In these populations, the nutritional status of the infants was generally poor, and their growth was faltering, which may explain why an effect was found on weight but not on height. The mechanism behind this growth-limiting effect of iron is not yet known. It is possible that the gut microflora of iron-replete infants given iron was different than that of irondeficient infants given iron. Moreover, the gut microflora has recently been shown to affect energy utilization in the gut [10].