Underweight middle-aged people are a third more likely to develop dementia than those of the same age with a healthy weight.

A study's also found nearly 30 per cent of obese people in middle-age are less likely to develop the condition.

This contradicts previous findings which suggest obesity leads to an increased risk.

The study also warned people against smoking and to partake in exercise and a healthy diet.

"Yes, it is a surprise," lead researcher Dr Nawab Qizilbash told the BBC.

"The controversial side is the observation that overweight and obese people have a lower risk of dementia than people with a normal, healthy body mass index.

"That's contrary to most if not all studies that have been done, but if you collect them all together our study overwhelms them in terms of size and precision."

Dr Clare Walton from the UK Alzheimer's Society says more research is needed: