Quick Answer: Most supplement labels list milligrams — but milligrams don’t equal absorption. Reported figures vary between studies, but magnesium oxide typically absorbs at only ~4% while magnesium glycinate is reported in the 50–80% range. In controlled comparisons ferrous bisglycinate is absorbed roughly 2–4× better than ferrous sulfate, and both outperform cheap ferric (non-heme iron oxide) forms. This guide summarizes the real absorption numbers for every major supplement form so you can stop wasting money on pills your body can’t use.
Key Takeaways
- Magnesium oxide (the most common supplement form) absorbs at roughly 4% — many times worse per milligram than magnesium glycinate (reported 50–80%).
- Ubiquinol CoQ10 outperforms ubiquinone in most head-to-head trials, especially in older adults whose conversion of ubiquinone to ubiquinol declines. Absolute absorption percentages vary widely across studies.
- Standard curcumin extract has very low bioavailability (~1%); formulated versions (phytosome, piperine co-ingestion, solid lipid particles) multiply it many-fold. Specific multipliers (e.g. 20× for piperine) come from individual trials — they are directional, not a universal constant.
- Vitamin D3 is consistently more efficient than D2 at raising serum 25(OH)D per IU; the Tripkovic 2012 meta-analysis and follow-ups confirm this.
- B12 form matters: hydroxocobalamin and methylcobalamin (sublingual) have near-complete retention; cyanocobalamin at pharmacologic oral doses (500 mcg+) passively diffuses a meaningful fraction even without intrinsic factor.
- Calcium carbonate requires stomach acid — it performs meaningfully worse when taken fasted vs. with food, which is why calcium citrate is preferred for older adults and people on acid-suppressing medication.
Why Bioavailability Matters More Than Dose
When a supplement bottle says “500mg,” that’s the amount going in. What matters is what actually crosses your intestinal wall and reaches your bloodstream — and those numbers are dramatically different.
Bioavailability refers to the fraction of an administered dose that reaches the systemic circulation in an active form. For supplements, it’s shaped by:
- Solubility — fat-soluble vs. water-soluble, crystalline vs. amorphous forms
- Particle size — nanoparticle formulations dramatically increase surface area
- Chelation — amino acid chelates (glycinate, bisglycinate) act as carriers
- Competitive absorption — calcium and iron share the same transporter
- Stomach environment — some forms need acid; others are acid-independent
- Food matrix — fat-soluble vitamins need dietary fat; others need fasting
The numbers below come from pharmacokinetic studies, not manufacturer claims. Where ranges exist, we’ve used midpoints from the best-controlled trials.
Iron Forms: Bioavailability Rankings
Iron deficiency is the most prevalent nutritional deficiency worldwide, yet most iron supplements use cheap forms your body barely absorbs.
| Iron Form | Absorption Rate | Notes |
|---|---|---|
| Ferrous Bisglycinate | High (2–4× ferrous sulfate in controlled trials) | Best-tolerated, gentle on GI |
| Ferrous Sulfate | ~10–15% (fasted, iron-replete) | Standard prescription form; high GI side effects |
| Ferrous Fumarate | Similar to ferrous sulfate | Common in prenatal vitamins |
| Ferric (non-heme) salts | Low (often single-digit %) | Cheapest forms; typically avoid in supplements |
| Carbonyl Iron | Lower than ferrous sulfate per mg; slow release | Less GI upset than sulfate |
| Heme Iron (from food) | 15–35% | Range depends on body iron status |
| Non-heme plant iron | 2–20% | Varies widely; enhanced by vitamin C |
Clinical note: Iron absorption is regulated by body iron status. When stores are low, absorption can increase up to 3× the baseline rates above. Vitamin C co-ingestion increases non-heme iron absorption by 2–3×.
Magnesium Forms: Bioavailability Rankings
Magnesium is a cofactor in over 300 enzymatic reactions. Yet the most popular supplement form (oxide) is essentially a laxative with negligible absorption.
| Magnesium Form | Absorption Rate | Best Use |
|---|---|---|
| Magnesium Glycinate | ~50–80% (reported range) | Sleep, anxiety, general deficiency |
| Magnesium Malate | High; better than oxide/citrate in comparative studies | Energy, muscle function |
| Magnesium Citrate | Good — better than oxide, often cited near ~60% | General use, constipation relief |
| Magnesium Chloride | Good oral/topical absorption | Versatile |
| Magnesium L-Threonate | Comparable to other chelates; unique CNS delivery | Crosses blood-brain barrier |
| Magnesium Taurate | Good; combined with taurine | Cardiovascular |
| Magnesium Oxide | ~4% | Laxative effect; poor supplemental value |
Why oxide dominates despite 4% absorption: It’s cheap to manufacture and sells by the milligram. A 500mg oxide pill sounds impressive vs. a 200mg glycinate — but the glycinate delivers 160mg absorbed vs. 20mg from oxide.
