The first mystery is the epidemic itself. For most of history, the obesity rate was about 1%, even for people who had all the food they wanted. Today, many countries are 40% obese or more. Even in lean countries like Italy, France, and Sweden, the obesity rate is around 20%.
The second mystery is how quickly the shift occurred. In 1975, there wasn’t a single country in the world with an obesity rate higher than 15%. In most countries, obesity was steady at about 10% until around 1980, when it suddenly began rising.