Maintaining a certain amount of body fat is crucial for survival, as it provides energy reserves to defend against infections and disease.

“if we look at the primary causes of mortality in children under five in low income settings what we see is that it's strongly related to their weight for height which is kind of a different way of measuring bmi and and it's also strongly related to disease pressure especially diseases like diarrhea that interfere with nutrition and so if you look at the correlation between weight for height and mortality there's a massive correlation so kids who have malnutrition moderate or severe malnutrition that's what we call being underweight to a certain degree they have massively increased mortality because basically if you don't have those energy stores you can't defend yourself against infections”