I've been reading about Vitamin D at the Life Extension Foundation lately, and this source looks like it may help confirm their research as well.
Vitamin D 'is mental health aid'
Vitamin D, found in fish and produced by sun exposure, can help stave off the mental decline that can affect people in old age, a study has suggested.
UK and US researchers looked at 2,000 people aged 65 and over.
They found that compared to those with the highest vitamin D levels, those with the lowest were more than twice as likely to have impaired understanding.