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Research Paper Volume 16, Issue 9 pp 7856—7869

Role of vitamin B12 and folic acid in treatment of Alzheimer’s disease: a meta-analysis of randomized control trials

Chih-Ying Lee1, , Lung Chan1,2, , Chaur-Jong Hu1,2, , Chien-Tai Hong1,2, , Jia-Hung Chen1, ,

  • 1 Department of Neurology, Shuang-Ho Hospital, Taipei Medical University, Taipei, Taiwan
  • 2 Department of Neurology, School of Medicine, College of Medicine, Taipei Medical University, Taipei, Taiwan

Received: September 5, 2023       Accepted: March 28, 2024       Published: May 2, 2024      

https://doi.org/10.18632/aging.205788
How to Cite

Copyright: © 2024 Lee et al. This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Abstract

Vitamin B12 and folic acid could reduce blood homocysteine levels, which was thought to slow down the progression of Alzheimer’s disease (AD), but previous studies regarding the effect of vitamin B12 and folic acid in treatment of AD have not reached conclusive results. We searched PubMed and Embase until January 12, 2023. Only randomized control trials involving participants clearly diagnosed with AD and who received vitamin B12 and folic acid were enrolled. Five studies that met the criteria were selected for inclusion in the meta-analysis. Changes in cognitive function were measured based on either the Mini-Mental State Examination (MMSE) or the Alzheimer’s Disease Assessment Scale-Cognitive Subscale (ADAS-Cog). Changes in daily life function and the level of blood homocysteine were also investigated. After a 6-month treatment, administration of vitamin B12 and folic acid improved the MMSE scores more than placebo did (SMD = 0.21, 95% CI = 0.01 to 0.32, p = 0.04) but did not significantly affect ADAS-Cog scores (SMD = 0.06, 95% CI = −0.22 to 0.33, p = 0.68) or measures of daily life function. Blood homocysteine levels were significantly decreased after vitamin B12 and folic acid treatment. Participants with AD who received 6 months of vitamin B12 and folic acid supplementation had better MMSE scores but had no difference in ADAS-Cog scores. Daily life function did not improve after treatment.

Introduction

Alzheimer’s disease (AD) is a progressive neurological disorder characterized by a decline in cognitive function, memory loss, and changes in behavior and personality [1]. It is the most common neurodegenerative disorder worldwide and the leading cause of dementia [2]. From 1990 to 2016, the global prevalence of AD increased 117%, and mortality from AD increased 148% [3]. With these increases come additional economic expenses, AD and AD-related dementia were estimated to cost 2.8 trillion USD in 2019 and are expected to have annual costs of 4.7 trillion and 16.9 trillion USD by 2030 and 2050, respectively [4]. This economic burden is especially large in low- and middle-income countries, which are estimated will bear 65% of all AD costs in 2050 [4]. Therefore, how to slow the progression or reverse the course of AD is imperative and critical.

Currently, there is no cure for AD, and available treatments can only temporarily alleviate symptoms rather than halt or reverse disease progression. Although the cholinesterase inhibitor (ChEI) is the established method of treating AD, it is not always effective in improving cognitive function or slowing disease progression. Many subjects may experience side effects such as nausea, diarrhea, and dizziness, and not all subjects respond well to these medications [5, 6]. Additionally, ChEIs do not address the underlying pathology of AD, such as the accumulation of beta-amyloid plaques and tau tangles in the brain. Hence, there is a need for alternative treatments that can target the underlying disease mechanisms and improve the overall outcomes for subjects with AD. This includes immunotherapies, neuroprotective agents, and lifestyle interventions that can help slow disease progression and improve quality of life for subjects with AD.

Vitamin B12 and folic acid have been discovered to possess neuroprotective properties, as well as being vital for the growth and functioning of the nervous system [7]. Deficiency of both vitamins has been linked to neurodegenerative disorders such as AD, Parkinson’s disease, and multiple sclerosis [8–12]. Therefore, consuming adequate amounts of vitamin B12 and folic acid through a balanced diet or supplements is considered to help protect the nervous system and prevent such conditions. However, there is currently no conclusive evidence that vitamin B12 and folic acid can effectively treat or prevent AD. Some studies have suggested that low levels of vitamin B12 and folic acid may be linked to cognitive decline and an increased risk of developing Alzheimer’s disease, but more research is needed to confirm this association [8]. This meta-analysis analyzed published randomized control trials (RCTs) to explore the effects of vitamin B12 and folic acid on the treatment of AD; these nutrients may offer a supportive management tool for slowing cognitive decline in people with AD.

Results

Search strategy and eligibility criteria

A total of 893 records were initially identified from PubMed and Embase; 597 remained after duplicates were removed, 24 remained after title and abstract screening, and 14 remained after full-text review. Two of these studies were excluded due to unclear diagnosis of AD, one was excluded because it investigated vascular dementia, five were excluded due to adding other nutrients to their vitamin B12 and folic acid formulas, and another was excluded due to a lack of reporting on the change in cognitive function. In total, five studies were used for the final meta-analysis [13–17]. The study selection flow chart is shown in Figure 1.

Study selection flow chart.

Figure 1. Study selection flow chart.

