Professor Davey Smith said that children of lower incomes tended to be normal weight until they reached young adulthood and that this was when the difference became more marked.

BBC: Obesity 'starts in the womb'

And sure enough, Mathur found that among the nearly 800 participants she tested, those with higher levels of methane (three or more parts per million over 90 minutes) and hydrogen gases (20 or more parts per million) in their breath also tended to be heavier, with a BMI about 2.4 points greater than those with normal levels of the gases and about 6% more body fat on average.

CNN: Can you smell obesity?