In this issue, Papakostas and colleagues report the successful use of adjunctive l-methylfolate in treatment-resistant patients with major depressive disorder (1). The study employs a novel trial design, the sequenced parallel comparison design (2), and adds to the growing literature suggesting that the one-carbon cycle may moderate antidepressant treatment response.

The study involved two trials. The first compared l-methylfolate at 7.5 mg/day with placebo in 148 patients with nonpsychotic unipolar depression. All patients had a prior failed 8-week trial of a selective serotonin reuptake inhibitor (SSRI). l-Methylfolate at 7.5 mg/day was not more effective than placebo, but efficacy was suggested for a small group of patients in the trial who were treated with 15 mg/day. Based on that finding, a second trial was conducted in 75 patients using 15 mg/day or placebo. The difference in response rates between l-methylfolate at 15 mg/day and placebo (32.3% and 14.6%, respectively) was significant and meaningful. The number needed to treat, six, is quite respectable. Side effects were no more common with l-methylfolate than with placebo, which suggests that side effects were not likely to unblind the investigators or patients to treatment assignment.

The novel sequenced parallel comparison design, described in detail elsewhere (2), consists of two phases, each 4 weeks in length. In phase 1, patients are randomly assigned to drug or placebo, with more patients in the placebo arm. In the second phase, patients who did not respond to placebo in phase 1 are randomized to drug and placebo again. This latter group is pooled with patients from phase 1 for the analysis of outcome. The design is intended to enhance the power to detect differences and thus allow for smaller samples.

The association of depressive symptoms and folate deficiency has been known for five decades. Numerous studies have found low serum folate levels or low RBC folate concentrations in depressed patients (3, 4). Other studies suggested that low folate levels are associated with reduced response to antidepressants (3, 4), which in turn suggested that folic acid might be used to augment antidepressants. Coppen and Bailey (5) conducted a double-blind placebo-controlled study of adjunctive folic acid (500 μg/day) added to fluoxetine at the beginning of treatment in patients with major depressive disorder. They found that adjunctive folic acid was effective in women but not in men. Resler et al. (6) also observed greater improvement with folic acid in 27 patients with major depression who were being treated with fluoxetine (20 mg/day) and were randomly assigned to receive to folic acid (10 mg/day) or placebo. Alpert et al. (7) noted modest improvement in depressive symptoms after they administered folinic acid in an open-label study to 22 patients who had failed to respond to 4 weeks of SSRI treatment. In those three studies, patients with major depression were selected without regard to folate deficiency.

Trials of both monotherapy and adjunctive l-methylfolate have been reported. Four monotherapy trials (reviewed elsewhere [4]), suggested efficacy. Two were open trials and two were double-blind trials comparing l-methylfolate with antidepressants. The first adjunctive trial was a double-blind placebo-controlled trial in 24 patients with major depression who had deficient RBC folate levels (8). l-Methylfolate at 15 mg/day was added to ongoing antidepressant treatment. At 3 months and at 6 months, patients receiving adjunctive l-methylfolate exhibited greater improvement than those in the placebo group.

Folate is a member of the one-carbon cycle involving folate, B₁₂, and homocysteine. Abnormalities of each of these compounds have been associated with depression, although low levels of folate appear to be most common (3). Synthetic folic acid (in supplements) or dihydrofolate (in foods) are first converted to 5-methyltetrahydrofolate (l-methylfolate). Methionine synthetase uses l-methylfolate as a methyl donor to convert homocysteine into l-methionine. Vitamin B₁₂ facilitates this reaction. Methionine combines with ATP to form S-adenosylmethionine (SAMe). SAMe is widely distributed in the CNS, where it acts as a methyl donor. Through the synthesis of tetrahydrobioterin (BH4) and the hydroxylation of phenylalanine and tryptophan, SAMe facilitates the synthesis of the neurotransmitters—dopamine, norepinephrine, and serotonin—that are thought to mediate antidepressant effects.

The conversion of folates to l-methylfolate is accomplished by 5,10-methylenetetrahydrofolate reductase (MTHFR) (9). The gene for MTHFR has a genetic polymorphism that affects this conversion. Individuals who are homozygous for the T variant have about 30% of the enzyme activity of individuals with the wild-type (CC) variant (9). Heterozygous (CT) individuals have about 65% of the enzyme activity of CC individuals. In Caucasian North Americans, 8%–20% of the population has the TT genotype. (Thus, 40%–50% of this population would have the CT genotype.) The T allele frequency is higher in some ethnic groups (e.g., California Hispanics) and lower in others (e.g., African Americans). A recent meta-analysis of 10 studies found a modest but significant elevation of depression in individuals with the TT genotype (9). Administration of l-methylfolate essentially bypasses the enzymatic step that may be affected.

Folate deficiency has a variety of causes—inborn errors of metabolism, various drugs (certain anticonvulsants, antibiotics, oral contraceptives, some anticancer agents), malabsorption syndromes, chronic diseases, alcoholism, and dietary deficiency (3). In 1998, the U.S. Food and Drug Administration required that grain products be fortified with folic acid. Since that time, the prevalence of folate deficiency in the United States has been greatly reduced. As Alpert et al. argued (3), given the low prevalence of true folate deficiency, the use of folate and its various forms in depression is usually better thought of as folate supplementation than folate replacement.

Because depression can be associated with substantial anorexia and weight loss, the question arises as to whether low folate levels are a cause or an effect in depression. While the issue is not resolved, both may be involved. Depression leads to reduced appetite and dietary restriction, which lowers folate levels, which exacerbates the depression.

Folic acid is included in most multivitamin supplements; 400 μg is a typical amount. This is close to the dose used in the Coppen and Bailey study (5). l-Methylfolate is a “medical food.” It differs from dietary supplements, which are vitamins or minerals given to healthy individuals to maintain health. Medical foods are prescribed by a physician and are intended for the dietary management of a disease or condition for which nutritional requirements are established. Two large chain pharmacies report that a 30-day supply of l-methylfolate at 15 mg/day costs $90 to $94, and it is not likely to be covered by insurance.

In the Papakostas study (1), side effects were no more common with l-methylfolate than with placebo. The study is small, however, and limited for assessment of safety. One concern is that folate administration may mask B₁₂ deficiency (3). A possible association with an increased risk of cancer has been suggested, but this remains controversial (4). Increased mortality was observed in the Iowa Women’s Health Study in women taking multivitamins, vitamin B₆, folic acid, iron, magnesium, or copper (10). The authors examined mortality rates in 38,772 older women who were taking or not taking various vitamin and mineral supplements between 1986 and 2004. During that period, supplement use increased from 62.7% to 85.1%. Folic acid supplementation was associated with an increase of 5.9% in mortality risk. If supplementation is used to treat a disorder for which efficacy is established, the benefit will likely outweigh the harm, but the data suggest that these compounds are not harmless. Although the FDA asks the manufacturers of dietary supplements and medical foods to attest to their safety, these agents are not systematically studied in the manner that pharmaceutical agents are.

In summary, the Papakostas et al. study suggests that l-methylfolate is a useful treatment for depression that has proved to be resistant to a course of SSRI treatment. Previous studies of folic acid, folinic acid, and l-methylfolate support this contention. l-Methylfolate was well tolerated and may be preferred by patients for that reason. It may be particularly helpful in patients with the TT genetic variant. The efficacy of l-methylfolate in resistant depression has not been compared with that of other adjunctive agents, nor has long-term use of the agent been reported in major depression. The potential value of long-term administration of l-methylfolate in individuals with recurrent depression and the genetic enzyme deficiency is particularly intriguing.