[tracker1]

About 2/3 of caloric intake is simply for energy... and excess will lead to weight gain. That said, there is a hormonal component that can dramatically effect how absorption works and lead to a higher or lower chance of metabolic syndrome. There's also a genetic factor as well in terms of what does and doesn't work well. Pretty much all foods have a portion of the population that reacts badly to them.

You're probably right that it's highly related to body fat. But I would suggest it's also highly correlated to sugar (particularly fructose) intake as well as other simple starches. There was a controlled study reported just the other day that showed that even maintaining a given weight, macro nutrient changes can dramatically effect metabolic syndrome and related issues.