Calcium
Also called: calcium carbonate, calcium citrate, calcium and vitamin D.
A bone mineral whose absorption caps out per dose, depends on the form and on stomach acidity, and also plays out on your plate.
What it is
Calcium is the most abundant mineral in the body and is found almost entirely in bones and teeth. It also serves muscle contraction, nerve conduction and blood clotting. People buy it mostly for bone health after 50.
What the label does not say
- Absorption is highest at 500 mg or less per dose: about 36% at 300 mg, versus about 28% at 1,000 mg. A 1,000 mg tablet swallowed in one go does not deliver twice what a 500 mg tablet does.
- The NIH notes that in people with low stomach acidity, the solubility rate of calcium carbonate is lower. The choice between carbonate and citrate is therefore not neutral, depending on your stomach context.
- Spinach shows a lot of calcium but delivers only about 5% of it, versus 27% for milk, because of oxalic acid. Broccoli, kale and green cabbage do not have that brake.
- The upper limit rests on an observed link between 1,000 mg per day of supplemental calcium for 7 years and an increased risk of kidney stones, a link that two later systematic reviews did not find.
The forms on the shelf
- Calcium carbonate: 40% elemental calcium, so less bulk per dose, but its solubility depends on stomach acidity and it is better absorbed with a meal.
- Calcium citrate: 21% elemental calcium, so more bulk per dose, but less dependent on stomach acidity and it can be taken without food.
- Products combining calcium and vitamin D: vitamin D is required for active transport of calcium in the gut.
Published reference values
- Recommended intake: 1,000 mg per day from 19 to 50; 1,200 mg for women from 51 onward and for men from 71 onward.
- Upper limit: 2,500 mg per day from 19 to 50, 2,000 mg after that.
- On the cardiovascular side, the data are contradictory: some studies suggest a 15 to 22% increase in risk, others no association. An expert panel concludes that intakes that do not exceed the upper limit are safe from that standpoint.
When
Split into doses of 500 mg or less. Carbonate is taken with a meal; citrate does not require one.
Keep apart from
- Levothyroxine: avoid taking it within 4 hours of calcium carbonate.
- Quinolones (ciprofloxacin, gemifloxacin, moxifloxacin): the antibiotic 2 hours before or 2 hours after calcium.
- Dolutegravir: 2 hours before or 6 hours after calcium.
- Iron: calcium reduces iron bioavailability, so it is better not to take them at the same meal.
To mention to a professional
- History of kidney stones, kidney disease, bisphosphonate use, prolonged use of a proton pump inhibitor, cardiovascular disease, pregnancy: check with a health care professional.
From the plate first
Milk, yogurt and cheese remain the most absorbable sources. Broccoli, kale and green cabbage show bioavailability close to that of milk. Spinach, rhubarb, sweet potato and beans are high in oxalates.
Sources
- NIH Office of Dietary Supplements, Calcium (Health Professional)
- Health Canada, Dietary Reference Intakes, reference values for elements
Educational information, drawn from the sources above. It does not replace the advice of a health professional, who remains the reference for your situation, your medications, pregnancy or breastfeeding.