The last poster came very close to a proper treatment of depression.
Scientists have known for a long time that there is an association between Depression and hypoglycemia. This is because hypoglycemia is a Sugar handling disease that interferes with the production of biological energy called ATP . This energy is essential in the production of serotonin, the feel good neurotransmitters that makes u feel happy and content when we should.
Without that energy the body cannot manufacture serotonin and we feel depressed. Thus when the body cannot convert tryptophan into serotonin for lack of energy it causes the mind to experience the symptoms of depression.
For a full explanation of the nutritional aspects of Depression see: