Calcium
Calcium (Calcium Citrate/Carbonate)
Calcium is an essential macromineral critical for bone mineralization, muscle contraction, nerve transmission, and vascular function. It is primarily used to prevent and treat calcium deficiency states, support bone density (particularly in postmenopausal women and older adults), and as adjunct therapy for osteoporosis prevention. Calcium citrate and calcium carbonate are the two most common supplemental forms, differing mainly in bioavailability and required conditions for absorption.
Mechanism of Action
Calcium is absorbed in the small intestine via both active transport (mediated by vitamin D-dependent calcium-binding proteins such as calbindin) and passive paracellular diffusion. Once in circulation, it acts as a second messenger in numerous cellular processes, regulates muscle contraction via interaction with troponin-C, and is deposited into the hydroxyapatite matrix of bone under the influence of osteoblasts. Parathyroid hormone (PTH) and calcitriol (active vitamin D) tightly regulate serum calcium homeostasis through effects on intestinal absorption, renal reabsorption, and bone resorption.
Evidence by Health Goal(17 goals)
Dosing Protocols
With meals for calcium carbonate (requires gastric acid); calcium citrate can be taken with or without food
Do not exceed 500-600mg elemental calcium per single dose as absorption efficiency declines significantly above this threshold. Calcium carbonate is more cost-effective but less bioavailable in those with low stomach acid or on PPIs; calcium citrate is preferred in those cases. Total dietary plus supplemental intake should generally not exceed 2000-2500mg/day.
Safety & Side Effects
Calcium supplementation at recommended doses (1000-1200mg elemental calcium/day total intake) is generally well-tolerated and safe for most adults; however, evidence suggests that high supplemental doses (>1000mg/day from supplements alone, independent of diet) may be associated with modest increases in cardiovascular events and kidney stone risk in susceptible individuals, warranting individualized dosing based on dietary intake assessment.
Possible Side Effects
- !Constipation, especially with calcium carbonate at higher doses
- !Gastrointestinal bloating and gas, more common with carbonate form
- !Nausea and stomach discomfort when taken on an empty stomach (carbonate form)
- !Hypercalcemia with chronic excessive intake: fatigue, confusion, kidney stones, polyuria
- !Increased risk of nephrolithiasis (calcium oxalate kidney stones) with high supplemental doses
- !Potential increased cardiovascular risk with high supplemental doses in some epidemiological studies
- !Rebound gastric acid hypersecretion with very high carbonate doses (milk-alkali syndrome at extreme intakes)
Interactions
- -Significantly reduces absorption of thyroid medications (levothyroxine) - separate by at least 4 hours
- -Impairs absorption of bisphosphonates (alendronate, risedronate) - must be taken at separate times
- -Reduces absorption of quinolone and tetracycline antibiotics - separate by 2-4 hours
- -May reduce absorption of iron supplements - avoid co-administration; separate by 2 hours
- -Thiazide diuretics increase renal calcium reabsorption, raising hypercalcemia risk when combined with high-dose supplementation
Cost & Where to Buy
Calcium carbonate is significantly cheaper than calcium citrate; generic store-brand options are widely available at very low cost. Premium brands with added vitamin D3, magnesium, or microcrystalline hydroxyapatite forms cost more. Price based on 1000-1200mg elemental calcium daily.
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