Young people must eat less and exercise more than their parents did to avoid gaining weight, a new study finds.

The Canadian study found that people who are 25 years old have to eat even less and exercise more than those who are older to prevent putting on weight.

The researchers analyzed the data of nearly 36,400 American adults between 1971 and 2008 as well as the available physical activity frequency data, of 14,419 adults in the 1988 to 2006 period.

The researchers found that for the same amount of food consumed, people will be about 10 percent heavier in 2008 than in 1971, and about five percent heavier for a given amount of physical activity level in 1988 than 2006.

But the tendency to gain weight goes beyond food intake and exercise level, the researchers said, noting that changes in environmental and lifestyle factors over the years might also account for the weight gain. These include medication use, environmental pollutants, genetics, timing of food intake, stress, gut bacteria and even nighttime light exposure.

"Ultimately, maintaining a healthy body weight is now more challenging than ever,” said Jennifer Kuk, of York University, in reference to the study, which is published in Obesity Research & Clinical Practice.