Normal bone metabolism is a sequence of bone turnover (osteoclast activity) and formation (osteoblast activity).
Bone Metabolism functions largely involve the homeostasis of calcium and phosphate.
Bone Metabolism Regulators
Bone metabolism is regulated by:
- Parathyroid hormone (PTH)
- Sex Hormones (estrogen, androgens)
- Growth Hormone
- Thyroid Hormones
- Vitamin D
- More than 99% of the body’s calcium is stored in bones.
- Approximately 400 mg of calcium is released from bone daily.
- Calcium absorption:
- Calcium is absorbed in the duodenum by active transport.
- Calcium is absorbed in the jejunum by passive diffusion.
- Primary homeostatic regulators of serum calcium are PTH and 1,25(OH)2-vitamin D3.
Dietary requirement of calcium:
- Approximately 600 mg/day for children.
- Approximately 1300 mg/day for adolescents and.
- young adults (ages 10-25 years).
- 750 mg/day for adults (ages 25-50 years).
- 1200-1500 over the age of 50.
- 1500 mg/day for pregnant women.
- 2000 mg/day for lactating women.
- 1500 mg/day for postmenopausal women and for.
- patients with a healing fracture in a long bone.
- A key component of bone mineral:
- Approximately 85% of the body’s phosphate stores are in bone.
- Also important in enzyme systems and molecular interactions as a metabolite and buffer.
- Daily requirement is 1000 to 1500 mg.
Parathyroid hormone (PTH)
- PTH is synthesized in and secreted from chief cells of the (four) parathyroid glands.
- PTH helps regulate plasma calcium.
- Decreased calcium levels in extracellular fluid stimulate β2 receptors to release PTH, which acts at the intestines, kidneys, and bones.
- PTH directly activates osteoblasts.
- Modulates renal phosphate filtration.
- May accentuate bone loss in elderly persons.
- PTH-related protein and its receptor have been implicated in metaphyseal dysplasia.
- It’s activated by ultraviolet radiation from sunlight or utilized from dietary intake.
- Vitamin D hydroxylated to 25(OH)-vitamin D3 in the liver, and hydroxylated a second time in the kidney to one of the following:
- 1,25(OH)2-vitamin D3, the active hormone.
- 24,25(OH)2-vitamin D3, the inactive form.
- 1,25(OH)2-vitamin D3 works at the intestines, kidneys, and bones.
- It maintains normal serum calcium levels by activating osteoclasts for bone resorption and increasing intestinal absorption of calcium (increase serum Ca++)
- And promotes the mineralization of osteoid matrix.
- Calcitonin produced by clear cells in the parafollicles of the thyroid gland.
- Increased extracellular calcium levels cause secretion of calcitonin.
- Calcitonin inhibits osteoclastic bone resorption:
- Osteoclasts have calcitonin receptors.
- Calcitonin decreases osteoclast number and activity.
- It Decreases serum calcium level.
- May also have a role in fracture healing and in reducing vertebral compression fractures in highturnover osteoporosis.
- Estrogen prevents bone loss by inhibiting bone resorption.
- Estrogen therapy decreases bone formation:
- Supplementation is helpful in postmenopausal women only if started within 5 to 10 years after onset of menopause.
- Corticosteroids affects bone metabolism by increase bone loss and decrease bone formation.
- Decrease gut absorption of calcium by decreasing binding proteins.
- Decrease bone formation (especially cancellous) by inhibiting collagen synthesis and osteoblast productivity.
- Thyroid hormones increase bone resorption and can lead to osteoporosis.
- Regulates skeletal growth at the physis by Stimulates:
- Chondrocyte growth
- Type X collagen synthesis
- Alkaline phosphatase activity
- Causes positive calcium balance by increasing gut absorption of calcium more than it increases urinary excretion.
- Insulin and somatomedins participate in this effect.
- Transforming growth factor (TGF)-β, PDGF, monokines, and lymphokines have roles in bone and cartilage repair.