Ok, just saw the comments after my quote of Faith above. Wow. I like to watch from the sidelines with popcorn, but not to participate in threads gone sideways. Since I already wrote this, before I bow out into lurkerville for another year...I will respond to Faith...
Junk food is food with little nutritional value, imho (that means in my humble opinion, not in my opinion with which you must agree). Food that has stuff in it that is not needed for the product to be edible, but that is added to extend it's shelf life, make it sweeter, make it saltier, make it a prettier color, but that adds no nutritional value...is what I think of as junk. This includes things such as dyes, preservatives, flavor enhancers such as MSG and derivatives, dough softeners, high fructose corn syrup, coating on fruit that makes it shiny. This includes alot of old school fast food. Please don't judge me as a bad parent for trying to minimize these things and I won't judge you if you choose to maximize them.
Simplest example: I like half n half in my coffee that is made of just cream and milk. Only one of the 6 containers labeled 'half n half' at our local large grocery store chain is made of these two ingredients exclusively. So i pick that one. I don't see any 'added value' of the other stuff in there, so I don't want to consume it.
Faith, let me ask you- and I'm not try to stir the pot - do you do any endurance activity? Have you tried to eat say a typical fast food meal and then exert a boatload of energy? Do you feel optimal doing so? I am a distance runner...I'm a middle age mom, not an elite athlete by any stretch! but I've run three marathons in the last year, sorta on the fast side, to give you an idea of my activity level. I cannot eat crap food and feel optimal running afterward. The immediate issues related to the 'fuel' are many and varied but have included fluid retention - gastro issues- headaches - dry mouth - fatigue not related to the activity itself..all of which make for a less than optimal workout. Granted this does not qualify as a double blind study with empirical data
. It's what I learned for myself tweaking my pre-workout and post-work out diet over a number of years and so it turns out, I can run alot faster (and alot easier) than I could 10 - 20 years ago because I have none of these issues anymore. My husband has experienced the same. We started cutting out 'junk food' years apart, but with similar results. I have many athletic friends who have experienced the same. There's plenty of research on additives/preservatives/salt/excess sugar and the impact on ever cell and organ process you can think of but you can google just as well as I can. There's alot of stuff that is not simply carbs-fat-protein included in food, and if it makes no impact on your body...I would consider that pretty fortunate.
Perhaps my family is the exception and everyone else eating what I call 'junk food' feels great when they are doing an endurance activity. Or... maybe not every one has actually experimented to see if they feel differently eating 'non-junk' vs 'junk' food before an endurance activity?
We're a typical family. We love good food. We indulge in lots of yummy things. If we're going to a fabulous restaurant and I want double fudge cheesecake for dessert, great! I won't plan on going for a nice long run the next morning and I'll have my cheesecake. But we generally avoid food that I have described above as what we consider 'junk' as part of pre-endurance workouts and regular meals. So this is why I said above that my daughter thinks of junk food as fast food. That was her interpretation, and I'm fine her assessment. Maybe she's been influenced not only by me, but by two of her grandparents who have chronic, debilitating health conditions related to largely preventable diseases, in their cases (heart disease and diabetes). Those grandparents tell their grandchildren to be active and to protect their health. Those loving folks will tell their grandkids to eat good food mostly and not too much junk food... those grandparents don't want their grandkids to suffer as they have.
Nothing but planks of wood for mine to sleep on 
