The dose on the label is not the dose your body gets
You could take 500mg of a supplement and absorb 400mg — or 20mg. The difference is the form. Bioavailability measures the fraction of an ingested substance that reaches systemic circulation in an active state. For supplements, this means the percentage of the stated dose that your cells can actually use.
This is not a minor technicality. Choosing magnesium oxide over magnesium glycinate means absorbing roughly 4% instead of 80% of the stated dose — a 20-fold difference that determines whether you are addressing a deficiency or flushing money through your digestive tract.
Magnesium Forms
Magnesium is the most form-sensitive mineral supplement. The elemental magnesium content and absorption rate vary dramatically:
| Form | Elemental Mg (%) | Bioavailability | Best For | Notes |
|---|---|---|---|---|
| **Glycinate** (bisglycinate) | 14% | ~80% | Sleep, anxiety, general repletion | Best tolerated; minimal GI effects |
| **Threonate** | 8% | ~80% | Cognitive function | Crosses blood-brain barrier; lower elemental content |
| **Citrate** | 16% | ~30% | Constipation, general use | Mild laxative effect; good balance of cost and absorption |
| **Taurate** | 9% | ~25% | Cardiovascular health | Taurine adds cardioprotective synergy |
| **Malate** | 15% | ~25% | Energy, muscle pain | Malic acid supports ATP production |
| **Oxide** | 60% | ~4% | Laxative use only | High elemental content is misleading — almost none is absorbed |
| **Sulfate** (Epsom salt) | 10% | ~4% | Topical/bath use | Poor oral absorption; effective transdermally |
Recommendation: [Magnesium glycinate](/supplements/magnesium-glycinate) for most people. Threonate for cognitive focus. Citrate as a budget option with decent absorption.
Iron Forms
Iron absorption is complicated by both the form and what you eat alongside it:
| Form | Bioavailability | GI Side Effects | Best For |
|---|---|---|---|
| **Iron bisglycinate** (chelated) | ~4x ferrous sulfate | Minimal | Most people with iron deficiency |
| **Ferrous sulfate** | Baseline reference | Common (nausea, constipation) | Cost-sensitive; clinical standard |
| **Ferrous gluconate** | ~12% | Moderate | Alternative when sulfate causes GI issues |
| **Ferrous fumarate** | ~33% | Moderate-High | High elemental iron content |
| **Heme iron polypeptide** | 10-15x nonheme | Minimal | Severe deficiency; best absorption |
| **Carbonyl iron** | Slow-release | Low | Reduced acute toxicity risk |
Recommendation: Iron bisglycinate (Ferrochel) for the best absorption-to-side-effect ratio. Take with [vitamin C](/supplements/vitamin-c) to enhance nonheme iron absorption by 2-3x.
B-Vitamin Forms
| Vitamin | Inferior Form | Optimal Form | Why It Matters |
|---|---|---|---|
| [B12](/supplements/vitamin-b12) | Cyanocobalamin | Methylcobalamin or adenosylcobalamin | Cyano requires conversion; methyl is directly active |
| [Folate](/supplements/folate) (B9) | Folic acid | 5-MTHF (methylfolate) | 40% of people have MTHFR variants that impair folic acid conversion |
| [B6](/supplements/vitamin-b6) | Pyridoxine HCl | Pyridoxal 5'-phosphate (P5P) | P5P is the active coenzyme form; no liver conversion needed |
| [B1](/supplements/vitamin-b1) | Thiamine HCl | Benfotiamine | Benfotiamine is fat-soluble with 5x higher bioavailability |
| [B2](/supplements/vitamin-b2) | Riboflavin | Riboflavin 5'-phosphate (R5P) | R5P is the active coenzyme form |
Recommendation: Look for B-complex products listing methylcobalamin, methylfolate, and P5P — this indicates a formulator who prioritizes bioavailability over cost.
