1 INTRODUCTION

Depression is recognized as one of the most common mental disorders that has substantial impacts on public health (Bender et al., 2017; Musazadeh et al., 2023). Based on the report of the World Health Organization, over 264 million people suffer from depressive disorders worldwide (James et al., 2018). It is related to significant complications like changes in mood, appetite, and weight, loss of interest, lack of energy, anhedonia, and feelings of worthlessness. Untreated depression will result in increase in morbidity and mortality (Altaf et al., 2021; Smith et al., 2021). Although the disease burden and mortality associated with depressive disorders are substantial, current medications and psychotherapies are not beneficial to almost 60% of patients with depressive disorders (Bender et al., 2017; Strawbridge et al., 2017). Additionally, antidepressants could have adverse effects and, as a result, lead to refusal to continuing the treatment process (Davies & Read, 2019; Lincoln et al., 2016). Thus, it would appear that current treatments for depression might not be completely effective for some individuals, and it becomes imperative to provide complementary treatments for it.

It has been shown that there is an association between dietary intake and depression (Yu et al., 2023). Folate or vitamin B9 is one of the water-soluble vitamins that may play a role in depression (Lyon et al., 2020). Folate plays a fundamental role in maintaining one-carbon metabolism for cell division and nucleic acid synthesis, regulation of gene expression, interconversion of amino acids and neurotransmitter synthesis, and neurological development during pregnancy and infancy (Gianfredi et al., 2022; Himayda et al., 2018; Hoepner et al., 2021; Jin et al., 2022; Xie et al., 2019; Yang et al., 2021). Growing research demonstrated that depression might be linked to folate deficiency. Particularly, studies indicate a link between folate deficiency and an increased risk of depression, more severe depressive symptoms, more prolonged episodes of depression, increased risk of depressive symptom relapse, and poor response to antidepressants (Bender et al., 2017; Liwinski & Lang, 2023; Martínez-Cengotitabengoa & González-Pinto, 2017). However, some studies also showed that low-serum folate levels have been positively associated with depression (Huang et al., 2018; Kaner et al., 2015; Khalili et al., 2022; Lin et al., 2020; Seppälä et al., 2013), whereas others have found no difference (Gargari et al., 2012; Gilbody et al., 2007; Kendrick et al., 2008). As a result, a substantial body of evidence suggests the introduction of supplemental folate in preventing and treating depression at the population and individual levels (Himayda et al., 2018). Figure 1 shows how folate could be related to depression.

The aim of the current meta-analysis of meta-analyses was to determine the effectiveness of folate supplementation or association of higher serum levels of folate on/with depressive disorders to provide insight into the subject and resolve controversies.

2 METHODS

This study was conducted according to the guiding principle of the Preferred Reporting Items for Systematic Reviews and Meta-analysis (PRISMA) (Moher et al., 2015). The protocol of the study was registered on PROSPERO (Code: CRD42023403671).

3 RESULTS

3.1 Study selection

Overall 477 studies were retrieved from searching databases among which 68 studies were duplicates. After screening remaining 409 articles by titles and abstracts, 13 articles went under careful evaluation by full-text version among which 8 articles (Al Maruf et al., 2022; Altaf et al., 2021; Gilbody et al., 2007; Jin et al., 2022; Khalili et al., 2022; Petridou et al., 2016; Roberts et al., 2018; Taylor et al., 2004) with 11 datasets fully met the inclusion criteria for quantitative synthesis. Considering the qualitative synthesis, 11 studies were included in total. The process of study selection is demonstrated in Figure 2.

3.3 The effect of folate supplementation on depression based on interventional studies (SMD analyses)

Folate supplementation significantly reduced depression symptoms based on SMD analyses (SMD: −0.42; 95% CI: −0.57, −0.27, p < .001), based on the pooled analysis of three meta-analyses. There was no trace of significant between-study heterogeneity (I² = 0.0%, p-heterogeneity = 0.554; Figure 3).

