One of my clients clued me into this odd story.
Junk Food Diet
Short version: a college professor goes on a junk-food-only (plus 1 serving of vegetables a day) 1800 kcal diet for a month. The next month he'll do 1800 kcals of health foods only.
From the latest news stories I've been able to find, he lost some weight.
I'm not surprised - food quantity and caloric totals matter - otherwise you'd lose weight or gain weight solely based on what you eat, not how much. You go further on a full tank of gas than a half-tank, no matter if it's 76-octane sludge or 93-octane super.
I'd not be surprised if he felt awful and generally run down on the junk food, either - there isn't a lot of nutrition in those kcals, even if there are a lot in a little amount of food.
I can't recommend this diet to anyone, but it's worth looking at. It can help demonstrate that total food intake does, in fact, matter. But with both halves done I'd expect it would show that what those calories consist of affects your health and your overall well-being.
I'm on the lookout for a complete summary of his results - the stories I found are old, and he should be done with this by now . . .