Effects depend strongly on whether the person is deficient. Probably helpful for muscle strength in older adults, immune-related effects, asthma or COPD, skin conditions, and blood sugar control, but many headline claims still land in mixed territory. Subgroup framing matters more here than for almost any other supplement.
Each row is a different health effect. A means the support looks strongest, B likely helpful, C mixed or modest, and — not enough human evidence yet.
Papers are individual studies. Research reviews summarize evidence from multiple studies, such as systematic reviews and meta-analyses.
Dose, form and deficiency thresholds based on trials in the database.
Products that pass third-party quality testing.
Grades come from 767 unique research papers indexed from PubMed — including 142 meta-analyses, 78 systematic reviews, and 547 randomized trials. Subgroup-dependent effects (deficient vs. sufficient populations) make pooled grades less informative than per-subgroup grades. Where data permitted, subgroup framing leads. Each supplement is graded by health effect using a certainty × effect matrix applied across PubMed evidence per health effect. Cochrane reviews and high-quality systematic reviews carry the most weight. From abstract-only data, "high certainty" is rare — most A grades come from moderate certainty plus consistent meaningful-positive evidence. See Methodology for full detail.
Read the full methodology → · Download all 767 citations (PDF)