Of course people can buy fruits and vegetables instead of junk food, but the fact that junk/processed food is more filling plays a big role too.
Here in one of the large supermarkets, for 2€ you can buy a 500-gram bag of spinach (total of 120 calories) or a 900-gram box of 3 frozen cheese pizzas (total of 1926 calories). For a shopper on a very tight budget, which choice will keep the kids from leaving the table feeling hungry?
Seriously--I am genuinely curious--are there low-cost alternatives to frozen pizza that might be no less unhealthful but equally filling?
Families in the past, without access to frozen prepared foods, might have served potatoes, gravy, butter, along with vegetables, milk, and perhaps meat. (Bear with me for speaking only of my personal experience, not all of which might be transferable. I understand that you are from Paris--lucky you! ^ ) Anyhow, families have faced the problem of hungry teenagers before. It seems that if someone is nutritionally deprived, that person is going to feel hungry and to seek out more and more food--with, unfortunately, that food being deficient in nutrition. I don't know and am not an expert. I do believe that foods need to feel "filling," and that fast foods meet that need in terms of bulk and calories, but they seem to set up a cycle if they scant on the nutritional requirement as well. I.e., that foods, or at least one of the meals a day, need to have (1) bulk, consisting of both fiber and some degree of fat, (2), calories (a given, if the meal contains any fat), and (3), nutrition. which I assume generally means a variety of vegetables and fruit as well as milk and protein.
Anyhow--Just wondering.