"Food groups" refer to the food pyramid which the USDA stopped using in 2011 because it gave people way wrong ideas about how to eat healthily.
Foods are not inherently bad or good, it's all dependent on lifestyle and diet. For most people who eat pizza, it is very easy to overeat and consume way more calories than appropriate for a single meal. 2 slices would be a normal size meal. How many people do you think eat *only 2 slices* when eating pizza? That's the problem.
In a small quantity pizza is an excellent food. It's low in fiber and some vitamins, but high in lots of other things. If you eat a modest portion of pizza together with something that has fiber and vitamins -- say, a big salad -- that's a healthy meal.
The main problem is that people eat a shitload of pizza and nothing else. That's too high in calories, carbs, fats, and salt, and too low in fiber and vitamins.
4 slices of Pizza Hut medium (12") hand tossed pizza have 880 calories, mostly from simple carbs (100 cal vs 90 from fat and 30 from protein), which is almost half of a 2000 calorie diet's daily caloric intake, and very few vitamins.
What does having some (potentially small) part of each food group have to do with being healthy?
Most pizza is greasy and full of carbs, and even pizza with vegetables on it typically doesn't have a lot of vitamin or other nutrient content relative to the fats and carbs.
As a type 1 diabetic, I love but hate pizza. It's great, it's easy to figure out how many carbs in a slice, and you can get it in so many forms. The problem is, there's a delayed hit on the carbs due to the cheese and oils, so a few hours later I get a sudden spike in my bloodsugar.
I remember when I was a kid watching a Barney episode on the food groups, and at the end they made pizza because it had an item from all the food groups. My mom was not amused.
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This question seems disingenuous. It is like asking why chocolate cake isn't healthy since it contains eggs and wheat.
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Look up nutritional guides for chain pizza places. Little caesars, for example, lists their supreme as having 320 calories per slice. 120 of those calories are from fat. There is a good amount of protein, vitamin c, and calcium, but the sodium and fats are whats bad. We arent even getting to the flavor of that shit. Their large is like a medium at a real pizza place so i can eat the whole damn thing without any trouble. Their latest extra pepperoni and cheese tastes decent but is over 3000 calories for the whole thing. I had to stop eating it even though it was only once every couple months or so. Like anything else, its about portion control and making it part of the meal instead of the entire meal. Who does that shit though? Pizza night is about pizza.