Calcium Forms: Bioavailability Rankings
Calcium absorption is more nuanced than most minerals because it depends heavily on stomach acid levels and co-factors.
| Calcium Form | Absorption Rate | Notes |
|---|---|---|
| Calcium Citrate Malate | ~40–45% | Best overall; acid-independent |
| Calcium Citrate | ~30–35% | Good choice for older adults (low acid) |
| Calcium Carbonate (with food) | ~25–30% | Requires stomach acid; take with meals |
| Calcium Carbonate (fasted) | Meaningfully lower than with food | Significantly reduced without acid |
| Calcium Gluconate | Lower elemental Ca per pill; moderate absorption | Not a first-choice form |
| Calcium Phosphate | Moderate absorption | Common in antacids |
| Calcium Lactate | Comparable to citrate | Less common commercially |
Critical caveat: Calcium competes with magnesium, zinc, and iron for absorption. Never take calcium and iron supplements together. Splitting doses (500mg max per sitting) improves absorption by 10–20%.
CoQ10: Ubiquinol vs. Ubiquinone
CoQ10 bioavailability is one of the most studied — and most manipulated — supplement claims on the market.
| CoQ10 Form | Absorption Rate | Notes |
|---|---|---|
| Ubiquinol (reduced) | Highest in most head-to-head comparisons | Bioactive form; preferred for age 40+ |
| Ubiquinone (softgel / oil base) | Good; better than powdered/crystalline | Converted to ubiquinol in the body |
| Ubiquinone (powder/tablet) | Lowest | Crystalline form; poor absorption |
| Ubiquinone + piperine | Higher than plain powder | Piperine inhibits metabolism |
| Nano-/solubilized CoQ10 | High | Specialized formulations |
Why it matters: If you’re over 40, your body’s ability to convert ubiquinone to ubiquinol declines. The ubiquinol form bypasses this conversion step, delivering the bioactive molecule directly.
Curcumin Formulations: Bioavailability Rankings
Standard curcumin extract has approximately 1% bioavailability. The supplement industry has invested heavily in fixing this — with mixed success.
| Curcumin Form | Relative Bioavailability | Notes |
|---|---|---|
| Phytosome (Meriva®) | Manufacturer/trial data: ~20–30× standard | Phospholipid complex; well studied |
| Nanoparticle Curcumin | High multiplier in PK studies | Small particle size |
| Curcumin + Piperine (BioPerine®) | Shoba 1998 reported ~20× (2000%) serum AUC vs. curcumin alone | Piperine inhibits glucuronidation |
| CurcuWIN® (water-dispersible) | Manufacturer PK study: ~45× vs. standard | Hydrophilic; high AUC in trials |
| Longvida® (SLCP) | Manufacturer data: up to ~65× standard (free curcumin) | Solid lipid curcumin particles |
| BCM-95® (Biocurcumax) | Reported several-fold increase | Self-emulsifying; volatile oils |
| Standard extract (95%) | 1× (baseline) | Reference form; very poor without enhancer |
Bottom line: “Turmeric 500mg” with standard extract delivers roughly the same curcumin as a properly formulated 50mg phytosome dose. Price accordingly — the cheapest curcumin is often the most expensive in terms of actual dose delivered.
Vitamin D: D3 vs. D2
The D2 vs. D3 debate was largely settled by a 2012 Cochrane analysis and subsequent meta-analyses.
| Vitamin D Form | Efficacy vs. D2 | Half-Life | Notes |
|---|---|---|---|
| Vitamin D3 (Cholecalciferol) | Reference standard (more efficient per IU) | Longer | From animal or lichen sources; converts to 25(OH)D3 |
| Vitamin D2 (Ergocalciferol) | Less efficient at raising 25(OH)D per IU | Shorter | Plant/fungi-derived |
The Tripkovic et al. 2012 meta-analysis (Am J Clin Nutr 2012;95:1357–1364) concluded that D3 raises serum 25(OH)D more effectively than D2 across a range of dosing regimens. For vegans avoiding animal-derived D3, lichen-derived D3 is available and is understood to behave equivalently to the animal-sourced form.