Detailed characteristics of the enrolled studies are presented in Table 1. Each of the five studies used randomized, placebo-controlled, and single- or double-blind designs; was published between 2007 and 2021; had a sample size ranging from 41 to 409; and was conducted in Taiwan, the United Kingdom, the United States, or China. Two studies treated participants with only folic acid, and three studies treated participants with vitamin B12 and folic acid. Each study enrolled participants who were clinically diagnosed with AD and had a mean age ranging from 67 to 77 years. Among participants, 429 were allocated to receive vitamin B12 or folic acid, and 351 were allocated to receive placebo; all were under ChEI treatment during the study period. Four studies reported changes in MMSE score, three reported changes in ADAS-Cog score, and four reported changes in daily life functions after 6 months of treatment. All studies measured the change in blood homocysteine levels before and after treatment.

Table 1. Characteristics of enrolled studies.

Author, yearDesignInclusion criteriaInterventionNo. of patients (male, %)Age (mean ± SD)Baseline cognitive function (mean ± SD)Main outcome measure
Sun, 2007 [13]RCT-DBAge >50, mild-to-moderate AD, MMSE 10–26 and CDR 1–2Mecobalamin 0.5 mg, folic acid 1 mg, and pyridoxine hydrochloride 5 mg for 26 weeksI: 45 (21, 46.7%)
C: 44 (24, 54.5%)		I: 74.9 ± 7.1
C: 74.6 ± 7.5		(I) ADAS-Cog: 24.0 ± 12.3
(I) MMSE: 18.7 ± 4.6
(C) ADAS-Cog: 21.2 ± 10.5
(C) MMSE: 18.6 ± 5.3		Change in ADAS-Cog after 26 weeks
Connelly, 2008 [14]RCT-DBProbable AD (fulfilling NINCDS-ADRDA criteria)Folic acid 1 mg for 6 monthsI: 23 (NA)
C: 18 (NA)		I: 75.65 ± 5.94
C: 77.60 ± 6.89		(I) MMSE: 23.48 ± 4.10
(C) MMSE: 23.50 ± 2.75		Change in MMSE and IADL after 6 months
Aisen, 2008 [15]RCT-DBAge >50, probable AD, MMSE 14–26Folic acid 5 mg, vitamin B12 1 mg, and vitamin B6 25 mg for 18 monthsI: 240 (102, 42.5%)
C: 169 (78, 46.1%)		I: 75.7 ± 8.0
C: 77.3 ± 7.9		(I) ADAS-Cog: 22.43 ± 9.0
(I) MMSE: 20.98 ± 3.4
(C) ADAS-Cog: 22.63 ± 8.6
(C) MMSE: 20.91 ± 3.7		Change in ADAS-Cog after 18 months
Chen, 2016 [16]RCT-SBAge 40–90, possible or probable AD of mild-to-moderate severity, MMSE 3–26Folic acid 1.25 mg for 6 monthsI: 61 (33, 54.10%)
C: 60 (28, 46.67%)		I: 68.10 ± 8.50
C: 67.63 ± 7.92		(I) MMSE: 18.56 ± 6.23
(C) MMSE: 17.63 ± 7.77		Change in MMSE and ADL after 6 months
Chen, 2021 [17]RCT-SBAge >45, clinically diagnosed as probable AD, MoCA <22Folic acid 1.2 mg and vitamin B12 50 μg for 6 monthsI: 60 (30, 50.00%)
C: 60 (26, 43.33%)		I: 68.58 ± 7.29
C: 68.02 ± 8.34		(I) ADAS-Cog: 24.50 ± 13.79 (n = 24)
(C) ADAS-Cog: 20.89 ± 9.83 (n = 15)		Change in MoCA and ADAS-Cog after 6 months
Abbreviations: RCT: randomized controlled trial; DB: double-blind; SB: single-blind; AD: Alzheimer’s disease; CDR: Clinical Dementia Rating; MoCA: Montreal Cognitive Assessment; I: intervention; C: control; MMSE: Mini-Mental State Examination; ADAS-Cog: Alzheimer’s Disease Assessment Scale-Cognitive Subscale; ADL: Activities of Daily Living; IADL: Instrumental Activities of Daily Living.

Risk of bias

Each of the five RCTs was determined to have a low risk of bias in the randomization process, deviation from the intended interventions, measurement of the outcome, and selection of the reported result. One RCT was classified as having some concerns of bias due to missing outcome data, and the others were classified as having low risk of bias. The detailed risk of bias assessment is presented in Table 2. There was no publication bias through visual inspection of funnel plots and utilization of Egger’s and Begg’s test, which is provided in the Supplementary Materials section (Supplementary Figures 1–4).

Table 2. Risk of bias assessment.

Author, yearBias arising from the randomization processBias due to deviations from the intended interventionsBias due to missing outcome dataBias in measurement of the outcomeBias in selection of the reported resultOverall risk of bias
Sun, 2007 [13]LowLowLowLowLowLow
Connelly, 2008 [14]LowLowLowLowLowLow
Aisen, 2008 [15]LowLowLowLowLowLow
Chen, 2016 [16]LowLowLowLowLowLow
Chen, 2021 [17]LowLowSome concernsLowLowSome concerns

Effect of vitamin B12 and folic acid on cognitive function

The change in cognitive function after vitamin B12 and folic acid treatment after 6 months was measured with the MMSE and ADAS-Cog scales. Among the five included studies, four reported changes in MMSE scores, and three reported changes in ADAS-Cog scores. Improvement in MMSE scores was observed in the treatment arm in three studies, while one study reported a decline in MMSE scores. In the placebo arm, two studies demonstrated improved MMSE scores, while two did not observe this effect. The changes in MMSE scores were found to be lower in the treatment group than in the control group (SMD = 0.2116, 95% CI = 0.01 to 0.32, I2 = 0%, p = 0.04, Figure 2). However, the changes in ADAS-Cog scores after 6-months of treatment were non-significant between the treatment and control groups (SMD = 0.06, 95% CI = −0.22 to 0.33, I2 = 38%, p = 0.68, Figure 3). Worsening of ADAS-Cog scores was observed in the treatment arm in two studies, as well as in the placebo arm in two studies.

