Metabolic Bone Disease

Introduction 

Metabolic bone disease is an umbrella term for a variety of diseases that result in a deterioration of bone structure and strength. These disorders affect a large number of the population, particularly amongst the elderly, namely because of a reduced bone mineral content.¹ Metabolic bone diseases can be caused by nutritional deficiencies, genetic disorders, or appear as a side-effect from other conditions.²

The Different Types of Metabolic Bone Disease

Osteoporosis

Osteoporosis is the most common form of metabolic bone disease and is characterised by a decrease in bone mass. The bone becomes thin and porous because of the loss of bone minerals. Osteoporosis becomes problematic when it starts to cause the sufferer to be prone to fractures, which can lead to falls. These falls can be dangerous, especially in the elderly as they are associated with significant pain and a loss of independence.³

Osteomalacia & Rickets

Osteomalacia is a disease where the bone is unable to mineralise and strengthen, resulting in the softening of bones. Children who suffer from osteomalacia commonly also suffer from rickets – which is characterised by bone weakening.⁴

Paget's Disease of Bone

This is a metabolic bone disease that occurs when the body breaks down too much bone. Although the disease can affect almost all bones in the body, particularly the spine and pelvis, it is rare to affect bones in the hands and feet.⁵ 

Signs and Symptoms?

Osteoporosis

Women are particularly prone to developing osteoporosis after menopause because they lack the hormone oestrogen; statistically, 1 in 3 women over the age of 50 are affected by an osteoporotic fracture in their lifetime. Men are less likely to suffer from osteoporosis as they have a larger initial bone mass and hormones that help protect their bones; statistically, 1 in 5 men are affected by an osteoporotic fracture in their lifetime.³ Symptoms includes⁹:

• Bone pain – as the bone becomes weaker, it is easily fractured
• Loss of height – weakened bones cause the spine to curve
• Fractures – most common fractures occur in the forearm, hip, pelvis and ribs 

Osteomalacia

Osteomalacia is a disease that can occur in children and adults, with highest prevalence found in the elderly.⁶ Symptoms includes:

• Musculoskeletal pain
• Skeletal deformity
• Muscle weakness

Rickets

Rickets is only prevalent in children and adolescents⁷ Symptoms includes:

• Bone deformities
• Bone pain
• Impaired growth

Paget’s Disease

It is the second most common metabolic bone disease, commonly found in people over 50 years of age and is most notable in Caucasians.^{5, 8} Symptoms includes:

• Bone pain
• Bone fractures
• Bone deformity

Causes and Risk Factors

Osteoporosis

Calcium is vital for bone and teeth formation and for normal muscle function. We can modify our calcium levels through diet and naturally through several hormones in our bodies. When these hormones are out of balance, metabolic diseases can occur. These hormones include the parathyroid hormone, which is released by small glands in the neck that help to raise calcium levels. 

Some conditions (e.g. a cancer) can cause there to be too much parathyroid being released and can cause bone cysts and bone pain. Osteitis Fibrosa Cystica (OFC), although not very common, is a disease that is caused by excessive parathyroid hormone. Symptoms include fractures, kidney stones, and weight loss.¹⁰

Surgical removal of the tumour or removal of the parathyroid are methods to treat excessive hormone release. 

Sometimes the opposite can happen where too little parathyroid hormone is being released and this is characterised by dental abnormalities and muscle cramps. Vitamin D supplements are usually given as a form of treatment.

Calcitonin: released by the thyroid gland in the neck, serving to stop bone breaking down (reabsorption) and thus lowers calcium levels in the blood. 

A metabolite of Vitamin D: helps bones reabsorb calcium, thus strengthening them. 

There are two important types of cells within the bone that manage bone turnover:¹¹

• Osteoblasts: respond to the parathyroid hormone and are responsible for making new bone 
• Osteoclasts: line the surface of bone and remove old bone (resorption)

When the function of these cells are out of balance and more bone is removed than being made, metabolic bone diseases can occur. 

Risk Factors for Osteoporosis:¹²

• Gender and age – greater risk for women and the elderly
• Low body weight
• Low physical activity
• Alcohol consumption and smoking
• Certain medicines 
• Conditions such as Inflammatory Bowel Disease, Type I Diabetes, Parkinson’s Disease

Risk Factors for Rickets & Osteomalacia

The largest known cause of Rickets and Osteomalacia is Vitamin D deficiency. Vitamin D is important as it helps the body absorb more calcium and is known to aid in bone mineralisation.¹²  

Risk Factors for Paget’s Disease

Both genetic and environmental factors can cause the disease, however the exact triggers of the disease remain uncertain.⁸

Treatments

Osteoporosis

Health professionals can measure a patient’s risk of fractures by taking into account their risk factors and the resulting score will indicate whether a patient should start treatment.¹² 

To date, a DEXA scan is the most common bone densitometry imaging technique to diagnose osteoporosis and it works by measuring the amount of bone in different areas of the body.

Nutrition is an important aspect of treatment in people with osteoporosis and other metabolic bone diseases. It is necessary to have adequate calcium and vitamin D as they are vital for bone health:

• Vitamin D can be obtained through good sun exposure, supplementation (e.g. vitamin D3, also known as cholecalciferol) and various food sources,such as cod liver oil, fatty fish (e.g sardines), and eggs.^{13, 14} 
• Calcium can be obtained through a calcium-rich diet, such as milk, green leafy vegetables (e.g. spinach and broccoli), grains, cereals and through calcium supplementation.¹⁵
• Phosphorus is another mineral important in bone health and can easily be obtained through diet. Meat and dairy products are good sources of phosphorus.¹⁴ 

Other Treatment Methods

• Moderate exercise and reduced smoking and alcohol consumption – helps to maintain bone mass

• Oestrogen replacement – helps to decrease bone loss in post-menopausal woman
• Consumption of oral bisphosphonates (e.g. Alendronic acid)

Rickets & Osteomalacia

Replacement of vitamin D through supplementation is given; the amount varies depending on the severity of the condition.

Paget’s Disease

The most effective treatments are ensuring adequate levels of vitamin D and bisphosphonate drugs.⁸

Summary

It is important to take a preventative approach in order to slow down the progression of many metabolic bone diseases. The most effective ways are through good lifestyle and diet habits. If you suffer from bone problems, it is best to seek help from a doctor who can help determine the cause; an important step in finding the best treatment pathway.