Zinc is an important factor for testosterone production, however, most of the evidence points to deficiency being a problem, but once you have sufficient zinc your testosterone will not go up and up if you supplement with more.
Review articles
systematic review and meta-analysis http://www.nature.com/articles/srep22386
https://www.ncbi.nlm.nih.gov/pubmed/26932683 Zinc levels in seminal plasma and their correlation with male infertility
Literature review https://www.innovativepublication.com/admin/uploaded_files/IJOGR_3(2)_167-171.pdf
"Zinc deficiency first impairs angiotensin converting enzyme (ACE) activity, and this in turn leads to depletion of testosterone and inhibition of spermatogenesis. Defects in spermatozoa are frequently observed in the zinc-deficient rat. Zinc is thought to help to extend the functional life span of the ejaculated spermatozoa."
https://www.ncbi.nlm.nih.gov/pubmed/8033970
The importance of folate, zinc and antioxidants in the pathogenesis and prevention of subfertility.
https://www.ncbi.nlm.nih.gov/pubmed/17099205/
https://www.ncbi.nlm.nih.gov/pubmed/1989405/
Zinc deficiency in human subjects https://www.ncbi.nlm.nih.gov/pubmed/6361778
https://www.ncbi.nlm.nih.gov/pubmed/3914098
https://www.ncbi.nlm.nih.gov/pubmed/2419703
Human Studies
250 Zinc Sulfate per day raises serum testosterone and LH in hemodialysis
https://www.ncbi.nlm.nih.gov/pubmed/20446777
- No effect on FSH or prolactin.
- Admittedly, not a particularly representative group.
he results indicated that intervention treatment significantly increased sperm concentration in subfertile males. Other semen and endocrine parameters were not affected
https://www.ncbi.nlm.nih.gov/pubmed/16533356/
The mean value of semen volume, progressive sperm motility percentage and total normal sperm count were increased after zinc sulfate supplementation.
https://www.ncbi.nlm.nih.gov/pubmed/23145537
Dietary zinc restriction in normal young men was associated with a significant decrease in serum testosterone concentrations
Zinc supplementation of marginally zinc-deficient normal elderly men for six months resulted in an increase in serum testosterone.
https://www.ncbi.nlm.nih.gov/pubmed/8875519
Uncontrolled study in the elderly - Zinc boosts testosterone
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1141561/?page=1
statistically significant improvement of semen parameters compared with controls.
https://www.ncbi.nlm.nih.gov/pubmed/24383664
Volume of semen, progressive sperm motility and total normal sperm count were increased after zinc supplementation.
https://www.ncbi.nlm.nih.gov/pubmed/26780395
Effects of folic acid and zinc sulfate on male factor subfertility: a double-blind, randomized, placebo-controlled trial.
https://www.ncbi.nlm.nih.gov/pubmed/11872201
The original ZMA study
https://www.asep.org/asep/asep/BrillaV2.PDF
Diet records (3-day) showed that mean values of selected nutrients exceeded the RDA for Zn (17.0±7.4 mg), Mg (539±272 mg), and vitamin B-6 (3.6±1.6 mg).
- Serum zinc increased as a result of supplementation.
ZMA does not increase testosterone when zinc is sufficient.
Participants: Fourteen healthy, regularly exercising men aged 22–33 years
Baseline zinc intake: between 11.9 and 23.2 mg per day day (average in experimental group 18mg ±3.4)
Serum zinc increased.
https://www.ncbi.nlm.nih.gov/pubmed/17882141
Zinc supplementation raises testosterone when deficient
https://www.ncbi.nlm.nih.gov/pubmed/16648789
22mg of Zinc reduces copper and iron satus in adolescents
https://www.ncbi.nlm.nih.gov/pubmed/19277992
https://www.ncbi.nlm.nih.gov/pubmed/1375078?dopt=Abstract&holding=npg
Human observational studies
seminal plasma zinc positively correlated with serum free testosterone
https://www.ncbi.nlm.nih.gov/pubmed/27437207
Zinc non-significantly correlated with testosterone
https://www.ncbi.nlm.nih.gov/m/pubmed/18990371/
But may be protective against molybdenum
Zinc significantly correlated with testosterone in nationally representative sample
https://www.ncbi.nlm.nih.gov/m/pubmed/25439796/?i=2&from=/18990371/related
Urinary zinc inversely related to testoterone in Chinese men
https://www.ncbi.nlm.nih.gov/pubmed/23791927
Observational study: Decreased serum testosterone is significantly associated with a high level of Cu and elevated Cu/Zn ratio in hair tissue
https://www.ncbi.nlm.nih.gov/pubmed/21671089
Interactions with Vitamin A
I've seen it mentioned that zinc is needed to covert vitamin A into its active form. This warrants further investigation.
Rat study evidence
Rat evidence all seems to point in one direction. You need zinc to produce testosterone in the testes, but it will not effect things higher up the chain in the HPA axis. This means you won't get more and more testosterone from more and more zinc.
Rat study: Zinc deficiency reduces testosterone. LH elevated
https://www.ncbi.nlm.nih.gov/pubmed/376793
Rat study: Zinc deficiency reduces testosterone in rats. LH remained fine. Problem is in responsiveness to LH
https://www.ncbi.nlm.nih.gov/pubmed/779495
Rat study: Zinc deficiency reduces testosterone in rats. Problem seems to be in the leydig cells in the testes
https://www.ncbi.nlm.nih.gov/pubmed/6438259?dopt=Abstract&holding=npg
Rat study: Zinc deficiency suppresses 5AR and increases aromatase in the liver
https://www.ncbi.nlm.nih.gov/pubmed/8613886?dopt=Abstract&holding=npg
Unclear whether this is replicated in the testes - but this if it is, then sufficient zinc may also be neccessary for conversion of T to DHT.
Also a marked loss of spermatogenic cells was observed in Zn and Cu deficient rats.
https://www.ncbi.nlm.nih.gov/pubmed/2599557
Effect of marginal or severe dietary zinc deficiency on testicular development and functions of the rat.
https://www.ncbi.nlm.nih.gov/pubmed/9140621
Sheep Study Evidence
Zinc seems to operate differently in sheep than rats. There is evidence that it does affect the HPA axis.
https://www.ncbi.nlm.nih.gov/pubmed/1339830
General
https://www.ncbi.nlm.nih.gov/pubmed/21248196
https://www.ncbi.nlm.nih.gov/pubmed/19541612
https://www.ncbi.nlm.nih.gov/pubmed/19222874
https://www.ncbi.nlm.nih.gov/pubmed/17984944-