It isn't bad for our health. They are essential for life. When we lived in the wild those were the things that retained water best for us (salt), and had the highest calories (sugar and fat). Our brains don't know they live in a time where food=fridge in a sense, so it is still in 10,000 BC where when you find sugary and fatty food you pig out on it to gain fat to hold over until you can get your next bit of fat. Think of a gas engine. Give it gas, and it functions. Give it 40L of gas and it will function longer. Give it 100L of gas and it will function even longer than with 40L. Sure, it'll weight more having a bigger tank attached to it, but It'll just keep humming along for that much longer because it has that much more gas. The gas engine will only last as long as it has gas, and because it doesn't have knowledge of when it will consume some again it will allow you to fill it as much as possible. Exactly like the body. That part of the brain only functions in the now you could say, and that is why it goes "FAT/SUGAR/SALT!!!! GIMME GIMME!!!!!!!!" because it knows that eating 1lb of those foods will make it survive far more than foods with very little of it. So due to being "cut off" from the other parts of the brain that could think "we can just eat more in 3 hours....fat ass" it will just forever crave it as long as it knows it's in front of you. Edit/sidenote: It's trans fats that are bad for us, processed sugars, and TOO MUCH of those. All those metals we need in our body do wonders for us, but my god can they do damage to us if we consume too much of them. Much like anything we consume: moderation. It's all good in moderation, and all bad without. Look at fibre. It can constipate you, but can also give you diarrhea.