Adjunctive Zinc for Major Depression : A Systematic Review of RCTs

 In Men's Health, Mind/Body

Student Scholarship Honorable Mention

Georgi Stoychev, BSc
Baljit Khamba, ND, Mph

Mental disorders are the leading cause of non-fatal disease burden, globally and nationally. It is estimated that 1 in 4 adults in the United States suffers from anxiety and depression disorders.1 Major depressive disorder (MDD) is the most common type of depression, affecting 6.7% of the population in a given year.2

Depression is related to chronic disease. Clinical obesity raises the risk of developing MDD by 55%.3 Individuals who are depressed are more likely to subsequently develop obesity, type 2 diabetes, and coronary heart disease.3-5 MDD is also independently associated with poor treatment adherence, which is crucial to the success of naturopathic recommendations.6-8

The Diagnostic and Statistical Manual of Mental Disorders (DSM) sets diagnostic criteria for mental disorders in the United States. MDD is characterized by the following: (1) persistent low mood and (2) loss of interest or pleasure, and accompanied by somatic symptoms such as significant changes in (3) weight, (4) appetite, (5) sleep, and (6) cognition.9

The monoamine deficiency hypothesis is a widely accepted explanation for the pathophysiology of depression. Low levels of serotonin, norepinephrine, and dopamine are characteristic of depression, and replenishment with antidepressants improves symptoms in severe MDD.10,11 However, minimal benefit is reported in mild-to-moderate depression.12 This is further complicated by low response and remission rates and high recurrence (“treatment-resistance”).13-15

Rationale for Using Zinc in MDD

Zinc saturates the hippocampus and the grey matter of the cerebral cortex.16 In MDD, moderate-to-severe shrinkage of these areas is observed on imaging studies.17,18 On a molecular level, zinc is a partial N-methyl-D-aspartate (NMDA) receptor antagonist, thus preventing the excessive neuron stimulation that may be present in MDD.19 Individuals with depression have lower plasma zinc levels compared to healthy controls.20 An earlier systematic review found adjunctive zinc administration with antidepressants to be beneficial in improving depressive symptoms; however, conclusions were limited by inadequately powered randomized controlled trials (RCTs).21

Literature Review

In order to assess the effects of zinc compared to placebo as an add-on therapy to antidepressants for treating MDD in adults, we conducted a systematic-type review of existing research. Table 1 lists our criteria for selecting studies in our review. Consistent with an evidence-based medicine approach, we utilized a PICO approach in our analysis (P=population; I=intervention; C=control; O=outcome).

Table 1. PICO Analysis & Eligibility Criteria for Review

Category Inclusion/Exclusion Criteria
Study type RCTs with human subjects, published and unpublished ≥1 week

English language studies



Men and women ≥18 years

MDD diagnosis, with or without psychosis and other comorbidities, according to DSM-IV/V or International Statistical Classification of Diseases and Related Health problems, 10th revision (ICD-10)




Zinc as add-on therapy to antidepressants

Excluded zinc as monotherapy or as part of multivitamin or mineral complex



Placebo as control


Primary outcomes:

1. Changes in mean score on depression scales, from baseline to end

2. Participant drop-out rates

Secondary outcomes:

3. Response (50% or greater reduction in depression score at study endpoint)

4. Remission (depression score within normal range at study conclusion)

5. Number and nature of adverse effects

Search Methods

Seven databases were searched, from all years of record until January 2017 (Figure 1). The databases included the Cochrane Central Register of Controlled Trials, the Cochrane Database of Systematic Reviews, PubMed, Web of Science, Google Scholar, PsychINFO, and Trip. International trial registries, including and WHO International Clinical Trials Registry Platform, were also searched for unpublished or ongoing trials. Reference lists were checked and experts in the field were contacted.

Figure 1. PRISMA Diagram of Search & Study Selection Process


Data Collection & Analysis

One author independently conducted data extraction, risk of bias, and quality-of-evidence assessment. This was done through the Cochrane Collaboration’s data extraction form, risk of bias tool, and the GRADE framework.22,23 Studies were judged as high, low, or unclear risk of bias (Figures 2,3). Evidence for each outcome was graded as high, medium, low, or very low quality.

Figure 2. Risk of Bias; Percentages


Figure 3. Risk of Bias: Category Judgments


Three studies were selected for review. Most of these studies were determined to be at low risk of performance, detection, and attrition bias, unclear risk of selection bias, and mixed risk of reporting bias. Two trials ensured rigorous measures of minimizing bias overall,24-27 while the third study had poor reporting of methodology and was ascribed unclear risk of bias for most categories.28 The quality of evidence23 for all outcomes ranged from low to very low (see Table 2).

