The current systematic review and meta-analysis suggests supplementation with B vitamins does not substantially improve overall mood, but may offer a small benefit for reducing stress. These data also indicate that B vitamin supplementation is likely an effective way to increase circulating levels of B vitamins. However, given the high levels of heterogeneity present, the wide range of doses and types of B vitamins, and the presence of other nutrients in some studies, it is difficult to discern a true effect in the present study.
As depicted in Figure 3, the study under review included randomized controlled trials that included several B vitamins, along with many other ingredients. It is possible that the other ingredients either competed with the B vitamins or masked any effect due to opposite and compensatory effects. It is also possible that the other ingredients were responsible for the positive effects observed on stress. Given the wide range of interventions included in the studies, it is difficult to determine the effects attributed to B vitamins compared to the effects of other ingredients, as well as which B vitamins may have been more effective than others. The dosages were also incredibly varied, from two-fold to 300-fold more than the daily recommended intake.
The one positive finding that was observed in this study was a reduction in stress. However, the reduction in stress observed in the present study may also not be linked to B vitamins per se. Three of the studies (Armborst et al., Carrol et al., Kennedy et al.), that heavily weighted the results toward a significant reduction in stress all utilized ingredients other than B vitamins in their interventions. For example, one study utilized a multi-ingredient intervention that included tyrosine and magnesium, which have also been shown to reduce stress. Another one of these studies also included magnesium. So, even the reduction in stress may have been due to confounding by ingredients other than B vitamins. Another consideration is whether or not B vitamins would have any effect in individuals who are not deficient in B vitamins.
The study also used scales to provide a quantification of mental states. This method has shortcomings and does not provide highly quantifiable or accurate measures of mental states. That said, a true gold standard measure of measuring mood in a healthy population or in people at risk for vitamin deficiency does not exist, and this was reflected in the heterogeneity of measures used in the studies included in this review.
The study under review has several strengths. The study followed PRISMA guidelines and included both healthy and at risk populations. Mood was also measured using three different facets of mood (i.e. depressive symptoms, stress, and anxiety). The study also had some key limitations. First, there was a high level of heterogeneity across studies, which is reflective of the varied dosages and interventions used in the individual studies. Second, B vitamins were not isolated variables and were included as components of a larger list of ingredients/interventions used in each study. Third, there is no gold standard measure for mood measurements, making it difficult to reliably and accurately assess the primary outcome of the study. Finally, the study was not preregistered.
B vitamins play known roles in neurotransmitter metabolism, and there is rationale behind their use for mood disorders. The present study found that supplementation with B vitamins did not substantially alter mood, but may have a small benefit for stress. However, there are substantial limitations to the study that make clean interpretations difficult. As such, the study did not provide robust evidence as to whether B vitamins do, or do not, have an effect in humans on mood disorders.