The best explanation so far that I've heard as to why people have gotten fatter since the 1970s goes like this:

1. In the early 1970s and prior, people ate three meals a day and didn't snack - it would ruin your next meal, we were told.

2. In the 1970s, the medical establishment told people to eat a low-fat diet, to eat more carbohydrates.

3. Carbohydrates, epsecially refined ones, do not trigger the same satiety signal that protein and fat do, and they spike insulin, and then you have a crash.

4. People couldn't make it 5 hours to the next meal because of the blood sugar crash, so people started snacking.

5. Became everybody felt they had to snack given the blood sugar crash, it became culturally acceptable to eat outside of meals, and eventually all day long.

6. It turns out that eating more often screws up your hormonal hunger signals and causes you to gain weight over time.

7. Eventually we learned protein and fat were important and carbs were causing a problem, and people started eating fat and protein again, however,

8. Even though we fixed that problem, people are still eating all day long, and our hormonal hunger signals are still screwed up.