Is our weight mostly controlled by hormones or by the brain? Is it about normalizing our fat-storing hormones (mainly insulin) or is it just about deciding not to overeat?
The second answer has been the most commonly believed one, and it's been a giant failure. We need new ideas that actually work. So we need to find the truth.
The old arguments in this interminable debate are nicely packaged by the formerly popular blogger Stephan Guyenet, PhD, at Whole Health Source: Always Hungry? It's Probably Not Your Insulin
As a reply Professor David Ludwig just published this: Ludwig Responds to Whole Health Source Article
Who wins?
So who wins? The way I see it they are both wrong, but Professor Ludwig is much less wrong.
Insulin levels and future weight gain
The argument from Guyenet that "High insulin levels do not predict future weight gain" and calling it "a basic prediction of the hypothesis" is simply based on a misunderstanding. Of course it does not. High insulin levels just predict (very accurately) already being obese.
I don't think Ludwig answers this quite right. This argument from Guyenet is silly and needs no high science to answer.
If high insulin levels predicted future weight gain then obese people (who almost always have high insulin) would blow up like balloons. They would never stop gaining. In fact they would gain weight faster and faster until they exploded like balloons.
Conversely, if low insulin levels predicted weight loss then thin people (who almost always have low insulin) would continue losing weight forever, until they disappeared.
Of course, neither of these ludicrous predictions are ever going to happen. Instead the body quickly reaches an equilibrium, where a certain average insulin level corresponds to a given fatness level. This is why obese people often maintain about the same body fatness over years or decades.
So: High insulin does not predict future weight gain, it predicts already being obese.
I discuss these misconceptions in more detail in a 2015 presentation (click image to the right).
Where Ludwig may be wrong
I think Professor Ludwig (as well as Taubes in GCBC) over-simplify the idea of "internal starvation" as a driver of weight gain. As Guyenet rightly points out, blood levels of fat and glucose on average tend to be higher in people with obesity.
Professor Ludwig does not have a fully satisfactory answer here, even if he correctly points out that episodes of low blood sugar is common some time after consuming high GI carbohydrates.
Probably a more correct way to look at this is to not only consider absolute levels of fat and glucose, but also changes in these nutrients. I.e. rapidly falling levels of nutrients in the blood may trigger hunger. Even if they are only falling lower than what is normal in the obese individual, not necessarily lower than what is normal in thin people. The body only knows itself.
Bottom Line: What works?
While it's highly exciting to watch scientists disagree and quote studies, there is something more important: What actually works? How do you eat to lose weight?
People have been losing weight for hundreds of years using low carb. Conversely people have been gaining unprecedented amounts of weight for decades, just trying to eat less.
At least 20 high-quality weight loss studies back this up: low carb simply works better. More weight loss - with no need for hunger or calorie restriction.
It just works.