Infants whose weight does not increase as normally expected or those who lose weight should be investigated for four different causes1: insufficient intake, inability to absorb, increased caloric need, and inability to metabolise.
History, examination, and bedside investigations can often identify which group(s) of causes should be considered
1.Krugman SD, Dubowitz H. Failure to thrive. Am Fam Physician 2003;68:879-84.