The vegetables' key nutrient is vitamin K, which not only protects against dementia and Alzheimer's, it also keeps us mentally sharp, says Martha Clare Morris from Rush University.
She tracked the diets and mental health of more than 950 older adults, with an average age of 81, for around five years. She saw a "significant decrease" in the rate of cognitive decline for those who ate more leafy vegetables. Those who ate one to two servings a day had the cognitive ability of a person 11 years younger who didn't eat any of the vegetables.
Other sources of vitamin K include coloured fruits and vegetables.
(Source: proceedings of the Federation of American Societies for Experimental Biology (FASEB), March 30, 2015)