I really have tried with my DC. I believe the best diets have everything in moderation, lots of fruit and veg, plenty of protein, fats so long as it's not so much it's you overweight, avoid processed food and artificial rubbish and include minimal sugar and refined carbs.
I'm in no way obsessive about it, but this is what we aim for.
The message from schools is all low fat and replace sugar with sweetners. At the school where I work they serve an ice lolly that is basically coloured flavoured water as dessert. It's low in fat, sugar and salt and therefore must be healthy. 
DS1 is just back from cadet camp and thrilled to tell me how unhealthy the food has been because he's had a cooked breakfast every morning before going out on the moors for a long active day. OTOH, he thinks (despite me continually telling him otherwise) that the fruit cola they sell at school is healthy because it says on the bottle it contains one of your five a day 