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Hormone replacement theraphy: what causes osteoporosis? HORMONE REPLACEMENT THERAPHY: WHAT CAUSES OSTEOPOROSIS?
As a simple definition, osteoporosis is a condition in which bone becomes so fragile and brittle that it breaks comparatively easily. Bone is a living, changing thing, containing two main types of cells called, confusingly, 'osteoclasts' and 'osteoblasts'. All through our lives, the osteoclasts wear away microscopic craters in the bone, and then the osteoblasts fill these craters with newly formed bone, exactly matching the space dissolved away by the osteoclasts. That way, bone is constantly renewed — a sort of repair-and-maintenance system. Oestrogen is thought to reduce the rate at which osteoclasts dissolve bone, and to increase the rate at which osteoblasts build it up. Once oestrogen levels fall, the osteoclasts dissolve the tiny craters at a faster rate than before, while the osteoblasts don't replace the bone so efficiently. Eventually, the bone becomes less dense and strong, and more liable to fracture.
Bone is built up during childhood and teenage years, and reaches a peak content (called 'peak bone mass') in the early twenties. For the next 15 years or so, the bones thicken and strengthen, but then from about the age of 35 onwards, bone mass starts to fall gradually. In a man it continues in -this gradual fall for the rest of his life, and a man of 90 can expect to have lost about 25 per cent of his total quantity of bone. In women, however, bone density drops dramatically in the years immediately after the menopause — about 3-5 per cent every year in the vertebrae of women who have had a natural menopause, and as high as 7-9 per cent in the vertebrae of women who have had an early oophorectomy. (The hip joint loses bone density at a slightly lower rate.) To lose bone at about 3 per cent a year may not seem much, but if you get a calculator, start with 100 (to represent the amount of bone you have at the menopause), then subtract 3 per cent from that figure, then 3 per cent again from the next figure, you will find that after doing this seven times you get a figure of 80. In other words, after losing 3 per cent of your bone every year for seven years, you are left with just 80 per cent of what you had at the start of the menopause - and you are probably still only in your fifties. Many women lose bone mass at a faster rate than 3 per cent, and sometimes for as long as 15 or 20 years; it is not unusual for them to end up having lost one-third or even half of their bone mass by about the age of 70. No wonder fractures occur!
Bones are not solid things, like an iron bar, as this would make them very heavy. Each bone contains an outer shell of 'cortical' bone, which is strong, compact and dense, and an inner area of 'trabecular' bone, which is brittle and fragile. The bones that are most vulnerable to fracture in osteoporosis are those that have a higher proportion of trabecular bone, such as the hip joints and the vertebrae of the spine.
Trabecular bone is made up of tiny vertical pillars, joined together with horizontal cross-ties, giving it strength with the minimum of weight. In osteoporosis, as the osteoclasts wear away bone faster than the osteoblasts can build it up again, these pillars and cross-ties lose their connections with each other, and the bone therefore loses its strength. Eventually, it becomes so fragile that it can fracture while you are doing such everyday things as lifting a casserole out of the oven, opening a stuck window, putting shopping bags into the boot of the car, doing up the back zip of a dress, or even coughing, laughing or sneezing.
The main component of bone is calcium, which is held in a soft substance called collagen; as collagen levels fall, so do die levels of the calcium held within it. Oestrogen helps the body to absorb calcium effectively, making it available to the osteoblasts, and as oestrogen levels fall after the menopause, so calcium is stored less effectively. Much of it is just excreted in the urine instead of being used to build bone.
As oestrogen is needed to get the 'bone building' balance right, and as it also helps the body to absorb calcium and preserve collagen, you can see why oestrogen therapy is so effective in preventing osteoporosis. It can't rebuild damaged bone, but it can help prevent further bone loss. It has been said of HRT that it 'stops osteoporosis in its tracks', because oestrogen therapy at any stage after the menopause halts bone loss.
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