Research updates
Osteoporosis   p 9
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4. What causes osteoporotic fractures? Link to the Medical Research Council web site
4.3 Determinants of rate of bone loss
In this section we will examine the factors that can affect the rate of bone loss:
Thinness
Thinness is not always a good thing. Studies have shown that the possession of a fuller figure has a protective effect against bone loss. The reasons for this association are not understood. The mechanisms proposed include oestrogen production in adipose tissue (adipose tissue is thought to be a major site of oestrogen production in post-menopausal women) and metabolic effects on body composition. Mechanical loading is likely to contribute since the mass of the body itself is bone-loading.

Photo of old people

Figure 16. Keeping active can maintain bone mass.
Inactivity
There is much epidemiological evidence linking inactivity to the risk of hip fracture. The link is due both to an increase in the risk of falling with lack of exercise and to an increase in the rate of bone loss. There is a dynamic balance between the resorption and rebuilding of bone (see 'The turnover of bone mineral' on page 2) and the effect of load-bearing exercise is to encourage bone formation. Lack of exercise tilts the balance towards resorption causing the net rate of bone loss to increase.

Adopting a more active lifestyle is generally helpful. Everyday bone loaders, such as stair climbing, are good for improving bone density in the spine and hip in older women. A variety of brief, energetic weight-bearing activities stimulate bone to improve, although not all forms of exercise (e.g. swimming) are useful for increasing bone density.

Calcium intake
If the calcium balance in the plasma is to be maintained without calcium being resorbed from the bones there must be an adequate amount of calcium in the diet. A calcium-rich diet, such as that recommended by the National Osteoporosis Society helps to prevent bone loss. Calcium supplements are also available.
Vitamin D
Vitamin D deficiency is an important cause of bone loss in the elderly. The role of vitamin D in calcium homeostasis has been explained in Box 2 on page 4. It is necessary for the absorption of calcium from the intestines and inhibits PTH release from the parathyroid glands. A fall in the level of calcium in the plasma stimulates the secretion of parathyroid hormone.

Lack of vitamin D has a further effects – e.g. a deficiency of vitamin D causes muscle weakness, so increasing the risk of falling.

Smoking
There is a well-established relationship between cigarette smoking and low peak bone mass. It has been shown that smoking increases the rate of bone loss in women after the menopause. However, the impact of cigarette smoking on the risk of osteoporotic fracture is not nearly as great as its impact on lung or heart disease.
Alcohol
Studies of alcoholics suggest that alcohol in large quantities may be detrimental to bone. This could be because excess alcohol has a direct toxic effect on osteoblasts or it may have an indirect effect - e.g. adverse effect on calcium metabolism, mobility. Moderate alcohol consumption is not associated with bone loss.
Corticosteroids
Corticosteroids are used in the treatment of a range of diseases but they have an adverse effect on bone, which may lead to osteoporosis. They cause the rate of bone loss to increase by suppressing the rebuilding function of the osteoblasts. A large study in Southampton has shown that fracture risk increases rapidly with the use of oral corticosteroids. The study also showed that there is no ‘safe’ dose. However, the risk diminishes rapidly when corticosteroid treatment stops.

The new types of inhaled corticosteroids prescribed for the treatment of asthma seem to have less effect on bone. A study shows that, when compared with other types of inhalant treatment, they do not increase the risk of fracture.

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Question 8
a) Suggest a mechanism in terms of the bone cells in the BMU (section 2.1) by which load-bearing exercise could encourage bone formation.
b) List mechanisms by which asthma may lead to low bone mass.