Losing weight is hard — and it’s getting harder.

That’s not an excuse, a group of researchers say, it’s science. A study from York University published recently in the journal Obesity Research & Clinical Practice looked at dietary and exercise data for tens of thousands of Americans over the past four decades and found an unsettling but perhaps not so surprising trend: Even when he had the same diet and same activity level, a given adult in 2006 had a higher BMI than a counterpart of the same age in 1988.

In other words, “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 York and co-author of the paper, said in a statement. “Ultimately, maintaining a healthy body weight is now more challenging than ever.”

Just how much more challenging? When comparing people with the same diets in 1971 and 2008, the more recent counterpart was on average 10 percent heavier. Looking at physical activity data, which was only available between 1988 and 2006, those born later were five percent heavier even if they exercised just as much people two decades earlier.

In a couple of charts, the paper complicated years of conventional wisdom on weight loss, which has stressed diet and exercise and blamed problems with both for increasing rates of obesity.

“Weight management is actually much more complex than just ‘energy in’ versus ‘energy out,’” Kuk said in the statement. “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.”

In the case of weight, the “other things” affecting our balance might have to do with our environment — both outside our bodies and within them. Discussing her theories with the Atlantic and in her statement (the explanations are still only hypotheses), Kuk said that the world we live in today makes it harder to manage our weight than it was for people a generation ago.