Zinc

Bisglycinate and picolinate are the two top-tier forms — both chelated, both 20-30% absorbable depending on the study and the gut environment. Bisglycinate is gentler on the stomach; picolinate has a slight edge in some absorption trials. Citrate is the value pick — meaningfully better than oxide, often under $0.10 per dose. Sulfate is the historical research form (Prasad 1996 used zinc sulfate), bioavailable but more nausea-prone. AVOID oxide — it's the cheapest form, dominant in big-box multivitamins, and has roughly half the absorption of the chelated forms. "Zinc oxide, 50 mg" delivers less elemental zinc than "zinc bisglycinate, 15 mg."

Yes — but only in deficient men. Prasad 1996 (PMID 8702195) measured a near-doubling of serum testosterone (8.3 → 16.0 nmol/L) at 30 mg/day for 6 months in marginally zinc-deficient men. The mechanism is aromatase inhibition (less T leaks into estradiol) plus restored Leydig-cell synthesis. In zinc-replete men, the effect is essentially zero — zinc is a cofactor-restoration play, not a stimulant. Get a serum zinc test before assuming you need it: if you're already at 90+ μg/dL, supplementation isn't going to move your T much. If you're below 80 μg/dL, especially with a low-meat or vegan diet, the upside is real.

The RDA is the minimum to prevent symptomatic deficiency in 97% of the population, not the optimal therapeutic dose. The testosterone, acne, and immune trials all used doses meaningfully above RDA — Prasad 1996 used 30 mg, Cervantes 2019 catalogued 30-50 mg for acne, Hemilä's cold meta-analysis used 80+ mg acute via lozenges. The Tolerable Upper Intake Level (40 mg/day chronic) is the safety ceiling for daily long-term use; 25-30 mg sits comfortably within it and matches the trial record. If you're running a multivitamin that already contains 15 mg zinc, factor that into your total — don't stack to 50+ mg/day chronically without copper co-supplementation.

Zinc and copper compete for the same intestinal transporter (DMT1 / Menkes protein), so chronic high-dose zinc (>40 mg/day) competitively blocks copper absorption. Long-term excess causes copper deficiency, which manifests as anemia, neutropenia, and in extreme cases neurological symptoms (the documented failure mode in long-term denture-cream zinc-toxicity cases at gram-level doses). The protocol fix: if dosing 50+ mg zinc chronically, co-supplement 2 mg copper bisglycinate at a separate meal. If dosing 15-30 mg, no copper supplementation needed — the antagonism is dose-dependent and clinically negligible at sub-40 mg/day.

Different protocols, different mechanisms. Tablets/capsules (15-30 mg/day with food) are for chronic supplementation — testosterone support, acne, structural deficiency repletion. Lozenges (zinc acetate or zinc gluconate, 75-80 mg/day total dissolved slowly in the mouth) are for ACUTE cold symptoms — they work by bathing the throat in zinc ions that inactivate rhinovirus locally, plus systemic immune support. The Hemilä 2017 meta-analysis (PMID 28515951) of 18 RCTs found cold duration cut ~33% when lozenges were started within 24 hours of symptom onset and dosed every 2-3 hours while awake. Don't take lozenges chronically (the daily dose is far above the UL) and don't expect tablets to shorten an active cold meaningfully — they're different tools.

With food, always — specifically a meal containing protein and fat. Zinc on an empty stomach causes nausea in roughly 30% of users (the rate is higher with sulfate and gluconate, lower with bisglycinate). The trade-off: food slightly reduces absorption (calcium and phytates in the meal bind some zinc), but the tolerability gain massively outweighs the absorption hit for sustained dosing. Separate zinc from your highest-calcium meal (don't co-dose with a large dairy serving) and from iron supplements by 2 hours minimum — the three minerals all compete for the same transporter.

The COVID-specific zinc story turned out weaker than the early 2020 hype. The initial mechanistic case was strong — zinc inhibits RNA-dependent RNA polymerase, the enzyme SARS-CoV-2 replicates with — but the controlled trials (notably the COVID A to Z study) didn't find meaningful clinical benefit at the doses tested. The broader cold + URI evidence remains intact: Hemilä's lozenge meta-analysis is robust for rhinovirus-driven colds. For COVID specifically, the data don't support zinc as a primary intervention. For routine cold prevention + duration reduction, zinc lozenges started early remain one of the most-evidenced over-the-counter tools.

Three approaches, in order of reliability. (1) Serum zinc test — most accurate single marker. Aim for >90 μg/dL; below 80 is functional deficiency. Available through any standard blood panel; cheap to add. (2) Dietary audit — if you're vegetarian/vegan, eat <2 servings of red meat per week, or are an endurance athlete, you're statistically likely deficient even with a multivitamin. (3) Symptom cluster — persistent acne, slow wound healing, frequent colds, hair shedding, suppressed libido in younger men, white spots on fingernails (leukonychia), reduced sense of taste/smell. None of these are zinc-specific in isolation, but the cluster suggests trial supplementation is reasonable. The cheapest path: 8-week trial at 25 mg/day with food, recheck symptoms; if no shift, you weren't deficient.