In 30 seconds
- Bioavailability is the share of a nutrient that your body absorbs and can use or store: almost 100% of creatine monohydrate; between 30% and 40% of the magnesium in food.
- The chemical form matters: your body makes better use of magnesium citrate, aspartate, lactate and chloride than of magnesium oxide.
- So does what you take with it: with a meal containing fat, vitamin D3 reached a blood peak 32% higher than with a fat-free meal.
- A higher dose does not always mean more: about 50% of vitamin B12 is absorbed in small doses and around 2% with 500 µg.
Bioavailability is the share of a nutrient that your body absorbs and can use or store 1. Not everything stated on the label gets where it needs to go. It depends on four things: the chemical form, what you take with it, the dose and your own body. Knowing them helps you choose better and get more out of what you already take.
In three figures
Not everything you take is used equally
What bioavailability is
It is not just about absorption. A nutrient has to be released from the food or the capsule, pass from the gut into the blood and then be available to be used or stored in the tissues. Bioavailability measures the fraction that completes that journey 1.
The differences between nutrients are huge:
| Nutrient | How much your body uses | What it depends on |
|---|---|---|
| Creatine monohydrate | Almost 100% | It is the most studied form |
| Omega-3 (EPA and DHA) | Around 95%, like other fats | The chemical form changes absorption slightly |
| Vitamin B12 from supplements | About 50% in doses below 1-2 µg; 2% with 500 µg | The dose in each serving |
| Magnesium from food | Between 30% and 40% | The form, your age and your magnesium status |
| Iron from a mixed diet | Between 14% and 18% (in a vegetarian diet, 5% to 12%) | What comes with it in the meal |
Sources: Kreider et al., 2022; NIH, Office of Dietary Supplements (omega-3, vitamin B12, magnesium and iron).
That is why two products with the same figure on the label may not give you the same. It is also why some nutrients are provided in amounts far above what you need each day: it is not a mistake, it is because only a small share is used.
1. The chemical form
Many nutrients are sold bound to another molecule, and that pairing changes how much of them your body uses.
| Nutrient | What the studies say |
|---|---|
| Creatine | Monohydrate is almost 100% bioavailable. No other form has been shown to raise muscle creatine further. |
| Magnesium | Forms that dissolve well are absorbed better: citrate, aspartate, lactate and chloride, better than oxide and sulphate. |
| Omega-3 | Triglycerides and free fatty acids are absorbed slightly better than ethyl esters, but all forms raise blood EPA and DHA. |
| Vitamin D | D3 raises blood levels more than D2. |
Sources: Kreider et al., 2022; NIH, Office of Dietary Supplements (magnesium, omega-3 and vitamin D).
Creatine. Monohydrate is almost 100% bioavailable: what you take is absorbed and either reaches the tissues or is excreted in the urine, and less than 1% breaks down into creatinine during digestion 3. Many alternatives have been put forward, but none has been shown to raise muscle creatine further. Hydrochloride dissolves better in water, but there is no evidence that it is better absorbed in people; ethyl ester actually led to less creatine in the muscle 3. You will find it form by form in types of creatine.
Magnesium. Forms that dissolve well in liquid are absorbed better than poorly soluble ones. Citrate, aspartate, lactate and chloride are more bioavailable than oxide and sulphate 2. We compare all the forms, bisglycinate included, in which magnesium to take and in which magnesium is best absorbed.
Omega-3. EPA and DHA are absorbed efficiently, around 95%, like the rest of the fats you eat 4. Triglycerides, natural or re-esterified, and free fatty acids are absorbed slightly better than ethyl esters, but all forms raise blood EPA and DHA 4. We explain how to check this on the label in which omega-3 to buy.
Vitamin D. Both forms are well absorbed, but D3 raises blood levels more than D2 5.
2. What you take with it
A nutrient does not travel alone. What you eat at the same time can help or get in the way.
Fat helps fat-soluble vitamins. Vitamin D is better absorbed when there is fat in the gut, although some is absorbed without it 5. In a trial with 50 people, the same dose of vitamin D3 taken with a meal containing fat reached a blood peak 32% higher than with a fat-free meal 6. Dietary fat also helps you absorb vitamin A, vitamin K and carotenes 1.
Iron is the classic example of a nutrient that is sensitive to what comes with it. Vitamin C and meat, fish or poultry improve the absorption of iron from plant sources. The phytates in cereals and pulses and some polyphenols reduce it, and calcium may reduce it too 8. That said, in a typical Western diet these effects cancel each other out and barely change iron status in most people 8.
The food the nutrient comes in also counts. A nutrient trapped in the structure of a vegetable is released less easily: this is one of the steps that most limits how much beta-carotene your body uses. Fibre can reduce the absorption of some nutrients, and phytates and oxalates decide how much calcium you get from vegetables. On the other hand, some casein and whey peptides help you absorb calcium 1. In a supplement, the nutrient comes almost on its own, which is why the chemical form and the dose carry so much weight.
Some nutrients compete with each other, or with medicines. Very high doses of zinc, of the order of 142 mg a day, can interfere with magnesium absorption. And some antibiotics should be taken at least 2 hours before or 4 to 6 hours after a supplement containing magnesium 2. If you take medication, check with your doctor or pharmacist.
