Right, it could be that 'high-educated' people have been raised to snack on health food, say cucumber, while low-educated people were not and snack on whatever.

Right, it could be that 'high-educated' people have been raised to snack on health food, say cucumber, while low-educated people were not and snack on whatever. Then you would find that eating health food correlates with high-education. The correlation is real: if you see someone snacking cucumber, he is likely highly educated. It doesn't mean that eating more cucumber will make you smart though.