Effect of vitamin B12 and folic acid on change in MMSE score. In the pooled treatment group, MMSE score was better than did the pooled control group (SMD = 0.16, 95% CI = 0.01 to 0.32, p = 0.04).

Figure 2. Effect of vitamin B12 and folic acid on change in MMSE score. In the pooled treatment group, MMSE score was better than did the pooled control group (SMD = 0.16, 95% CI = 0.01 to 0.32, p = 0.04).

Effect of vitamin B12 and folic acid on change in ADAS-Cog score. In both the pooled treatment and control groups, there was non-significant change in ADAS-Cog score after 6-months treatment (SMD = 0.06, 95% CI = −0.22 to 0.33, p = 0.68).

Figure 3. Effect of vitamin B12 and folic acid on change in ADAS-Cog score. In both the pooled treatment and control groups, there was non-significant change in ADAS-Cog score after 6-months treatment (SMD = 0.06, 95% CI = −0.22 to 0.33, p = 0.68).

Effect of vitamin B12 and folic acid on daily life function

Two studies reported the change in ADL score, two reported the change in IADL score, and one reported the change in Alzheimer’s Disease Co-operative Study-activities of daily living (ADCS-ADL) score. The change in daily life function score in the treatment group was not significantly different from that of the control group after the 6-month treatment, regardless of whether it was measured with the ADL (SMD = −0.04, 95% CI = −0.31 to 0.23, I2 = 0%, p = 0.79), IADL (SMD = 0.31, 95% CI = −0.32 to 0.94, I2 = 64%, p = 0.34), or ADCS-ADL (SMD = −0.05, 95% CI = −0.25 to 0.14, p = 0.60, Figure 4).

Effect of vitamin B12 and folic acid on change in daily life functions. The daily life functions were not significantly different between the pooled treatment and control groups in either ADL (MD = −0.04, 95% CI = −0.31 to 0.23, p = 0.79), IADL (MD = 0.31, 95% CI = −0.32 to 0.94, p = 0.34), or ADCS-ADL (MD = −0.05, 95% CI = −0.25 to 0.14, p = 0.60).

Figure 4. Effect of vitamin B12 and folic acid on change in daily life functions. The daily life functions were not significantly different between the pooled treatment and control groups in either ADL (MD = −0.04, 95% CI = −0.31 to 0.23, p = 0.79), IADL (MD = 0.31, 95% CI = −0.32 to 0.94, p = 0.34), or ADCS-ADL (MD = −0.05, 95% CI = −0.25 to 0.14, p = 0.60).

Change in blood homocysteine level after treatment

Four studies reported the change in blood homocysteine level before and after the 6-month treatment. The blood homocysteine level in the treatment group was reduced more significantly than that of the control group (SMD = −0.76, 95% CI = −1.05 to −0.47, I2 = 44%, p < 0.00001, Figure 5).

Effect of vitamin B12 and folic acid on change in blood homocysteine level. Blood homocysteine level was reduced more significantly in the treatment group than that of the control group (MD = –0.76, 95% CI = –1.05 to –0.47, p

Figure 5. Effect of vitamin B12 and folic acid on change in blood homocysteine level. Blood homocysteine level was reduced more significantly in the treatment group than that of the control group (MD = –0.76, 95% CI = –1.05 to –0.47, p < 0.00001).

Discussion

This meta-analysis included five RCTs that investigated the efficacy of vitamin B12 and folic acid on AD. The results demonstrated that AD subjects receiving vitamin B12 and folic acid exhibited less change in MMSE scores but exhibited no significant difference in ADAS-Cog score. The effect of the vitamin B12 and folic acid intervention on the change in daily life function scores did not differ between the treatment and control groups. The blood homocysteine level significantly decreased after treatment with vitamin B12 and folic acid.

Cognitive impairment is the hallmark and most troublesome symptom of AD. Loss of cholinergic neurons in the nucleus basalis of Meynert, which is found in the early stage of AD, is thought to be responsible for cognitive decline [18, 19]. Vitamin B12 and folic acid play a vital role in many physiological processes, including the development and maintenance of the nervous system. It is particularly important for the development of the cholinergic system, which is essential for memory, attention, and other cognitive functions [20]. There is some evidence to suggest that vitamin B12 may have a modulatory effect on the cholinergic system and could play a role in restoring blood-brain barrier integrity in AD [21]. The blood-brain barrier is a protective layer of cells that separates the brain from circulating blood, and disruptions in this barrier have been linked to the development of AD [22, 23]. Also, folic acid deficiency can lead to a decrease in acetylcholine synthesis and cholinergic neuron degeneration, which can result in cognitive impairment as well [24, 25].

