Dementia is usually thought of as a disease of the elderly.

But a small but growing number of studies have emerged which indicate early warning signs might appear as soon as childhood.

Research this week found children who regularly have nightmares are up to twice as likely than their peers to develop the memory-robbing disorder by age 50.

It is thought that nightmares disrupt the brain-regenerating elements of sleep, speeding up the decay of neurons.

But research has also indicated that children who sustain head injuries playing sports or who regularly crave sugar may also be more at risk of dementia.

While the true cause of dementia is still a mystery, scientists have attributed it to a build-up of inflammation in the body and genetic factors.

These can lead to more amyloid beta and tau protein tangles forming, disrupting brain cells' proper functioning.

Scientists say that because dementia is in part driven by genetic factors, warning signs of the disease could also crop up among children.

About 7million Americans have dementia, but studies suggest this figure could surge to nearly 12million by 2040. 

Regularly having bad dreams in childhood could be a warning sign of dementia in later life.

The findings were revealed in a study that tracked 7,000 people from birth to age 50 this week.

Scientists found those who had nightmares persistently as children were nearly twice as likely to develop dementia in later life. 

They were also seven times more likely to be diagnosed with Parkinson's — a brain disorder that causes unintended or uncontrollable movements, such as shaking, stiffness, and difficulty with balance and coordination.

The scientists — from Birmingham University in the UK — said that nightmares disrupt deep sleep, which is crucial for keeping the brain sharp and healthy.

The nightmares could also be linked to genetics.

One gene — PTPRJ — is has been associated with nightmares and Alzheimer's disease in the past.

Previous research published last year has already linked having regular nightmares as adults with a higher likelihood of dementia.

Scientists point out, however, that not every child experiencing regular bad dreams will develop dementia. 

In their paper published this week, out of the 268 children with nightmares, 17 — or six percent — developed dementia by age 50 years. 

For comparison, among the 5,470 who did not have nightmares, 199 — 3.6 percent — developed dementia.

Although the dreams may be a warning sign, the results suggest that other factors may also be at play.

It is well known that being fat as a senior is a major risk factor for dementia and, increasingly, studies have shown that being overweight in middle-age raises the risk.

But scientists now believe that fat children may also be at higher risk of the neurodegenerative disease in old age.

Being fat raises inflammation levels, which can negatively impact the brain.

It is also usually an indicator of too little exercise, meaning there is less blood flowing to the brain which keeps it healthy.

And it raises the risk of suffering diabetes and high blood pressure, which both raise the risk of dementia. 

A study published last year which tracked 1,200 children for 30 years found fitter and skinnier youngsters had better thinking skills in later life.

Scientists suggested that their enhanced cognitive ability could go on to shield them from being robbed of their memories in old age.

Dr Keith Vossel, a neurologist at the University of California, Los Angeles, agreed that being fat in childhood raised the risk of dementia.

He told DailyMail.com: 'The reason for that is it raises the risk of diabetes and high blood pressure, and both confer a higher risk of dementia.'

America's childhood obesity rates are set to surge over the coming decades.

Currently, 14.7million children — or one in five — are obese according to estimates. This rate has tripled in the past three decades alone.

And scientists suggest it will continue to rise, estimating that rates of type 2 diabetes in children — which is linked to unhealthy lifestyles — could rise by as much as 600 percent by 2060.

Being fat has been linked to an avalanche of negative health consequences including a higher risk of heart disease, cancer — and even a shorter lifespan. 

Children and adolescents may knock their heads while playing American football, ice hockey and soccer.

But there are concerns among scientists that this may raise their risk of neurodegenerative disease later in life.

Rigorous scientific studies to prove how sports injuries in childhood could cause dementia later in life are ongoing.

But papers have already suggested a link between head injuries sustained by professional athletes and dementia later in life.

Dr Vossel told DailyMail.com that it was certainly plausible that head injuries from playing sports in childhood could raise the risk of dementia.

In research that tracked 7,000 people from birth to age 50, a team at the University of Birmingham, in the UK, found those who has persistent nightmares as children are twice as likely to develop dementia.

But he said the link had only been established to date in children who suffer severe head injuries, where they are in a coma for at least 24 hours.

Studies showing the link in adults include a report from 2014 which looked at National Football League (NFL) players.

It found they were about twice as likely to get dementia compared to the general population and that 14 percent will develop the disease later in life.

Players diagnosed with dementia include former NFL star Mark Gastineau, who retired as defensive end for the New York Jets after a 10-season career.

There is also evidence from soccer, where players may 'head' a soccer ball.

One major study published in 2019 tracked 7,600 professional soccer players and 23,000 people from the population for about two decades.

It found that professional soccer players were five times as likely to die from Alzheimer's disease compared to others. They were also three-and-a-half times more likely to die from neurodegenerative diseases.

The UK banned heading the soccer ball during practice for all children under 11 years old in 2020.

And the US put the block in place for youngsters in 2015 over concerns about the risk of head injuries.

Dr Willie Stewart, a neuropathologist at the University of Glasgow, said: 'With the current data we're now at the point to suggest that [soccer] football should be sold with a health warning saying repeated heading in [soccer] football may lead to an increased risk of dementia.

'That's where we are now, that cannot be ignored.'

Regularly craving sugar could raise someone's risk of dementia later in life, scientists say.

Proposing the new theory this week, experts at the University of Colorado Anschutz scientists did not specify what age groups this may apply to — but Dr Vossel said it may well apply to children too.

They suggest that an ancient survival mechanism is triggered when the body starts to metabolize fructose, a type of sugar common in juices, sugary drinks and sauces.

This mechanism is thought to lead to blood flow being restricted to the cerebral cortex, the part of the brain located at the outer layer.

The cerebral cortex is involved in self-control, as well as the hippocampus, which is linked to learning and memory, and the thalamus, which is important for filtering information between brain and body.

The reduction in blood flow to these areas helps someone to become more focussed without being distracted by recent memories and time, they said. 

This could help make foraging more efficient in the wild, they said, and the system could then be switched off once the individual was satiated.

But these days the system is stuck permanently in the 'on' position because of the onslaught of sugar from people's diets. 

Dr Richard Johnson, the lead author behind the theory, said: 'We believe that initially the fructose-dependent reduction in cerebral metabolism in these regions was reversible and meant to be beneficial.

'But chronic and persistent reductions in cerebral metabolism driven by recurrent fructose metabolism leads to progressive brain atrophy and neuron loss with all of the features of Alzheimer's disease.'

The theory was published this week in the American Journal of Clinical Nutrition.

Dr Vossel, who was asked about the research by DailyMail.com, said it 'made sense' that poor eating habits early in life could raise the risk of dementia later on.

'If sugar craving is not well controlled it leads to a higher risk of diabetes, and that is a strong risk factor for dementia.'

He added: 'The explanation of how high intake of fructose can trigger Alzheimer's-like changes in the brain is biologically plauysible and deserves further study.

'Foods that children are exposed to influence their cravings and food preferences later in life, so it is important to start good dietary habits early in life.'

Previous research has already found that tau and amyloid beta proteins build up in the brains of rats over time when they are exposed to fructose for long periods.

Regularly drinking sodas has also previously been linked to a higher risk of Alzheimer's disease.

But all these papers suggest an association but have not definitively proved that high-sugar diets cause neurodegenerative decline.

Other factors could also be at play, including bodyweight, age and exercise. Scientists agree that more research is needed into this area.

Estimates suggest about three in five American children have a sugary drink at least once a week.