A study finds that people today who eat and exercise the same amount as people 20 years ago are still fatter.

The researchers sought to determine whether the relationship between obesity and caloric intake, macronutrient intake, and physical activity has changed over time.

Amazingly, the researchers found that it's harder for adults today to maintain the same weight as those 20 to 30 years ago did, even at the same levels of food intake and exercise.

They used dietary data from 36,377 U.S. adults from the National Health and Nutrition Survey between 1971 and 2008, as well as physical activity frequency data from 14,419 adults between 1988 and 2006. They grouped the data sets together by the amount of food and activity, age, and BMI.

Between 1971 and 2008, BMI, total caloric intake and carbohydrate intake increased 10–14%, and fat and protein intake decreased 5–9%. Between 1988 and 2006, frequency of leisure time physical activity increased 47–120%. However, for a given amount of caloric intake, macronutrient intake or leisure time physical activity, the predicted BMI was up to 2.3 points higher in 2006 than in 1988. 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.

“Our study results suggest that if you are 25, you’d have to eat even less and exercise more than those older, to prevent gaining weight,” Jennifer Kuk, a professor of kinesiology and health science at Toronto's York University, said in a statement. “However, it also indicates there may be other specific changes contributing to the rise in obesity beyond just diet and exercise.”

What could those changes be, you might ask. That is still a matter of speculation. In an interview, Kluk suggested three possibilities:

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.
 
Second, the use of prescription drugs has risen dramatically since the 1970s and 1980s. Prozac, the first blockbuster SSRI, came out in 1988. Antidepressants are now one of the most commonly prescribed drugs in the U.S., and many of them have been linked to weight gain.
 
Finally, Kuk and the other study authors think that the microbiomes of Americans might have somehow changed between the 1980s and now. It's well known that some types of gut bacteria make a person more prone to weight gain and obesity. Americans are eating more meat than they were a few decades ago, and many animal products are treated with hormones and antibiotics in order to promote growth. All that meat might be changing gut bacteria in ways that are subtle, at first, but add up over time. Kuk believes the proliferation of artificial sweeteners could also be playing a role.

The researchers concluded that factors other than diet and physical activity may be contributing to the increase in BMI over time. They said that further research is necessary to identify these factors and to determine the mechanisms through which they affect body weight.

Sources:

Olga Khazan, The Atlantic Oct 3, 2015

Secular differences in the association between caloric intake, macronutrient intake, and physical activity with obesity, Obesity Research & Clinical Practice, volume 10, issue 3 pages 243 – 255 (May-June 2016).