Calcium Metabolism – Function, Regulation and Disorders
Calcium metabolism refers to the processes regulating the absorption, distribution, and excretion of calcium in the body. It is essential for bones, muscles, and nerve function.
Things worth knowing about "Calcium Metabolism"
Calcium metabolism refers to the processes regulating the absorption, distribution, and excretion of calcium in the body. It is essential for bones, muscles, and nerve function.
What Is Calcium Metabolism?
Calcium metabolism describes all the biological processes that regulate the uptake, transport, storage, and excretion of calcium (Ca²⁺) in the human body. Calcium is the most abundant mineral in the body and plays a vital role in nearly every organ system. Approximately 99% of all body calcium is stored in the bones and teeth, while the remaining 1% circulates in the blood and soft tissues.
Biological Functions of Calcium
Calcium performs a wide range of essential functions throughout the body:
- Bone and tooth formation: Together with phosphate, calcium is the main structural component of bone tissue (hydroxyapatite).
- Muscle contraction: Calcium triggers contraction in skeletal, cardiac, and smooth muscle.
- Nerve conduction: Calcium is involved in the transmission of nerve impulses.
- Blood clotting: Several coagulation factors depend on calcium.
- Cellular signaling: Calcium acts as an intracellular second messenger.
- Enzyme activation: Many enzymes require calcium as a cofactor.
Regulation of Calcium Metabolism
Blood calcium levels (normal range: 2.2–2.6 mmol/l) are tightly regulated through a complex hormonal interplay. The three main regulators are:
Parathyroid Hormone (PTH)
Parathyroid hormone is secreted by the parathyroid glands when blood calcium levels fall. It raises calcium levels through three mechanisms: mobilizing calcium from bone (bone resorption), increasing calcium reabsorption in the kidneys, and activating vitamin D in the kidneys.
Vitamin D (Calcitriol)
Vitamin D is synthesized in the skin under UV light and subsequently converted in the liver and kidneys into its active form, calcitriol. Calcitriol promotes calcium absorption in the intestine and supports the effects of PTH on bone and kidney.
Calcitonin
Calcitonin is secreted by the thyroid gland when blood calcium levels are too high. It lowers calcium levels by inhibiting bone resorption and increasing calcium excretion via the kidneys.
Calcium Intake and Requirements
Calcium is primarily absorbed through food. The World Health Organization (WHO) and most national health authorities recommend a daily calcium intake of 1000 mg for adults. Good dietary sources of calcium include:
- Milk and dairy products (cheese, yogurt)
- Green leafy vegetables (broccoli, kale, arugula)
- Calcium-rich mineral water (over 150 mg/l)
- Legumes and nuts (almonds, white beans)
- Tofu (made with calcium sulfate)
Intestinal calcium absorption ranges from approximately 25–35%, depending on vitamin D status, age, and dietary composition. Absorption can be inhibited by oxalates (found in spinach), phytates (found in grains), and vitamin D deficiency.
Disorders of Calcium Metabolism
Hypercalcemia (elevated calcium levels)
Hypercalcemia occurs when blood calcium levels exceed 2.6 mmol/l. Causes include primary hyperparathyroidism, malignant tumors, sarcoidosis, or vitamin D overdose. Symptoms may include nausea, constipation, muscle weakness, increased urination, and in severe cases, cardiac arrhythmias.
Hypocalcemia (low calcium levels)
Hypocalcemia occurs when blood calcium levels fall below 2.2 mmol/l. Causes include hypoparathyroidism, vitamin D deficiency, renal insufficiency, or malabsorption syndromes. Typical symptoms include muscle cramps, tingling sensations (paresthesia), tetany (involuntary muscle contractions), and in severe cases, cardiac arrhythmias.
Diagnosis
The following tests are used to assess calcium metabolism:
- Serum calcium: Measurement of total and ionized calcium in the blood
- Parathyroid hormone (PTH): Measurement of hormone levels
- Vitamin D (25-OH vitamin D): Assessment of vitamin D status
- Urinary calcium: 24-hour urine calcium excretion
- Bone density scan (DXA): Evaluation of bone health
- Phosphate and magnesium: Supporting parameters in mineral metabolism
Treatment of Disorders
Treatment depends on the underlying cause. Hypocalcemia is treated with calcium supplements and vitamin D. Hypercalcemia may be managed with bisphosphonates, calcitonin, corticosteroids, or surgical removal of overactive parathyroid glands, depending on the cause. A balanced, calcium-rich diet and adequate vitamin D intake form the foundation of prevention.
References
- World Health Organization (WHO): Calcium supplementation in pregnant women. WHO Guidelines, Geneva, 2013.
- Shoback D. et al.: Metabolic Bone Disease. In: Gardner D.G., Shoback D. (eds.): Greenspan's Basic and Clinical Endocrinology. 10th edition, McGraw-Hill, New York, 2018.
- Peacock M.: Calcium Metabolism in Health and Disease. Clinical Journal of the American Society of Nephrology, 5(Suppl 1): S23–S30, 2010. DOI: 10.2215/CJN.05910809.
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