Okay, I'll admit I was too vague when I said it's "needed for energy". I should have said that it is optimal for bursts of energy. That's why athletes use sugar as part of their strategy in competition. That's why Gatorade has simple sugars in it. That was the point I was attempting to make.
Energy sources in the body are not equal. They come with their drawbacks - yes, even sugar.
Carbs, proteins and fats form the macronutrient category. They're in separate categories because they provide functions for different mechanisms in the body. At a very, very high level - proteins help in building muscle tissue, fats help with hormone production, and carbs provide energy.
Ketogenesis can only happen when you take in a very low level of carbs consistently over a long period of time, and it's sure as
hell not ideal for everyone. Not everybody responds to keto the same way, either. There's a lot of research on this topic that I just don't have the room to put here, and it'd derail this point anyway, so I'll leave that reading up to you.
Amino acids will only be used to aid in muscle recovery - it's absolutely nothing like the energy you get from carbs, and this is extremely well studied at this point. It has no effects on performance directly, like say, creatine monohydrate. Better recovery can lead to better performance, but it's not from it being used as an energy source. Proteins will only be used as a primary energy source in extreme situations where you're basically starving and not training.
Also, where the hell did you "learn" that sugar is only used by the brain? Lol! Every cell in your body utilizes sugar. You completely goofed on that point.
Point of all this being - sugar has a place in a well rounded diet, and is used by people who take nutrition to a competitive level. It's not "bad" unless you
overeat sugar. Fast food is perfectly fine if you know what you're doing. Moderation, and all that.
Did you ever read about the nutrition professor who ate only McDonalds for 6 months and lost weight? It was all to prove this very point:
https://www.businessinsider.com/how-to-lose-weight-eating-only-mcdonalds-2015-10#:~:text=Science teacher John Cisna after,on the McDonald's-only diet.
Also maybe check out some bodybuilders' diets. You'll notice a lot of them have ice cream, milkshakes, etc. as part of their rotation. Of course, they don't overdo it, because they know what they're doing, so it helps them more than it hinders them.
This isn't even up for debate, dude. Nutrition has never been about "cut out this food because it's bad". That's not how developing a healthy diet works.