While dementia is much more common in older adults, hundreds of thousands of people are diagnosed with young-onset dementia (YOD) each year – and an extensive new study sheds some considerable new light on why.

Most previous research in this area has looked at genetics passed down through generations, but here, the team was able to identify 15 different lifestyle and health factors that are associated with YOD risk.

"This is the largest and most robust study of its kind ever conducted," says epidemiologist David Llewellyn from the University of Exeter in the UK.

"Excitingly, for the first time it reveals that we may be able to take action to reduce risk of this debilitating condition, through targeting a range of different factors."

The research team analyzed data collected on 356,052 people aged under 65 in the UK. Low socioeconomic status, social isolation, hearing impairment, stroke, diabetes, heart disease, and depression were all associated with a higher risk of YOD.

Vitamin D deficiency and high levels of the C-reactive protein (produced by the liver in response to inflammation) also meant a higher risk, as did having two of the ApoE4 ε4 gene variants (a genetic scenario already linked to Alzheimer's disease).

The researchers describe the relationship between alcohol and YOD as "complex". While alcohol abuse led to an increased risk, moderate to heavy drinking correlated with a reduced risk – possibly because people in this second group are usually healthier in general (bear in mind that those who abstain from alcohol often do so on medical grounds).

Higher levels of formal education and lower physical frailty (measured through higher handgrip strength) were also associated with a lower YOD risk. This all helps to fill in some of the knowledge gaps around YOD.

"We already knew from research on people who develop dementia at older age that there are a series of modifiable risk factors," says neuroepidemiologist Sebastian Köhler from Maastricht University in the Netherlands.

"In addition to physical factors, mental health also plays an important role, including avoiding chronic stress, loneliness and depression."

While the results don't prove dementia is caused by these factors, they help build a more detailed picture. As always in this kind of research, knowing more about the causes can help develop better treatments and preventative measures.

Many of these factors are modifiable, which offers more hope for those working to find ways to beat dementia rather than just manage it. Ultimately, dementia may be something we can reduce the risk of by living healthier lives.

"Young-onset dementia has a very serious impact, because the people affected usually still have a job, children, and a busy life," says neuroscientist Stevie Hendriks, from Maastricht University.

"The cause is often assumed to be genetic, but for many people we don't actually know exactly what the cause is. This is why we also wanted to investigate other risk factors in this study."

The research has been published in JAMA Neurology.