Calcium

Calcium serves five essential roles: (1) bone and tooth mineralisation — 99% of body calcium is stored as hydroxyapatite crystal, providing structural strength to skeleton and enamel; (2) muscle contraction — calcium triggers the interaction of actin and myosin filaments (cross-bridge cycle) to allow muscles to contract; (3) nerve signal transmission — calcium enters nerve terminals and triggers neurotransmitter release to transmit signals between neurons; (4) blood clotting — calcium is a required cofactor for multiple enzymes in the coagulation cascade; (5) enzymatic and signalling functions — calcium activates hundreds of enzymes and regulates vascular tone, blood pressure, and gene expression. The body maintains blood calcium in a narrow range (8.5–10.5 mg/dL) through tight hormonal control; when dietary intake is low, the parathyroid hormone triggers bone resorption to maintain this level — over years, this depletes bone mineral and increases fracture risk.

Acute hypocalcemia (serum calcium <8.5 mg/dL) causes muscle cramps, twitches, and spasms (tetany), particularly in legs and feet. Numbness and tingling in fingers, toes, and lips occur due to nerve hyperexcitability. Irregular heartbeat may develop. In more severe deficiency, confusion, memory problems, irritability, depression, and anxiety appear. Seizures can occur if serum calcium drops below 6.5 mg/dL. Chronic subclinical deficiency (low intake over years) does not produce acute symptoms but leads to progressive bone loss (osteopenia, then osteoporosis), increased fracture risk, weak teeth, and in advanced cases, cataracts. An estimated 1.3 billion people worldwide have inadequate calcium intake, making subclinical deficiency common.

The FDA Daily Value is 1,300 mg/day. Recommended Dietary Allowances (RDA) vary by age: adolescents aged 14–18 need 1,300 mg/day; adults aged 19–50 need 1,000 mg/day; adults aged 51+ need 1,200 mg/day (due to increased bone loss with oestrogen decline in postmenopausal women, and reduced absorption efficiency). The Tolerable Upper Intake Level (UL) is 2,500 mg/day for adults aged ≤50 years and 2,000 mg/day for those 51+. A critical point: the intestine can absorb only ~500 mg of calcium per dose due to saturation of calcium transporters — splitting intake into 2–3 smaller doses throughout the day significantly improves total absorption compared to single large doses.

Top sources per serving are parmesan cheese (331 mg per 28 g / 1 oz), yogurt plain (415 mg per 245 g / 1 cup), milk (305 mg per 240 mL / 8 fl oz), and sardines with bones (325 mg per 92 g / 1 can). Fortified plant-based milks (soy, almond, oat) provide 300–450 mg per 240 mL cup (equivalent to dairy milk). Non-dairy whole food sources include white beans (131 mg per 179 g / 1 cup cooked), bok choy cooked (158 mg per 170 g / 1 cup), broccoli (70 mg per 156 g / 1 cup cooked), and edamame (98 mg per 155 g / 1 cup). Spinach and rhubarb are calcium-rich but contain high amounts of oxalates, which bind calcium and reduce bioavailability to ~5%; bok choy and broccoli are superior leafy green sources. Food sources provide calcium alongside complementary nutrients (lactose, vitamin K2, magnesium) that support bone health synergistically.

Calcium carbonate is the most common and cheapest form (40% elemental calcium); it requires stomach acid and should be taken with meals. It may cause constipation in some people. Calcium citrate (21% elemental calcium) has better absorption without food and is gentler on the stomach; it is ideal for those on proton-pump inhibitors (PPIs), antihistamines (H2-blockers), or older adults with reduced stomach acid. Calcium gluconate and malate have weak absorption evidence. Avoid calcium from oyster shell, dolomite, or bone meal due to potential lead contamination. Elemental calcium content is critical to check: a 500 mg supplement of calcium carbonate contains only 200 mg elemental calcium (40%). When supplementing, split doses to ≤500 mg elemental calcium per dose to maximise absorption.

Adequate calcium intake is necessary but not sufficient alone for osteoporosis prevention. Achieving the RDA throughout life supports peak bone mass in young adulthood (ages 20–30) and slows age-related bone loss afterwards. However, calcium + vitamin D together are more effective than calcium alone — a landmark study (Chapuy 1992, NEJM) showed that women receiving both vitamin D (cholecalciferol 17.5 mcg / 700 IU daily) and calcium (1,200 mg/day) had 43% fewer hip fractures than placebo over 18 months. Systematic meta-analyses show calcium supplementation reduces vertebral fracture risk modestly (about 12% reduction). Weight-bearing exercise, adequate protein intake (0.8–1.2 g/kg body weight), and sufficient magnesium are equally important. Excessive supplemental calcium (>1,500 mg/day from supplements alone) may slightly increase cardiovascular risk in some observational studies; food sources are preferred.

Vitamin D is absolutely essential for efficient calcium absorption. Without adequate vitamin D (serum 25-OH-D <50 nmol/L / <20 ng/mL), the intestine absorbs only 10–15% of dietary calcium; with sufficient vitamin D, absorption rises to 30–40% — a 3–4 fold difference. Mechanistically, vitamin D binds to vitamin D receptors in intestinal epithelial cells and stimulates the synthesis of calbindin (the calcium-binding transport protein) and alkaline phosphatase, both required for active calcium transport. Vitamin D deficiency is one of the most common causes of subtle calcium malabsorption and "calcium resistance" — a person can eat plenty of calcium but absorb very little if vitamin D is deficient. This is why any serious calcium and bone health strategy must include vitamin D testing (target ≥50 nmol/L / ≥20 ng/mL) and supplementation if necessary.

The Tolerable Upper Intake Level from supplements is 2,500 mg/day (adults ≤50) and 2,000 mg/day (51+); dietary calcium has no upper limit because excess is excreted. Consuming >2,000 mg/day from supplements can cause hypercalcemia (high blood calcium), leading to nausea, constipation, confusion, and kidney stones. "Milk-alkali syndrome" is rare but serious — it occurs when very high calcium and absorbable alkali (antacids) are consumed together, causing severe hypercalcemia, alkalosis, and kidney damage. The cardiovascular risk from supplemental calcium remains debated: some meta-analyses (Bolland 2010, BMJ) found a modest increase in myocardial infarction risk with supplements ≥1,000 mg/day, whilst others found no effect when vitamin D is adequate. Current evidence favours meeting calcium needs from food whenever possible and using supplements only to fill documented gaps of ≤500 mg/day. Always inform your doctor of high-dose calcium supplementation, especially if taking medications or have kidney disease.