It may be the final straw that kicks off intergenerational war. Hard-pressed millennials already resent their parents’ generation for their free university education, generous pensions, higher employment rates and ownership of mansions they bought for £18.50.

Now it turns out baby boomers even had it easier when it came to dieting. A new study has found those consuming a given number of calories were 10% heavier in 2008 than 1971.

The difference, it turns out, is not down to Generation Y spending all its time sat on their well-padded nether regions playing computer games and sexting. Those with the same calorie intake and physical activity levels had an average body mass index 2.3kg/m⁲ higher in 2006 than in 1988. While average food and energy intake around the world has risen in recent decades, research has undermined the notion that weight gain is simply the result of people consuming more calories than they expend.

“Weight management is actually much more complex than just ‘energy in’ versus ‘energy out’,” says Professor Jennifer Kuk, of York University, Toronto, one of the new study’s authors.

“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.”

The diet and physical activity data analysed by the researchers was based on questionnaire answers, so perhaps people have become more or less honest when being asked what they’ve eaten. But Kuk and her colleagues, whose study was published in the journal Obesity Research and Clinical Practice, say self-report bias has remained pretty constant over time.

So, if it’s not down to more eating, slobbing around and lying, just why is slimming getting harder?

The Canadian research can’t tell us, but its authors have plenty of suggestions. These include changes in sleep patterns, stress levels, night-time light exposure, more pollutants in our food, higher maternal ages, reduced variability of ambient temperatures or changes to our gut microflora.

The complexity of disentangling these potential causes probably means millennials will have become the object of resentment from their own offspring’s generation for their good fortune before scientists figure it out.