Human beings are adapted to an uncertain diet. We can live for a short time on a great variety of different foods. In pre-history our ancestors were opportunistic. They feasted when a feast was available, and if they had to struggle to find food for a few days, weeks or even months, they were able to do that. Being a bit fat, was an advantage, if famine came most years.
With supermarkets, those famines never happen. We buy the cheapest food we can. That means that our diet has become overloaded with carbohydrates. The lack of food variety, (Most of us only eat 12 different things a day, and 20 different things in a week.) and the constant supply of food making us sick.