Do Mineral Supplements Work? Six Reviews, Narrower Answers

You've seen the posts. Magnesium for sleep. Zinc for testosterone. Iron for energy. And somewhere underneath the confident advice, a nagging question: does any of this actually work?

The supplement industry pulls in roughly $50 billion per year in the US alone, and minerals are its fastest-growing segment. The marketing says "everyone needs them." The science says something more nuanced, more useful, and cheaper.

Each mineral here has a real finding behind it, and each finding is narrower than the label. Zinc raised testosterone mainly in infertile men and people low in zinc. Iron supplements, tested only in iron-deficient, high-level female athletes, were followed by endurance gains. Magnesium helped older adults fall asleep faster, at low certainty.

And the daily multivitamin? A review of 19 meta-analyses found no effect on mortality, benefits in specific groups, such as better memory in older adults and lower blood pressure in at-risk people, and one harm signal. Not the zero some posts promise, and not the cure-all on the label either.

The question isn't whether supplements work. It's which ones, for whom, and how much of the answer depends on where you start.

This is not a guide that tells you which supplements to buy. It reads six evidence reviews and reports what each one found, including what none of them tested.

What went into this: six major evidence reviews covering vitamin D, zinc, magnesium (sleep and soreness, tracked separately), iron, and multivitamins. Sixteen supporting studies. More than five million people total. Every finding checked through our Skeptic Protocol.

The most-shared claim in supplement marketing is that ZMA boosts free testosterone by 33.5%. That number comes from one study, published in an unindexed journal, with 53% of participants dropping out before the trial ended. The co-author held the patent on ZMA, and later pled guilty in a federal steroid distribution case.

Two independent research teams replicated the experiment. Their combined result: zero effect.

One detail the supplement industry exploits: the original study measured free testosterone, which is more volatile and easier to show short-term changes in. No replication has confirmed that finding. The two clean attempts found nothing.

But zinc itself is not the fraud. The same 45-year evidence base that destroys the ZMA claim confirms a different finding. A 2023 systematic review of 38 papers, 30 of them animal studies and 8 in people, concluded that zinc deficiency reduces testosterone and zinc supplementation improves it. How much may depend on starting zinc and testosterone levels, the zinc form, the dose and the duration.

In the human studies, the rise showed up mainly in infertile men and in people low in zinc. In one trial in men with idiopathic infertility, testosterone improved when it started below 4.8 ng/ml, and zinc had no effect when it was higher.

Why low zinc lowers testosterone: in zinc deficiency, the review explains, the cells of the testes convert less of the body's raw material into active hormone.

How would you know if you're low? A serum zinc blood test costs about the same as a month of ZMA. If you're low, plain zinc at $5–8 per month is the cheap way to correct it. If you're not, the review's human evidence says little about you: its testosterone gains came mainly from infertile men and people low in zinc. The test costs less than two months of the supplement it might replace.

One finding makes this personal for anyone who trains: athletes consistently eat more zinc than non-active people, yet carry less of it in their blood. Training depletes zinc faster than diet replaces it. The people least likely to worry about zinc status are the most likely to be deficient. And they're the ones spending the most on ZMA.

Vitamin D has its own version of the story. In a 2024 meta-analysis of ten trials, 354 athletes took vitamin D3 or a placebo. Their blood levels climbed significantly. Their overall muscle strength did not improve significantly.

One test did: quadriceps contraction. But only two of the ten trials measured it, both in athletes who started low in vitamin D (about 11 and 13 ng/mL), and the authors call the result a potential trend that falls short of conclusive evidence. The review did not compare strength by starting level.

Two minerals, two reviews, and in both the clearest signal came from people who started low. Whether people who start with normal levels gain nothing, neither review settles: the zinc review reports testosterone rising with zinc in normal rats, and the vitamin D review never split strength by starting level.

Up to 60% of female athletes have low iron levels. A 2024 systematic review of 23 studies, 669 high-level female athletes in 16 sports found that iron deficiency costs endurance: in four of the seven studies that compared iron-deficient with iron-sufficient athletes, the iron-sufficient athletes finished time-trials 1% to 19% faster.

Iron has three stages of shortage, and this is where the problem hides. Hemoglobin stays normal until Stage 3, full anemia. In Stages 1 and 2, ferritin, the iron stores, falls while hemoglobin hasn't budged, and the review's iron-deficient athletes were mostly presumed to be at those earlier stages when their endurance came out lower.

Low iron stores slow the enzymes that use oxygen to make energy. The athlete feels heavier, recovers slower, can't finish intervals the way she used to, and has a hemoglobin result that says everything is fine.

This staging matters because it changes what "normal" means. A female distance runner with ferritin at 20 µg/L and hemoglobin at 12.5 g/dL is not anemic by the review's cut-off. She's in Stage 1 or early Stage 2, the range where the review's athletes had slower endurance.

Iron carries oxygen AND helps muscles turn it into energy. In the early stages, the review says, oxygen delivery is usually intact and the hit is thought to be in energy production; once hemoglobin falls, less oxygen arrives too. Either looks like overtraining, poor recovery, or bad programming. All three get blamed before anyone checks ferritin.

