HEALTH: On a diet? It's no wonder you're feeling miserable

Dieting to lose weight may lead to depression by altering brain chemistry, research showed yesterday. Women experienced clinical depressive symptoms when a protein component called tryptophan was removed from their diets, said the researchers. Tryptophan is an amino-acid precursor of serotonin, a chemical messenger in the brain which is linked with mood.

A standard 1,000-calorie carbohydrate restricted diet would lower blood plasma levels of tryptophan enough to alter serotonin function in healthy women, the researchers, led by Dr Katy Smith from the psychopharmacology research unit at Oxford University, wrote in The Lancet.

In the study, 15 women prone to episodes of depression were asked to drink a nutritionally balanced mixture, or one that contained no tryptophan. They then spent seven hours alone in the laboratory. Each volunteer drank both mixtures, separated a week apart, but did not know which was which.

After drinking the tryptophan-free mixture there was a 75 per cent reduction of tryptophan in the blood plasma, and 10 of the women suffered clinically significant depressive symptoms. No mood changes occurred after they drank the balanced mixture.