulin sensitivity was unaffected by exercise.
Researcher Professor Barbara Kahn said: "Collectively, these findings tell us that RBP4 is a useful marker for therapeutic improvement and that this protein could play a causal role in insulin resistance in humans, just as our lab previously showed in mice.
"Being able to determine diabetes risk well before the onset of symptoms could provide an important opportunity for patients to take preventive measures.
"For those who are overweight or sedentary, this could mean making changes to their diet and fitness routines.
"For those who are lean and fit, but have a family history of type 2 diabetes, this could mean taking anti-diabetic medication.
"Either way, these findings could help clinicians to better manage this growing epidemic."
Dr Iain Frame, research manager at the charity Diabetes UK, said: "The causes of type 2 diabetes are very complex and still not fully understood.
"This research could offer a new line of enquiry which could help to understand the condition and help to produce new ways of battling its effects."