A paper in Obesity Research & Clinical Practice claims that it is now harder to keep the pounds off than it was 30 years ago, even given the same amount of exercise and food consumption.
Olga Khazan reports:
The authors examined the dietary data of 36,400 Americans between 1971 and 2008 and the physical activity data of 14,419 people between 1988 and 2006. They grouped the data sets together by the amount of food and activity, age, and BMI. …
They found a very surprising correlation: A given person, in 2006, eating the same amount of calories, taking in the same quantities of macronutrients like protein and fat, and exercising the same amount as a person of the same age did in 1988 would have a BMI that was about 2.3 points higher. In other words, people today are about 10 percent heavier than people were in the 1980s, even if they follow the exact same diet and exercise plans.
The suggestion is plainly that "there may be other specific changes contributing to the rise in obesity beyond just diet and exercise," but it's not clear what.
The three main suspects: exposure to chemicals that alter hormonal processes (think pesticides and food packaging), the dramatic increase in prescription drugs linked to weight gain (such as SSRIs), and changes in gut bacteria.