Will Smith got it right when in 1988 he rapped, “Parents Just Don’t Understand”—and no one knows that better than adults in their early 20s to mid 30s who struggle to achieve and maintain their body weight. A study published in the journal Obesity Research & Clinical Practice found that despite having comparable eating and workout habits, millennials are 10 percent heavier than their parents’ generation, with a BMI (body mass index) around 2.3 points higher.

The authors of the study analyzed National Health and Nutrition Survey (NHANES) dietary data from between 1971 and 2008 for more than 36,000 American adults, and physical activity data for 14,419 adults between 1988 and 2006. They found that the total caloric and carbohydrate intakes increased up to 14 percent, despite a large spike (47 to 120 percent) in how often participants were exercising.

“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,” study co-author Jennifer Kuk 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.”

Kuk offered The Atlantic three theories that may explain the findings of the study: an increase in exposure to environmental chemicals like pesticides that could result in weight gain; an increase in the use of prescription drugs with weight gain as a possible side effect; and lastly, bacteria in American stomachs that may have changed over the years as people began to eat more hormone-treated meat and artificial sweeteners.

More research will need to be done before researchers can better explain the changes. In the meantime, don’t cancel that gym membership.