Dose-absorption note: At doses over 10,000 IU/day, absorption efficiency begins to decline. Stay under 4,000 IU/day without physician supervision.
Vitamin B12 Forms: Bioavailability Rankings
B12 bioavailability is complicated by two factors: intrinsic factor (IF) dependency for passive absorption, and route of administration (oral vs. sublingual vs. injection).
| B12 Form | Absorption Rate | Notes |
|---|---|---|
| Hydroxocobalamin (IM injection) | Effectively complete | Highest retention; long half-life |
| Methylcobalamin (sublingual) | High | Bioactive form; widely used orally |
| Adenosylcobalamin | High | Mitochondrial form |
| Hydroxocobalamin (oral) | Good | Better retained than cyano |
| Cyanocobalamin (oral, high dose ≥500 mcg) | Meaningful fraction via passive diffusion | Standard pharmaceutical form |
| Cyanocobalamin (oral, 1–10 mcg physiologic dose) | ~1–3% (IF-dependent) | Tiny amounts reach circulation |
Why form and dose both matter: At low doses (1–10mcg), B12 relies on intrinsic factor — a protein secreted by the stomach. At high doses (500mcg+), passive diffusion bypasses IF entirely, which is why high-dose cyanocobalamin can still work despite its lower per-molecule bioavailability.
People over 50, those with gastric issues, or those on metformin or PPIs should strongly consider methylcobalamin or hydroxocobalamin rather than standard cyanocobalamin.
Bioavailability by Nutrient: The Master Table
| Supplement | Best Form | Absorption | Worst Form | Absorption |
|---|---|---|---|---|
| Magnesium | Glycinate | ~50–80% | Oxide | ~4% |
| Iron | Ferrous Bisglycinate | 2–4× ferrous sulfate | Ferric (non-heme) | Single-digit % |
| Calcium | Citrate Malate | ~40–45% | Carbonate (fasted) | Notably lower than with food |
| CoQ10 | Ubiquinol | Highest in most comparisons | Ubiquinone (powder) | Lowest |
| Curcumin | Longvida SLCP | Up to ~65× standard (mfr data) | Standard extract | 1× |
| Vitamin D | D3 | More efficient than D2 per IU | D2 | Less efficient per IU |
| B12 | Methylcob. sublingual / hydroxocob. IM | High retention | Cyanocobalamin at 1 mcg | ~1–3% |
| Zinc | Bisglycinate/picolinate | High | Oxide | Low |
| Folate | 5-MTHF | High; does not require DHFR | Synthetic folic acid* | Moderate; DHFR-limited |
| Vitamin C | Liposomal | High at high doses | Ascorbic acid (very high dose) | Saturation-limited |
*Folic acid requires MTHFR enzyme conversion. Individuals with MTHFR mutations (≈40% of the population) convert folic acid significantly less efficiently.
How to Use This Guide: Practical Buying Rules
Rule 1: Prioritize form over dose. A 200mg glycinate pill beats a 500mg oxide pill every single time for magnesium.
Rule 2: Check the “other ingredients” for form. Manufacturers sometimes list a premium form in the marketing but use a mix in the actual formula.
Rule 3: Fat-soluble vitamins need fat. Vitamins A, D, E, K, and CoQ10 all need dietary fat present for absorption. Take with a meal containing at least 10g of fat.
Rule 4: Separate calcium and iron. They compete for the same transporter (DMT1 and ferroportin pathways). Space them by at least 2 hours.
Rule 5: For curcumin, verify the formulation. The words “95% curcuminoids” without a delivery technology attached means standard extract — i.e., ~1% bioavailability.
Frequently Asked Questions
What does bioavailability mean for supplements?
Bioavailability refers to the percentage of a supplement dose that enters systemic circulation in an active form. A supplement with 10% bioavailability means only 10% of what you swallow actually gets used by your body — the rest is excreted. It’s influenced by the chemical form of the nutrient, presence of food or fat, stomach acid levels, and competing nutrients.
Why is magnesium oxide so common if it absorbs poorly?
Magnesium oxide contains 60% elemental magnesium by weight (vs. 14% for glycinate), so manufacturers can put “500mg magnesium” on the label at very low cost. The label milligrams look impressive. In practice, you’d absorb more from a 200mg glycinate than from a 2000mg oxide dose. The economics of supplement labeling favor cheap forms.
Is more expensive always more bioavailable?
