Researchers have discovered a protein in blood that can be used to diagnose chronic fatigue syndrome, a breakthrough that could help detect the ailment during physical checkups.

There are diagnostic criteria for chronic fatigue syndrome - a disorder involving extreme fatigue of unknown cause that continues for at least six months - that rely primarily on subjective symptoms, but there have been no objective markers such as blood tests.

The research team led by Hiroshi Kiyama, a professor of anatomy at Osaka City University, examined the intermediate lobes of the pituitary glands of rats in which they induced extreme fatigue by making them exercise for five consecutive days. They found that the lobes excreted extraordinarily high amounts of a protein called alpha-MSH and that alpha-MSH levels in the animals' blood also increased.

The neurotransmitter dopamine inhibits the secretion of alpha-MSH, but the rats' ability to produce dopamine declined as their fatigue grew.

The group also tested the levels of alpha-MSH in the blood of 57 people diagnosed with chronic fatigue syndrome and the blood of 30 healthy people. The average level among the 37 people who had been diagnosed with chronic fatigue syndrome less than five years before was about 50 percent higher than the healthy people.