Roberts et al. (2018) performed a meta-analysis regarding adjunctive folate supplementation in depressive illness, with seven RCTs of 904 participants. Four trials reported investigation of folic acid compared to placebo, two with low dosage (<5 mg/day) and two with high dosage (≥5 mg/day). Three trials reported investigation of methyl folate compared to placebo, two with optimal dose (15 mg/day) and one with a suboptimal dose (<15 mg/day). The overall effect size indicated that depressive symptoms significantly decreased following folic acid supplementation. However, it was of a low quality and considerable heterogeneity. Interestingly, subgroup analysis revealed that there were not any significant changes in depressive symptoms after folate or methyl folate supplementation. Finally, Roberts et al. (2018) recommend not offering either folate or methyl folate as a monotherapy in patients with major depressive disorders. Abdullah Al Maruf et al. (2022) performed a meta-analysis that included clinical trials evaluating L-methylfolate supplementation in depressive disorders. There were three studies with four effect sizes in this study. Except for one study, others used 15 mg adjunctive L-methylfolate. Results revealed that L-methylfolate supplementation might have modest effect in adults with major depressive disorder (MDD) under treatment with antidepressants. Altaf et al. (2021) evaluated folate as an adjunct therapy to SSRI/SNRI for major depressive disorder with 584 participants. This meta-analysis, which included five studies with six effect sizes, reported that vitamin B9 significantly improved depression scores evaluated by HAM-D or BDI-II. Altaf et al. (2021) reported that participants who were treated with SSRI/SNRI along with folate responded to treatment 36% more than those who were on monotherapy. Furthermore, patients who received folate along with routine medication had a 39% increase in achieving remission in comparison with patients who were on SSRI/SNRI alone (Altaf et al., 2021).

3.4 The effect of folate supplementation on depression based on interventional studies (WMD analyses)

Folate supplementation had a significant impact on relieving depressive symptoms based on WMD analysis (WMD: −3.20; 95% CI: −4.00, −2.41, p < .001) of three studies with four effect sizes. No considerable between-study heterogeneity was found (I² = 14.8%, p-heterogeneity = 0.318; Figure 4). Subgroup analysis revealed that folate supplementation significantly decreased depression symptoms according to both HAM-D and BDI scales, with a more robust effect in meta-analyses with more than 10 studies (See Table 3).

TABLE 3. Results of subgroup analyses based on duration of intervention and assessment tool of depression for the effect folate supplementation on depression.

	Effect size n	ES (95% CI)	p-within	I2 (%)	P-heterogeneity
Effect of folate supplementation on depression (WMD)
Overall	3	−3.20 (−4.00, −2.41)	≤.001	14.8	0.318
Duration
>10	2	−3.54 (−4.78, −2.31)	≤.001	0.0	0.359
≤10	2	−2.95 (−4.34, −1.57)	≤.001	56.4	0.130
Assessment tool
HAM-D	3	−2.96 (−3.90, −2.03)	≤.001	16.7	0.301
BDI	1	−3.92 (−5.40, −2.44)	≤.001	–	–

Khalili et al. (2022) performed a meta-analysis on the effects of folic acid supplementation on depression in adults. This meta-analysis, with 6 RCTs and a sample size of 308 patients, showed that folate supplementation (0.5–10 mg/day) exerted beneficial impacts on depression test scores, in 6–24 weeks. Taylor et al. (2004) which included only two studies, reported that Hamilton Depression Rating Scale scores were significantly reduced following folate supplementation. One of the included studies used 500 μg/day of folic acid with 20 mg fluoxetine, and the other used 15 mg/day of methyltetrahydrofolate supplementation in addition to the standard psychotropic treatments.

3.5 The relationship between low levels of folate and depression based on observational studies

In three meta-analyses of observational studies with four effect sizes, the pooled analysis reported that low levels of folate increase odds of depression by 35% (OR: 1.35; 95% CI: 1.27, 1.42, p < .001). There was no significant between-study heterogeneity (I² = 8.7%, p-heterogeneity = 0.350) (Figure 5).

Jin et al. (2022) reported that supplementation with folate and folic acid levels are both associated with a reduction in risk of depression symptoms. In another study, Gilbody et al. (2007) included 11 relevant observational studies with 15,315 patients: 3 case–control studies, 7 population surveys, and 1 cohort study. It was reported that there was a significant association between low folate levels and increase in risk of depression. In addition, a meta-analysis by Petridou et al. (2016) studied the association of folate and vitamin B12 serum levels with depression in the aged population. They reported that “low folate and B12 serum levels are associated with depression in the older individuals.” This meta-analysis comprised 11 observational studies with 12 effect sizes (9195 participants).

