Lisa Amir spoke about breastfeeding and obesity at the Gold 09 conference.  I didn't take notes but I seem to remember her speaking about breastmilk of some women who had had weight loss surgery being deficient in some components, of course I can't remember which ones.  Of course, that does not answer your initial question.

Here is a small study http://pediatrics.aappublications.org/cgi/content/abstract/113/5/e465 that shows that may answer your question.  

Results. As expected, prolactin values decreased from 48 hours to 7 days postpartum. Women who were overweight or obese (using the Institute of Medicine’s cutoff for women of a BMI >26 kg/m2) before conception had a lower prolactin response to suckling than normal-weight women at 48 hours but not atday 7. In multivariate analyses, overweight/obesity, primiparity, and birth weight were negatively associated with the prolactin response to suckling at 48 hours. After adjustment for confounding by time since delivery and the duration of the nursing episode, only overweight/obesity remained a significant negative predictor of prolactin response to suckling at day 7. Concentrations of progesterone decreased dramatically from 48 hours to 7 days postpartum but did not differ between normal-weight and overweight/obese women at either time. In addition, the decreases in progesterone concentrations from 48 hours to 7 days postpartum did not differ between the prepregnant BMI groups.

Cordelia Merritt RN BSN IBCLC
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