Do zinc and ZMA raise strength and testosterone?

Short answer

Not unless you're low in zinc. In 9 people low in zinc, testosterone nearly doubled, but in 14 who already ate enough and 42 who trained for 8 weeks, ZMA made no difference to testosterone or strength.

Do zinc and ZMA raise strength and testosterone?
'More zinc → more testosterone → more strength' — does this formula hold?
ZMA is a supplement mixing zinc, magnesium and vitamin B6, sold with the claim that taking it before bed raises testosterone and strength. We checked step by step whether zinc really raises testosterone and whether that adds strength.
Zinc and testosterone are linked
Cutting zinc took 100 down to 27
When 4 people cut zinc from their food for 20 weeks, testosterone fell from 100 to about 27. The other way round, when 9 people low in zinc took zinc for 3-6 months, testosterone nearly doubled (8.3 → 16.0).
But what if you're not low?
Blood zinc rose, hormones stayed put
14 people who already got 11.9-23.2 mg of zinc a day from food took ZMA. Zinc in the blood and in urine went up, but testosterone did not change, neither the total level nor the active (free) level.
Strength showed no difference either
8 weeks of training, bench 5.6 kg vs 3.6 kg
42 people did the same strength training for 8 weeks while taking ZMA (30 mg zinc, 450 mg magnesium) or a placebo. Bench press max rose 5.6 kg vs 3.6 kg and leg press 25.4 kg vs 24.6 kg, with no difference. Testosterone and body composition showed no difference either.
What about the magnesium?
No strength effect in 215 active people
An analysis that pooled 14 randomized trials found that in 215 active people, magnesium did not raise strength or muscle power. An effect showed up only in people low in magnesium.
Does it improve sleep, then?
No difference even after 4-hour nights
16 people slept only 4 hours for two nights while rotating ZMA, a placebo and nothing. ZMA changed neither sleep quality nor lifting power and speed the next morning.
Bottom line: only people low in zinc saw a rise
If your diet covers it, ZMA adds nothing
Testosterone rose only in the 9 people low in zinc. In the 14 who already ate 11.9-23.2 mg a day and the 42 who trained for 8 weeks, neither testosterone nor strength differed. Getting enough zinc from your regular diet is enough, rather than a supplement.