Fat-Soluble Vitamin Forms
| Vitamin | Forms | Optimal | Notes |
|---|---|---|---|
| [Vitamin D](/supplements/vitamin-d) | D2 (ergocalciferol) vs D3 (cholecalciferol) | **D3** | D3 raises 25(OH)D levels ~87% more effectively than D2 |
| [Vitamin E](/supplements/vitamin-e) | dl-alpha-tocopherol (synthetic) vs d-alpha-tocopherol (natural) | **d-alpha (natural)** | Natural form has 2x the biological activity of synthetic |
| [Vitamin K2](/supplements/vitamin-k2) | MK-4 vs MK-7 | **MK-7** | MK-7 half-life is 72h vs MK-4 at 1-2h; one daily dose sufficient |
| [Vitamin A](/supplements/vitamin-a) | Beta-carotene vs retinyl palmitate vs retinol | **Retinol** (preformed) | Beta-carotene conversion varies 3-28x between individuals |
Recommendation: D3 in oil-based softgels (fat enhances absorption). Vitamin E as mixed tocopherols (not just alpha). K2 as MK-7.
Mineral Forms Comparison
| Mineral | Inferior Form | Optimal Form | Difference |
|---|---|---|---|
| [Zinc](/supplements/zinc) | Zinc oxide | Zinc picolinate or bisglycinate | Picolinate: ~4x better absorption |
| [Calcium](/supplements/calcium) | Calcium carbonate | Calcium citrate | Citrate absorbed on empty stomach; carbonate requires acid |
| [Selenium](/supplements/selenium) | Sodium selenite | Selenomethionine | Organic form with ~90% bioavailability vs ~50% for selenite |
| [Chromium](/supplements/chromium) | Chromium chloride | Chromium picolinate | Picolinate is the form used in glucose metabolism RCTs |
| [Copper](/supplements/copper) | Copper oxide | Copper bisglycinate | Chelated form avoids zinc competition at absorption sites |
Nutraceutical Forms
| Compound | Standard Form | Optimal Form | Absorption Increase |
|---|---|---|---|
| [Curcumin](/supplements/turmeric) | 95% curcuminoids extract | Meriva (phytosome) or C3 + BioPerine | 29x (phytosome) or 20x (BioPerine) |
| [CoQ10](/supplements/coq10) | Ubiquinone | Ubiquinol | 2x in adults over 40 (reduced form, no conversion needed) |
| [Resveratrol](/supplements/resveratrol) | Trans-resveratrol | Micronized or liposomal | 2-5x (standard resveratrol has ~1% oral bioavailability) |
| [Quercetin](/supplements/quercetin) | Quercetin dihydrate | Quercetin phytosome (Quercefit) | 20x (phytosomes bypass poor gut absorption) |
| [Berberine](/supplements/berberine) | Standard berberine HCl | Dihydroberberine (GlucoVantage) | 5x (active gut metabolite, skips first-pass conversion) |
Absorption Enhancers
Some forms are inherently well-absorbed, but others benefit from co-factors:
| Enhancer | Works With | Mechanism | Dose |
|---|---|---|---|
| BioPerine (piperine) | Curcumin, CoQ10, resveratrol | Inhibits glucuronidation in gut/liver | 5-20mg |
| Vitamin C | Iron (nonheme) | Reduces Fe³⁺ to Fe²⁺ for better gut absorption | 100-200mg with iron |
| Fat/oil | Vitamins D, E, K, A; CoQ10 | Fat-soluble compounds require bile salt emulsification | Take with a meal containing fat |
| Black pepper | Curcumin, beta-carotene, selenium | Piperine increases thermogenic absorption | 5mg piperine |
How to Use This Cheat Sheet
1. Find your supplement in the tables above
2. Check your current product's label for the specific form
3. If you are using an "inferior" form, consider switching — the absorption difference often justifies a modest price increase
4. When comparing prices, calculate cost per absorbed dose, not cost per total dose