In terms of the possible mechanism …
The speculations vary but one popular one is Lustig’s blaming the fructose.
Fructose is present pretty much equally in both table sugar and high-fructose corn syrup (HFCS), so in this regard HFCS is only a specific villian because it has been cheap and thus added more.
The idea is that fructose is unlike glucose:
How is the fructose in real foods, like fruit, different (in this way of thinking)? First because usually there is less of it there. But per Lustig mainly because in fruit and real foods fructose comes packaged with what he thinks of as “the antidote,” fiber.
That said one does have to remember that one of two things are happening as added sugar intake goes up (either alone or in some mixed fashion):
-
Other foods are replaced and intake otherwise decreases enough that there is no fat gain. In this case (and to the degree that other foods are replaced) the impact might not be by what is eaten but by what is not eaten: less vegetables, fruits, beans, whole grain, fish, less real food in general.
-
Added sugar may represent additional calories. In this case the impact might be the impact of extra fat mass.
Of course all of the above - a direct negative impact of large loads of fructose suddenly delivered to the liver, with its limited ability to maetabilize fructose both quickly and safely, after a single Big Gulp; the impact of of what it replaces in the diet; and the impact of the additional calories consumed - may all contribute. And the added sugar usually comes in foods that have a whole host of other crap in it - maybe the mechanism is the other crap too.
Teasing out which plays what role may be difficult but easy to say that eating a diet high in the foods that are high in added sugar is very very likely very very bad.