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British doctors reported on Thursday that they had discovered why diets don't work - it's all in the head.

Dieting upsets the level of chemicals in the brain, which in turn gives people an urge to overeat, E.M. Clifford and colleagues at Littlemore Hospital in Oxford found.They cited studies on rats showing that those lacking the ability to use the neurotransmitter serotonin become overweight. Neurotransmitters are chemicals that help carry electrical signals along a nerve.

The doctors tested 12 women age 20 to 39 and found that those on a restricted diet of 1,000 calories a day had lower levels of the essential amino acid tryptophan, which the body uses to make serotonin.

"Thus, during dieting, subjects are likely to experience urges to overeat which will compromise efforts at continued food restriction," they wrote in the science journal Nature.

In other words, dieting can give people the urge to binge.

"Humans have evolved powerful adaptive mechanisms for maintaining food intake. Trying to overcome these by voluntary food restriction is not only difficult but may be counterproductive in some individuals," they added.

But they said doctors could deliberately manipulate levels of these chemicals for those found to be clinically obese.