Half a year of high-dose zinc did not move testosterone
Eighty-seven men took 66 mg of zinc sulphate daily for 26 weeks against placebo. Testosterone did not move. Sperm concentration did rise in the subfertile men.
What the study found
Six months of 66 mg of zinc a day left testosterone alone
Eighty-seven men took 66 mg of zinc sulphate daily for 26 weeks against placebo, six times the recommended intake and well over the 40 mg ceiling. Testosterone did not move. Sperm concentration, the outcome the trial was actually built around, did rise in the subfertile men.
The caveat · Zinc came with folic acid
The zinc came with 5 mg of folic acid a day and there was no zinc-only arm, so neither result can be pinned on the zinc by itself. The men were fertile and subfertile patients at a fertility clinic, not chosen for their zinc status, so this shows what happens when you supplement a mixed group rather than a group known to have enough. Eighty-seven men across four arms is thin enough that this is an effect not detected rather than an effect ruled out.
What we don't know
The zinc came with 5 mg of folic acid a day and there was no zinc-only arm, so neither result can be pinned on the zinc by itself. The men were fertile and subfertile patients at a fertility clinic, not chosen for their zinc status, so this shows what happens when you supplement a mixed group rather than a group known to have enough. Eighty-seven men across four arms is thin enough that this is an effect not detected rather than an effect ruled out.