Vitamin D

The mechanism is real

Vitamin D receptors are expressed in testicular tissue and in the pituitary, and vitamin D is involved in the regulation of steroidogenic enzymes. This is not a case where a plausible mechanism has been invented after the fact to justify a supplement.

Observational versus intervention data

Cross-sectional studies consistently find that men with higher serum 25-hydroxyvitamin D have higher testosterone. That association is robust but cannot establish direction: better overall health, more time outdoors and lower adiposity all track with both.

Intervention trials are more mixed. The most cited is a randomized controlled trial in overweight men undergoing a weight reduction programme, where 3,332 IU daily for one year significantly increased total testosterone, free testosterone and bioactive testosterone compared with placebo.¹ Other trials, particularly in men who were already replete, have found no effect.²

The pattern is the same one we describe for zinc: correcting a deficiency produces measurable change, adding more on top of sufficiency does not.

Deficiency is common

This is what makes vitamin D worth including despite the mixed intervention data. Insufficiency is widespread, particularly at higher latitudes, in winter, in men with darker skin, in those who work indoors and in those carrying excess weight. A meaningful share of the population is starting from a state where correction is the relevant question.

Unlike most ingredients in this category, this one is measurable. A serum 25-hydroxyvitamin D test is inexpensive and widely available, and it converts a guess into a fact.

Dosing

Supplemental doses of 1,000 to 4,000 IU daily of vitamin D3 are typical and sit within the tolerable upper intake level of 4,000 IU. Vitamin D3 is better than D2 at raising serum levels. Because it is fat soluble, absorption improves when taken with a meal containing fat.

Very high doses are not better. Intermittent megadosing has performed poorly in trials, and toxicity, though rare, is real.

Bottom line

Vitamin D is worth having in a formula, and worth measuring rather than assuming. If you are deficient, correction is one of the higher-value interventions available and it costs very little. If you are already replete, do not expect a hormonal effect from adding more.