For the study, published in Obesity Research & Clinical Practice​, they looked at self-reported dietary data from about 36,400 American adults between 1971 and 2008. Then they compared that to physical activity data, which was available for 14,400 adults between 1988 and 2006. (This info was collected for the National Health and Nutrition Survey; everyone provided their age as well as height and weight, which was used to calculate body mass index, or BMI.) Depressingly, the authors found that the BMI of a person in 2006 was up to 2.3 points higher than someone of the same age in 1988, even if they both ate the same number of calories, consumed the same percentages of protein and fat, and exercised the same amount.
"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, one of the authors, said in a release. But the findings also underscore that weight management and weight loss aren't as simple as calories eaten versus calories burned. "That's similar to saying your investment account balance is simply your deposits subtracting your withdrawals and not accounting for all the other things that affect your balance, like stock market fluctuations, bank fees, or currency exchange rates," Kuk said.
What are those other things when it comes to weight? The authors aren't exactly sure, but they do have some theories. Kuk told The Atlantic that it could be a combination of increased exposure to hormone-altering chemicals, the rise in prescription meds like antidepressants that can lead to weight gain, and changing gut bacteria thanks to our meat- and sugar-heavy diets. Plus, stress, eating later, and exposure to light at night, which can mess with your sleep rhythms.
So who wants to time-travel back to the '80s? You'll have to leave your iPhone, but you can experience the Jane Fonda insanity firsthand.