Anatomy and Metabolism of Bone
Bone composition: highly metabolic tissue consisting of type I collagen (40%) + hydroxyapatite (45%) + water (15%)
Bone mass: function of
- peak bone mass attained in 3rd decade of life influenced by dietary calcium intake during adolescence, sex hormone status (male >female), nutrition, physical activity, genetic factors (Blacks >Whites >Asians)
- rate of age-related bone loss = endosteal + Haversian resorption + loss of cancellous bone (particularly in vertebrae) without replacement by new bone
Bone resorption: in the elderly due to
- estrogen deficiency = rate increased at menopause
- diminished fractions of bioavailability for testosterone
- vitamin D insufficiency → secondary hyperparathyroidism
- declining levels of physical activity
- reduced serum levels of insulin-like growth factors
- 99% in bone
- serum calcium
- protein-bound fraction (albumin)
- ionic (pH-dependent) 3% as calcium citrate / phosphate
Absorption: facilitated by vitamin D
Excretion: related to dietary intake; >500 mg/24 hours (= hypercalciuria)
Absorption: requires sodium; decreased by aluminum hydroxide gel in gut
Excretion: increased by estrogen, parathormone decreased by vitamin D, growth hormone, glucocorticoids