The main dangers with sugar (table sugar, high fructose corn syrup) is how your body absorbs it, and the amount of fructose in it. Fats and proteins are broken down to glucose, yes, however this process is done over time, and it is almost ALL glucose, which is what your body uses as fuel. However when you drink sugary beverages like soda or orange juice (yes even orange juice is bad for you), you get an extreamely high dose of fruit sugar, very quickly (fruit sugar is different than sugar because it has a higher concentration of fructose in it which is bad).

Also, the reason fruits are healthy while fruit juice is not is that fruits have sugar in them, but between all the sugars, there is fiber, cell walls, and other parts of the fruit that slow down the absorption of the sugar. Another reason is that there is simply less sugar in 1 apple than there is in 1 glass of apple juice (there is around 8 apple squeezed into 8 ounces of juice).

Here is an amazing lecture which goes into amazing detail of the chemistry of sugars, and how they affect your body (also this is where I got most of my information): "Sugar the Bitter Truth" https://www.youtube.com/watch?v=dBnniua6-oM

Here is a short clip from a show called "Adam Ruins Everything" where he talks about how unnatural "natural orange juice" is: https://www.youtube.com/watch?v=ZuYPdTvqitg