We assessed whether serum concentrations of folate, vitamin B-12 and tHcy predicted depressive symptoms among US adults (aged 20–85 years) using data from NHANES (2005–06). Overall, mean PHQ scores were significantly higher among women compared to men. Second, in multivariate models adjusting for socio-demographic, lifestyle and dietary factors, elevated depressive symptoms were inversely associated with folate status but not tHcy or vitamin B-12, and significant associations were restricted to women. Among women, odds of elevated depressive symptoms in the upper tertile of folate status was almost one-third of those in the lowest tertile. tHcy and vitamin B-12 levels did not interact with folate status to affect its inverse association with depressive symptoms among women.
Several studies previously examined associations of folate status (or folate intake) with depressive symptoms (or diagnosis of depression). Similar to our main finding in the total population, case-control studies showed that psychiatric patients with major depression had lower serum and erythrocyte levels of folate compared to controls (23
) and large population-based cross-sectional studies corroborated this evidence for both serum and dietary folate (27
), with inconsistent effect modification by sex found in some. (27
) In two US studies, however, this inverse association between folate status and depressive symptoms was mostly found among women, which was also the case in our present study. (37
) This can either be due to sample size differences between men and women in our present study and previous ones or a real difference in the association and in serum folate’s ability to affect mental health to a greater extent among women compared to men.
A power analysis of our multiple logistic regression model was done, taking into account the following: (A) The average correlations between predictor variables (assuming R2=0.2), (B) The proportion with elevated depressive symptoms among men and women in the uppermost tertile of serum folate (6.33% among women and 10.67% among men) and (C) The observed odds ratio point estimates [Model 2, (0.37 among women and 0.85 among men)], (D) An alpha level of 0.05 and (E) an equal balance between referent group and index group (i.e. lowest vs. uppermost tertile). A power curve was obtained based on those parameters and for a power of 0.80, the sample size needed to detect the odds ratio observed among women was n=1,084, when the actual one was n=1,358 and thus was adequate. However, among men, the minimum sample size needed for a power of 0.80 to detect the odds ratio that was observed was n=16,196 whereas the actual sample size available was only n=1,030. When R2 between predictors was reduced to 0.10, the minimal sample size was still inadequate among men. Thus, in fact, the study was under-powered among men, given the small difference in proportion “with elevated depressive symptoms” between lowest and uppermost tertiles.
Moreover, at least two cross-sectional studies were unable to detect a significant association (70
). Among large population-based cohort studies, (26
) an inverse association between folate status (or folate intake) and depressive symptoms (or depression) was observed in most, (26
) with inconsistent effect modification by sex found in some. (43
) Null findings with respect of folate-depressive symptoms associations in some of the observational studies (30
) may be due to the selection of older age groups and differences in measurements of depressive symptoms, including the use of specific instruments (e.g. Geriatric Depression Scale instead of CES-D or PHQ). In particular, the proportion of somatic items relative to items addressing strictly depressed affect differs in various instruments. In this instrument (PHQ), five out of the ten items were somatic items (i.e. sleep, appetite, trouble concentrating, trouble moving/speaking and poor energy). Depressed affect was elicited using the five other items (i.e. loss of interest, feeling down, feeling bad about self, feeling better off dead, difficulty these problems have caused). In fact, when a sensitivity analysis was further conducted using binary outcomes of elevated somatic complaints sub-scale (1: score≥6; 0: score<6, cut-point corresponding to the 90th
percentile) and a similar binary outcome for the depressed affect sub-scale, only somatic complaints were found to be inversely related to folate tertiles after controlling for vitamin B-12, tHcy, socio-demographic, lifestyle and dietary factors as was done in Model 2 of in the total population [T3
: OR=0.56; 95% CI=0.36–0.88; p=0.02] and among women only [T3
: OR=0.52; 95% CI=0.28–0.96; p=0.04], without a significant effect modification by sex for that sub-scale.
Similar to our study, previous research did not find any association when considering tHcy (62
) and vitamin B-12 (40
) levels as exposures in relation to depressive symptoms. However, a number of observational studies found that depressed individuals had higher tHcy levels (21
) and/or lower vitamin B-12 levels. (25
), among which many were conducted in older adults. (21
) Thus, their study population differed markedly from ours which had a wider age range (20–85 years, mean in the mid-40s) and from those who found null associations between those two exposures and depressive symptoms. Our sensitivity analysis on those aged 50 years or older demonstrated that in fact there was a statistically significant and positive association between tHcy status and elevated depressive symptoms.
Our findings suggest that serum folate level may reflect brain tissue folate, which in turn may be playing a primary role in neuroprotection with the BH4 pathway as a possible mechanism (See introduction). In fact, levels of tHcy and vitamin B-12 did not seem to play a direct or interactive role with the levels of folate in protecting against elevated depressive symptoms.
Our study has several strengths. First, to our knowledge, it is the only large nationally representative observational study to assess the associations among serum folate, vitamin B-12, tHcy and elevated depressive symptoms in U.S. adults after mandatory folic acid fortification of foods was implemented in the U.S. Second, the study examined associations stratifying by sex and assessed interactions between exposures in the main association of interest. However, our study is limited by its cross-sectional design which precludes temporality ascertainment and reverse causality is a possibility whereby folate status is indicative poor dietary quality among depressed individuals compared to their non-depressed counterparts. Nevertheless, it was observed in at least one previous cohort study that serum folate was inversely related to incident depression (26
). Another limitation is the lack of control for several potential confounders such as participant’s family history of depression, social isolation or personality dimensions commonly associated with depression, in addition to the presence of chronic illness. Adjusting for those potential confounders would attenuate the observed association given their positive relationship with the outcome (elevated depressive symptoms), only if their association with serum folate was an inverse one. The use of depressive symptoms as outcome and not a clinical diagnosis of depression is also an important limitation to be considered when interpreting our findings, although the CES-D has been used for similar purposes in a number of previous studies. (37
) Finally, residual confounding caused by measurement error in dietary and other factors including in regression models cannot be ruled out.
To ascertain temporality of those main associations, future longitudinal studies should further examine whether trajectories in folate status are associated with trajectories in depressive symptoms and whether those associations differ between men and women and across various age groups. Future interventions and randomized controlled trials for improving mental health outcomes should take into account dietary and other factors that would increase levels of serum folate, and stratification by age and sex is important, and possibly race/ethnicity, to uncover possible differences in the effects of folate status-enhancing lifestyles (including folate dietary supplementation) on various socio-demographic groups.
In conclusion, our results suggest that depressive symptoms may be associated with low serum folate status, despite folate fortification efforts, specifically for women, but not by increasing vitamin B-12 or reducing tHcy serum level. Thus even with a lower prevalence of folate deficiency in the US, serum folate level may still be an important factor to consider for the prevention of elevated depressive symptoms.