If you've ever reached for chocolate as a cure for the blues or a tired brain, you probably know that food affects mood and intellect.

But most foods comprise more than one nutrient, and we know very little about how specific nutrients affect the brain. However, a few hard facts stand out amidst the bewildering array of myths and old maid's tales.

"It's brain," I said; "Pure brain! What do you do to get like that, Jeeves? I believe you must eat a lot of fish or something. Do you eat a lot of fish, Jeeves?"

Fish may not be `brain food', but a protein rich diet, especially at breakfast, maintains alertness throughout the day. Tyrosine, an amino acid in protein, helps form dopamine, adrenaline and noradrenalin-neurotransmitters that raise energy levels and sharpen wits.

Ever wondered why missing out on rice adds a blue shade to your mood?

Carbohydrates such as rice, pasta and potatoes are comfort foods because they result in higher tryptophan levels in the blood. Tryptophan is the precursor of serotonin, a neurotransmitter that reduces pain, decreases appetite, and calms the mind. In large amounts, serotonin induces sleep.

Folic acid deficiency decreases brain serotonin levels and can cause depression. Green leafy vegetables, dried beans and peas, and many other types of vegetables and fruits are rich in folate.

Next to water, tea is the most popular drink in the world. In moderate amounts 2 or 3 cups a day, tea, coffee and cocoa are harmless larder anti-depressants that act by slowing the breakdown of neurotransmitters.

Choline is a little-known vitamin found in eggs and liver. It is a precursor of acetylcholine, a vital neurotransmitter. Low brain levels of acetylcholine are associated with Alzheimer's disease and poor memory. A diet rich in fresh fruit and vegetables and essential fatty acid-rich foods like fish and nuts is best for positive mental health. Too much sugar, chocolate, caffeine and alcohol can depress the mind after an initial burst of euphoria and mental energy.

Emotional factors affect our response to foods as much as foods affect our emotional responses. Chocolate, candy and alcohol may be pleasurable, but guilt, brought on by childhood and social conditioning, can determine our mood after consuming these `foods'.