Calcium

mineralcalcium atom

What is it

Calcium is the most abundant mineral in the human body, with roughly 99 percent stored in bones and teeth. It is required for skeletal strength, muscle contraction, nerve signaling, hormone secretion, and blood clotting.

How it works

Calcium absorption occurs mainly in the small intestine and depends on vitamin D. The body tightly regulates blood calcium through parathyroid hormone, calcitonin, and active vitamin D. When dietary calcium is insufficient, the skeleton acts as a reservoir — calcium is pulled from bone to maintain blood levels, which weakens bone over time. Beyond skeletal function, calcium triggers muscle contraction (including the heartbeat), supports nerve impulse transmission, and is essential for blood clotting cascades. Absorption is most efficient at single doses of 500 mg or less, so larger daily totals are best split into multiple doses.

Evidence for 5 uses

AI-assisted evidence assessment — talk to your doctor before relying on any single supplement.

Bone density and fracture prevention

Grade A

Strong evidence

Adequate calcium plus vitamin D is foundational for bone health and reduces fracture risk in older adults with low intake. Benefit is most pronounced when baseline intake is low.

Rickets and osteomalacia prevention

Grade A

Strong evidence

Together with vitamin D, calcium prevents these diseases of impaired bone mineralization.

Preeclampsia prevention

Grade B

Good evidence

Calcium supplementation in pregnant women with low intake reduces preeclampsia risk. WHO recommends supplementation in low-intake settings.

Blood pressure reduction

Grade B

Good evidence

Modest blood pressure reductions with supplementation, especially in people with low baseline intake.

Colorectal adenoma prevention

Grade C

Moderate evidence

Mixed evidence for reduction in colorectal adenomas; effects on cancer incidence are uncertain.

3 commercial forms

Calcium carbonate

40 percent elemental, requires stomach acid

Most concentrated form. Take with food. Inexpensive and effective for people with normal stomach acid.

Calcium citrate

21 percent elemental, absorbs with or without food

Preferred for older adults, people with low stomach acid, and those on proton pump inhibitors.

Calcium phosphate

well absorbed, common in fortified foods

Frequently used in fortified beverages. Comparable absorption to carbonate when taken with food.

Dosage

The RDA is 1,000 mg per day for most adults, rising to 1,200 mg for women over 50 and men over 70. Pregnancy and breastfeeding follow the same age-based guidance. Aim to meet needs from food when possible; supplement only what your diet does not cover. The tolerable upper limit is 2,500 mg per day for adults under 50 and 2,000 mg for adults over 50.

When and how to take it

Take calcium in doses of 500 mg or less for best absorption — split larger daily totals into morning and evening. Calcium carbonate needs stomach acid and works best with food; calcium citrate absorbs well with or without food and is the better choice for people taking acid-suppressing medications. Avoid taking calcium at the same time as iron, thyroid medication, or certain antibiotics. If you take calcium and magnesium, splitting them between morning and evening can support absorption of each.

Food sources

Food	Amount	%DV
Yogurt (plain, low-fat), 8 oz	415 mg	32%
Sardines (canned with bones), 3 oz	325 mg	25%
Milk (low-fat), 1 cup	305 mg	23%
Cheddar cheese, 1.5 oz	307 mg	24%
Tofu (calcium-set), 1/2 cup	253 mg	19%
Fortified orange juice, 1 cup	350 mg	27%
Kale (cooked), 1 cup	94 mg	7%
Almonds, 1 oz	76 mg	6%

Safety

Calcium from food is safe. Supplemental calcium can cause constipation, bloating, and gas. Total intake above 2,000 mg per day increases kidney stone risk and may be associated with slightly higher cardiovascular event rates, though that evidence is debated. Excess calcium can also interfere with absorption of other minerals.

Who should be cautious

People with hypercalcemia, kidney stones, sarcoidosis, or hyperparathyroidism should not supplement without medical guidance. Those on thiazide diuretics should be cautious with high-dose calcium. Most adults should fill the diet-to-RDA gap from food first and supplement minimally.

Interactions

Calcium reduces absorption of bisphosphonates, levothyroxine, tetracycline and quinolone antibiotics, and iron — separate them by at least 2 hours. High doses can interfere with magnesium and zinc absorption. Thiazide diuretics raise blood calcium and require care with supplementation.

Frequently asked questions

How much calcium do I need?⌄

1,000 mg per day for most adults; 1,200 mg for women over 50 and men over 70. Aim to meet needs from food when possible.

Can I get enough calcium without dairy?⌄

Yes. Calcium-set tofu, fortified plant milk, fortified orange juice, leafy greens, beans, and canned sardines or salmon with bones all provide significant calcium.

Is too much calcium harmful?⌄

Long-term intake above 2,000 mg/day increases kidney stone risk and may have cardiovascular concerns. Stay within the RDA range unless directed otherwise.

Should I take calcium with vitamin D?⌄

Yes — vitamin D is required for calcium absorption. Many calcium supplements include vitamin D.

When is the best time to take calcium?⌄

Carbonate with meals; citrate any time. Avoid taking with iron or thyroid medication. Split doses if total is above 500 mg per dose.

References

NIH ODS Calcium Fact Sheet — NIH Office of Dietary Supplements link

Track Calcium with Pilora

Set up dose reminders, check interactions, and join the community in the Pilora iPhone app.

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Disclaimer: These statements have not been evaluated by the FDA. This page is educational, not a substitute for personalized medical advice. Evidence grades are AI-assisted assessments — talk to your doctor before starting any new supplement, especially if you're pregnant, breastfeeding, on medications, or managing a chronic condition.