Bones are continually being remodelled throughout life and, with ageing, more bone is broken down than is rebuilt and the mineral content of bones diminishes, causing them to become less dense and less able to resist normal stresses. This type of bone weakness is called osteoporosis, meaning ‘porous bones’, and causes increased risk of fractures. Hip fractures are the most serious outcome because they may lead to permanent disability and even death.
In women, oestrogen enhances calcium absorption and maintains bone strength. When oestrogen levels decline at menopause, bone loss accelerates and bones can lose 1-5% of their mineral content each year.
Calcium
Calcium combines with phosphate and is incorporated into the mineral matrix of bones as hydroxyapatite to provide rigidity and strength. When blood levels of calcium drop, parathyroid hormone is released and stimulates the kidneys to produce 1,25 dihydroxycholecalciferol (calcitriol), the active form. This causes calcium to move out of bones into the blood, increases calcium absorption from foods and reduces losses in urine to ensure blood levels of calcium are adequate for normal heart, muscle and nerve function. When blood levels of calcium are restored, bone is rebuilt. A low calcium intake over time causes hyperparathyroidism and weakens bones. Older people absorb less dietary calcium because their intestines are no longer as responsive to active vitamin D and this can be compensated for by increased calcium intake and maintaining adequate levels of vitamin D.
Vitamin D
Vitamin D3 (cholecalciferol) is a steroid-derived hormone that is made naturally in the skin when exposed to solar ultraviolet B (UVB) radiation of 290-315 nm wavelength, which is within the spectrum that causes sunburn. The precursor is a cholesterol metabolite, 7-dehydrocholesterol, that is converted to previtamin D3, which then undergoes thermal isomerisation to form cholecalciferol. It is transported to the liver, where it is converted to 25-dihydroxycholecalciferol, which is then converted to calcitriol in the kidneys by the action of parathyroid hormone.
During winter in higher latitudes, the sun is too weak to make vitamin D effectively and the body relies on stores built up over summer. Vitamin D deficiency is becoming more common because exposure to sunlight is restricted by excessive clothing, sunscreen use and indoor lifestyles. Having a dark skin colour reduces skin synthesis and vitamin D metabolism is impaired by ageing.