Calcium
Calcium (Ca²⁺)
Also known as: Calcium Carbonate, Calcium Citrate
Calcium is an essential mineral required for strong bones and teeth, muscle contraction, nerve signaling, and blood clotting. It is obtained through dietary sources and supplements, and adequate intake is vital for skeletal integrity throughout life.
Introduction
Calcium is the most abundant mineral in the human body, with 99% stored in bones and teeth. It functions as a signaling molecule in muscle contraction, nerve transmission, and hormone secretion. Dietary calcium is absorbed in the small intestine under the influence of active vitamin D. When intake is insufficient, the body mobilizes calcium from bone, increasing fracture risk over time. Calcium supplements—often in forms such as calcium carbonate and calcium citrate—help individuals meet recommended intakes when diet alone is inadequate. Optimal calcium intake supports peak bone mass during growth and helps prevent osteoporosis and fractures in older adults. Calcium also participates in vascular contraction, vasodilation, and hormone release. While high doses can cause adverse effects, most individuals benefit from balanced calcium intake combined with vitamin D and weight-bearing exercise to maintain bone health.
Main Benefits
Supports bone health by providing the primary mineral component for bone mineralization and strength.
Aids muscle function, including contraction and relaxation, essential for mobility and cardiovascular health.
Facilitates nerve transmission and signaling by maintaining appropriate extracellular calcium levels.
Contributes to blood clotting by activating clotting factors and platelet aggregation.
Mechanism of Action
Calcium is absorbed in the small intestine in its ionized form (Ca²⁺), a process enhanced by active vitamin D (calcitriol). Once in circulation, calcium is deposited in bone by osteoblasts to form hydroxyapatite crystals, providing structural integrity. In muscle cells, calcium binds to troponin to initiate contraction. In neurons, calcium influx triggers neurotransmitter release at synapses. Calcium also acts as a cofactor for clotting factors in the coagulation cascade. When serum calcium levels drop, parathyroid hormone (PTH) stimulates bone resorption by osteoclasts and increases renal calcium reabsorption, maintaining homeostasis.
Natural Sources
Calcium occurs naturally in dairy products (milk, yogurt, cheese), leafy green vegetables (kale, collard greens), fortified plant milks, tofu set with calcium, and small fish with edible bones (sardines, salmon).
Examples:
Milk
Yogurt
Cheese
Kale
Collard greens
Broccoli
Fortified plant milks
Tofu (calcium-set)
Sardines (with bones)
Salmon (with bones)
Many common foods are rich in calcium; fortified products help those avoiding dairy.
Deficiency Symptoms
Calcium deficiency can lead to osteopenia and osteoporosis, characterized by low bone density and increased fracture risk. In severe cases, hypocalcemia causes muscle cramps, tingling in extremities, and cardiac arrhythmias. Chronic low intake impairs peak bone mass attainment in youth and accelerates bone loss in older adults.
Common Symptoms:
Low bone density
Increased fracture risk
Muscle cramps
Numbness and tingling in fingers and toes
Irregular heartbeat
Osteoporosis (long-term)
Suboptimal intake is common, especially in populations avoiding dairy and older adults.
Prolonged deficiency leads to osteoporosis and significant morbidity in later life.
Recommended Daily Intake
The RDA for calcium varies by age, sex, and life stage. Guidelines from NIH and EFSA provide reference values to ensure adequate intake and prevent deficiency.
Reference Values:
| Infants (0–6 months) | 200 mg/day |
| Infants (7–12 months) | 260 mg/day |
| Children (1–3 years) | 700 mg/day |
| Children (4–8 years) | 1,000 mg/day |
| Adolescents (9–18 years) | 1,300 mg/day |
| Adults (19–50 years) | 1,000 mg/day |
| Adult men (51–70 years) | 1,000 mg/day |
| Adult women (51–70 years) | 1,200 mg/day |
| Adults (71+ years) | 1,200 mg/day |
| Pregnant and lactating women (19–50 years) | 1,000 mg/day |
Sources for RDI/AI:
- https://ods.od.nih.gov/factsheets/Calcium-Consumer/
- https://www.efsa.europa.eu/en/efsajournal/pub/4101
These values are general guidelines. Individual needs may vary; consult a healthcare provider for personalized advice.
Effectiveness for Specific Focuses
Calcium is essential for bone mineral density and joint support, preventing fractures.
Calcium enables muscle contraction and relaxation, improving strength and performance.
Adequate calcium helps prevent age-related bone loss, supporting healthy aging.
Safety Information
Potential Side Effects
Constipation
Gas and bloating
Hypercalciuria (increased urinary calcium)
Contraindications
Hypercalcemia (e.g., primary hyperparathyroidism)
History of kidney stones
Certain medications that raise serum calcium
Overdose Information
Excessive intake can cause hypercalcemia, kidney stones, and impaired kidney function.
Chronic high calcium intake may lead to nausea, vomiting, polyuria, kidney stones, and renal failure due to hypercalcemia.
Documented Overdose Symptoms:
Hypercalcemia
Vomiting
Polyuria (frequent urination)
Kidney stones
Renal dysfunction
Toxicity Thresholds: UL: 2,500 mg/day for adults up to age 50; 2,000 mg/day for adults over 50 (NIH)
Toxicity generally arises from supplements; dietary calcium rarely causes overdose.
Interactions
Drug Interactions:
Bisphosphonates e.g., alendronate, take calcium at least 2 hours after bisphosphonate to avoid reduced absorption.
Tetracycline antibiotics, Calcium can bind tetracyclines, reducing antibiotic efficacy.
Fluoroquinolone antibiotics, Calcium may decrease absorption of fluoroquinolones.
Thyroid medications e.g., levothyroxine, Calcium can interfere; separate intake by ≥4 hours.
Moderate risk; timing adjustments can mitigate interactions.
Other Supplement Interactions:
Vitamin D Enhances calcium absorption and utilization.
Magnesium Required for calcium metabolism, deficiency can impair calcium balance.
Often beneficial interactions; monitor total mineral intake.
Individuals with kidney disease, hyperparathyroidism, or history of kidney stones should consult a healthcare provider before supplementation.
Forms and Bioavailability
Calcium supplements are available in forms such as calcium carbonate, calcium citrate, calcium lactate, and calcium gluconate. Bioavailability varies by form and conditions (e.g., stomach acidity).
Calcium Carbonate
Contains 40% elemental calcium; best absorbed with food in an acidic environment.
High elemental content; requires stomach acid for optimal absorption.
May cause more gastrointestinal discomfort than citrate.
Calcium Citrate
Contains 21% elemental calcium; absorbed well on an empty stomach and in low-acid conditions.
Superior absorption in older adults and those on antacids.
Lower elemental calcium; may require larger pill doses.
Calcium Lactate
Contains ~13% elemental calcium; more soluble and well tolerated.
Good absorption but lower elemental concentration than carbonate and citrate.
Often used in fortified foods.
Warnings & Suitability
Did You Know...?
The human body contains about 1–2% calcium by weight, mostly in bones and teeth.
Peak bone mass is achieved by age 30; adequate calcium intake during youth is crucial to prevent osteoporosis.
General Scientific Sources
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Content Verification
Last Medical Review: 5/29/2025
Reviewed by: Editorial Team