mineral
Calcium: Carbonate vs Citrate, Absorption and Timing
Also known as calcium carbonate, calcium citrate, calcium supplement, calcium with vitamin D, calcium citrate malate, calcium gluconate
Short answer
Calcium is the mineral that builds bones and teeth and helps muscles, nerves and blood clotting work. Calcium carbonate absorbs best with food; calcium citrate can be taken without food. The body absorbs the most from doses of 500 mg or less. Experts suggest taking calcium and iron supplements at different times of day [1][3].
Last reviewed · 5 sources
What is Calcium?
Calcium is the most abundant mineral in the body, and 98% of it is stored in the bones [1]. It makes up much of the structure of bones and teeth [1]. The small amount in blood and tissues is used for blood vessel contraction and dilation, muscle function, blood clotting, nerve signals and hormone release [1]. Vitamin D is needed for the gut to absorb calcium by active transport [1].
Forms of Calcium you'll see on labels
Calcium carbonate
About 40% calcium by weight. It needs stomach acid, so absorption may be lower unless it is taken with a meal [1]. It causes more gas, bloating and constipation than citrate, especially in older adults [1].
Calcium citrate
About 21% calcium by weight. It depends less on stomach acid, so it can be taken without food [1].
Calcium citrate malate
A well-absorbed form used in some fortified juices [1].
Other forms
Supplements may also use calcium sulfate, ascorbate, gluconate, lactate, phosphate or microcrystalline hydroxyapatite [1].
How much Calcium do adults need?
NIH lists these Recommended Dietary Allowances (RDA) for adults, in mg, from food and supplements combined. Reference values, not a personal recommendation.
| Group | Amount per day |
|---|---|
| Men 19–70 | 1,000 mg |
| Men over 70 | 1,200 mg |
| Women 19–50 | 1,000 mg |
| Women 51+ | 1,200 mg |
| Pregnancy 19–50 | 1,000 mg |
| Lactation 19–50 | 1,000 mg |
Upper limit: 2,500 mg a day for adults 19–50 (including pregnancy and lactation) and 2,000 mg a day for adults 51+, from food and supplements [1].
How Calcium appears on a label
The Supplement Facts panel lists elemental calcium, so you do not need to work it out from the compound (carbonate is about 40% calcium, citrate about 21%) [1]. Amounts are in mg. The Daily Value (DV) used for % DV is 1,300 mg for adults and children 4 and older [1]. Some antacids also contain calcium carbonate, about 270 to 400 mg calcium per chew [1].
When to take Calcium
NIH says calcium supplements are absorbed better when taken with food, and calcium carbonate in particular is best taken with a meal; calcium citrate can be taken without food [1]. Absorption is highest at 500 mg or less at one time, for example about 36% of a 300 mg dose versus 28% of a 1,000 mg dose [1]. MedlinePlus advises taking calcium carbonate supplements with food or after meals [2].
What to keep apart from Calcium
Iron
Calcium might reduce iron absorption, so experts suggest taking calcium and iron supplements at different times of day [3]. The NHS also says calcium can reduce the effect of iron tablets [4].
Zinc
The NIH calcium and zinc fact sheets do not describe a calcium–zinc interaction or give a spacing rule. If you take both, ask your pharmacist.
Vitamin D
Vitamin D is needed to absorb calcium [1]. Low vitamin D status reduces calcium absorption [1].
Levothyroxine
Calcium carbonate can interfere with its absorption. The FDA label says not to take levothyroxine within 4 hours of calcium carbonate [1]. Ask your pharmacist.
Quinolone antibiotics
Calcium can reduce absorption of antibiotics such as ciprofloxacin. NIH says taking the antibiotic 2 hours before or 2 hours after calcium prevents this [1]. Ask your pharmacist.
Dolutegravir (HIV medicine)
Its FDA labels advise taking it 2 hours before or 6 hours after calcium supplements [1]. Ask your pharmacist.
Lithium
Long-term lithium can raise blood calcium, and calcium supplements could add to this risk [1]. Ask your doctor.
Other medicines
MedlinePlus advises not taking calcium carbonate within 1 to 2 hours of other medicines, as it may make them less effective [2]. Ask your pharmacist.
Foods that contain Calcium
- Milk, yogurt and cheese
- Canned sardines and salmon with bones
- Kale, broccoli and Chinese cabbage (bok choy)
- Calcium-set tofu
- Fortified soy and almond drinks
- Fortified juices
- Fortified breakfast cereals
Who should ask a doctor or pharmacist first
Talk to a doctor or pharmacist first if you take levothyroxine, quinolone antibiotics, dolutegravir or lithium [1]. Ask too if you have kidney disease or a stomach condition, or if you are pregnant or breastfeeding [2]. If you have had kidney stones, ask before taking calcium supplements, since higher supplement intakes might raise that risk [1].
Keep Calcium in a routine that fits
Supin schedules what you take by time of day, reminds you, and tells you when a bottle runs low.
Calcium: questions people ask
Calcium might interfere with iron absorption, so experts suggest taking calcium and iron supplements at different times of day [3].
The NIH fact sheets for calcium and zinc do not describe an interaction between them or a spacing rule. Ask your pharmacist if you take both.
Spinach contains oxalic acid, which binds calcium. The body absorbs only about 5% of the calcium in spinach, compared with 27% from milk [1].
Sources
- Calcium: Fact Sheet for Health Professionals — NIH Office of Dietary Supplements
- Calcium Carbonate — MedlinePlus (U.S. National Library of Medicine)
- Iron: Fact Sheet for Health Professionals — NIH Office of Dietary Supplements
- Taking ferrous sulfate with other medicines and herbal supplements — NHS
- Calcium - Vitamins and minerals — NHS
Educational information, not medical advice. The amounts above are official reference values, not a recommendation for you. Check with your doctor or pharmacist before starting or changing supplements, especially if you take medication, are pregnant or have a health condition. Supin sells a subscription, not supplements. Last reviewed .