In people with MCI or senile dementia, treatment with vitamin B12 and folic acid has been demonstrated to improve cognitive function. The Folic Acid and Carotid Intima-media Thickness (FACIT) trial enrolled 818 older adults and randomly assigned them to receive folic acid or placebo; taking folic acid supplements for 3 years significantly improved cognitive function [26]. However, Nikita et al. investigated the effect of vitamin B12 and folic acid in 2,919 elderly participants with elevated homocysteine levels. They revealed 2-year supplementation did not enhance cognitive function, although it slightly slowed the rate of cognitive decline [27]. This controversial result was also demonstrated in many studies in which people with AD were treated vitamin B12 and folic acid. In this review, Aisen et al. reported a noteworthy slow decline in MMSE scores after 6 months of treatment, while Sun et al., Connelly et al., and Chen et al. performed three additional RCTs that revealed no significant difference in change of scores [13–16]. Nevertheless, the meta-analysis of all four studies displayed a meaningful less change in MMSE scores for the treatment group. Interestingly, the meta-analysis of three RCTs that reported ADAS-Cog scores showed no significant difference between pretreatment and post-treatment scores [13, 15, 16].

Our findings demonstrated that MMSE scores were influenced by vitamin B12 and folic acid treatment, whereas ADAS-Cog scores were not. One potential explanation for this difference could be attributed to the varying numbers of studies included in the meta-analysis. Specifically, the analysis for MMSE involved four studies, whereas the analysis for ADAS-Cog encompassed only three studies. Although lack of ADAS-Cog scores from the study conducted by Connely et al. may not alter the final result of meta-analysis since its smaller sample size and non-significant difference observed in the change of MMSE scores, it should still be considered as a potential confounding factor. Moreover, subtle disparities were observed in the capacity of MMSE and ADAS-Cog to discern cognitive alterations. While both MMSE and ADAS-Cog assess memory function, MMSE places relatively more emphasis on other cognitive domains, such as orientation, calculation, and visual-spatial skills. The potential influence of vitamin B12 and folic acid on MMSE scores improvement may extend beyond enhancing memory to encompass other cognitive domains. These broader effects could translate into improved MMSE scores.

Despite the MMSE assessing various aspects of cognitive function, the ADAS-Cog offers a more comprehensive evaluation tailored specifically to assess cognitive function in individuals with AD. Previous studies have demonstrated a strong association between the MMSE and the ADAS-Cog, suggesting that they could be utilized for comparable purposes and indicate similar conditions [28, 29]. Although this meta-analysis demonstrated that supplementation of vitamin B12 and folic acid had inconsistent effects in terms of cognitive function assessment, these supplements significantly improved MMSE scores; however, caution is warranted in interpreting these MMSE scores because the better performance was only found in one RCT, which had a larger sample size than others.

The hypothesis that explains why vitamin B12 and folic acid improve cognitive function is that cognitive function is associated with blood homocysteine levels. Vitamin B12 and folic acid are important enzymes cofactors that are essential in remethylation of homocysteine to methionine through methylmalonyl-CoA mutase and methionine synthase [30]. Lack of Vitamin B12 and folic acid lead to elevated blood homocysteine levels, which further cause endothelial dysfunction and vascular injury [31] and result in diseases such as cardiovascular disease, thrombosis, chronic kidney disease, osteoporosis, and neuropsychiatric illness [32–37]. Additionally, altered homocysteine metabolism has been demonstrated to be a consequence of cholinergic disruption, which can be reversed by vitamin B12 supplementation [21]. In all studies of this meta-analysis, treatment with vitamin B12 and folic acid significantly reduced blood homocysteine levels, which was considered to be the mechanism for improving cognitive function, as measured by MMSE performance.

Another hypothesis for why vitamin B12 and folic acid improve cognitive function is related to the pathophysiology of AD. Extracellular deposits of amyloid beta (Aβ) and flame-shaped neurofibrillary tangles are thought to contribute to AD expression [38, 39]. Vitamin B12 and folic acid have been demonstrated to reduce the burden of Aβ deposition in some studies. Lam et al. found vitamin B12 can alleviate mitochondrial fragmentation, bioenergetic defects, and oxidative stress, which protects against Aβ induced proteotoxicity [40]. Li et al. found that folic acid inhibits Aβ peptide accumulation in mice models by enhancing methyltransferase activity [41]. In a study conducted by Chen et al., AD participants receiving folic acid supplementation had a higher Aβ-42 to Aβ-40 ratio [16], which might indicate better cognitive performance because a lower plasma Aβ-42 to Aβ-40 ratio was demonstrated to be associated with greater cognitive decline among older individuals [42].

Although vitamin B12 and folic acid treatment appear to enhance cognitive function in subjects with AD, these nutrients did not significantly improve daily life functions in most studies enrolled in this meta-analysis. In the study conducted by Aisen et al., AD participants exhibited improved MMSE performance after vitamin B12 and folic acid treatment, but ADCS-ADL scores did not improve [15]. Another three studies also demonstrated no difference in the change in daily life functions [13, 16, 17]. However, a study conducted by Connelly et al. reported better IADL scores in the treatment group than in the control group, but cognitive function did not improve in either group [14]. Overall, the results of this meta-analysis did not demonstrate that vitamin B12 and folic acid improve daily functions, although these results might be due to the small number of participants in the studies that were analyzed.

