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, it 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 bone thinning 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.

The Royal Australian College of General Practitioners Guidelines describe osteoporosis as a condition characterized by both low bone mineral density and micro-architectural deterioration of bone tissue, leading to decreased bone strength, increased bone fragility and a consequent increase in fracture risk. Osteoporotic fractures usually result from falls from a standing height or less in individuals with decreased bone strength.

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 even cause mortality.

Osteoporosis and 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. This is 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 resorption and reduced bone formation.

What can I do to prevent osteoporosis?

You can prevent osteoporosis by taking low-dose estrogen, as delivered in hormone replacement therapy. This protects against bone loss, but only for as long as you take it. 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. These include 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.

References

  1. http://www.racgp.org.au/download/Documents/Guidelines/Musculoskeletal/osteoporosis-guidelines.pdf
  2. Schuit SC, van der Klift M, Weel AE, et al. Fracture incidence and association with bone mineral density in elderly men and women: The Rotterdam Study. Bone 2004;34(1):195–202
  3. Bliuc D, Alarkawi D, Nguyen TV, Eisman JA, Center JR. Risk of subsequent fractures and mortality in elderly women and men with fragility fractures with and without osteoporotic bone density: The Dubbo Osteoporosis Epidemiology Study. J Bone Miner Res 2015;30(4):637–46
  4. Eisman J, Clapham S, Kehoe L. Osteoporosis prevalence and levels of treatment in primary care: The Australian Bone Care Study. J Bone Miner Res 2004;19(12):1969–75

Last reviewed 24/Apr/2017

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Dr Merlin Thomas

Professor Merlin Thomas is Professor of Medicine at Melbourne’s Monash University, based in the Department of Diabetes. He is both a physician and a scientist. Merlin has a broader interest in all aspects of preventive medicine and ageing. He has published over 270 articles in many of the worlds’ leading medical journals