Bioavailability explained: how much of a nutrient your body actually absorbs

The package says 100 mg of magnesium. Inside your body — depending on the form, what you eat alongside it, and how your metabolism is running — maybe 30 mg, maybe 70 mg actually arrive. This difference is called bioavailability, and it's the reason why amount labels alone say nothing about your real supply. What determines absorption, what you can influence in everyday life, and what to look for when buying — coming up. The overall view of all micronutrients is in the pillar Micronutrients.

What bioavailability actually measures

Bioavailability describes the share of a consumed nutrient that the body can actually use for its functions. Scientifically defined, it's the concentration of a substance in blood or at its site of action relative to the amount consumed. Simplified: what really arrives from the tablet or food?

The values vary widely — depending on substance, source, and person. The most prominent example is iron:

Iron source	Absorption rate (typical)
Heme iron (meat, fish)	15–35%
Non-heme iron (plants)	2–20%

More in Iron and tiredness. With many other substances the spread is less dramatic but still relevant — with magnesium, for instance, between 30% (oxide) and 80% (citrate or bisglycinate).

Three factors that account for almost everything

1. Chemical form

Substances can come in different compounds, and the form determines a large part of bioavailability:

• Magnesium: citrate and bisglycinate tend to be absorbed better than oxide. More in Magnesium and muscle function.
• Iron: heme iron from animal sources is bound in a form well accessible to the body; plant non-heme iron has to be converted first, losing portions in the process.
• Calcium: calcium carbonate needs gastric acid for absorption — with reduced acid (age, proton pump inhibitors), calcium citrate is better suited.
• Vitamin D: D3 (cholecalciferol) is metabolized better than D2 (ergocalciferol).
• Folate: synthetic folic acid is more bioavailable than natural folate — see B vitamins overview.

2. Meal context

What you eat at the same time can shift absorption noticeably:

Absorption promoters:

• Vitamin C with an iron-rich meal (peppers, lemon, fresh sauerkraut with lentil soup) — clearly improves non-heme iron uptake
• Some fat with fat-soluble vitamins (A, D, E, K) — olive oil on carrot salad makes carotenoids usable
• Acid with minerals (lemon juice) — improves uptake
• Fermented foods (sourdough bread, sauerkraut) — partly break down phytates and improve mineral absorption

Absorption inhibitors:

• Coffee, black and green tea with an iron-rich meal — tannins block iron uptake. Rule of thumb: 30–60 minutes apart.
• Calcium-rich foods and iron tablets — compete for the same uptake pathways. Calcium supplements ideally not together with the iron meal.
• Phytates from unprocessed legumes and whole grains — bind minerals. Soaking, sprouting, or fermenting reduces this.
• Oxalic acid (spinach, rhubarb, chard) — binds calcium and iron.

3. Individual factors

Even when everything aligns, absorption ultimately depends on your metabolism:

• Gastric acid: declines with age; reduces absorption of B12, calcium and some minerals. More in Micronutrients after 50.
• Gut health: chronic intestinal conditions (celiac disease, Crohn's) can significantly limit absorption.
• Age: protein utilization declines from middle age (anabolic resistance), see Amino acids and sport.
• Medications: proton pump inhibitors reduce B12 and mineral uptake; metformin lowers B12 levels long-term; antibiotics can affect gut flora.
• Pregnancy, growth, illness: increased need with potentially worse utilization at the same time.

Why "mg on the package" says little

Anyone seeing "600 mg per tablet" on a magnesium product and thinking that covers more than the daily requirement is wrong twice:

1. Bioavailability: 600 mg of magnesium oxide may deliver only 200 mg of actually absorbed magnesium.
2. Upper limit: BfR recommends a maximum of 250 mg of magnesium per day from supplements — higher amounts can cause diarrhea.

So: the label amount is a gross figure. The relevant numbers are the DGE daily recommendation (gross need including food) and the BfR upper limit from supplements. A thousandfold excess on the package isn't an advantage but often a marketing trick.

What to watch for when buying supplements

If you want to supplement specifically, a few simple criteria help:

• Check the form: for magnesium, citrate or bisglycinate instead of just oxide; for iron, bisglycinate or sulfate instead of oxide; for calcium, citrate instead of just carbonate.
• Dose by DGE need — not by advertising figures.
• Timing of intake matters: fat-soluble vitamins with a fatty meal, iron in the morning on an empty stomach (if your stomach tolerates it without complaints), B12 independent of food.
• Consistency in everyday life — a moderate daily amount over several weeks almost always works better than a high-dose boost followed by a pause.
• With multi preparations watch for interactions: calcium supplements ideally timed apart from iron or zinc.

What you don't need: the premium powder for €60 promising "revolutionary absorption technology." The difference from a proper standard form is marginal in most cases — if measurable at all.

Bioavailability is also a question of diet

The biggest lever for good micronutrient supply isn't the best supplement but a diet that takes absorption promoters and inhibitors into account in everyday life:

• Iron-rich meal + vitamin C — lentils with peppers, oats with kiwi, legumes with lemon vinaigrette.
• Coffee with distance from the main meal, not directly with it.
• Carrots or sweet potato with olive oil — carotenoids need fat for absorption.
• Fermented foods as a standard in everyday life — sourdough, sauerkraut, yogurt, kefir.
• Soak whole grains and legumes overnight — softens phytates.

If you want to know whether your supply fits

With diffuse complaints or uncertainty: a blood panel at your GP is the only way that really brings clarity — hemoglobin, ferritin, vitamin D and B12 are the most common markers. For a structured take stock in everyday life (nutrition, medications, life stage), the free Vital-Check helps as a first written orientation in a few minutes — without diagnosis, without sales pressure, with a pointer to the doctor's appointment where it belongs.