The study of obesity is the study of mysteries.
This is a nearly book-length multi-part blog series but every part of it is fascinating to read. I recommend you give it a chance even if some of it might contradict what you previously believed.
If you can't be bothered, here's a very simplified summary:
Type of food intake does not cause obesity. The author points out that tribal societies vary massive in diet. There are some that eat 70% carbs, some that eat Western levels of sugar because of fruit and honey diets, many who eat almost nothing but fat, and so on. The obesity rate is basically 1% across all of them.
Carb and sugar consumption tracked with obesity until around 2000, when both began falling even as obesity continued to skyrocket.
Amount of food intake is only weakly associated with obesity. The author cites a study that tried to force feed normal weight people with substantially more food than they normally ate. It ended up requiring hugely more calories than expected to induce even minor weight gain, and after the study ended, almost every participant immediately returned to their previous weight. The author also points out that historically the obesity rate has been 1% across social classes, including classes that should have had more access to food.
Obesity is an entirely contemporary phenomenon. We are used to obesity increasing every year, but it's only really happened since around 1980. The obesity rate slowly increased from 1900 to 1980 and then absolutely exploded beginning in 1980.
A lot of people blame high fructose corn syrup consumption, which also exploded beginning in 1980. However, HFCS consumption also stalled and fell slightly beginning in 2000. Just like sugar and carbs, it's diverged from the obesity trend.
Pets are also becoming more obese. When scientists switch test animals from animal chow to human food with the exact same macronutrient profile, the animals put on a large amount of weight.
Anorexia has also exploded since 1980. The author identifies this as a paradoxical effect -- taking too much of a drug can occasionally cause completely inverted effects to occur.
All of this leads the author to conclude that obesity is primarily caused by chemical contamination. He uses a broken thermostat as an analogy. If the body's fat thermostat is disrupted by chemicals (the author focuses heavily on lithium), it will store more (or less, in the case of anorexia) fat for the same amount of calories.
He brings further evidence to bear for this hypothesis:
Firstly, increase in contaminants tracks obesity better than macronutrient trends.
Secondly, according to historical data, people tended to get less obese as they got older. The inverse is now true. This would be congruous with older people having accumulated more contaminants in their system.
Thirdly, obesity rates on a map track shockingly well with water flows. High-altitude states such as Colorado have lower obesity rates. Low-altitude states such as most of the South have the highest obesity rates. This would be because water is least contaminated at its source and becomes increasingly contaminated as it flows down to its terminus. This trend also holds in China, where water originates in the Tibetan plateau; the western provinces have almost no obesity, while the coasts have rates approaching 20%.
Of course, I already knew that chemical contaminants are responsible for half of modern society's ills (the other half being caused by Jews or both, of course), but this post has convinced me that it's much more important than even diet and nutrition.