Finally, the studies included in this meta-analysis reported few instances of adverse effects (AEs) after administration of vitamin B12 and folic acid. Sun et al. reported five participants with dizziness, diarrhea, and muscle pain among 45 participants under treatment [13]. Connelly et al. reported no AEs in the treatment group [14]. Participants receiving vitamin B12 and folic acid were reported to have higher levels of depression in a study conducted by Aisen et al. [15]. This effect was opposite to that of previous studies that suggested vitamin B12 and folic acid are beneficial for treating depression [43, 44]. However, the analysis of changes in the depression item of the Neuropsychiatric Inventory scale in their study only showed a trend toward supporting this finding. Consequently, it cannot be conclusively stated that vitamin B12 and folic acid supplementation directly causes depression. Overall, the infrequency of AEs in the included studies suggesting that vitamin B12 and folic supplementation are quite safe.

The strength of this study is its rigorous selection criteria, such as the requirements for a clear diagnosis of AD and randomized control design. The results of this meta-analysis of five RCTs yielded evidence that vitamin B12 and folic acid treatment can improve MMSE scores in subjects with AD, which can be considered for use as an adjuvant therapy. However, this study has several limitations. First, several of the enrolled studies had fewer than 100 participants. The one study that enrolled more cases than others may have skewed the results of the meta-analysis, but this effect was mitigated by the random-effects model used in this study. Second, the reports of clinical outcomes were heterogeneous across studies. Different measurement scales were not able to be entered into the meta-analysis, which limited the number of cases that could be compared and thus limits the power of the results. Third, although vitamin B12 and folic acid are both members of the vitamin B group, they are different substances. In this meta-analysis, we examined the change in blood homocysteine level as mediated by both vitamin B12 and folic acid because we hypothesized that blood homocysteine level is associated with cognitive decline in AD subjects. Finally, vitamin B12 and folic acid dosage variability in the studies may have affected the results. The limited duration of the trials also restricted the outcomes, as a more extended exposure to vitamin B12 and folic acid may be necessary. Yet, despite this variability, all the studies demonstrated decreased blood homocysteine levels, which suggests the related findings are valid.

In conclusion, the administration of vitamin B12 and folic acid in subjects with AD improved MMSE scores, but ADAS-Cog scores and daily life function were not improved. Blood homocysteine level significantly decreased after treatment with vitamin B12 and folic acid. The result should be interpreted with caution due to the use of varying studies for analysis. Further RCTs with larger samples are warranted to provide more evidence on the effect of vitamin B12 and folic acid as AD treatments.

Methods

This meta-analysis was performed in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines. The review protocol was registered with PROSPERO (CRD42023394167).

Search strategy and study selection

The literature search and review were conducted by two independent reviewers (C.Y.L. and J.H.C.). PubMed and Embase were searched with no language restrictions until Jan 12, 2023, using keywords related to vitamin B, folic acid, Alzheimer’s, and dementia. The reference sections of prior systematic reviews and meta-analyses were also screened for related studies. The details of the search strategy and results are provided in the Supplementary Materials section (Supplementary Table 1).

Studies were included if they (1) researched a population with a clinical diagnosis of AD, (2) reported on the use of vitamin B12, folic acid, or both as treatment, (3) used a randomized controlled design with at least two comparator arms, and (4) reported outcomes related to changes in cognitive function. All non-RCTs, crossover trials, uncompleted clinical trials, review articles, and studies that did not use original data were excluded. There were no limitations placed on the publication languages. After the duplicate studies were removed, the titles and abstracts of the remaining were screened for eligibility, and the full texts of eligible studies were evaluated.

Data collection

Baseline characteristics, intervention strategies, and outcome data were independently extracted by two reviewers (C.Y.L. and J.H.C.). Information on study design, study population, and inclusion and exclusion criteria were also retrieved. Disagreements between the two reviewers were resolved through a panel discussion with a third reviewer (C.T.H.) until consensus was reached.

Outcome measures

The primary outcome was the change in cognitive function after 6 months, which was measured using the Mini-Mental State Examination (MMSE) or the Alzheimer’s Disease Assessment Scale-Cognitive Subscale (ADAS-Cog), of vitamin B12 and folic acid treatment. The secondary outcome was changes in daily life functions, which were measured by the activities of daily living (ADL) scale or the instrumental activities of daily living (IADL) scale. The change in blood homocysteine level after 6 months treatment was also measured.

Risk of bias assessment

The risk of bias assessment of included studies was performed by two independent reviewers (C.Y.L. and J.H.C.) using version 2 of the Cochrane Risk of Bias Assessment Tool, which categorizes bias risk into low risk of bias, some concerns, or high risk of bias. The publication bias was evaluated through visual inspection of funnel plots and calculated with Egger’s and Begg’s test. Disagreements were resolved through a panel discussion involving all three reviewers (C.Y.L., J.H.C. and C.T.H.).

Statistical analysis

Analysis was conducted using Review Manager 5.4 (The Cochrane Collaboration, Oxford, UK). The summary effect sizes for outcomes used to compare vitamin B12 and folic acid with placebo were estimated with the DerSimonian and Laird random-effects model. The effect sizes of continuous outcomes are expressed in terms of the standardized mean difference (SMD). The standard deviation (SD) was calculated with the provided confidence interval (CI) limits or interquartile ranges, if the original study SD was not available [45, 46]. Statistical significance was defined by a 95% CI that did not cross zero in the weighted mean difference estimation. Heterogeneity and inconsistency across studies were assessed using the I2 statistic.

