What is the mode of action of calcium in the body?

Mode of Action of Calcium in the Body

Calcium serves as an essential mineral with multiple critical physiological functions, including maintenance of bone tissue, skeletal muscle and cardiac contraction, nerve conduction, and as an intracellular messenger. 1

Primary Physiological Functions

• Calcium is the most abundant mineral in the human body, accounting for approximately 1-2% of total body weight 2
• More than 98% of all calcium in the body is contained within the skeleton, serving as both structural support and as a reservoir for calcium that can be stored and released when needed 1
• Calcium has two key physiological functions in adults:
  • Acts as an intracellular messenger in virtually all body cells 1, 3
  • Serves as a key component of hydroxyapatite, which is present in the organic matrix of bone and provides strength and rigidity to the skeleton 1

Calcium Distribution and Metabolism

• Only 1% of total body calcium is distributed within extracellular fluids and soft tissues 4
• In serum, calcium exists in three fractions:
  • Ionized calcium (~50%) - the biologically active form 4, 5
  • Protein-bound calcium (~40%) - primarily bound to albumin 4
  • Small amount complexed with other molecules such as citrate and phosphate 5
• Calcium metabolism involves three main mechanisms:
  • Bone turnover (resorption and formation)
  • Intestinal absorption
  • Renal reabsorption 5

Hormonal Regulation of Calcium

• Serum calcium concentration is tightly regulated by several hormones regardless of acute calcium intake 1
• Key regulatory hormones include:
  • Parathyroid hormone (PTH) - increases serum calcium by promoting bone resorption, renal calcium reabsorption, and activation of vitamin D 5, 6
  • Calcitonin - decreases serum calcium by inhibiting bone resorption 1, 5
  • Vitamin D (calcitriol) - enhances intestinal calcium absorption 7, 5
• When dietary intake is inadequate, the skeleton is mobilized through the action of parathyroid hormone, leading to demineralization of bone tissue 1

Calcium Absorption and Bioavailability

• Calcium absorption occurs primarily in the small intestine through a vitamin D-regulated saturable pathway 7
• Bioavailability of dietary calcium depends on both physiological and dietary factors 3, 8
• Physiological factors affecting calcium absorption include:
  • Age (absorption decreases with aging)
  • Physiological status (pregnancy, lactation)
  • Vitamin D status
  • Presence of diseases affecting absorption 8
• The gut cannot absorb more than 500 mg of calcium at once, so calcium intake should be spread throughout the day 7

Calcium Excretion

• Calcium is eliminated primarily through urinary excretion 4
• Studies have shown a relationship between urinary calcium excretion and intravenous administration of calcium, with significant increases in urinary calcium excretion observed after IV calcium administration 4
• Calcium may also be lost through sweat, although modestly 1
• Dermal calcium losses have been reported following prolonged exercise alongside an increase in parathyroid hormone 1

Clinical Implications

• Insufficient calcium intake over prolonged periods may affect important physiological processes due to obligate losses in urine, sweat, and stool 1
• Chronic calcium deficiency can lead to reduced bone mass and osteoporosis 2
• When calcium intake is inadequate, the body uses calcium from skeletal stores, which can weaken existing bones 7
• Calcium supplementation may be necessary for individuals who don't consume adequate calcium from their diet 1
• Calcium intake from food and supplements that does not exceed 2000-2500 mg/day is considered safe from a cardiovascular standpoint 1, 7

Calcium Requirements

• Recommended dietary allowances for calcium vary by age and sex, ranging from 1000-1300 mg/day for adults 1
• Higher intake of 1500 mg/day may be recommended for athletes to optimize bone health, particularly in cases of relative energy deficiency in sport 1