4 DISCUSSION

Although some effective strategies for treating depression have been expanded, remission was not observed in all patients; hence, new treatment strategies focus on adjuvant therapies (Schefft et al., 2017). Previous studies showed increased folate deficiency in patients with depression, suggesting that folate supplements therapy can be a promising treatment strategy (Taylor et al., 2004; Young & Ghadirian, 1989). In this scenario, several meta-analyses of observational and randomized controlled trial (RCT) studies were conducted to assess the relationship between serum folate levels with depression and whether folate supplements can relieve patients' symptoms; however, conflicting results were obtained (Gilbody et al., 2007; Jin et al., 2022; Khalili et al., 2022; Roberts et al., 2018). The present study is an umbrella review of eight meta-analyses comprising five meta-analyses of RCTs and three meta-analyses of observational studies. This study evaluated how folate supplements as adjuvant therapy can improve depression symptoms and how folate serum levels correlate negatively with depression.

Depression is evaluated by many scales including the Montgomery–Åsberg Depression Rating Scale (MADRS), Profile of Mood States (POMS), Geriatric Depression Scale (GDS), Edinburgh Postnatal Depression Scale (EPDS), Depression Anxiety Stress Scales (DASS), Beck Depression Inventory (BDI), Hamilton Depression Rating Scale (HAM-D), Leiden Index of Depression (LEID), Quick Inventory of Depressive Symptomatology (QIDS), and Hospital Anxiety and Depression Scale (HADS) (Lee et al., 2010). Each scale is suitable for different functions like disease severity, treatment response, and differentiated diagnosis; for example, BDI is considered the best criterion for moderate depressed patients' severity assessment, while HAM-D is a gold standard tool for depression evaluation (Carrozzino et al., 2020; Richter et al., 1998).

In the present study, we showed folate supplements as adjuvant therapy can significantly reduce depression symptoms based on the results of both WMD and SMD. Moreover, no significant heterogeneity was observed between meta-analyses. The results of subgroup analysis indicated folate supplements could enhance depression disorder based on both HAM-D and BDI criteria; however, their effect was more substantial regarding the BDI criterion. Although we should consider only one effect size assessed BDI, hence the results should be interpreted cautiously. In addition, the results of subgroup analysis regarding treatment duration revealed that both treatment durations with folate supplementation over and below 10 weeks were significantly associated with depression alleviation; however, treatment duration over 10 weeks showed more robust effects.

Although the pooled effect of previous meta-analyses was significant, there were some controversies among them. Variations in the results can be attributed to different treatment doses and duration, variations in depression scales, different types of analysis, different quality of meta-analyses, and different sample sizes. For example, in a meta-analysis of six RCTs, Khalili, Asbaghi, Aghakhani, Clark, & Haghighat (2023) showed that folate supplementation can be promising in depressed patients. Like our study, they revealed that folate supplements are more effective in reducing symptoms based on the BDI than the HAM-D criterion. They also pointed out that heterogeneity of the analysis was less in BDI compared to HAM-D, showing more robust results in this regard. Contrary to our study, they showed no significant relationship between duration and dose of treatment with depression symptoms in the HAM-D criterion. In another meta-analysis by Altaf et al. (2021) on five studies, folate supplements could significantly improve depression symptoms based on HAM-D and BDI criteria. Although treatment duration for 4, 6, and 8 weeks significantly affected patients' symptoms, 6-week treatment had the best response. Another meta-analysis by Roberts et al. (2018) on seven RCTs showed only low dose of folic acid (<5 mg/day) and methyl folate optimal dose (15 mg/day) had significant effects on alleviating depression symptoms. On the other hand, high dose of folic acid (>5 mg/day) and methyl folate suboptimal dose (<15 mg/day) showed non-significant results. They reported that their results had low epidemiological strength regarding GRADE (Grading of Recommendations Assessment, Development, and Evaluation) criteria.