Abbreviations

AD: Alzheimer’s disease; RCTs: randomized control trials; MMSE: Mini-Mental State Examination; ADAS-Cog: Alzheimer’s Disease Assessment Scale-Cognitive Subscale; ChEI: cholinesterase inhibitor; ADL: activities of daily living; IADL: instrumental activities of daily living; SMD: standardized mean difference; SD: standard deviation; CI: confidence interval; ADCS-ADL: Alzheimer’s Disease Co-operative Study-activities of daily living; AEs: adverse effects.

Author Contributions

C.Y.L. and J.H.C performed the literature search, data extraction, and drafted the manuscript. L.C. and C.J.H. participated in data analysis. C.T.H and J.H.C. contributed to interpretation of results and the intellectual content of the manuscript. All authors read and approved the final manuscript.

Acknowledgments

This manuscript was edited by Wallace Academic Editing.

Conflicts of Interest

The authors declare no conflicts of interest related to this study.

Funding

No funding was used for this paper.

References

  • 1. Knopman DS, Amieva H, Petersen RC, Chételat G, Holtzman DM, Hyman BT, Nixon RA, Jones DT. Alzheimer disease. Nat Rev Dis Primers. 2021; 7:33. https://doi.org/10.1038/s41572-021-00269-y [PubMed]
  • 2. Cao Q, Tan CC, Xu W, Hu H, Cao XP, Dong Q, Tan L, Yu JT. The Prevalence of Dementia: A Systematic Review and Meta-Analysis. J Alzheimers Dis. 2020; 73:1157–66. https://doi.org/10.3233/JAD-191092 [PubMed]
  • 3. GBD 2016 Neurology Collaborators. Global, regional, and national burden of neurological disorders, 1990-2016: a systematic analysis for the Global Burden of Disease Study 2016. Lancet Neurol. 2019; 18:459–80. https://doi.org/10.1016/S1474-4422(18)30499-X [PubMed]
  • 4. Nandi A, Counts N, Chen S, Seligman B, Tortorice D, Vigo D, Bloom DE. Global and regional projections of the economic burden of Alzheimer's disease and related dementias from 2019 to 2050: A value of statistical life approach. EClinicalMedicine. 2022; 51:101580. https://doi.org/10.1016/j.eclinm.2022.101580 [PubMed]
  • 5. Thompson S, Lanctôt KL, Herrmann N. The benefits and risks associated with cholinesterase inhibitor therapy in Alzheimer's disease. Expert Opin Drug Saf. 2004; 3:425–40. https://doi.org/10.1517/14740338.3.5.425 [PubMed]
  • 6. Tan CC, Yu JT, Wang HF, Tan MS, Meng XF, Wang C, Jiang T, Zhu XC, Tan L. Efficacy and safety of donepezil, galantamine, rivastigmine, and memantine for the treatment of Alzheimer's disease: a systematic review and meta-analysis. J Alzheimers Dis. 2014; 41:615–31. https://doi.org/10.3233/JAD-132690 [PubMed]
  • 7. Reynolds E. Vitamin B12, folic acid, and the nervous system. Lancet Neurol. 2006; 5:949–60. https://doi.org/10.1016/S1474-4422(06)70598-1 [PubMed]
  • 8. Shen L, Ji HF. Associations between Homocysteine, Folic Acid, Vitamin B12 and Alzheimer's Disease: Insights from Meta-Analyses. J Alzheimers Dis. 2015; 46:777–90. https://doi.org/10.3233/JAD-150140 [PubMed]
  • 9. Smith AD, Refsum H. Homocysteine, B Vitamins, and Cognitive Impairment. Annu Rev Nutr. 2016; 36:211–39. https://doi.org/10.1146/annurev-nutr-071715-050947 [PubMed]
  • 10. Smith AD, Refsum H, Bottiglieri T, Fenech M, Hooshmand B, McCaddon A, Miller JW, Rosenberg IH, Obeid R. Homocysteine and Dementia: An International Consensus Statement. J Alzheimers Dis. 2018; 62:561–70. https://doi.org/10.3233/JAD-171042 [PubMed]
  • 11. Dong B, Wu R. Plasma homocysteine, folate and vitamin B12 levels in Parkinson's disease in China: A meta-analysis. Clin Neurol Neurosurg. 2020; 188:105587. https://doi.org/10.1016/j.clineuro.2019.105587 [PubMed]
  • 12. Dardiotis E, Arseniou S, Sokratous M, Tsouris Z, Siokas V, Mentis AA, Michalopoulou A, Andravizou A, Dastamani M, Paterakis K, Bogdanos D, Brotis A. Vitamin B12, folate, and homocysteine levels and multiple sclerosis: A meta-analysis. Mult Scler Relat Disord. 2017; 17:190–7. https://doi.org/10.1016/j.msard.2017.08.004 [PubMed]
  • 13. Sun Y, Lu CJ, Chien KL, Chen ST, Chen RC. Efficacy of multivitamin supplementation containing vitamins B6 and B12 and folic acid as adjunctive treatment with a cholinesterase inhibitor in Alzheimer's disease: a 26-week, randomized, double-blind, placebo-controlled study in Taiwanese patients. Clin Ther. 2007; 29:2204–14. https://doi.org/10.1016/j.clinthera.2007.10.012 [PubMed]
