| Clinical pearl | Prophylactic supplementation (200 mg or more per day) reduces cold duration by 8% in adults and 14% in children but does not prevent colds [3]. Bioavailability plateaus above 200 mg per dose — split larger doses [1]. Co-prescribe with iron-rich meals to enhance non-heme iron absorption 2–3 fold [1]. Smokers require an additional 35 mg/day [1]. | Zinc lozenges (gluconate or acetate, 75 mg/day total) started within 24 hours of cold onset reduce duration by approximately one-third [2]. Chronic supplementation above 40 mg/day induces copper deficiency — co-prescribe 1–2 mg copper if using high doses long-term [1]. Vegetarians require 50% more dietary zinc due to phytate inhibition [1]. Avoid zinc nasal sprays (permanent anosmia risk). | Check 25(OH)D in all at-risk patients (BMI >30, age >65, dark skin, northern latitude) [21]. D3 is 87% more potent than D2 at raising serum levels [4]. Prescribe with a fat-containing meal for optimal absorption [7]. Co-supplement with K2 (MK-7) for bone and vascular benefit [17]. Target 30–50 ng/mL; >50 rarely adds benefit and >150 risks toxicity [1][21]. Large bolus dosing (>300,000 IU) paradoxically increases falls — use daily/weekly dosing [11][25]. The VITAL trial showed no cancer or CVD prevention at 2,000 IU/day but did reduce autoimmune disease by 22% [12][13]. |