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Bioavailability: absorbed vs label iron/zinc/B12

Five nutrients — iron, zinc, calcium, magnesium and vitamin B12 — are scored on the amount your body is estimated to take up, not the amount printed on the label. That is why a day can show 20 mg of iron eaten and still sit below target: the target is an absorbed number, and so is the one it is compared with.

Why these five are different

For most nutrients, what you eat is close enough to what you get. For these five it is not — uptake swings several-fold with the meal around them and with your stage of pregnancy. Scoring them on the label number would quietly flatter some days and punish others.

So each one carries two figures: the gross amount (what the food contains) and the absorbed amount (what the model estimates you take up). Targets for these nutrients are absorbed-basis too, so you are always comparing like with like.

What moves the absorbed number

Nutrient	Raised by	Lowered by
Iron	Vitamin C in the same meal; meat (heme) iron; later trimesters	Phytate from grains and legumes; very large single doses
Zinc	Animal protein in the meal; pregnancy and breastfeeding	Phytate, scaled to how much zinc is present
Calcium	Later trimesters (uptake roughly doubles); adequate vitamin D	Very large amounts in one sitting — spreading it out gets you more
Magnesium	—	Phytate
B12	—	A per-meal ceiling: only so much can be taken up at once

Two consequences worth knowing. Splitting helps where a ceiling applies — the same calcium or B12 across two meals beats one large hit. And large single iron doses give you less per milligram than small ones, so a very big dose does not buy proportionally more iron.

Pregnancy changes the maths, and we do not double-count it

Uptake genuinely rises as pregnancy progresses — most steeply for iron in the third trimester. The model applies that rise to the absorbed figure, so your targets do not also get inflated to compensate. If you compare our iron target with a label RDA you will see a much smaller number; that is the point, not an error.

Choosing the calibration (Environment setup)

Published measurements of pregnancy iron uptake span a wide range, mostly because single test meals read higher than whole diets. Rather than hide that behind one number, Environment setup lets you pick:

• Conservative — the default, and deliberately so. It assumes less uptake, so a thin day looks thin. Most people have no blood results on file, and an optimistic setting would mark an inadequate diet as fine.
• Moderate — mid-range of the published estimates.
• Isotope-max — the upper bound from single-meal isotope studies.

The same panel exposes finer controls, including the calcium–iron interaction (off by default, because whole-diet studies show little effect at food-level intakes) and the iron dose-saturation point.

Changing these changes your scores. They are honest positions inside measured evidence, not a difficulty slider.

What the model does not claim

• Tea and coffee are not modelled. They inhibit iron in a single meal, but the whole-diet effect on stores is modest and we do not hold the data per food.
• Supplemental iron is treated as non-heme, whichever form it is.
• These are population estimates. They cannot know your own iron stores. A ferritin result logged in Medical records is better evidence than any model default — which is exactly why the app uses it when you have one.

For the biology behind any of this, see Learn — Help explains the product UI and links out for science depth.