Testosterone fell when men were fed almost no zinc for five weeks
Eleven men ate 1.4 mg of zinc a day for 35 days and testosterone fell from 26.9 to 21.9 nanomoles per litre. Every arm sat at or below what a man is meant to get.
What the study found
Starving men of zinc for five weeks lowered testosterone
Eleven men on a metabolic ward ate 1.4 mg of zinc a day, an eighth of the male requirement, for 35 days. Testosterone fell from 26.9 to 21.9 nanomoles per litre, roughly a fifth, and semen volume fell with it. The evidence behind zinc and testosterone is an experiment in taking it away.
The caveat · Tests going short, never extra zinc
Eleven men, and every arm of the study sat at or below the 11 mg of zinc a day a man is meant to get, so it compares degrees of going short and never tests giving anyone extra. Thirty-five days at each intake says nothing about years at one. Nothing here shows that a man who already has enough zinc gains anything from more.
What we don't know
How far zinc shortage lowers testosterone is unsettled. An uncontrolled study of four men reported a 73% fall; a controlled feeding study that cut zinc far lower saw 19%. The weaker design produced the figure that travelled.