You sometimes see articles in newspapers stating that eating a handful of wallnuts per day will prevent some ailment while a bowl of porridge will prevent another.
Is it the foods that prevent the illnesses or is it that someone who is careful about their diet probably wouldn't get them anyway?
Depends which foods.....depends which ailments.
As you quite rightly inferred,a balanced healthy diet is all that is required in the majority of cases.
The walnuts and porridge are both good for lowering cholesterol. The porridge bit really works and I am now having some walnuts each day as well. I brought my cholesterol level from 9.9 down to 6.7 and eating porridge most days was the only dietary change I made to do that.