Vitamin B2 (riboflavin) is essential for oxidative pathways, monoamine synthesis, and the methylation cycle. B2 is needed to create the essential flavoprotein coenzymes for synthesis of L-methylfolate—the active form of folate—and for proper utilization of B6. Deficiency can occur after 4 months of inadequate intake.
Although generally B2 deficiency is rare, surveys in the United States have found that 10% to 27% of older adults (age ≥65) are deficient.4 Low intake of dairy products and meat and chronic, excessive alcohol intake are associated with deficiency. Marginal B2 levels are more prevalent in depressed patients, possibly because of B2’s role in the function of glutathione, an endogenous antioxidant.5 Top dietary sources of B2 are dairy products, meat and fish, eggs, mushrooms, almonds, leafy greens, and legumes.
Vitamin B6 refers to 3 distinct compounds: pyridoxine, pyridoxal, and pyridoxamine. B6 is essential to glycolysis, the methylation cycle, and recharging glutathione, an innate antioxidant in the brain. Higher levels of vitamin B6 are associated with a lower prevalence of depression in adolescents,6 and low dietary and plasma B6 increases the risk and severity of depression in geriatric patients7 and predicts depression in prospective trials.8 Deficiency is common (24% to 56%) among patients receiving hemodialysis.9 Women who take oral contraceptives are at increased risk of vitamin B6 deficiency.10 Top dietary sources are fish, beef, poultry, potatoes, legumes, and spinach.
Vitamin B9 (folate) is needed for proper one-carbon metabolism and thus requisite in synthesis of serotonin, norepinephrine, dopamine, and DNA and in phospholipid production. Low maternal folate status increases the risk of neural tube defects in newborns. Folate deficiency and insufficiency are common among patients with mood disorders and correlate with illness severity.11 In a study of 2,682 Finnish men, those in the lowest one-third of folate consumption had a 67% increased relative risk of depression.12 A meta-analysis of 11 studies of 15,315 persons found those who had low folate levels had a significant risk of depression.13 Patients without deficiency but with folate levels near the low end of the normal range also report low mood.14 Compared with controls, patients experiencing a first episode of psychosis have lower levels of folate, B12, and docosahexaenoic acid.15
Dietary folate must be converted to L-methylfolate for use in the brain. Patients with a methylenetetrahydrofolate reductase (MTHFR) C677T polymorphism produce a less active form of the enzyme. The TT genotype is associated with major depression and bipolar disorder.16 Clinical trials have shown that several forms of folate can enhance antidepressant treatment.17 Augmentation with L-methylfolate, which bypasses the MTHFR enzyme, can be an effective strategy for treating depression in these patients.18
Vitamin B12 (cobalamin). An essential cofactor in one-carbon metabolism, B12 is needed to produce monoamine neurotransmitters and maintain myelin. Deficiency is found in up to one-third of depressed patients11 and compromises antidepressant response,21 whereas higher vitamin B12 levels are associated with better treatment outcomes.22 B12 deficiency can cause depression, irritability, agitation, psychosis, and obsessive symptoms.23,24 Low B12 levels and elevated homocysteine increase the risk of cognitive decline and Alzheimer’s disease and are linked to a 5-fold increase in the rate of brain atrophy.26
B12 deficiencies may be seen in patients with gastrointestinal illness, older adults with achlorhydria, and vegans and vegetarians, in whom B12 intake can be low. Proton pump inhibitors such as omeprazole interfere with B12 absorption from food.
Psychiatric symptoms of B12 deficiency may present before hematologic findings.23 Folic acid supplementation may mask a B12 deficiency by delaying anemia but will not delay psychiatric symptoms. Ten percent of patients with an insufficiency (low normal levels of 200 to 400 pg/mL) have elevated homocysteine, which increases the risk of psychiatric disorders as well as comorbid illnesses such as cardiovascular disease. Top dietary sources include fish, mollusks (oysters, mussels, and clams), meat, and dairy products.
Vitamin C is vital for the synthesis of monoamines such as serotonin and norepinephrine. Vitamin C’s primary role in the brain is as an antioxidant. As a necessary cofactor, it keeps the copper and iron in metalloenzymes reduced, and also recycles vitamin E. Proper function of the methylation cycle depends on vitamin C, as does collagen synthesis and metabolism of xenobiotics by the liver. It is concentrated in cerebrospinal fluid.