This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Large scale studies have suggested that childbearing is an important contributor to the development of obesity in many women, and that breastfeeding may be protective against risk of overweight or obesity in the mom and in the infant. It is possible that ghrelin and polypeptide YY (PYY), two recently discovered gut hormones involved in appetite regulation, are altered after child birth and with breastfeeding and are associated with body weight/body fat retention (or loss) after childbirth. It is also plausible that ghrelin and PYY are present in human milk where they may regulate appetite in the breastfed infant. The specific aims of this project are to: 1) determine whether fasting or meal-induced blood levels of these appetite regulating hormones differ between breastfeeding, non-breastfeeding and non-pregnant controls;2) assess whether these hormones are involved in body weight regulation after childbirth;and 3) determine whether breast milk contains these two hormones. The study will enroll 15-20 breastfeeding women and 15-20 non-breastfeeding women four weeks after delivery;and 15-20 -never pregnant controls. Volunteers will have measurements of their body weight, body composition and resting metabolism, as well as their ghrelin and PYY responses to a standardized breakfast meal. Volunteers'body weight and body composition will be followed for 1 year after delivery. The breastfeeding women will also provide samples of breast milk for analysis.