Why Are People Getting Fatter?

One theory is that it's due to changes in exposure to pesticides:

First, people are exposed to more chemicals that might be weight-gain inducing. Pesticides, flame retardants, and the substances in food packaging might all be altering our hormonal processes and tweaking the way our bodies put on and maintain weight.

If pesticide changes are the cause, does that mean that DDT prevents obesity? Or is it due to the cyclamate ban? There's even a possibility it's due to the absence of leaded gasoline. All of these might explain why humans are getting fatter and also why lab animals are getting fatter.

On the other hand, could it be due to the decline in red-meat consumption? On the gripping hand, that would not explain the lab-animal data.