Most of us find that our sense of smell declines as we get older. Most of the time it's a natural part of the ageing process, but sometimes it can be a sign of something more serious.
See also: 'Supersmeller' woman can sniff out Parkinson's
See also: 10 signs of dementia you need to know
An early warning sign of dementia?
While a poor sense of smell can be a part of ageing, it can also be an early sign of Alzheimer's disease.
According to a study published in the medical journal JAMA Neurology, elderly people who performed badly in smell tests were more likely to show signs of memory loss over the next three and a half years. And those who had the weakest sense of smell had the highest risk of developing Alzheimer's.
Researchers believe that the deterioration in the part of the brain that controls smell may coincide with degeneration in the part that controls memory.
Scientist Dr Mark Albers has been studying the link between smell and dementia and believes that a reduced sense of smell caused by Alzheimer's disease could be detected 10 years before patients start to experience memory loss.
Of course, there are a host of reasons why a person might lose their sense of smell – including the common cold, hay fever, and sinusitis.
If you've lost your sense of smell suddenly without there being an apparent cause, see your GP. They may be able to diagnose a cause and offer treatment to restore your sense of smell.
Some people who develop a smell disorder may have experienced a recent illness or injury or be suffering from an underlying medical condition, such as
• underactive thyroid
• hormonal disturbances
• Cushing's syndrome
• a head injury
• a brain tumour
• radiotherapy to the head and neck
• Parkinson's disease
• Alzheimer's disease
• liver or kidney disease
• vitamin B12 deficiency
• granulomatosis with polyangiitis – an uncommon disorder of the blood vessels
• sarcoidosis – a rare disease that causes body cells to form into clumps
In addition, a number of medications, such as antibiotics like metronidazole, and recreational drug use such as cocaine or amphetamines, can affect your ability to smell – as can smoking and long-term alcohol misuse.
A nose abnormality, such as a crooked nose or a nasal septum or exposure to a chemical that burns the inside of the nose can also be to blame.
A harbringer of death?
One study found that older people with a poor sense of smell were four times as likely to die within five years as those who could accurately identify odours.
Researchers at the University of Chicago assessed 3,000 people aged 57 to 85 to test their sense of smell. The five odours, in order of increasing difficulty, were peppermint, fish, orange, rose and leather. Five years later, the team confirmed which participants were still living.
When the scientists adjusted for variables - such as age, gender, socio-economic status, overall health, and race - those with greater smell loss when first tested were substantially more likely to have died five years later.
Experts aren't sure why a poor sense of smell indicated fading health, but say that it's a more accurate indicator than many other measures – including a diagnosis of heart failure, cancer or lung disease.
The study's lead author Doctor Jayant Pinto, associate professor of surgery at the University of Chicago, said: "We think loss of the sense of smell is like the canary in the coal mine.
"It doesn't directly cause death, but it's a harbinger, an early warning system, that something has already gone badly wrong, that damage has been done.
Another good reason to see your GP.