What happens to our bones as we age?

Bone is not a lifeless skeleton, but rather a dynamic metabolic organ always on the go. On the outside, new bone is continuously built up, layer upon layer.  On the inside, bone is continuously destroyed and reabsorbed. Continuous remodelling allows the body to replace tiny cracks before they weaken the structure, as well as adapt its shape and strength according to how it is being used. Like many aging processes, this balance becomes more precarious as we get older, where increased re-absorption and slower formation of new bone leads to thinning of the bone, from the inside out.

In our early twenties, our bones are the thickest and strongest they will ever get. Subsequently, for every year that goes by, our bones become progressively thinner. Sometimes there comes a point where bone loss is so significant that the strength and integrity of the bone is compromised, leading to an increased risk of fractures. This threshold is called osteoporosis. About half of all women and one in eight men, aged 50 years or older, will have an osteoporotic fracture during their lifetime. While most of us have broken a bone at some time in our lives, when we are older, any fracture can have catastrophic effects on health and mortality.

Is osteoporosis an inevitable part of aging?

Not everyone will develop osteoporosis or fractures. The stronger our bones are in our early twenties, the more time it will take before they are sufficiently thin to increase the risk of fracture, if at all. Although the major determinant of this peak bone mass is our ‘thick boned’ genes, maybe a quarter of all the variability is determined by nutrition and physical activity during the growth years of childhood and adolescence, as well as other factors like medication and illness.

Women are more likely to develop osteoporosis than men partly because they have a lower peak bone mass (by about 10%). In addition, the major female sex hormone, estrogen, is an important regulator of bone formation and function. Loss of estrogen associated with menopause leads to enhanced bone re-absorption and reduced bone formation.

What can I do to prevent osteoporosis?

Low-dose estrogen, as delivered in hormone replacement therapy, offers protection against bone loss, but only for as long as the estrogen is taken. Estrogen derived from plants (phytoestrogens), especially soy isoflavones, may have beneficial effects on bone health when used in doses much higher than can be achieved by dietary choices (i.e. supplements). Declining testosterone levels associated with the andropause also have important consequences for our bones.

Apart from hormones, a number of other factors may tip the balance and result in fragile bones. Some of these factors include alcohol abuse, diabetes, smoking and prolonged bed rest. A number of prescription medications can also trigger bone loss, including chronic steroid therapy, Glitazones and those that suppress sex hormone production. In all cases, these factors bring aging individuals closer to a threshold at which bone strength is compromised.

Last Reviewed 02/Mar/2014

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Merlin Thomas is a physician and a scientist. His research laboratory is at the JDRF/ Danielle Alberti Memorial Centre for the study of Diabetes Complications at the Baker IDI Heart and Diabetes Institute in Melbourne.

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