  • 14. Connelly PJ, Prentice NP, Cousland G, Bonham J. A randomised double-blind placebo-controlled trial of folic acid supplementation of cholinesterase inhibitors in Alzheimer's disease. Int J Geriatr Psychiatry. 2008; 23:155–60. https://doi.org/10.1002/gps.1856 [PubMed]
  • 15. Aisen PS, Schneider LS, Sano M, Diaz-Arrastia R, van Dyck CH, Weiner MF, Bottiglieri T, Jin S, Stokes KT, Thomas RG, Thal LJ, and Alzheimer Disease Cooperative Study. High-dose B vitamin supplementation and cognitive decline in Alzheimer disease: a randomized controlled trial. JAMA. 2008; 300:1774–83. https://doi.org/10.1001/jama.300.15.1774 [PubMed]
  • 16. Chen H, Liu S, Ji L, Wu T, Ji Y, Zhou Y, Zheng M, Zhang M, Xu W, Huang G. Folic Acid Supplementation Mitigates Alzheimer's Disease by Reducing Inflammation: A Randomized Controlled Trial. Mediators Inflamm. 2016; 2016:5912146. https://doi.org/10.1155/2016/5912146 [PubMed]
  • 17. Chen H, Liu S, Ge B, Zhou D, Li M, Li W, Ma F, Liu Z, Ji Y, Huang G. Effects of Folic Acid and Vitamin B12 Supplementation on Cognitive Impairment and Inflammation in Patients with Alzheimer's Disease: A Randomized, Single-Blinded, Placebo-Controlled Trial. J Prev Alzheimers Dis. 2021; 8:249–56. https://doi.org/10.14283/jpad.2021.22 [PubMed]
  • 18. Liu AK, Chang RC, Pearce RK, Gentleman SM. Nucleus basalis of Meynert revisited: anatomy, history and differential involvement in Alzheimer's and Parkinson's disease. Acta Neuropathol. 2015; 129:527–40. https://doi.org/10.1007/s00401-015-1392-5 [PubMed]
  • 19. Ferreira-Vieira TH, Guimaraes IM, Silva FR, Ribeiro FM. Alzheimer's disease: Targeting the Cholinergic System. Curr Neuropharmacol. 2016; 14:101–15. https://doi.org/10.2174/1570159x13666150716165726 [PubMed]
  • 20. Hampel H, Mesulam MM, Cuello AC, Farlow MR, Giacobini E, Grossberg GT, Khachaturian AS, Vergallo A, Cavedo E, Snyder PJ, Khachaturian ZS. The cholinergic system in the pathophysiology and treatment of Alzheimer's disease. Brain. 2018; 141:1917–33. https://doi.org/10.1093/brain/awy132 [PubMed]
  • 21. El-Mezayen NS, Abd El Moneim RA, El-Rewini SH. Vitamin B12 as a cholinergic system modulator and blood brain barrier integrity restorer in Alzheimer's disease. Eur J Pharm Sci. 2022; 174:106201. https://doi.org/10.1016/j.ejps.2022.106201 [PubMed]
  • 22. Bell RD, Zlokovic BV. Neurovascular mechanisms and blood-brain barrier disorder in Alzheimer's disease. Acta Neuropathol. 2009; 118:103–13. https://doi.org/10.1007/s00401-009-0522-3 [PubMed]
  • 23. Kurz C, Walker L, Rauchmann BS, Perneczky R. Dysfunction of the blood-brain barrier in Alzheimer's disease: Evidence from human studies. Neuropathol Appl Neurobiol. 2022; 48:e12782. https://doi.org/10.1111/nan.12782 [PubMed]
  • 24. Crivello NA, Blusztajn JK, Joseph JA, Shukitt-Hale B, Smith DE. Short-term nutritional folate deficiency in rats has a greater effect on choline and acetylcholine metabolism in the peripheral nervous system than in the brain, and this effect escalates with age. Nutr Res. 2010; 30:722–30. https://doi.org/10.1016/j.nutres.2010.09.008 [PubMed]
  • 25. Bennett C, Green J, Ciancio M, Goral J, Pitstick L, Pytynia M, Meyer A, Kwatra N, Jadavji NM. Dietary folic acid deficiency impacts hippocampal morphology and cortical acetylcholine metabolism in adult male and female mice. Nutr Neurosci. 2022; 25:2057–65. https://doi.org/10.1080/1028415X.2021.1932242 [PubMed]
  • 26. Durga J, van Boxtel MP, Schouten EG, Kok FJ, Jolles J, Katan MB, Verhoef P. Effect of 3-year folic acid supplementation on cognitive function in older adults in the FACIT trial: a randomised, double blind, controlled trial. Lancet. 2007; 369:208–16. https://doi.org/10.1016/S0140-6736(07)60109-3 [PubMed]
  • 27. van der Zwaluw NL, Dhonukshe-Rutten RA, van Wijngaarden JP, Brouwer-Brolsma EM, van de Rest O, In 't Veld PH, Enneman AW, van Dijk SC, Ham AC, Swart KM, van der Velde N, van Schoor NM, van der Cammen TJ, et al. Results of 2-year vitamin B treatment on cognitive performance: secondary data from an RCT. Neurology. 2014; 83:2158–66. https://doi.org/10.1212/WNL.0000000000001050 [PubMed]
