Just to clarify ... what do you mean by "catch-up growth"? I have data on response to treatment for weight and MUAC. Response (in responders) tends to follows a typical growth curve with a rapid response followed by slower consolidation. This is the sort of curve we see in other fields (e.g. return of foveal sensitivity after retinal re-attachment surgery). I see this in both weight and MUAC. Is this what you mean?

Yes, this is correct. It is due to the different composition of the tissue that is laid down during catch-up (measured by impedence) - at first there is mainly lean tissue, which contains a higher water content during active protein synthesis - as the weight approaches normality the lean tissue is less hydrated and more fat is laid down. This process continues right up to around median weght-for-height where the exponential fall in rate of weight gain reaches an asymptote. You will not see this in many children as they are usually discharged at -1.5Z and so the final portion of the curve will not be observed. There is no change in the ratio of essential nutrients required during catch up - but there is a progressive reduction in the essential-nutrient to energy (nutrient density) needed. F100/RUTF is formulated to provide the required nutrients for rapid catch up with appropriate lean/fat tissue synthesis throughout recovery If CSB/family plate etc is given along with F100.RITF it slows the rate of catch up and there is a change in the ratio of lean/fat laid down so that insufficient functional tissue is made and although the children gain weight (less efficiently) they do not return physiologically or immunologically to normal.