It's one of the most pressing and controversial questions in modern medicine: just why are cases of autism skyrocketing?

The lifelong developmental disorder – which begins in early childhood, making it difficult for people to understand social cues and express themselves to others – was once considered rare.

However, over the past 20 years, the number of people in the UK living with autism has surged by almost 800 per cent, according to research published in the Journal of Child Psychology and Psychiatry. There are now about 700,000 people in the UK with a diagnosis.

Many experts believe this staggering increase is largely due to a better understanding of autism among doctors and parents. This means children with the symptoms who might once have gone undiagnosed are now being spotted at an early age.

But not everyone agrees. Some experts claim there is growing evidence that while genetics – DNA passed down by parents – are the primary trigger of autism, there are environmental factors that appear to be raising the risk of children developing the condition.

Some doctors believe that  ensuring pregnant women get enough folate could slash the risk of children being born with autism

Perhaps most intriguing are the claims made by Dr Richard Frye, a child autism specialist and researcher at the Rossignol Medical Centre in Arizona, who believes a simple vitamin deficiency is to blame.

Dr Frye argues as many as three-quarters of children with autism have dangerously low levels of folate – known as vitamin B9 – which research shows is crucial for brain development.

The paediatric neurologist is one of about 50 US doctors giving folate drugs to children with autism, some of whom appear to have experienced remarkable improvements within weeks of starting treatment.

Significantly, he also believes that ensuring pregnant women get enough folate could slash the risk of children being born with autism in the first place.

This would be a game-changer, as folate is in a common over-the-counter supplement called folic acid. Pregnant women are already recommended to take it, and they can be bought from most UK chemists for as little as 10p a pill.

Dr Frye’s research is controversial among autism experts – many argue there isn’t enough evidence to support his claims.

However, he is by no means the first to make the link.

Studies have shown expectant mothers who take a daily folic acid supplement during the first months of pregnancy are less likely to have a child with autism.

Now experts are calling for more research into the phenomenon, in an effort to conclude whether it can lower the number of new diagnoses.

‘Our research so far has looked at giving folate to children who already have an autism diagnosis,’ says Dr Frye.

‘However, we could be tackling this before birth, too, by giving it to pregnant women.

‘We believe that as many as 75 per cent of children with autism have these low folate levels, and much of that begins in the womb.’

So what is folate – and could it really hold such promise?

Research suggests that autism appears to affect how the brain works and its structure.

A study published in the medical journal Molecular Psychiatry found people with autism tend to have fewer synapses – connections that feed messages from one part of the brain to another.

But autism is a spectrum – patients are not equally affected. At its most severe, they may not be able to communicate at all and require lifelong care. Others may not need any special care but find forming relationships challenging, prefer predictability and dislike loud, chaotic spaces.

Experts believe those with a family history of autism are most likely to develop it. ‘If one of your parents has autism, there is a higher chance you will too,’ says Dr Michael Absoud, a paediatric neurodisability expert at King’s College London. ‘It’s unclear why this happens – there isn’t an individual gene that is passed down that leads to autism – but the link is strong.’

There is also a list of environmental factors that increase the risk of autism occurring.

The older parents are when they conceive, the more likely the child will be autistic, research suggests. ‘This might partly explain the rise,’ says Dr Absoud. ‘Parents are having children later than 20 years ago.’

The epilepsy drug sodium valproate has been linked to some birth defects when taken during pregnancy, including autism.

High levels of air pollution, infections during pregnancy, diabetes and obesity are also linked to higher rates of autism, according to the US National Institute of Environmental Health Sciences.

And now experts say there is strong evidence supporting a link between low folate levels and autism.

Folate is crucial for the development of the brain and spinal cord. It can be found in foods such as dark green leafy vegetables, legumes and citrus fruits.

Folate is crucial for the development of the brain and spinal cord. It can be found in foods such as dark green leafy vegetables, legumes and citrus fruits

Many women have low levels of folate, which is why the NHS recommends they take a daily folic acid supplement when pregnant.

Since the brain and spinal cord are some of the first parts to form during pregnancy, folate is needed most urgently during the first few months. However, many women are unaware they are pregnant at this point.

Without adequate levels of folate, children can be born with severe defects. The most serious is spina bifida, a debilitating condition where the spine and spinal cord do not develop properly.

From next year, in an effort to combat folate-related defects, manufacturers will be legally required to add folic acid to flour. Some have even called to extend this to foods such as rice.

‘Of the potential environmental factors connected to autism, the evidence supporting the use of folic acid in the very early stages of pregnancy does appear the strongest,’ says Dr Absoud. ‘More research is needed before we can say that with certainty. Even then, it is unlikely to be the main cause.’

Dr Frye is more enthusiastic. He argues pregnant women should be taking the form of folate he gives to his patients.

Called leucovorin, it costs just £2 per pill and is usually used to relieve the toxic effects of some forms of chemotherapy.

Dr Frye argues the prescription-only treatment produces more folate that reaches the brain than folic acid supplements, and says he has treated dozens of patients who have experienced ‘substantial improvements’.

He adds: ‘Leucovorin is an extremely safe drug to take and doesn’t have any side effects.’

But Dr James Cusack, chief executive of autism research charity Autistica, argues that many of the children Dr Frye treated may have seen improvement anyway. ‘Around the age of three, many children with autism see a burst in development – or their difficulties can increase,’ he says. ‘Parents often look for a reason. If they’ve just joined a medical trial, it might appear this was the cause.’

However, Dr Frye points to studies in the US, France and India that show autistic children see a larger improvement when taking leucovorin than those taking a placebo.

Some experts argue autism should not be treated as a problem that needs to be cured.

‘We are moving away from viewing autism as something that needs fixing,’ Dr Absoud says. ‘It simply means that someone’s brain functions differently.’

Dr Frye disagrees: ‘A lot of people now say it’s just a character quirk. But come to my clinic and you will see how severely it affects so many children.

‘Many can’t even communicate. That isolates them and raises the risk of them dying young. Some people have gone so long believing there is nothing you can do to fix autism they are unwilling to listen to what we’ve found.

‘If you could do something to reduce the risk of your child getting autism, why would you not take it?’

After two weeks of treatment, Ryan said ‘I love you, Dad’ for first time 

When Ryan Baldridge (pictured with his younger brother, Grayson) was diagnosed with autism at age four, his family were told he may never speak coherently

Until the age of four, Ryan Baldridge Jr was ‘basically non-verbal’, according to his family.

Ryan, from Missouri, was diagnosed with autism at a young age.

The development disorder meant he was only able to repeat back words his parents said to him, rather than form sentences on his own. After Ryan’s parents, Kim and Ryan Sr, came across Dr Richard Frye’s research online, they took him to see the autism expert in 2023.

Dr Frye carried out a test that found Ryan had low levels of folate, a vitamin crucial for brain development.

Ryan began taking two daily liquid doses of leucovorin, originally a chemotherapy drug which also boosts folate production in the body.

Two weeks later, Ryan was talking to his parents in full sentences and, for the first time, was able to use words to express his affection for his parents.

‘I had never experienced an unprompted “I love you” from my son in five years of his life,’ Mr Baldridge told DailyMail.com last month.

‘It was like an explosion of improvement all happening so quickly.

‘Now you can’t shut him up. He’s doing unbelievably.’