Calcium is the most abundant mineral in the human body — it constitutes approximately 1.5-2% of the total body weight of an adult, and it is the primary mineral component of bone (where it is present as hydroxyapatite, a crystalline complex of calcium phosphate and calcium carbonate that gives bone its mechanical rigidity and its compressive strength). Calcium is not only a structural mineral — it is also a critical signalling molecule in every cell in the body, where it regulates muscle contraction, nerve conduction, hormone secretion, cell division, and the immune response. This dual role of calcium as both a structural mineral and a cellular signalling molecule is one of the most important aspects of human physiology, and its dysregulation is implicated in some of the most common diseases of modern civilisation, including osteoporosis, hypertension, colorectal cancer, and the metabolic dysfunction that characterises the metabolic syndrome.
Bone Remodelling and Calcium Homeostasis
Bone is a dynamic tissue that is constantly being remodelled throughout life — the old bone is removed by the osteoclasts (the bone-resorbing cells) and new bone is laid down by the osteoblasts (the bone-forming cells), in a tightly regulated process called bone remodelling that maintains the structural integrity of the skeleton and regulates the calcium concentration in the blood. The bone remodelling process is regulated by the hormones that control calcium homeostasis — primarily parathyroid hormone (PTH, which is secreted by the chief cells of the parathyroid gland when blood calcium is low and which stimulates osteoclast activity and renal calcium reabsorption), calcitonin (which is secreted by the parafollicular C cells of the thyroid gland when blood calcium is high and which inhibits osteoclast activity), and calcitriol (the active form of vitamin D, which is 1,25-dihydroxyvitamin D3, which increases calcium absorption from the gut and which works alongside PTH to maintain blood calcium levels). This calcium-PTH-calcitonin-calcitriol axis is the primary regulatory system for calcium homeostasis and for bone remodelling, and its dysregulation is the primary mechanism by which osteoporosis develops.
Osteoporosis is the clinical condition that results from the dysregulation of the bone remodelling process — it is characterised by low bone mineral density (BMD), microarchitectural deterioration of bone tissue, and an increased risk of fractures (particularly of the hip, the vertebra, and the distal radius). The pathophysiology of osteoporosis involves an imbalance between bone resorption and bone formation — when the osteoclasts resorb more bone than the osteoblasts can form, the net bone loss produces the progressive reduction in BMD that characterises osteoporosis. This osteoclast-osteoblast imbalance is driven by the hormonal changes that occur at the menopause (the decline in oestrogen, which normally suppresses the activity of the osteoclasts), by the ageing-related decline in the osteoblast activity, and by the calcium and vitamin D deficiency that is common in older adults.
Calcium Absorption
The efficiency of calcium absorption from the gut declines with age — from approximately 40-50% in young adults to approximately 20-30% in older adults — and this decline in calcium absorption is one of the primary mechanisms by which osteoporosis develops in older age. The calcium absorption efficiency is regulated by calcitriol (the active form of vitamin D) through the activation of the calcium-binding protein calbindin in the intestinal epithelial cells, and by the calcium-sensing receptor (CaSR) on the surface of the parathyroid chief cells, which regulates the secretion of PTH in response to changes in blood calcium concentration. When calcitriol levels are low (as in vitamin D deficiency, which is very common in older adults who have limited sun exposure), calcium absorption is reduced and the bone resorption that is driven by PTH is increased, leading to the progressive bone loss that characterises osteoporosis.
Practical Application
For general calcium supplementation, the evidence-based dose is 1,000-1,200mg of elemental calcium daily (from calcium citrate or calcium carbonate, the two most commonly used forms), divided into 2 doses and taken with meals for optimal absorption. The calcium citrate form is preferred for people over the age of 50 who have reduced gastric acid production, because it does not require gastric acid for absorption; the calcium carbonate form is preferred for younger people who have normal gastric acid production, because it contains more elemental calcium per tablet (40% vs 20%) and is more cost-effective. The RDA for calcium is 1,000-1,200mg daily for adults, and most people do not achieve this from diet alone, making supplementation necessary for the maintenance of calcium balance and for the prevention of osteoporosis. For comprehensive bone health support, calcium pairs well with vitamin D (which is required for calcium absorption — the RDA is 600-800 IU daily of vitamin D3, and many experts recommend 1,000-2,000 IU daily for optimal bone health), with vitamin K2 (which directs calcium to bone and away from the arterial wall by activating the osteocalcin matrix Gla protein system), with magnesium (which is required for the activity of the osteoblasts and for the bone mineralisation process), and with strontium (which at 1-2g daily has been shown in multiple RCTs to increase BMD and reduce fracture risk, possibly by stimulating the osteoblasts and inhibiting the osteoclasts).
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