Obese and inactive children are more likely to have poorer brain function in middle age and a higher risk of dementia later in life, an Australian-led study has found.

The world-first study followed the development over 30 years of 1244 people from across Australia who were children in 1985.

It was led by Dr Jamie Tait and Associate Professor Michele Callisaya from the Melbourne-based National Centre for Healthy Ageing.

Study participants, aged from seven to 15 at the time, were assessed for fitness including cardiorespiratory, muscular power and muscular endurance as well as anthropometry through their waist-to-hip ratio.

Three decades on, the same people - now adults aged 38 to 50 - were run through a series of computerised cognitive tests between 2017 and 2019.

The children who recorded the highest levels of cardio-respiratory and muscular fitness and lower average waist-to-hip ratio had higher mid-life test results for processing speed and attention, as well as global cognitive function.

The findings, unaffected by academic ability and socio-economic status at childhood, or smoking and alcohol consumption at midlife, were published in the Journal of Science and Medicine in Sport this week.

The longitudinal study demonstrates it is key to identify factors in early life that may protect against cognitive decline later in life, including for dementia, Associate Professor Callisaya said.

"Developing strategies that improve low fitness and decrease obesity levels in childhood are important because it could contribute to improvements in cognitive performance in midlife," she said.

"Importantly the study also indicates that protective strategies against future cognitive decline may need to start as far back as early childhood, so that the brain can develop sufficient reserve against developing conditions such as dementia in older life."

It was already known children with greater muscular strength, cardiorespiratory fitness and endurance from sport and activity have better health outcomes as they age and higher adult fitness can lead to better cognition and reduced dementia risk in older age, the researchers acknowledged.

However, the National Health and Medical Research Council and Heart Foundation-funded study is the first to establish a link between objectively measured fitness and obesity in childhood with cognition in middle age.