Two supplements can list the same nutrient at the same milligrams and deliver completely different results. The variable is the chemical form — and most brands hide it. This page explains, ingredient by ingredient, what to look for and why our standard is what it is.
Oxide / Sulfate
Magnesium oxide has ~4% bioavailability vs ~30%+ for glycinate or citrate. Oxide is cheap because most of it isn't absorbed — it's commonly used as a laxative, not a magnesium supplement.
NIH ODS — Magnesium Fact Sheet for Health Professionals →Cyanocobalamin (contains a cyanide molecule the body must remove)
Methylcobalamin is the active coenzyme form your body uses directly. Cyanocobalamin is synthetic and must be converted — a step impaired in roughly 25% of the population with MTHFR variants.
NIH ODS — Vitamin B12 Fact Sheet →Folic acid (requires hepatic conversion)
Up to 40% of people carry MTHFR polymorphisms that limit folic acid → 5-MTHF conversion. Methylfolate skips the bottleneck and is the form your cells actually use.
NIH ODS — Folate Fact Sheet →D2 (ergocalciferol)
D3 raises serum 25(OH)D more efficiently and lasts longer than D2. The Endocrine Society and most clinical guidelines specify D3 for routine supplementation.
NIH ODS — Vitamin D Fact Sheet →Oxide / Sulfate
Zinc oxide is poorly absorbed and frequently used because it's cheap. Picolinate and bisglycinate forms are absorbed at substantially higher rates, especially when taken with food.
NIH ODS — Zinc Fact Sheet →Ferrous sulfate (high GI side effects, lower tolerability)
Bisglycinate is gentler on the GI tract and shows comparable or superior absorption to sulfate at lower doses. Ferrous sulfate's well-documented constipation and nausea profile reduces adherence.
NIH ODS — Iron Fact Sheet →Carbonate (poorly absorbed without stomach acid)
Calcium carbonate requires acidic gastric pH to dissolve, which makes it a poor choice for older adults and anyone on PPIs/antacids. Citrate absorbs well regardless of stomach acid.
NIH ODS — Calcium Fact Sheet →Ubiquinone (oxidized form, lower absorption above age 40)
Ubiquinol is the body-ready reduced form. Adults over ~40 lose efficiency at converting ubiquinone → ubiquinol, so the active form delivers higher serum levels per dose.
NIH ODS / NCCIH — Coenzyme Q10 →Plain curcumin (essentially zero oral bioavailability)
Plain curcumin is poorly absorbed and rapidly metabolized. Piperine increases bioavailability ~20×; phytosomal/liposomal carriers offer multi-fold improvements through enhanced uptake.
PubMed — Shoba 1998; Cuomo 2011 →On every Supplement Facts panel, the meaningful information is in the form noted in parentheses — “Magnesium (as Magnesium Glycinate),” not just “Magnesium.”
We map each form to the National Institutes of Health Office of Dietary Supplements bioavailability guidance. If a form isn't in the cited ‘preferred’ range, we don't claim it's high-bioavailability.
We will not call a supplement ‘highly bioavailable’ if its disclosed form is oxide, carbonate, cyanocobalamin, or folic acid. The chemistry settles it, not our marketing copy.
Educational reference only
The information on this page is sourced from the National Institutes of Health Office of Dietary Supplements, NCCIH, MedlinePlus, and peer-reviewed clinical research. It is provided for education and is not medical advice. Talk to your physician or pharmacist before starting any supplement, especially if you take medications or have a health condition.
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