A recent major theory was that a meal high in carbohydrate increased the rate that tryptophan enters the brain, leading to an increase in the level of the neurotransmitter serotonin that modulates mood. Although such a mechanism may be important under laboratory conditions it is unlikely to be of significance following the eating of any typical meal. As little as 2-4% of the calories of a meal as protein will prevent an increased availability of tryptophan. Arguably the food with the greatest impact on mood is chocolate. Those who crave chocolate tend to do so when they feel emotionally low. There have been a series of suggestions that chocolate's mood elevating properties reflect 'drug-like' constituents including anandamines, caffeine, phenylethylamine and magnesium. However, the levels of these substances are so low as to preclude such influences. As all palatable foods stimulate endorphin release in the brain this is the most likely mechanism to account for the elevation of mood. A deficiency of many vitamins is associated with psychological symptoms. In some elderly patients folate deficiency is associated with depression. In four double-blind studies an improvement in thiamine status was associated with improved mood. Iron deficiency anaemia is common, particularly in women, and is associated with apathy, depression and rapid fatigue when exercising.