 Table 2. Findings & Quality of Evidence

Outcome Number of Participants in Studies Result

(Zinc compared to placebo)

Quality of Evidence (GRADE)
Change in mean scores on depression scales 103 (3 studies)24-28 Zinc significantly superior ⊕⊕⊝⊝


Participant drop-outs 103 (3 studies)24-28 Non-significantly lower in zinc ⊝⊝⊝⊝

very low

Response rate 52 (1 study)26,27 No significant difference ⊕⊝⊝⊝

very low

Remission rate 52 (1 study)26,27 No significant difference ⊕⊝⊝⊝

very low

Adverse effects 52 (1 study)26,27 No significant difference ⊝⊝⊝⊝

very low


The quality of the evidence for every outcome was rated as follows: risk of bias, indirectness, inconsistency, imprecision, and publication bias. (See Table 3)

Points were downgraded for risk of bias (high for 2 categories), indirectness (only 2 countries represented), and imprecision (small sample size and number of events).

Table 3. Completeness & Applicability of Findings

Category Comments
 P Studies conducted in Poland and Iran only

Depression subgroups not considered

Treatment-resistant subgroup explored in only 1 study

Male-to-female ratio in studies matched depression gender distribution in general population

No ICD-10-diagnosed depression; however, high concordance between DSM-5 and ICD-1029



Only zinc hydroaspartate and sulphate examined

Limited classes of antidepressants used

C Compared to placebo in all studies
O Insufficient information on clinically relevant outcomes, eg, response and remission rates, adverse effects

Biases in Review Process

  • Strengths: Detailed search strategy; use of “gold-standard” methodology for data extraction, risk of bias, and quality-of-evidence assessment; multiple outcome measures; Cochrane Collaboration Group structure
  • Limitations: No grey literature search; single study author (limits search, data extraction, risk of bias, and quality-of-evidence assessment); no meta-analysis; however, mini meta-analysis informed quality-of-evidence assessment

 Agreement with Other Reviews/Studies

  • Similarities: Similar PICO, eg, adults with MDD, zinc as add-on therapy, placebo as comparator, reduction in scores on depression scales21
  • Differences: 2012 review also examined zinc in healthy adults and as part of multivitamins; this review included 1 new study24,25; more clinically relevant outcomes (Table 2); more rigorous and up-to-date methodology for risk of bias and quality-of-evidence assessment


Given the absence of moderate-to-high-quality evidence, firm conclusions about zinc as an add-on therapy to antidepressants for treating MDD cannot be made at this time. Further evidence from larger, higher-quality RCTs is necessary, with particular focus on adequate sequence generation, allocation concealment, handling of missing data, and avoidance of selective outcome reporting. Trials should encompass a variety of participants, settings, antidepressant subclasses, and patient-centered outcome measures. Healthcare professionals in mental health, including naturopathic doctors, should be mindful of zinc status in patients, given the higher prevalence of zinc deficiency in MDD, and address any imbalances found.


  1. Anxiety and Depression Association of America. Facts & Statistics. August 2017. Available at: Accessed September 7, 2017.
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  17. Koolschijn P, van Haren N, Lensvelt-Mulders G, et al. Brain volume abnormalities in major depressive disorder: a meta-analysis of magnetic resonance imaging studies. Hum Brain Mapp. 2009;30(11):3719-3735.
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Thanks to Dr Dimple Radia, lecturer at London Metropolitan University, for her extensive support and supervision while conducting this undergraduate research project.

Georgi Stoychev, BSc, is a dietetics graduate from London, UK; he obtained his BSc at London Metropolitan University. While working toward his dietetics degree, he was able to see a variety of patients with nutrition-related disorders. Presently, Georgi is a student of naturopathic medicine at Bastyr University California. He has a particular interest in naturopathic approaches to mental health. As part of broadening our understanding in this area, he stays abreast of research on naturopathic treatment options for mental health conditions.


Baljit Khamba, ND, MPH, is a licensed naturopathic doctor in California. Dr Khamba completed her (honors) Bachelor of Science degree (specializing in psychology), as well as her Masters in Public Health degree, at York University, in Toronto, Canada. She received her naturopathic doctoral degree from the Canadian College of Naturopathic Medicine (CCNM), also in Toronto. She was also involved with research projects at the University of Alberta on natural health product safety. Dr Khamba is a member of the American Osteopathic Association of Prolotherapy Regenerative Medicine. 


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