In contrast, many studies suggest that diets which are rich in trans- and saturated fatty acids, such as those containing a lot of deep-fried foods and butter, have bad effects on cognition.

ECONOMIST: Eat your way to a better brain

Fast food often contains high levels of saturated- and trans-fatty acids, which are known to affect immunity, while fruit is rich in antioxidants and other beneficial compounds, say the researchers.

BBC: Fast-food 'linked to childhood asthma and eczema'

Now the company plans to cut the trans-fatty acids in its fries by 48% and saturated fat by 16%.

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