Bioavailability — what the science says

In a nutshell

Organic forms of magnesium (citrate, glycinate) are absorbed better than inorganic ones such as oxide.¹
Vitamin D3 raises blood vitamin D more than D2 and is absorbed better with a meal that contains fat.^2,3
At higher doses, methylfolate is slightly more available than folic acid.⁴

1Magnesium

Bioavailability tells you how much of a substance from a supplement or food is absorbed so that the body can use it. For some substances the form really matters, for others less so.

Moderate evidence 14 studies A systematic review of 14 studies: inorganic forms of magnesium are absorbed worse than organic ones, and the share absorbed falls as the dose rises. In healthy people without a deficiency, however, any supplement maintains normal levels.¹

Weak evidence — small studies 4% In one study only about 4% of magnesium oxide was absorbed. In another, 60-day study, oxide did not differ from placebo, while citrate and an amino-acid chelate raised magnesium in the body.^5,6

2Vitamin D

Strong evidence D3 > D2 A summary of randomised trials: vitamin D3 raises blood vitamin D more than D2, especially as a single large dose. With daily dosing, the difference disappeared.²

Moderate evidence — one study +32% People who took vitamin D3 with a meal containing fat had 32% higher blood levels than after a fat-free meal.³

3Folic acid and methylfolate

EFSA recommendation 1.7 vs. 2 Folic acid from a supplement is absorbed better than folate from food — EFSA counts it 1.7 times. Methylfolate counts the same below 400 µg a day; from 400 µg up it is even more available and counts double.⁴

4Iron and vitamin C

From an ordinary mixed diet, 14–18% of iron is absorbed; from a vegetarian diet, 5–12%.⁷

Moderate evidence 15 studies A summary of 15 randomised trials: vitamin C-rich fruit with meals probably raises iron stores (ferritin) and haemoglobin slightly.⁸

Weak evidence Adding a vitamin C tablet to iron for treating anaemia, however, did not improve anything in a summary of 7 studies; the quality of the evidence was very low.⁹

For curcumin, piperine from black pepper boosts absorption — more in the article Turmeric.

5Forms of each vitamin and mineral

Each article on a vitamin or mineral has a part on its forms in supplements and how they are absorbed. Vitamins: vitamin A, vitamin B1 (thiamin), vitamin B2 (riboflavin), vitamin B3 (niacin), vitamin B5 (pantothenic acid), vitamin B6, vitamin B7 (biotin), vitamin B9 (folate), vitamin B12, vitamin C, vitamin D, vitamin E, vitamin K.

Minerals: calcium, magnesium, potassium, sodium and salt, phosphorus, iron, zinc, copper, manganese, selenium, iodine, chromium, fluoride, molybdenum.

What to take from this

For magnesium, prefer citrate or glycinate; oxide is cheap but poorly absorbed.^1,6
Take vitamin D as D3 and with a meal that contains fat.^2,3
Expensive "super forms" often don't matter — ordinary forms maintain levels in healthy people.¹
In ForkRep you record the form of a supplement too — the app counts methylfolate from 400 µg double, as EFSA does.

Sources

Written on 9 Oct 2026, based on studies available at that date. Newer research may change the conclusions.

Pardo M. R. et al. (2021). Bioavailability of magnesium food supplements: a systematic review. Nutrition 89:111294. doi.org/10.1016/j.nut.2021.111294
Tripkovic L. et al. (2012). 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 95:1357–1364. doi.org/10.3945/ajcn.111.031070
Dawson-Hughes B. et al. (2015). Dietary fat increases vitamin D-3 absorption. J Acad Nutr Diet 115:225–230. doi.org/10.1016/j.jand.2014.09.014
EFSA NDA Panel (2022). Conversion of calcium-L-methylfolate and (6S)-5-methyltetrahydrofolic acid glucosamine salt into dietary folate equivalents. EFSA Journal 20:e07452. doi.org/10.2903/j.efsa.2022.7452
Firoz M., Graber M. (2001). Bioavailability of US commercial magnesium preparations. Magnes Res 14:257–262. pubmed.ncbi.nlm.nih.gov/11794633
Walker A. F. et al. (2003). Mg citrate found more bioavailable than other Mg preparations in a randomised, double-blind study. Magnes Res 16:183–191. pubmed.ncbi.nlm.nih.gov/14596323
Hurrell R., Egli I. (2010). Iron bioavailability and dietary reference values. Am J Clin Nutr 91:1461S–1467S. doi.org/10.3945/ajcn.2010.28674F
Nyakundi P. N. et al. (2026). Does consuming ascorbate-rich fruits and fruit juices with meals improve iron bioavailability? A systematic review and meta-analysis of interventions. Am J Clin Nutr 124:101418. doi.org/10.1016/j.ajcnut.2026.101418
Loganathan V. et al. (2023). Treatment efficacy of vitamin C or ascorbate given as co-intervention with iron for anemia — a systematic review and meta-analysis of experimental studies. Clin Nutr ESPEN 57:459–468. doi.org/10.1016/j.clnesp.2023.07.081

This is general information and does not replace a doctor. Food supplements are not medicines; buy them from trusted sellers and talk to a doctor if you are ill or take medicines.