This about sums it up: "A far more parsimonious hypothesis is that the same thing that makes our fat cells fat makes us fat: ‘high blood glucose’ and concomitan

This about sums it up: "A far more parsimonious hypothesis is that the same thing that makes our fat cells fat makes us fat: ‘high blood glucose’ and concomitant elevated levels of insulin and the insulin resistance itself, both caused by the carbohydrate content of our diets. Insulin is secreted in response to rising blood sugar, and rising blood sugar is a response to a carbohydrate-rich meal. Sugar is implicated, in particular, because its chemical structure includes a large proportion of the carbohydrate fructose, and fructose is preferentially metabolised in the liver. As such, it is a prime suspect for the fat accumulation in liver cells that is hypothesised to be the trigger of insulin resistance itself."