Losing weight is harder than it used to be even if people eat the same diet and exercise the same amount as someone 25 years ago, researchers say.

Even with that identical diet and identical exercise levels, American adults are heavier than they used to be, researchers at Canada's York University say.

Children born in the 1990s have higher body mass indexes (BMI) than those born in the 1970s and 1980s, the study published in the journal Obesity Research and Clinical Practice reports.

"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," says study leader Jennifer Kuk, a York professor of kinesiology and health science.

For the study, the researchers analyzed dietary data gathered between 1971 and 2008 on U.S. adults for the National Health and Nutrition Survey, and data on the physical activity frequency of adults between 1988 and 2006.

"We observe that for a given amount of self-reported food intake, people will be about 10 percent heavier in 2008 than in 1971, and about 5 percent heavier for a given amount of physical activity level in 1988 than 2006," explains researcher Ruth Brown, a York graduate student.

While conventional wisdom blames problems with diet and exercise for increasing rates of obesity, the new study suggests there's more at play, the researchers say.

Kuk explains that there's more to managing weight than just considering energy in versus energy out. She compares it to a investment account, where assuming a balance based on deposits in and withdrawals out doesn't take into account the effect on the balance of factors such as stock market fluctuations, currency exchange rates or bank fees.

While the study could not positively identify the reasons why weight loss has become more difficult, Kuk suggested several factors might be involved.

People are exposed to more environmental chemicals that could be causing weight gain; people are increasingly taking prescription drugs, many of which have been linked to weight gain; and finally, the gut bacteria in the average American may have been altered over time, making them more prone to putting on weight even with a diet identical to that of decades ago.

However, she acknowledges, these must at the present be considered only hypotheses.

Still, she says, there is no escaping the evidence that losing weight – something that has never been easy – is becoming harder, an that maintaining body weight at a healthy level a a greater challenge than it has ever been.

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