Consequently, over the last million years or so the genetic controls over the upper threshold of body fat have become more relaxed. While some people might by chance have kept these genes, in others the gene effect has weakened and the threshold has drifted upwards. This means that some of us will keep increasing our fat levels up to this raised variable threshold, and others – around a third of the population – will stay lean even if surrounded by food.13 This also makes sense as the genes for leanness overlap with those for increased physical activity.14
