Calcium

non-nutrient/non-botanicalcalcium atom

What is it

Calcium is the most abundant mineral in the body, with about 99 percent stored in bones and teeth. It is essential for skeletal structure, muscle contraction, nerve signaling, blood clotting, and many other cellular processes.

How it works

Calcium absorption in the small intestine is regulated by vitamin D — without enough vitamin D, calcium absorption drops sharply. The body tightly controls blood calcium levels through coordinated action of parathyroid hormone, vitamin D, and calcitonin. When dietary intake is insufficient, the body pulls calcium from bone to maintain blood levels, which over time weakens the skeleton. Beyond bones, calcium triggers muscle contraction (including the heartbeat), enables nerve impulses, and is involved in blood clotting and hormone secretion. Absorption efficiency decreases at single doses above about 500 mg, so spreading intake across the day is more effective.

Evidence for 5 uses

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

Bone density and osteoporosis prevention

Grade A

Strong evidence

Adequate calcium plus vitamin D is the foundation of bone health throughout life and reduces fracture risk in older adults with low intake. Effect is modest in well-fed populations.

Prevention of rickets and osteomalacia

Grade A

Strong evidence

Adequate calcium prevents these diseases of impaired bone mineralization. Vitamin D is also required.

Blood pressure (in deficient populations)

Grade B

Good evidence

Calcium supplementation modestly reduces blood pressure, with larger effects in people with low baseline intake.

Preeclampsia prevention

Grade B

Good evidence

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

Colorectal cancer prevention

Grade C

Moderate evidence

Some observational and trial data suggest modest reduction in colorectal adenomas with calcium supplementation. Effects on cancer incidence are uncertain.

5 commercial forms

Calcium carbonate

high elemental calcium, requires stomach acid

About 40 percent elemental calcium by weight — small pills can deliver large doses. Requires stomach acid; take with food. Inexpensive.

Calcium citrate

absorbs with or without food, lower elemental content

About 21 percent elemental calcium — larger pills needed. Absorbs well without stomach acid, preferred for people on acid-suppressing drugs or with low stomach acid.

Calcium phosphate (tricalcium phosphate)

well absorbed, often in fortified foods

Common in fortified beverages and some supplements. Well absorbed.

Calcium gluconate / lactate

lower elemental calcium per dose

Lower elemental calcium per pill, so multiple pills are needed for a meaningful dose. Used in IV formulations clinically.

Calcium from algae (Algaecal)

plant-source, marketed for bone health

Derived from marine algae with added trace minerals. Marketed as more effective for bone density but evidence for superiority over conventional forms is limited.

Dosage

The RDA is 1,000 mg per day for adults under 50, rising to 1,200 mg for women over 50 and men over 70. Pregnant and breastfeeding women have the same needs as non-pregnant women in their age group. 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

Calcium absorption tops out at about 500 mg per dose, so split larger total doses into two or three across the day. Calcium carbonate requires stomach acid for absorption and works best with food. Calcium citrate absorbs equally well with or without food, making it the better choice for people taking acid-suppressing medications. Avoid taking calcium at the same time as iron supplements or levothyroxine. If you take both calcium and magnesium, splitting them between morning and evening can improve absorption of each.

Food sources

Food	Amount	%DV
Yogurt (plain, low-fat), 8 oz	415 mg	32%
Milk (low-fat), 1 cup	305 mg	23%
Sardines (canned with bones), 3 oz	325 mg	25%
Cheddar cheese, 1.5 oz	307 mg	24%
Tofu (calcium-set), 1/2 cup	253 mg	19%
Kale (cooked), 1 cup	94 mg	7%
Fortified orange juice, 1 cup	350 mg	27%
Almonds, 1 oz	76 mg	6%
Bok choy (cooked), 1/2 cup	79 mg	6%
White beans (canned), 1/2 cup	63 mg	5%

Safety

Calcium from food is safe at high intakes. Supplemental calcium commonly causes constipation, bloating, and gas. Long-term high-dose supplementation (above 2,000 mg/day total) increases kidney stone risk and has been linked in some studies to increased cardiovascular events, though evidence is mixed. Take only what you need to fill the gap between diet and RDA.

Who should be cautious

People with kidney disease, hypercalcemia, or a history of calcium-containing kidney stones should not take calcium supplements without medical guidance. People taking thiazide diuretics should not take high-dose calcium because of additive effects. Aim to meet calcium needs from food first; supplement only the gap.

Interactions

Calcium reduces absorption of bisphosphonates, levothyroxine, certain antibiotics (tetracyclines, quinolones), and iron — separate by at least 2 hours. High-dose calcium can interfere with magnesium and zinc absorption. Some diuretics affect calcium balance.

Frequently asked questions

Do I need calcium supplements?⌄

Only if your dietary intake is below the RDA. Most adults can meet needs through dairy, calcium-set tofu, fortified plant milk, leafy greens, and canned fish with bones. Supplement only the gap.

What is the best form of calcium?⌄

Carbonate is cheapest and most concentrated but needs stomach acid (take with food). Citrate works without stomach acid, making it better for older adults and people on acid-suppressing medications.

Can calcium supplements harm the heart?⌄

Some studies have linked high-dose supplementation (above 1,000 mg/day) to slightly increased cardiovascular events. Evidence is mixed. Dietary calcium does not show this signal.

Should I take calcium with vitamin D?⌄

Yes — vitamin D is required for calcium absorption. Many calcium supplements include vitamin D; otherwise take vitamin D separately.

Why is single-dose calcium limited to 500 mg?⌄

Calcium absorption efficiency drops sharply at higher single doses. Spreading total daily intake over two or three doses absorbs more of it.

Track Calcium with Pilora

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

Coming to App Store

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.