Calcium is the most abundant mineral in the human body — the adult body contains approximately 1-1.2kg of calcium, of which 99% is stored in the skeleton (where it provides the structural rigidity and the mineral reservoir for the blood calcium homeostasis) and 1% is in the blood and the extracellular fluid (where it performs the essential physiological functions of the calcium as a signalling ion, as a cofactor, and as a blood clotting factor). The calcium is the essential component of the hydroxyapatite crystal — the mineral complex of calcium and phosphate that provides the compressive strength and the structural rigidity of the bone and the teeth. The bone is not a static mineral deposit — it is a dynamic, living tissue that is constantly being remodelled by the osteoclasts (which resorb the old bone) and by the osteoblasts (which form the new bone), and this remodelling is regulated by the parathyroid hormone (PTH, which raises the blood calcium by stimulating the bone resorption and the renal calcium reabsorption), by the vitamin D (which raises the blood calcium by stimulating the intestinal calcium absorption and the bone resorption), by the calcitonin (which lowers the blood calcium by inhibiting the bone resorption), and by the mechanical stress on the bones (which stimulates the osteoblast activity and the bone formation). Without adequate calcium and vitamin D, the bone mineralisation is impaired, the bone density is reduced, and the osteoporosis develops — the hallmark of the calcium deficiency and the primary risk factor for the fragility fractures, which are among the most common and most disabling injuries in the elderly.
Calcium and the Muscle Contraction
The calcium is the primary regulator of the muscle contraction — it is released from the sarcoplasmic reticulum (SR) in response to the neural stimulation (the acetylcholine binding to the motor endplate triggers the action potential propagation along the muscle fibre, which triggers the calcium release from the SR), and it binds to the troponin C (the regulatory protein of the thin filament), causing the conformational change that moves the tropomyosin away from the myosin-binding sites on the actin filament and enabling the cross-bridge cycling between the actin and the myosin filaments that produces the muscle contraction. The calcium is then pumped back into the SR by the calcium-ATPase (SERCA) to terminate the contraction and to allow the muscle to relax. Without adequate calcium, the muscle contraction is impaired — the skeletal muscle cannot contract normally (producing the muscle weakness, the tetany, and the seizures), and the cardiac muscle cannot contract and relax normally (producing the cardiac arrhythmias, the heart failure, and the cardiac arrest). The calcium is therefore as essential for the cardiac function as it is for the skeletal muscle function — and the blood calcium concentration is tightly regulated within the narrow range of 8.5-10.5mg/dL by the PTH, the vitamin D, and the calcitonin.
The clinical importance of the calcium for the muscle function is underscored by the observation that the hypocalcaemia (blood calcium below 8.5mg/dL) produces the neuromuscular irritability, the muscle spasms, the tetany, the Chvostek sign (facial muscle twitching when the facial nerve is tapped), the Trousseau sign (carpopedal spasm when the blood pressure cuff is inflated above the systolic pressure), and the seizures — which are the hallmark of the severe calcium deficiency and which are reversed by the calcium supplementation. The hypocalcaemia is one of the most common and most dangerous electrolyte disturbances, and it should always be considered in patients with the seizures, the tetany, and the cardiac arrhythmias.
Practical Application
For general calcium supplementation for the bone health and for the prevention of the osteoporosis, the evidence-based approach is to supplement with 1000-1200mg of calcium daily (as calcium citrate, calcium carbonate, or calcium hydroxyapatite — the forms that are well absorbed and well tolerated). The calcium citrate is preferred for people with the low stomach acid (because it does not require the gastric acid for the absorption), and the calcium carbonate is preferred for people with the normal stomach acid (because it provides the most calcium per gram and it is the most cost-effective form). The calcium should be taken in divided doses (500mg or less per dose) and with food (to enhance the absorption and to reduce the gastrointestinal side effects). The RDA of calcium is 1000mg daily for adults under 50 and 1200mg daily for adults over 50, and the tolerable upper intake level is 2500mg daily for adults — above which the calcium can cause the hypercalcaemia, the kidney stones, and the cardiovascular disease. For comprehensive bone health support, calcium pairs well with the vitamin D (which is required for the intestinal calcium absorption and for the bone mineralisation — the combination of calcium and vitamin D is more effective than either alone for the prevention of the osteoporosis and of the fractures), with the vitamin K2 (which activates the osteocalcin and the matrix Gla protein, the two proteins that are essential for the bone mineralisation and for the inhibition of the vascular calcification), with the magnesium (which is a cofactor for many of the enzymes of the bone matrix synthesis and which has complementary effects on the bone health), and with the strontium (which has complementary effects on the bone formation and on the inhibition of the bone resorption).
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