Clinical update: osteoporosis and eating disorders

 

People with eating disorders often have decreased bone density, osteoporosis and fragility fractures at much younger ages than would be expected. A report commissioned by Beat (Beating Eating Disorders) in 2015 estimated that 725,000 people in the UK are affected by an eating disorder. The National Institute for Health and Clinical Excellence estimates that about 11 per cent of those affected by an eating disorder are male. 

 

In 2007, NHS research reported that 6.4 per cent of adults in the UK display signs of an eating disorder. Figures published in 2014 showed an 8 per cent  rise in the number of inpatient hospital admissions for eating disorders in the 12 months since the previous year.

 

There are many risk factors affecting bone density, some of which are unavoidable and part of our genetic makeup. However, some medical conditions, medicines and/or lifestyle choices can also be risk factors for low bone density. Anorexia nervosa, and to a lesser extent bulimia nervosa, are risk factors for osteoporosis. Low bone density in anorexia nervosa and bulimia nervosa have several causes. The increased bone fragility and susceptibility to fracture can persist long after an eating disorder has been treated.  

 

Bone is alive and constantly changing. This happens through a process called remodelling. Osteoclast cells’ primary function is to remove bone. Bone forming cells are called osteoblasts. In normal bone, there is a balance between the actions of these two cell types. In osteoporotic bone, osteoclasts remove bone faster than the osteoblasts can form it. The result of this is a net bone loss.

 

Osteoporosis causes permanent changes in the bone. There are two types of bone: cortical bone is the hard, outer layer of the bone and trabecular bone is the sponge-like internal structure of the bone. Osteoporosis can lead to thinning of both cortical and trabecular bone, which makes bones more susceptible to breaks.

 

Up until a person is in their late 20s it is vital to maximise bone strength by ‘banking’ plenty of bone. This puts the skeleton in a better position to withstand the bone loss that occurs later. Plenty of weight-bearing exercise, specific resistance exercises and a healthy, well-balanced calcium-rich diet can help. After 30, and especially after the menopause in women, building bone density is not easy but small gains have been shown in post-menopausal women. Importantly, exercise at a healthy weight after the age of 30 can help reduce further bone loss, and cut the risk of fractures and the risk of falls. In men, bone density tends to stay stable until middle age, decreasing at a slower rate than is seen in women. 

 

A dual-energy X-ray absorptiometry (DXA) scan measures low bone density, from osteopenia (low bone density) to severe osteoporosis. However, bone density is only part of the story. What is most important is the ‘fragility risk assessment’ (FRAX) - how likely the bones are to break. The FRAX identifies the fracture risk over the next 10 years. The risk will be influenced by the existence of lack of other risk factors and will be included on the DXA scan results. A bone scan is normally recommended after menstruation has stopped for a year in women of menstrual age. Further scans may be repeated every two years if periods do not resume. The optimum frequency of repeat DXA scans is uncertain with some patients being scanned annually, while others may have three or more years between scans. People with eating disorders have decreased bone mineral density (BMD). The ‘T’ score compares a person’s BMD with the optimal BMD of a 30-year-old of the same gender. The ‘Z’ score compares a person’s BMD with that of someone the same age and gender – it is generally used in children and young adults. 

 

The most common fractures associated with osteoporosis and eating disorders are in the thoracic spine (T6-T8). This is due to the vertebral bodies being orientated towards flexion, meaning that force is transmitted through the weakest part of the vertebrae. Wrist and hip fractures are also common, often the result of a fall. Over-exercising in this population can cause such stress fractures in the feet and lower limbs.

 

Although exercise is known to have a beneficial effect on BMD at a healthy weight, studies are conflicting and it cannot be said that exercise in those with anorexia nervosa is beneficial to bone density. If exercise contributes to further weight loss then the overall result will be detrimental to bone density. With research and evidence in mind, physiotherapy advice will always be that return of menstruation and restoration of body weight are paramount and any exercise that prevents this occurring will be detrimental to bone density. fl

 

In an attempt to lose weight, people with anorexia nervosa may:

Where osteoporosis is present, certain movements should be avoided to lower the risk of fragility fractures. More caution is needed if people have already sustained an osteoporotic fracture.

 

The following activities may pose a particular risk:

 

Copies of the full article, titled Physiotherapy guidance notes for osteoporosis and exercise in anorexia nervosa and bulimia nervosa, are available through the Chartered Physiotherapists in Mental Health (CPMH) following the link to Eating Disorder Network.