Previous evidence from the 1960s suggested that depressed patients have lower-serum folate levels (Young & Ghadirian, 1989). This fact is not surprising, as depressed patients have low appetites, which results in lower folate absorption. On the other hand, lower folate levels exacerbate depression, and this defective cycle goes on (Young, 2007). Our study demonstrated that low-serum folate levels could aggravate depression symptoms by up to 35%. The results were accompanied by low heterogeneity. In another meta-analysis by Bender et al. (2017) depressed patients had lower-serum folate levels and dietary intake than healthy individuals; however, their finding was accompanied by high heterogeneity. In another meta-analysis by Petridou et al. (2016), lower folate serum level was significantly associated with depression in females, but a counter-direction was found in men. The effect of low-serum folate on other psychological diseases has also been observed. In a meta-analysis study by Hsieh et al. (2019) 481 bipolar patients were compared with 760 controls and had significantly lower-serum folate levels. Cao et al. (2016) reported significant serum folate decrease in schizophrenia in a meta-analysis of 1463 patients and 1276 controls. The results of these studies suggest folate is a crucial factor in brain function, and its deprivation can cause significant health problems.

Among the included observational meta-analyses, one study did not assess the quality of primary studies (Gilbody et al., 2007). The other two meta-analyses used the Newcastle Ottawa checklist, and most of their primary studies were considered highly qualified (Jin et al., 2022; Petridou et al., 2016). Five meta-analyses on RCTs used the Cochrane checklist and rated most of their primary studies as high quality; however, one study used The Grading of Recommendations Assessment, Development, and Evaluation (GRADE) criteria and reported their results as very low epidemiological strength. We assessed the quality of included meta-analyses in the present study using the AMSTAR2 checklist. This checklist includes 16 questions covering different aspects of meta-analyses. All the included studies had critically low qualities, indicating that our meta-umbrella review results may not be conclusive and should be interpreted cautiously; hence, more studies are needed to make our results conclusive. The most prevalent flaws in the included meta-analyses were as follows: no providing the list of excluded studies and exclusion justification, no assessment regarding the effect of risk of bias of included primary studies on the results, and no identification of the funding source of included primary studies.

Depression is defined as low mood. The present guidelines suggest MDD treatment with monotherapy antidepressants like mirtazapine, bupropion, selective serotonin reuptake inhibitors (SSRIs), and serotonin–norepinephrine reuptake inhibitors (SNRIs) (Clevenger et al., 2018; Mathys & Mitchell, 2011). The mechanism of action of these agents is increasing monoamine transmitters by preventing their reuptake in synapses. Traditional antidepressants have been the foremost choice of depression treatment for years; however, recent studies have shown inadequate response treatment, late response treatment, no response treatment, low efficacy, and significant side effects (Altaf et al., 2021; Blier, 2001; Ginsberg et al., 2011; Jain et al., 2020; Miller, 2008). A novel strategy for increasing the efficacy of depression treatment is the administration of micronutrients like folate (Altaf et al., 2021).

Folate is considered a crucial factor for proper brain and other organ functions (McGarel et al., 2015). Previous studies have shown that folate deficiency may increase depression risk and lower response to antidepressant treatment. There is accumulating evidence that depressed patients who have low-serum folate levels are more at risk of relapsing and cognitive function worsening (Gilbody et al., 2007; Martínez-Cengotitabengoa & González-Pinto, 2017). Supplements with folate can effectively prevent dementia and depression (Mischoulon & Raab, 2007). Animal studies have also shown that folate supplements alone or in combination with other nutrients like omega-3 fatty acids or zinc induce antidepressant effects (Dou et al., 2018; Gao et al., 2017; Réus et al., 2018).

