Us twenty-somethings can find it hard to shift those extra stubborn pounds, but it turns out it's not all down to diet and exercise.

If you've ever looked back at old photos of your mum/aunt/granny at your age and marvelled at how slim they were, then the following will make some sense to you.

Scientists in Canada claim that people today simply weight more, even those taking in the same amount of calories as people would have 35 years ago.

Researchers at Toronto's York University studied the dietary and physical activity of American adults, and found that they were 10% heavier in 2008 than in 1971, and 5% heavier than in 1988… despite having the same food intake and level of physical activity.

But if our heavier frames aren't down to our lapsed gym membership or sneaky takeaway, what's the cause?

According to researcher Jennifer Kruk, factors like environment, lifestyle, pollutants, stress, genetics, product ingredients and even the time of day we eat all probably play a role.

"If you think about it from an evolutionary standpoint, you wouldn’t eat when it was dark out," she explained.

"So maybe we are genetically programmed to eat during the day and not during the night.

"The reasons why we are seeing this increase in body weight might be because we can now do things that we are not programmed to do."

Okay, so that Friday night pizza might not be the sole cause of your weight gain, but the fact that you ordered it online at 1am certainly didn't help.

Jennifer adds that there's also "strong research" to indicate that pesticides, fossil fuel burning and cigarette smoke could be linked to adult obesity today.

Our mums had it so easy…