A new study on the relationship between human body mass and environmental problems has got a lot of attention. But, ironically, it’s a pretty lightweight study.

The new paper about human body mass and environmental damage in BCM Public Health is certainly a novel attempt to bring together obesity and climate change in a scare-story double-whammy. But far from being a heavyweight study, it’s more of a back-of-a-burger-box calculation.

The logic of the report is this. First, fat people burn more calories than slim people. (Forget all that guff about ‘slow metabolisms’.) Second, those calories have to come from somewhere - in other words, we’ll need to grow more food. So, if everyone was as fat as Americans are, how many extra calories would we need to produce? ‘If all countries had the BMI distribution of the USA, the increase in human biomass of 58million tonnes would be equivalent in mass to an extra 935million people of average body mass and have energy requirements equivalent to that of 473million adults.’

In other words, if fat people really do use extra calories in real-world conditions as the authors assume and if everyone became as fat as Americans (which would only happen decades from now, if ever, as the by-product of a good thing - economic growth), and if we don’t become more efficient with how we use food (like reducing food waste), then the world would need to produce more food. How much? Well, in this worst-case scenario, 473million divided by seven billion people on the planet would suggest about seven per cent more food. Since many commentators believe we may need to double food output by 2050 in any event, the effect of obesity on the problem looks pretty irrelevant.