Calcium by the Label: Citrate vs Carbonate, Elemental mg, D3 and K2

Calcium is the supplement where the number on the front deceives most often: «calcium 1000 mg» may mean 1000 mg of compound, of which calcium itself is 210. Below is how to read a calcium label: how carbonate, citrate, hydroxyapatite, lactate and gluconate differ, how much element each form gives, why a serving should not exceed 500 mg, what D3 and K2 next to it add, and how to compare two products by price per 500 mg of elemental calcium.

Why bones need calcium, who is prescribed it and how bone loss is treated are in the article on osteoporosis. Here — the label only.

Calcium carbonate, citrate, hydroxyapatite, lactate: forms on the label

The share of element is a constant of the compound. In absorption, carbonate with food and citrate differ little: NIH ODS estimates absorption of both at roughly 25–30 % at usual doses, and the difference appears with low stomach acid — on an empty stomach and in people taking acid-reducing drugs carbonate is absorbed markedly worse, citrate the same as with food. Hydroxyapatite and «algae calcium» are the same carbonate and phosphate with a surcharge for origin.

How many mg of elemental calcium per serving

Look for «elemental calcium» or «as Ca» on the label. If only «calcium carbonate 1250 mg» is stated, that is 500 mg of calcium; «calcium citrate 1000 mg» — 210 mg; «calcium gluconate 500 mg» — 45 mg. The reference intake is 1000 mg/day for adults and 1200 mg after 60 and in pregnancy (Russian MR 2.3.1.0253-21; NIH RDA 1000–1200 mg); most of it should come from food (a glass of milk ~240 mg, 100 g of hard cheese ~700–1000 mg), and a supplement covers the gap. The tolerable upper level by EFSA and NIH is 2500 mg/day in total, and 2000 mg for people over 50 by NIH.

The second rule is a serving of no more than 500 mg of element: fractional absorption falls as the single dose rises, so 1000 mg from a supplement is split into two doses. A «calcium 1000 mg» tablet taken at once is not convenience but loss of half the dose. The meta-analysis by Bolland and colleagues (2010) linked calcium supplements without vitamin D at ≥500 mg to a small rise in heart-attack risk — the data are debated, but that is one more argument not to take more than the dietary gap and to discuss intake with a doctor.

Calcium with D3 and K2: what that means on the label

Vitamin D3 with calcium is a sensible pair: without it calcium is absorbed worse, and the usual dose in such products is 200–800 IU per serving. If you already take D3 separately, add them up — how to read IU and mcg is on the page on vitamin D by the label. Vitamin K2 (menaquinone, MK-7) is the fashionable neighbour: it takes part in the work of bone proteins, but there is no clinical evidence that K2 in a calcium supplement reduces fractures or «directs calcium to bone»; by the label it adds to the price, not the outcome. Magnesium in the same tablet competes with calcium for absorption — they are usually taken at different times; how to read a magnesium label is on the page on magnesium by the label.

What else to check in the formula

• Serving. «500 mg» may be per 1 tablet or per 2; count per serving and per pack.
• Position in the list. With carbonate calcium always comes first — it is 40 % by mass; if with citrate a filler comes first, the dose per tablet is below the claim.
• Chewable and effervescent forms. Sugar, sorbitol, sodium (up to 300 mg per effervescent tablet) — check if you limit salt or sugar.
• Lead. Calcium from natural sources (oyster shell, dolomite, bone) historically contained lead impurities; the maker should control heavy metals.
• Registration. The registration number on the pack lets you cross-check the formula against the official register.

How to compare two products: price per 500 mg

Divide the pack price by the number of servings, then by mg of elemental calcium per serving, and multiply by 500. Carbonate is the cheapest per milligram; citrate costs 1.5–3 times more, and the premium is justified with low stomach acid, acid-reducing drugs and at 60+; hydroxyapatite and «algae» calcium cost 3–5 times more with no proven advantage. Compare «with D3 and K2» separately and only if you need the whole formula.

Checklist before buying

1. Find mg of elemental calcium — if only the compound mass is stated, recalculate by the form's share.
2. Serving ≤500 mg of element; split the daily supplement into two doses.
3. Form: carbonate with food and normal stomach acid; citrate with acid-reducing drugs, after 60, on an empty stomach.
4. D3 alongside is appropriate; K2 and «from algae» add to the price, not the outcome.
5. Do not take calcium in the same tablet as magnesium and iron if the goal is to absorb calcium.
6. Calculate the price per 500 mg and compare with plain carbonate.

Is this calcium right for you

The label tells you whether a product is honest. Whether you need calcium is told not only by blood calcium (it is tightly regulated and almost always normal even when bone is short of it) but also by parathyroid hormone, vitamin D, bone densitometry and diet; with low protein the level is recalculated in the corrected calcium calculator. Upload your report — the service will match the form and dose to your numbers. How products are scored in this section is explained on the method page. Products with calcium and their label score are in the catalogue.