It's pretty crazy that there's an argument whether the subcutaneous fat tissue or the brain ultimately regulates the metabolism. It's like asking a plant whether it would rather have sunshine or water. Without the liver the pancreas can't regulate blood glucose. And vice versa. I think Woo's best point is that insulin causing fattening is a direct observation. It might take more insulin to fatten one person up than another, but you can get there.

The question of postprandial insulin and getting fatter is another thing, the meal that raises insulin the most--is there something about it that is decreasing insulin sensitivity? If it is--where? Muscle, fat, liver? What about Dr. Fung's contention that time spent with elevated insulin matters, as opposed to just peak insulin or even total insulin under the curve? We know fructose plus fat is fattening, neither nutrient is very insulinogenic on its own.

There is also the simple observation that when things are working right, the oxidative hierarchy dictates that carbohydrate, since we can only store so much glucose as glycogen, at some point must cause a decrease in oxidation of fat, whatever the insulin levels might be that facilitate that. There are a lot of studies showing that if the respiratory quotient is higher than the food quotient, people are likely to get fatter. RQ is a measure of fat vs carb oxidation, a high quotient means more energy is produced vs. oxygen use, since carbohydrate has some anaerobic energy production, it has a higher RQ, food quotient is the RQ that would result if what you oxidized was exactly what you ate.

During the low fat heyday researchers interpreted RQ/FQ mismatches as an inability for the body to switch to burning fat in response to a fat load. Of course low carbers can and do turn this around and say, fat oxidation is pretty much the default, so take the carbs out of the picture. I think both are sort of right--once you're already overeating a carbohydrate meal/diet, adding fat past a certain point for essential fat/vitamin needs really doesn't improve things.