Thanks all for the responses. I wasn't concerned so much because of myself -- I will always do what I think is right regardless -- but because they are giving out incorrect and outdated info to other parents. I did some research and dug up the study they referenced and some subsequent ones. From an email I sent to my friend who is currently doing his pediatrics residency:
The study relied upon is over 20 years old and has been contradicted by a number of more recent studies. here is the link -- you can get access to the whole study, I cannot (being a lay person, lol): http://www.jpeds.com/article/S0022-3476%2805%2982677-0/abstract. And here we are back at pediatricians are not adequately educated about and do not adequately support breastfeeding. They are using an out of date study to tell mothers to stop breastfeeding at 12 months (and the fact that I'm probably the only mom who comes to them who would actually be doing so is beside the point). Gah! Besides which, the study they are citing says that breastfeeding beyond 5 months is associated with obesity at 6 years of age, which is OBVIOUSLY the result of some confounder in their study or a too-small sample size, because the bulk of the literature shows either no effect or a small protective effect regardless of duration. See, e.g., these two links:http://www.pediatricsdigest.mobi/content/113/2/e81.full; and http://www.nature.com/ijo/journal/v28/n10/full/0802758a.html. Now, I don't know the relative prestige of the various journals where all this work has been published, but the two above cite several of the relevant studies and this Agras character is the only one who has ever found a positive correlation between extended breastfeeding and obesity. I mean, seriously, given the relative wealth, education level, and privilege of mothers who breastfeed past 6 months these days, I'd be willing to bet that the correlation runs the other way but that either way, breastfeeding is less important than early childhood nutrition and activity level in predicting obesity.
Anyway, I would like to know more about how the study was done but it's not available for free online. My guess is that, based on the date of the study, the women who were breastfeeding their children at the time the data was collected were disproportionately poor, and that is what accounts for the study results. WIC started in 1974, but was only serving 26,000 mothers with infants at its inception. The study was submitted for publication in 1989, and the children in it were followed until age 6. So the data are likely from the late 70s or very early 80s, when many women who breastfed may well have been doing it because they could not afford formula.
even looking at the abstract a second time, the other factors they found that correlated with adiposity at age 6 were adiposity at birth and parental education level (low), so it is crystal clear to me that: (1) we are talking about a correlation that was more linked to poverty than to breastfeeding; and (2) what the data on breastfeeding and childhood obesity really show is that, as I said yesterday, any potential protective effect of breastfeeding against obesity can be cancelled out by poor nutrition later in childhood (e.g., some studies have found that the protective effect only applies to white children, but not to African Americans or Latinos, which suggests that childhood obesity can in part be attributed to cultural differences in child feeding OR that the protective effect only applies to exclusively breastfed children (who tend to be white)).
The other thing I should mention is that they are not singling out breastfeeding -- what the handout said was to wean from bottle and breast at 12 months. It seems entirely possible to me that they are trying to tell bottle feeding mothers to wean their babies from the bottle at that age because some other stuff I read indicated that kids who take bottles as toddlers drink more milk than those who don't and that's an easy way to cut excess calories. And that they only included breastfeeding in their sentence to seem evenhanded. I strongly doubt that they see very many babies still BF beyond the 1 year mark anyway.
It's definitely not anything I would switch peds over -- I chose this one because he is very laid back and not heavy handed about things like some others I have been to in the past. I honestly think they are trying to help prevent childhood obesity (the ped's father works with him, running a weight management clinic for kids) and that's great, but they are just misinformed on this specific issue. I just want to present the research and have them take that sentence off their handouts, that's all. (Well, and the part about starting solids at 4 months. But that's a separate story).