Calcium metabolism

• Calcium


  Essential mineral for the normal growth & maintenance of the body
• Functions of calcium


  • Calcification of bone
  • Blood coagulation
  • Neuromuscular irritability
  • Acid-base equilibrium
  • Fluid balance
  • Osmotic regulation
• Major elements/macro minerals


  Daily requirement more than 100 mg
• Micro minerals


  Daily requirement less than 100 mg
• Calcium in the human body

  • Most abundant mineral
  • 99% in bone together with phosphate
  • Small amounts in soft tissue
  • 1% in extracellular fluid
• Dietary sources of calcium

  • Milk
  • Egg
  • Fish
  • Cheese
  • Beans
  • Lentils
  • Nuts
  • Cabbage
  • Vegetables
• Daily calcium requirement


  • Adults = 500 mg/day
  • Children's = 1200 mg/day
  • Pregnancy & lactation = 1500 mg/day
  • After age 50 = 1500 mg/day
• Body distribution of calcium

  • About 99% in bones
  • Exists as carbonate or phosphate
  • About 0.5% in soft tissue
  • About 0.1% in extracellular fluid
• Types of calcium in plasma

  • Ionized/free/unbound calcium
  • Bound calcium
  • Complexed calcium
• Ionized/free/unbound calcium


  50% of plasma calcium, metabolically active, required for nerve function, membrane permeability, muscle contraction and hormone secretion
• Bound calcium


  40% of plasma calcium, bound to protein mostly albumin
• Complexed calcium


  10% of plasma calcium, complexed with anions including bicarbonate, phosphate, lactate & citrate
• Normal range of plasma calcium

  2.2-2.6 mmol/L
• Mechanism of calcium absorption

  1. Calcium taken in diet as phosphate, carbonate & tartarate

  2. About 40% of dietary calcium absorbed from gut

  3. Absorption occurs from first & second part of duodenum

  4. Absorbed against concentration gradient, requires energy
• Calcium absorption


  • Requires carrier protein, helped by calcium-dependent ATPase
  • 400 mg excreted in stool, 100 mg excreted in urine
  • Two mechanisms: simple diffusion and active transport
• 1,25 DHCC (Calcitriol)


  Regulates synthesis of Ca-binding proteins & transport, required for both absorption processes
• Factors causing increased calcium absorption

  • Vitamin D (Calcitriol)
  • Parathyroid hormone
  • Acidity
  • High protein diet
  • Amino acids (lysine & arginine)
  • Sugars and organic acids
• Factors causing decreased calcium absorption

  • Phytic acid
  • Oxalates
  • Fibres
  • Malabsorption syndromes
  • Glucocorticoids
  • High phosphate
  • High magnesium
• Biochemical functions of calcium

  • Development of bone and teeth
  • Muscles
  • Nerve conduction
  • Blood coagulation
  • Hormone release
  • Enzyme activation
  • Second messenger
• Calmodulin


  Calcium binding regulatory protein, activates enzymes when bound to calcium
• Organs regulating plasma calcium level

  • Bone
  • Kidney
  • Intestine
• Hormones regulating plasma calcium level

  • Calcitriol
  • Parathyroid hormone
  • Calcitonin
  • Growth hormone
  • Glucocorticoids
  • Estrogens
  • Testosterone
  • Thyroid hormones
• Regulation of plasma calcium level by calcitriol


  1. Stimulates calcium uptake in osteoblasts for bone formation

  2. Increases mobilization of calcium and phosphate from bone

  3. Minimizes calcium and phosphate excretion in kidneys

  4. Increases intestinal absorption of calcium and phosphate
• Regulation of plasma calcium level by parathyroid hormone (PTH)

  1. Causes bone demineralization by osteoclasts

  2. Increases calcium reabsorption in kidney tubules

  3. Promotes production of calcitriol in kidney
• Calcitonin


  Peptide secreted by thyroid, antagonistic to PTH, promotes bone calcification, decreases bone resorption, decreases blood calcium
• Phosphorus in the body

  • About 1 kg total
  • 80% in bones and teeth
  • 15% in soft tissues
  • 1% in extracellular fluid
  • Organic and inorganic forms
• Dietary sources of phosphorus

  • Milk
  • Cheese
  • Beans
  • Eggs
  • Cereals
  • Fish
  • Meat
• Daily phosphorus requirement


  800 mg/day, 1200 mg/day during pregnancy and lactation
• Biochemical functions of phosphorus

  • Bone and teeth formation
  • High energy phosphate compounds (ATP, ADP, etc.)
  • Phospholipids, phosphoproteins, nucleic acids
  • Phosphate buffer
  • Phosphorylation of enzymes and proteins
• Phosphorus absorption


  About 90% absorbed, stimulated by PTH and calcitriol
• Regulation of phosphorus


  1. Similar mechanisms to calcium regulation by PTH and calcitriol

  2. PTH increases phosphate excretion in urine
• Phosphorus excretion


  • 500 mg/day in urine, influenced by muscle mass, renal function and age
  • Renal threshold is 2 mg/dl
  • 90% of filtered phosphate reabsorbed
• Normal range of plasma phosphorus

  0.9 to 1.5 mmol/L in adults, 1.45 to 3 mmol/L in children
• Calcium and phosphorus relationship

  • Reciprocal, if phosphate rises, calcium falls
  • Fasting phosphate levels are higher
  • Postprandial decrease due to metabolism utilization