I usually limit my junk food consumption to holidays or special events only but I think it's ok to do it once in a while as long as your overall diet is a healthy one.
I'm interested in different diets that people use and one of the things I noticed about kosher food in particular is that it seems like if someone were to go on a kosher diet that fruits and vegetables would be one of the easiest foods to use to make kosher dinners, etc. because almost all of them are kosher. A lot of packaged foods have the OU symbol but I noticed that at least where I live you'd have to go to a specialty shop to get kosher meat. They don't sell it in the meat case at the regular grocery stores in Texas. So if someone were to go on a kosher diet here I bet that their meat consumption would go way down and their fruit and vegetable consumption would go way up. I don't know if that actually happens in practice or not (I guess they could eat a lot of canned tuna and crackers), but it seems like it would if they shop mostly at a regular grocery store.