The subtle eating habit that could be an early sign of the dementia suffered by Bruce Willis

Strange table manners could be more than just quirks, they may be an early sign of dementia, experts have warned.

Frontotemporal dementia, the disease affecting actor Bruce Willis and approximately 60,000 other Americans, is caused by a gradual decline in the areas of the brain linked to personality and language abilities.

Warning signs include shifts in personality and behavior, such as impulsivity or a lack of empathy, and language problems, where sufferers have difficulty finding words.

But experts say that a lesser-known symptom of the condition is 'obsessive and repetitive behaviors' that can extend to a patient's diet and eating habits.

Patients may become fixated on one food, refusing to eat anything else, researchers in DC said previously in a study. They may also eat non-food objects or steal from other people's plates.

Dr Marilena Aiello, a neuroscientist who researches the condition, said previously: 'These behaviors are problematic, of course, socially, but also with regard to patients' health as they tend to gain weight.

'[But], some people lose weight because they eat a narrow range of foods in an obsessive way.'

It was not clear why the condition may cause this behavior, but researchers say it is potentially linked to alterations in the body's signals for hunger and fullness.

It is not known whether Bruce Willis suffers from any compulsive eating behaviors as a result of his condition, but his family has said he is struggling with communication.

And earlier this month, his wife Emma Heming Willis revealed the subtle sign she noticed shortly before his diagnosis.

She told ABC News: 'For someone who is really talkative, very engaged, he was just a little more quiet, and when the family would get together he would kind of just melt a little bit.'

He also soon began losing words, she said, and a stutter he dealt with as a child returned.

'[He] felt very removed, a little cold, not like Bruce, who is very warm and affectionate.

'Going the complete opposite of that was alarming and scary,' she added.

In frontotemporal dementia, behavioral changes are normally the first noticeable sign of the disease. In Alzheimer's, for comparison, which affects up to 6.7 million people, the first signs are normally memory loss or confusion with time or place.

Over time, more areas of the brain degenerate, meaning patients with frontotemporal dementia often eventually mirror those with late-stage Alzheimer's.

Other symptoms of frontotemporal dementia include difficulty eating or swallowing, trouble walking and vulnerability to infections.

The condition is not normally fatal on its own, but weakens the body and raises the risk of life-threatening infections or other complications. This may include problems swallowing or eating and drinking.

There is no cure for the disease, although some drugs and therapies may help to slow the progression of the symptoms.

It is also not clear what causes frontotemporal dementia, although it has previously been linked to a build-up of protein clumps in the brain.

Some researchers say it can also be caused by damage to blood vessels in the brain, which they say can affect the supply of nutrients and oxygen to brain cells.