Not always. Calcium carbonate (cheap) absorbs reasonably well when taken with food. Cyanocobalamin (cheap B12) works fine at high doses. The key question isn’t price — it’s whether the form matches your biology. Check the form first, then price.
Does liposomal delivery really improve absorption?
For vitamin C and some B vitamins, yes — liposomal encapsulation can raise absorption from ~50–70% at high doses to ~90%, largely bypassing sodium-dependent transporters that become saturated. The evidence is strongest for vitamin C and glutathione. For minerals, chelation (amino acid bonding) is a better solution than liposomal delivery.
How does body iron status affect iron absorption?
Iron absorption is tightly regulated. When ferritin (iron stores) is low, the body upregulates expression of DMT1 (divalent metal transporter 1), increasing iron absorption by 2–3×. Conversely, when iron stores are high, absorption is downregulated to protect against iron overload — a serious condition, especially in men.
Can piperine (black pepper extract) really increase curcumin absorption 20×?
The Shoba et al. (1998, Planta Medica) study is the primary source of this number: piperine 20 mg co-ingested with curcumin 2 g raised serum curcumin roughly 2000% (≈20×) in human volunteers. Piperine inhibits intestinal and hepatic glucuronidation, the metabolic process that rapidly inactivates curcumin. This is a single study — the general phenomenon is well replicated, but the exact 20× figure should be understood as “one landmark trial,” not a universal multiplier. Piperine also increases absorption of many drugs, including some chemotherapy agents — consult a physician if you’re on prescription medications.
Should I take D2 if I’m vegan?
Ideally, seek out vegan D3 derived from lichen. It is understood to behave the same as animal-derived D3 at the same IU. D2 works, but requires larger doses to achieve equivalent serum 25(OH)D levels, and its shorter half-life means more frequent dosing is needed.
Sources
- Coudray C, et al. Study of magnesium bioavailability from organic and inorganic Mg salts. Magnesium Research. 2005.
- Hurrell R, Egli I. Iron bioavailability and dietary reference values. Am J Clin Nutr. 2010;91(5):1461S-1467S.
- Tripkovic L, Lambert H, Hart K, et al. Comparison of vitamin D2 and vitamin D3 supplementation in raising serum 25-hydroxyvitamin D status: a systematic review and meta-analysis. Am J Clin Nutr. 2012;95(6):1357-1364.
- Shoba G, Joy D, Joseph T, Majeed M, Rajendran R, Srinivas PS. Influence of piperine on the pharmacokinetics of curcumin in animals and human volunteers. Planta Med. 1998;64(4):353-356.
- Watanabe F, Yabuta Y, Bito T, Teng F. Vitamin B12-containing plant food sources for vegetarians. Nutrients. 2014;6(5):1861-1873.
- Weaver CM, Heaney RP, Nickel KP, Packard PI. Calcium bioavailability from high oxalate vegetables. Journal of Food Science. 1997;62(3):524-525.
- Anand P, Kunnumakkara AB, Newman RA, Aggarwal BB. Bioavailability of curcumin: Problems and promises. Mol Pharmaceutics. 2007;4(6):807-818.
- Nordøy A, Barstad L, Connor WE, Hatcher L. Absorption of the n-3 eicosapentaenoic and docosahexaenoic acids as ethyl esters and triglycerides by humans. Am J Clin Nutr. 1991;53(5):1185-1190.
- National Institutes of Health. Office of Dietary Supplements: Fact Sheets. ods.od.nih.gov (accessed 2026).
- Saw JT, Bahari MB, Ang HH, et al. Peer-reviewed PK/bioavailability reviews of calcium, iron, magnesium, and B12 forms (see NIH ODS fact sheets for current consolidated data).
Related Articles
- Best Magnesium Supplement: Glycinate vs. Citrate vs. Oxide Compared
- Iron Deficiency Symptoms and the Best Iron Supplements for Women
- CoQ10 Dosage Guide: How Much Do You Actually Need?
- Supplement Half-Life Guide: How Often Should You Really Take Each Supplement?
- Supplement Cost-Per-Dose Database: Are You Getting Ripped Off?
This article is for informational purposes only and does not constitute medical advice. Always consult your healthcare provider before starting any supplementation regimen.
Editorial note: this guide was drafted with AI assistance and then fact-checked against the listed primary sources and the NIH Office of Dietary Supplements fact sheets. Bioavailability percentages in the literature vary substantially between studies, study designs, and baseline nutrient status — the numbers above are directional, not exact constants.
Last updated: April 2026.
