In any diet / obesity research you have the "zero sum game" problem - if you want to see the effect of extra X then you need less Y to keep the calories the same and unless the macro composition of X and Y happen to be identical you'll change the macro ratios.

I don't have big hangups about vested interests funding studies because everyone is in my view a vested interest to some extent. At least with commercial entities this is obvious. We would lose a lot of good science if we had to rely on taxpayer funded studies and the money for those is equally open to being steered by the beliefs and biases of those on the funding panel.