The exact mechanism of how folate can enhance depression is yet to be understood, but one postulated mechanism is the monoamine hypothesis. Folic acid plays an essential role in synthesizing monoamines like norepinephrine, serotonin, and epinephrine (Rabjohn, 2014). Folic acid cannot be synthesized by the body and is supplied by foods. When entering the body, folic acid converts to its active metabolite, l-methyl folate. This process is done by a key enzyme called methylene tetrahydrofolate reductase (MTHFR). L-methyl folate crosses the blood–brain barrier and induces tetrahydrobiopterin (BH4). BH4 is an essential substrate for activating tyrosine hydroxylase and tryptophan hydroxylase, ultimately causing increased production of neurotransmitters like dopamine and 5-hydroxytryptophan (5-HTP) (Jain et al., 2020; Rabjohn, 2014). Previous studies illustrated 5-HT, dopamine, and norepinephrine's crucial role in brain function, cognitive performance, motivation, self-controlling, and stress reactions (Gu et al., 2018). Moreover, these neurotransmitters are decreased in the hypothalamus of depressed patients (Gu et al., 2019). The fact that folate supplements can increase levels of these neurotransmitters has been confirmed by animal studies (Zhou et al., 2020). Another postulated mechanism for the antidepressant effects of folate is its interaction with IL-6. Previous studies showed that an increased serum level of IL-6 is associated with depression (Dowlati et al., 2010; Liu et al., 2012). Moreover, higher levels of IL-6 can cause treatment resistance in depressed patients (Lanquillon et al., 2000; O'Brien et al., 2007). IL-6 can decrease BH4, which, as discussed above, is essential for neurotransmitter synthesis. Previous studies reported the anti-inflammatory effects of folic acid and its ability to reduce IL-6, resulting in increased neurotransmitter levels (Li et al., 2003; Zhou et al., 2020). Another mechanism of action of folic acid in depressed patients is its effect on homocysteine. Folic acid can reduce homocysteine, a toxic agent for the dopaminergic system (Lee et al., 2005). Previous studies showed that low levels of folic acid and high levels of homocysteine are associated with lower neurotransmitter production in patients with depression (Botez & Young, 1991; Reynolds, 2013). Brain-derived neurotrophic factor (BDNF) is another substrate folic acid supplements affect. Previous studies indicated that a low level of BDNF is associated with depression, and antidepressant therapy can stimulate BDNF production (Liu et al., 2015). Folic acid can enhance BDNF synthesis resulting in depression healing (Gao et al., 2017; Zhou et al., 2020).

Our study had some advantages. To the best of our knowledge, the present study is the first meta-umbrella analysis assessing the effect of folate supplements on depression. We assessed the relationship between folate and depression using observational and randomized control trial studies. We also separated the studies with different reporting results units. In addition, we conducted subgroup analysis to determine the effect of treatment duration on depression; however, we had some limitations in this study. The number of included studies was low, and we could not conduct publication bias. We recommend further meta-analysis studies in this regard. The quality of included primary studies in the meta-analyses was low and the quality of meta-analyses was critically low; hence, future highly qualified studies are needed. In addition, we recommend further meta-analysis to assess folate or methyl folate separately, as these two supplements may act differently. We also suggest further meta-analyses to assess different treatment doses. None of our included meta-analyses conducted trial sequential analysis to deal with type 1 and type 2 errors; hence, we suggest such analysis in future studies. As the main population of included meta-analyses was middle aged, we could not assess the effect of depression on young and old populations; hence, we recommend future studies on these two groups to better understand the effect of age on the folate's effects.

5 CONCLUSION

Present umbrella meta-analysis lends support to using folate supplementation as an efficient adjuvant agent for relieving depression symptoms along with routine medications. The improving effect could be more robust if the supplementation duration lasts for >10 weeks.

AUTHOR CONTRIBUTIONS

Shan Gao: Data curation (lead); investigation (equal); resources (equal). Awais Khalid: Data curation (equal); investigation (equal); resources (equal). Ehsan Amini-Salehi: Investigation (equal); writing – original draft (equal). Nima Radkhah: Data curation (equal); investigation (equal); writing – original draft (equal). Parsa Jamilian: Investigation (equal); writing – original draft (equal). Mohaddeseh Badpeima: Investigation (equal); writing – original draft (equal). Meysam Zarezadeh: Conceptualization (lead); formal analysis (lead); methodology (lead); project administration (lead); supervision (lead); writing – review and editing (lead).

FUNDING INFORMATION

This study is supported via funding from Prince Sattam bin Abdulaziz University project number (PSAU/2024/R/1445).

CONFLICT OF INTEREST STATEMENT

None.