Ten to 15 minutes of unprotected sun exposure during the day by older people may help them keep mental decline at bay.

Scientists have produced more evidence that vitamin D has an important role in keeping the brain in good working order in later life.

A study of over 3,000 European men aged 40-79 found those with high vitamin D levels performed better on memory and information-processing tests.

Vitamin D may protect cells or key signalling pathways in the brain, the findings in the Journal of Neurology Neurosurgery & Psychiatry suggested.

It follows research published in January which suggested that high levels of vitamin D can help stave off the mental decline that can affect people in old age.

Men with high vitamin D levels performed best, with those who had the lowest levels -- 35 nmol/litre or under -- registering poor scores.

The researchers said the reason why vitamin D -- found in fish and produced by sun exposure -- seemed to aid mental performance was unclear.

They suggested it might trigger an increase in protective hormonal activity in the brain.