3. How much you take at once
A larger amount does not always mean more is used. Vitamin B12 shows this most clearly: the mechanism that absorbs it becomes saturated at 1-2 µg 7.
- In small doses, below 1-2 µg, about 50% is absorbed.
- With 500 µg, only around 2%.
- With 1,000 µg, 1.3% 7.
That is why many B12 supplements contain doses far above the NRV: to make up for how little is absorbed in each serving. Interestingly, the B12 in supplements is about 50% more bioavailable than the B12 in food 7. If you want to go over what the % NRV means and why it can be over 100%, we explain it in how to read a food supplement label.
The practical rule: take the daily dose stated on the label. Taking more at once rarely pays off, and some nutrients have an upper level that is best not exceeded.
4. Your own body
The same dose is not used equally by everyone. Your nutritional status, your digestive health and your genetics all play a part 1.
Two examples:
- Magnesium: with age, less is absorbed in the gut and more is excreted by the kidneys. And when your magnesium level is low, your kidneys excrete less 2.
- Vitamin B12: between 8% and 9% of people over 65 have atrophic gastritis, which reduces the intrinsic factor and the stomach acid that B12 needs to be absorbed 7.
If you have a digestive condition, take medication regularly or are pregnant, check with your doctor before you start any supplement.
5. “More bioavailable”: what to ask yourself
It is one of marketing's favourite phrases. Before you pay more for it, ask yourself three questions:
- Was it measured in people? Dissolving better in a glass or working in an animal does not guarantee that it is better absorbed in you. The supposed 38-fold advantage of creatine hydrochloride, for example, came from solubility data and a rodent model, not from trials in people 3.
- More bioavailable than what? It is easy to compare against the least studied form. With magnesium, for example, several forms are absorbed better than oxide 2: what is useful is knowing whether it beats the good alternatives, not the worst one.
- Does it change anything that matters to you? A rise in a blood marker does not always mean a greater effect. With creatine, what counts is how much reaches the muscle, and there no form has outperformed monohydrate 3.
How to get more from your supplements
With all of the above in mind, five simple habits:
- Choose forms with studies behind them: monohydrate for creatine; for magnesium, forms that dissolve well 3 2.
- Take fat-soluble vitamins with food: ideally with some fat 5 6.
- Stick to the daily dose: taking more at once does not always mean more is used 7.
- Keep apart what competes: zinc in very high doses, some antibiotics and magnesium 2.
- Be consistent: creatine and omega-3 are taken every day, including the days you do not train.
If you are just starting out, we help you decide what to take in which supplements should I take.
Our creatine: monohydrate, the most studied form
Our Creatine monohydrate is just that: monohydrate micronised to 200 mesh, a single ingredient and unflavoured, so it dissolves easily in water, in your shake or in Nitrus Isotonic. One 3 g scoop a day, the form the studies point to as almost 100% bioavailable 3.
Frequently asked questions
What is the bioavailability of a supplement?
The share of what you take that your body absorbs and can use or store. Not everything stated on the label is put to use: it depends on the chemical form, what you take it with, the dose and you.
Which creatine has the highest bioavailability?
Monohydrate. It is almost 100% bioavailable and is the most studied form. Hydrochloride, ethyl ester and nitrate have not been shown to be absorbed better in people.
Which magnesium is absorbed best?
The forms that dissolve well in liquid. According to the US NIH, citrate, aspartate, lactate and chloride are more bioavailable than oxide and sulphate.
Should you take supplements with food?
Fat-soluble vitamins, such as A, D or K, are better absorbed with a meal that contains some fat. In one trial, vitamin D3 reached a blood peak 32% higher with a meal containing fat than with a fat-free one.
Why do some B12 supplements contain far more than 100% of the NRV?
Because the share of B12 that is absorbed gets smaller as the dose goes up: about 50% in doses below 1-2 µg and 1.3% with 1,000 µg.
Sources
- Melse-Boonstra A. Bioavailability of micronutrients from nutrient-dense whole foods: zooming in on dairy, vegetables, and fruits. Front Nutr. 2020;7:101. View publication
- Office of Dietary Supplements, National Institutes of Health. Magnesium: fact sheet for health professionals. View publication
- Kreider RB, Jäger R, Purpura M. Bioavailability, efficacy, safety, and regulatory status of creatine and related compounds: a critical review. Nutrients. 2022;14(5):1035. View publication
- Office of Dietary Supplements, National Institutes of Health. Omega-3 fatty acids: fact sheet for health professionals. View publication
- Office of Dietary Supplements, National Institutes of Health. Vitamin D: fact sheet for health professionals. View publication
- Dawson-Hughes B, Harris SS, Lichtenstein AH, Dolnikowski G, Palermo NJ, Rasmussen H. Dietary fat increases vitamin D-3 absorption. J Acad Nutr Diet. 2015;115(2):225-230. View publication
- Office of Dietary Supplements, National Institutes of Health. Vitamin B12: fact sheet for health professionals. View publication
- Office of Dietary Supplements, National Institutes of Health. Iron: fact sheet for health professionals. View publication
Information based on published studies. It does not replace the advice of a doctor or registered dietitian. The results describe what was measured in each study, not the properties of any specific product. A varied and balanced diet and a healthy lifestyle are important.
