Deficiency of the following Molecules may Cause Depression
Inositol (a B-vitamin)
CSF levels of Inositol have been reported to be lower than normal in depressed subjects. The authors administered 12 g/day of Inositol to depressed patients for 4 weeks in a double-blind, controlled trial. The overall improvement in scores on the Hamilton Depression Rating Scale was significantly greater for Inositol than for placebo at week 4. Although Inositol had a significant antidepressant effect in this study, replication is crucial.  Am J Psychiatry 1995; 152:792-794. - Read more
Omega-3 Fatty Acids
A relative deficiency of essential fatty acids in cellular membranes substantially impairs cell membrane function. Since the basic function of the cell membrane is to serve as a selective barrier that regulates the passage of molecules into and out of the cell, a disturbance of structure or function disrupts the cell's ability to control its internal environment. Because the brain is the richest source of fatty acids in the human body, and because proper nerve cell function is critically dependent on proper membrane fluidity, alterations in membrane fluidity impact behavior, mood, and mental function. Flaxseed oil and Fish Oil (DHA) are very rich in Omega-3 Fatty Acids.
Results of another study conducted by Dutch researchers provide additional evidence for a link between low cholesterol levels and an increased risk of depression in men. Investigators measured serum cholesterol levels in some 30,000 men, as part of a large screening program. They compared the presence of depressive symptoms, anger, hostility, and impulsivity in these men, to men with cholesterol levels in the normal range. They found that men with chronically low cholesterol levels showed a consistently higher risk of having depressive symptoms.
Cholesterol may affect the metabolism of serotonin, a substance known to be involved in the regulation of mood as the researchers have previously shown that serotonin levels are also reduced in men with low levels of cholesterol.  Psychosomatic Medicine 2000;62.
Vitamin B6   (Pyridoxine)
Vitamin B6 deficiency results in depressed immune function-both antibody-related and cell-mediated immunity are suppressed. This suppression is apparent as the number of white blood cells plummets, there is a tremendous reduction in quantity and quality of antibodies produced, and there is decreased thymic hormone activity. Vitamin B6 deficiency may result from low dietary intake of vitamin B6, excess protein intake, consumption of yellow food dyes (hydralazine), alcohol consumption, or use of oral contraceptives.
Vitamin B9   (Folic Acid)
In a preliminary study of depressed patients conducted by the Human Nutrition Research Center on Aging at Tufts University, Boston researchers found that high levels of homocysteine can potentially damage the brain by two paths. First, by increasing the risk for cardiovascular disease, which can impair blood function to the brain. Second, homocysteine is converted to an amino acid that stimulates brain cell receptors at normal levels, but can cause the cells to self-destruct at excess levels. Blood analyses of twenty-seven elderly patients showed a link between high levels of homocysteine and vascular disease. It was also found that those patients with no vascular disease but who scored lowest on cognitive tests had high homocysteine levels.
Excellent sources of folic acid include brusseles sprouts, cauliflower, broccoli, oranges, orange juice, bananas, liver, eggs, whole meal bread and fortified cereal. Behavioral effects of Folic acid deficiency: forgetfulness, insomnia, apathy, irritability, depression, psychosis, delirium, and dementia.
Vitamin B12   (Cyanocobalamin)
Many of the symptoms of depression that are found in the elderly, such as confusion, memory loss, and apathy, are actually due to a deficiency in vitamin B12. A deficiency of B vitamins, most notably B12, can cause Alzheimer-type symptoms. For many people over age sixty, a lack of B12 may contribute to certain neurologic symptoms such as mood changes, weakness, imbalance, disorientation, and loss of memory. Physicians at the University Hospital of Maastricht in the Netherlands discovered that otherwise healthy people with lower blood levels of B12 did not perform as well on mental tests as people with higher levels. Behavioral effects of B12 deficiency: psychotic states, depression, irritability, confusion, memory loss, hallucinations, delusions, and paranoia.