Eating plenty of folic acid - found in oranges, lemons and green vegetables - can halve the risk of Alzheimer's disease, a study has suggested.
Broccoli is one of the foods which contains folic acid
US National Institute on Aging experts monitored diets over seven years.
They found adults who ate the daily recommended allowance of folates (B vitamin nutrients) had a reduced risk of the disease.
UK researchers said the study added weight to previous suggestions folates could reduce Alzheimer's risk.
The study is published in Alzheimer's and Dementia: The Journal of the Alzheimer's Association.
Folates have already been proven to reduce birth defects, and research suggests that they are beneficial to warding off heart disease and strokes.
They have also been shown to help modify levels of homocysteine - an amino acid found in the blood.
Previous research has linked high levels of homocysteine to an increased risk of Alzheimer's disease.
In this latest US study, doctors analysed data on the diets of 579 people aged 60 or over from the Baltimore Longitudinal Study of Aging to identify the relationship between dietary factors and Alzheimer's disease risk.
None of the participants were showing signs of dementia when the study began.
Over the course of the study, participants provided detailed diaries documenting their eating habits, including supplement intakes and calorie amounts for typical seven-day periods.
Researchers examined the amounts of nutrients including vitamins E, C, B6, B12, carotenoids and folic acid in people's diets.
Fifty-seven of the original participants went on to develop Alzheimer's.
The researchers found those who consumed at least the recommended daily amount of 400 micrograms of folic acid had a 55% reduced risk of going on to develop Alzheimer's compared to those consuming under that amount.
However, most of those were taking folic acid supplements, suggesting they did not consume sufficient quantities of the nutrient in their diet.
It is estimated that the average person in Britain consumes around 200mcg per day.
The US study found no link between taking vitamin C, carotenoids (such as beta-carotene) or vitamin B-12 and decreased Alzheimer's risk.
Dr Maria Corrada, who led the research, said: "Although folates appear to be more beneficial than other nutrients, the primary message should be that overall healthy diets seem to have an impact on limiting Alzheimer's disease risk."
Dr Claudia Kawas, who also worked on the research, said: "It is still possible that other unmeasured factors also may be responsible for this reduction in risk.
"People with a high intake of one nutrient are likely to have a high intake of several other nutrients and may generally have a healthy lifestyle."
Dr Susanne Sorensen, head of research at the UK's Alzheimer's Society, said: "This study adds further weight to evidence that folates reduce the risk of people developing Alzheimer's disease.
She added: "Whereas the evidence for the benefit of other vitamins in changing the prospects for somebody at risk of developing Alzheimer's disease is not consistent; the evidence supporting folate intake is very convincing."