Science has discovered a mystery:

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.

Well, actually, calorie intake in the UK has declined over that period. But this paper is specifically looking at the US:

Between 1971 and 2008, BMI, total caloric intake and carbohydrate intake increased 10–14%, and fat and protein intake decreased 5–9%. Between 1988 and 2006, frequency of leisure time physical activity increased 47–120%. However, for a given amount of caloric intake, macronutrient intake or leisure time physical activity, the predicted BMI was up to 2.3 kg/m2 higher in 2006 that in 1988 in the mutually adjusted model (P < 0.05).

If that were a British result we would immediately "blame" central heating. Something unusual in 1971 and near universal now. As an American result we're less certain.

Factors other than diet and physical activity may be contributing to the increase in BMI over time. Further research is necessary to identify these factors and to determine the mechanisms through which they affect body weight.

But that is the first thing we would go and look at. Given that we are, in fact, mammals. And that the major use of calories in mammals is the regulation of body temperature?

Rather than, say, blaming the food industry for advertising yummy things to us which we regard as the inevitable outcome of this current approach.