  • 28. Wouters H, van Gool WA, Schmand B, Zwinderman AH, Lindeboom R. Three sides of the same coin: measuring global cognitive impairment with the MMSE, ADAS-cog and CAMCOG. Int J Geriatr Psychiatry. 2010; 25:770–9. https://doi.org/10.1002/gps.2402 [PubMed]
  • 29. Khandker R, Black C, Pike J, Husbands J, Ambegaonkar B, Jones E. The relationship between Mini-Mental State Examination (MMSE) & Alzheimer’s Disease Assessment Scale-cognitive subscale (ADAS-Cog) using real world data in US & Europe (P5.178). Neurology. 2018; 90:P5.178. https://doi.org/10.1212/WNL.90.15_supplement.P5.178
  • 30. Selhub J. Homocysteine metabolism. Annu Rev Nutr. 1999; 19:217–46. https://doi.org/10.1146/annurev.nutr.19.1.217 [PubMed]
  • 31. Esse R, Barroso M, Tavares de Almeida I, Castro R. The Contribution of Homocysteine Metabolism Disruption to Endothelial Dysfunction: State-of-the-Art. Int J Mol Sci. 2019; 20:867. https://doi.org/10.3390/ijms20040867 [PubMed]
  • 32. Refsum H, Ueland PM, Nygård O, Vollset SE. Homocysteine and cardiovascular disease. Annu Rev Med. 1998; 49:31–62. https://doi.org/10.1146/annurev.med.49.1.31 [PubMed]
  • 33. Cattaneo M. Hyperhomocysteinemia, atherosclerosis and thrombosis. Thromb Haemost. 1999; 81:165–76. [PubMed]
  • 34. Sjöberg B, Anderstam B, Suliman M, Alvestrand A. Plasma reduced homocysteine and other aminothiol concentrations in patients with CKD. Am J Kidney Dis. 2006; 47:60–71. https://doi.org/10.1053/j.ajkd.2005.09.032 [PubMed]
  • 35. Yang J, Hu X, Zhang Q, Cao H, Wang J, Liu B. Homocysteine level and risk of fracture: A meta-analysis and systematic review. Bone. 2012; 51:376–82. https://doi.org/10.1016/j.bone.2012.05.024 [PubMed]
  • 36. Morris MS. Homocysteine and Alzheimer's disease. Lancet Neurol. 2003; 2:425–8. https://doi.org/10.1016/s1474-4422(03)00438-1 [PubMed]
  • 37. Smach MA, Jacob N, Golmard JL, Charfeddine B, Lammouchi T, Ben Othman L, Dridi H, Bennamou S, Limem K. Folate and homocysteine in the cerebrospinal fluid of patients with Alzheimer's disease or dementia: a case control study. Eur Neurol. 2011; 65:270–8. https://doi.org/10.1159/000326301 [PubMed]
  • 38. Murphy MP, LeVine H3rd. Alzheimer's disease and the amyloid-beta peptide. J Alzheimers Dis. 2010; 19:311–23. https://doi.org/10.3233/JAD-2010-1221 [PubMed]
  • 39. Lacosta AM, Insua D, Badi H, Pesini P, Sarasa M. Neurofibrillary Tangles of Aβx-40 in Alzheimer's Disease Brains. J Alzheimers Dis. 2017; 58:661–7. https://doi.org/10.3233/JAD-170163 [PubMed]
  • 40. Lam AB, Kervin K, Tanis JE. Vitamin B12 impacts amyloid beta-induced proteotoxicity by regulating the methionine/S-adenosylmethionine cycle. Cell Rep. 2021; 36:109753. https://doi.org/10.1016/j.celrep.2021.109753 [PubMed]
  • 41. Li W, Liu H, Yu M, Zhang X, Zhang M, Wilson JX, Huang G. Folic acid administration inhibits amyloid β-peptide accumulation in APP/PS1 transgenic mice. J Nutr Biochem. 2015; 26:883–91. https://doi.org/10.1016/j.jnutbio.2015.03.009 [PubMed]
  • 42. Yaffe K, Weston A, Graff-Radford NR, Satterfield S, Simonsick EM, Younkin SG, Younkin LH, Kuller L, Ayonayon HN, Ding J, Harris TB. Association of plasma beta-amyloid level and cognitive reserve with subsequent cognitive decline. JAMA. 2011; 305:261–6. https://doi.org/10.1001/jama.2010.1995 [PubMed]
  • 43. Coppen A, Bolander-Gouaille C. Treatment of depression: time to consider folic acid and vitamin B12. J Psychopharmacol. 2005; 19:59–65. https://doi.org/10.1177/0269881105048899 [PubMed]
  • 44. Almeida OP, Ford AH, Flicker L. Systematic review and meta-analysis of randomized placebo-controlled trials of folate and vitamin B12 for depression. Int Psychogeriatr. 2015; 27:727–37. https://doi.org/10.1017/S1041610215000046 [PubMed]
  • 45. Cochrane handbook 16.1.3.2 Imputing standard deviations for changes from baseline.
  • 46. Wan X, Wang W, Liu J, Tong T. Estimating the sample mean and standard deviation from the sample size, median, range and/or interquartile range. BMC Med Res Methodol. 2014; 14:135. https://doi.org/10.1186/1471-2288-14-135 [PubMed]
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Corresponding Authors

Jia-Hung Chen
19587@s.tmu.edu.tw

Chien-Tai Hong
15004@s.tmu.edu.tw

Keywords
vitamin B12 folic acid homocysteine Alzheimer’s disease
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