Calcium
macro-mineral
Key Takeaways
- 98% of body calcium is stored in bones; the skeleton acts as a reservoir to maintain calcium homeostasis
- FDA daily value is 1,300 mg; adults aged 19-50 need 1,000 mg/day from diet and supplements combined
- Vitamin D is required for calcium absorption via active transport in the gut
- Calcium supplementation alone is not recommended for fracture prevention in the general population (pmid:29279934)
- Top dietary sources include fortified tofu, fortified orange juice, dairy products, and fortified soymilk
Evidence Spectrum
14 studies reviewed →Bone health and osteoporosis management
Calcium supplementation is considered key to osteoporosis management, but a 2020 narrative review found that calcium supplementation alone is not recommended for fracture prevention in the general postmenopausal population. Patients at risk of fracture with insufficient dietary intake may benefit, but supplementation needs to be customized. A 2018 meta-analysis of 33 RCTs (51,145 participants) found no significant association of calcium supplements with reduced hip fracture risk (RR 1.53, 95% CI 0.97-2.42).12
Fall prevention (with vitamin D)
A 2024 systematic review and network meta-analysis of 35 RCTs (58,937 participants) found that 800-1000 IU/day of vitamin D with or without calcium was more effective in preventing falls than calcium alone. Calcium combined with vitamin D showed no significant reduction in hip fracture risk versus placebo (RR 1.09, 95% CI 0.85-1.39).4
Colorectal cancer risk reduction
Protective effects on the risk of developing colorectal cancer have been suggested for calcium, but evidence remains limited and primarily observational.3
14
Studies Reviewed
200 mg
RDA (Infants 0-6 months (AI))
2500 mg
Upper Limit
Role in the Body
Calcium is the most abundant mineral in the human body, with more than 99% stored as calcium hydroxyapatite in bones and teeth. The skeleton serves as both a structural framework and a reservoir that the body draws upon to maintain calcium homeostasis in the blood and extracellular fluid. Bone undergoes continuous remodeling through resorption and deposition to maintain serum calcium levels and support growth and repair. Beyond its structural role, the small ionized pool of calcium in the circulatory system and extracellular fluid mediates critical physiological processes including blood vessel contraction and dilation, muscle contraction, blood clotting, nerve impulse transmission, and hormonal secretion. Calcium absorption occurs through both active transport (vitamin D-dependent) and passive diffusion across the intestinal mucosa, with active transport predominating at lower intakes. Calcium status is difficult to assess directly because serum calcium levels are tightly regulated and do not reflect overall body stores. Dual x-ray absorptiometry (DEXA) scans of bone mineral density provide the most meaningful assessment of cumulative calcium status over a lifetime.
- Structural component of bones and teeth as calcium hydroxyapatite
- Blood vessel contraction and dilation
- Muscle contraction
- Blood clotting (coagulation cascade)
- Nerve impulse transmission
- Hormonal secretion
- Cell signaling
Supplement Forms
Calcium carbonate
RecommendedBioavailability: 40%
Contains the highest percentage of elemental calcium (40%). Best absorbed with food. Common and inexpensive.
Calcium citrate
RecommendedBioavailability: 21%
Contains 21% elemental calcium. Can be taken with or without food. Better option for people with low stomach acid or those taking acid-reducing medications.
Calcium phosphate
Bioavailability: 38%
Contains 38% elemental calcium. Less commonly used in supplements.
Calcium lactate
Bioavailability: 13%
Contains 13% elemental calcium. Lower concentration requires more tablets to reach target doses.
Food Sources
Tofu prepared with calcium sulfate (dried-frozen)
2130 mg per 100g
Tofu, salted and fermented, prepared with calcium sulfate
1230 mg per 100g
Tofu, fried, prepared with calcium sulfate
961 mg per 100g
Tofu, raw, firm, prepared with calcium sulfate
683 mg per 100g
Frozen orange juice concentrate with added calcium
578 mg per 100g
Tofu, raw, regular, prepared with calcium sulfate
350 mg per 100g
Nonfat dry milk, calcium reduced
280 mg per 100g
Calcium-fortified hamburger rolls
233 mg per 100g
Calcium-fortified nonfat milk
204 mg per 100g
Calcium-fortified chocolate milk, reduced fat
194 mg per 100g
Deficiency
Prevalence: Common in certain populations. Vegans often have calcium intakes below recommendations (750 mg/day). Plant-based diets show lower calcium intake and lower bone mineral density compared to meat-eaters.
Symptoms:
- Osteoporosis (fragile bones, increased fracture risk)
- Rickets in children (impaired growth cartilage mineralization)
- Osteomalacia (bone softening in adults and children)
- Hypocalcemia (serum calcium <8.5 mg/dL): numbness, tingling, muscle cramps, cardiac arrhythmias
Risk Factors:
- Postmenopausal women
- Vegans and those with low dairy intake
- Vitamin D deficiency (impairs calcium absorption)
- Magnesium deficiency
- Impaired parathyroid hormone production (hypoparathyroidism)
- Malabsorptive disorders
- Use of certain medications (bisphosphonates, corticosteroids)
Safety & Interactions
Possible Side Effects:
- • Constipation and bloating at higher doses
- • Hypercalcemia at very high intakes
- • Possible increased risk of kidney stones (calcium supplements, not dietary calcium)
- • High-dose vitamin D (>1000 IU/day) combined with calcium may reduce rather than improve bone density (pmid:31454046)
Drug Interactions:
- • Vitamin D: required for calcium absorption; co-supplementation is standard practice
- • Vitamin K2: may improve bone quality and reduce fracture risk when combined with calcium (pmid:32972636)
- • Iron: calcium can inhibit iron absorption when taken simultaneously
- • Thyroid medications (levothyroxine): calcium can reduce absorption; separate by 4+ hours
- • Bisphosphonates: calcium can interfere with absorption; separate dosing required
Contraindications:
- • Hypercalcemia
- • Severe kidney disease (consult nephrologist)
- • History of calcium-containing kidney stones (requires individualized assessment, pmid:37049567)
Frequently Asked Questions
How much calcium do adults need daily?
Most adults aged 19-50 need 1,000 mg/day. Women over 50 and adults over 70 need 1,200 mg/day. The FDA daily value is 1,300 mg. This includes calcium from both food and supplements.
Does calcium supplementation prevent fractures?
Evidence is mixed. A 2018 meta-analysis of 33 RCTs found no significant association between calcium supplements and reduced fracture risk in community-dwelling older adults. Supplementation may benefit those with documented low dietary intake, but is not recommended routinely for fracture prevention.
What is the best form of calcium supplement?
Calcium carbonate (40% elemental calcium) is the most concentrated and cost-effective form but must be taken with food. Calcium citrate (21% elemental calcium) can be taken without food and is better for people with low stomach acid.
Can you take too much calcium?
Yes. The tolerable upper intake level is 2,500 mg/day for adults. Excessive calcium may cause constipation, hypercalcemia, and may increase the risk of kidney stones. High-dose supplementation has not been shown to provide additional bone benefits.
Research Sources
15 peer-reviewed studies analyzed from PubMed. 4 directly cited in this review.