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Having a big head may help protect against the worst ravages of dementia, say researchers.

They found that people with Alzheimer's with the largest craniums had better memory and thinking skills than patients with smaller skulls.

The Munich University team believe a larger head means there are greater brain reserves to buffer against dementia-related brain cell death.

Their findings, based on 270 patients, are published in the journal Neurology.

The patients were recruited through research registries or specialist memory clinics in the US, Canada, Germany and Greece.

They were given memory and cognitive skill tests and a brain scan to gauge the extent of their disease. They also had their head size measured.

A larger head was linked to better performance in the tests, even when patients had the same amount of Alzheimer's-related brain cell loss.

Specifically, for every 1% of brain cell death, an additional centimetre of head size was associated with a 6% greater score on the memory tests.

Although brain size is largely determined by genetics, the researchers say lifestyle can have an impact.

Poor nutrition or disease in early life can compromise growth, for example.

'Complex disease'

The researchers say the first few years of development are critical. By the age of six, the brain will have reached 93% of its final size.

"Improving prenatal and early life conditions could significantly increase brain reserve, which could have an impact on the risk of developing Alzheimer's disease or the severity of symptoms of the disease," lead researcher Dr Robert Perneczky said.

Dr Simon Ridley, head of research for the Alzheimer's Research Trust, said: "Alzheimer's is a very complex disease, so we should be careful not to focus too much on a single risk factor, particularly as there is little we can do about the size of our heads.

"The researchers have also posed the idea that nutrition, injury or infection in early life can have an impact on brain reserve, suggesting that we should look after our brain from day one.

"Research is the only answer to dementia. We must invest in research now to deliver the treatments needed to avert the coming dementia crisis."