The practical fix, when ferritin confirms deficiency, is specific: about 100 mg of elemental iron a day, by mouth for up to 56 days (or every second day by injection over 8–10 days), was followed by endurance gains of 2% to 20%. The largest, about 20% longer to exhaustion, came from two small trials (18 and 16 athletes) in which the iron group did not differ significantly from placebo. In three studies with less than 100 mg a day, iron-treated athletes were no faster than placebo in time-trials, though lower doses over longer periods improved work rates and speeds at the lactate threshold. That dose matters, and it reveals a quiet fact about your multivitamin.

Your multivitamin contains 18mg, the recommended daily intake for pre-menopausal female athletes. That is 18% of the dose behind the review's endurance gains. The daily pill that promises to "cover your bases" delivers less than a fifth of it.

The marker that catches iron deficiency early is ferritin, not hemoglobin. Hemoglobin is the last domino to fall. Ferritin is the first. By the time hemoglobin drops, endurance may already be lower. One test, one number, one question your next blood panel should include.

Magnesium is the most recommended mineral on social media for sleep. It's also gaining traction for post-training soreness. The evidence for both is real. The evidence for both is also remarkably thin.

The sleep case: a meta-analysis of magnesium for insomnia in older adults found three small trials. Pooled from the two that timed it (55 people), magnesium helped people fall asleep 17 minutes faster. Total sleep time rose 16 minutes, a difference that was not statistically significant. The trials took adults over 51 or over 60; none was in young adults.

The evidence quality was rated Low to Very Low by the review's own standards, and its authors call the clinical significance of the 17 minutes "debateable". The NIH's magnesium fact sheet covers 64 references and four health conditions. Sleep is not among them, despite being the number-one consumer reason for buying magnesium.

Why magnesium would shorten the time to fall asleep is not known: the review calls the mechanisms linking magnesium to sleep "not well understood".

The soreness case: of 1,254 records a 2024 systematic review found, four studies met its criteria, with 73 people in all. The two that measured soreness found less of it; the other two measured muscle-damage markers and found less damage. An early signal, not a proven fix.

Same mineral, opposite evidence for cramps: a Cochrane review of 735 people found magnesium unlikely to help cramps. A completely different condition that shares nothing but the supplement aisle.

What neither review tested: whether people low in magnesium gain more. The sleep review reports no analysis by magnesium status. The soreness review suggests extra magnesium for active people in general, as a capsule 2 hours before training. About 48% of adults fall below recommended magnesium intake.

And the form debate all over the internet (glycinate vs. oxide vs. threonate) gets no answer from either review: neither compared forms. The sleep trials used oxide and citrate; the soreness studies used glycinate, oxide, lactate and pure magnesium, and the soreness review leaves the right type to future studies.

The $300-per-year premium for branded sleep formulas buys marketing, not proof: the sleep trials used the cheap forms.

Three minerals, three reviews, and one shared caution: each found its clearest signal in a narrower group than the label suggests. Zinc raised testosterone mainly in infertile men and people low in zinc. Iron supplements were tested only in iron-deficient athletes. Vitamin D's strength gain came from two small trials of athletes who started low.

None of them shows a floor below which supplements work and above which nothing changes. The iron review looked only at iron-deficient athletes. The vitamin D review never split strength by starting level. The zinc review reports testosterone rising with zinc in normal rats. And the magnesium sleep review never analysed its results by magnesium status.

The multivitamin, which bundles small doses of many nutrients for everyone, is a different question with a different answer.

Nineteen meta-analyses, at least 5.5 million people, counting pregnancies and children whose mothers took supplements. A 2025 rapid review of them found no effect on all-cause mortality, COVID-19 outcomes or visual acuity. Its randomized trials did show benefits in specific groups: better global cognition and memory in older adults, less stress, fewer mild psychiatric symptoms and less fatigue in healthy adults, and a small drop in systolic blood pressure, larger in people with hypertension. Observational studies linked use to less colorectal cancer, coronary heart disease, cataracts and hip fractures, but the lower coronary heart disease did not show up in the trials, and the review flags that supplement users tend to live healthier lives. The authors call the effects mixed, with potential benefits mainly in older adults, people with lower dietary quality and people with chronic conditions. The review did not look at body composition or exercise performance. More than a hundred studies on mineral supplements in athletes specifically: zero performance benefit. The multivitamin question has a mixed answer.

The dose gap is real for iron. The iron endurance trials used about 100mg per day. The multivitamin delivers 18mg.

One harm signal stood out. In 13 trials of 85,321 adults with or at risk of age-related macular degeneration, progression was about twice as likely with multivitamins as with placebo (2.08 times). "At worst, expensive urine" understates it. There may be a cost beyond money.

The review's own conclusion points where the single-mineral reviews point: who takes the pill matters. It calls for trials in people with laboratory-confirmed micronutrient insufficiency. The review was funded by the National University of Singapore and Haleon; the authors say the funders had no role.

One more note for people who train: the ISSN position stand flags that antioxidant supplements may blunt training gains. The same molecules that mop up free radicals also mop up the stress signals muscles need to grow.