A Response to the New Breastfeeding and Obesity Study

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Wednesday, April 25, 2007

I've already got a pretty lengthy post up talking about the new study that claims breastfeeding has no impact on future obesity. In that post, I asked for some feedback from people that have a better understanding of studies.

Always one to lend a hand, Ohio Breastfeeding Coalition member (and epidemiologist) Jessica Lietz has graciously put together the following response. I thought my readers might find it interesting...

A study, "A longitudinal study of infant feeding and obesity throughout life course" by KB Michels, WC Willett, BI Graubard, RL Vaidya, MM Cantwell, LB Sansbury and MR Forman, was advance-released online by the International Journal of Obesity (International Journal of Obesity (2007) 1–8), claims that there is no association between breastfeeding and risk of obesity in adulthood.

Please hold on a minute while I put my epidemiologist hat on, and get my claws ready to tear this apart.

Okay, ready.

My preferred route of attack is with the methods section, as that is where the meat of the study is. The first concern I have is that the participants of the study are mainly Caucasian, as stated: "Participants are predominantly Caucasian white". The study provides no further background on the race or ethnicity of the participants. Many studies, such as "The Decision to Breastfeed in the United States: Does Race Matter?" (Pediatrics, Vol. 108 No. 2 August 2001, pp. 291-296) state that race is an important predictor of breastfeeding.

Second, they did not determine the SES (socio-economic status) of the study participants as infants or as adults in the way that most studies do (as percent of the Federal Poverty Level, which is based on family size and household income). A family of two earning $30,000 is much different from a family of 6 living on the same amount in the same locale. Moreover, many studies have shown a direct correlation between socio-economic status (SES) and duration of breastfeeding (examples: Effects of socioeconomic status on breastfeeding duration in mothers of preterm and term infants, from The European Journal of Public Health Advance Access published online on March 28, 2007; and Socioeconomic Status and Breastfeeding
Initiation Among California Mothers from http://www.publichealthreports.org/userfiles/121_1/121051.pdf).

In addition, in the tables of demographics, more than 28% of survey participants reported household incomes of $100,000 or more. This is not representative of the United States population.

The authors asked about the SES of the nurses' households, but not the households the nurses grew up in. I don't know about you, but most of my habits were formed well before I got married, and many habits- dietary and physical activity- are related to SES. For example, my mom had a factory laborer job and so did my dad and both have high school educations (less likely to breastfeed) and were poor enough to get WIC when I was a child (again, less likely to BF). It is basically impossible to breastfeed (or pump) when you stand on an assembly line all day.

They both had to work to pay for their rent, car, etc. It is a lot easier to breastfeed (and pump) when you have a white collar job. Here I am, 28 years old, with a Masters (husband has a BS) and we are not "poor". So asking my current SES would give you a much different picture than the SES I grew up in. I would also like to know why they recorded the nurses' husbands' education levels rather than those of the nurses (because we all know that the hubbies bring home the bacon, right?). As a side note, they did not use the same education level intervals for the nurses' parents as they did for the nurses' husbands.

Okay, so I don't like who they included in the study. So what? Let me dig in a little deeper. I also have some issues with what they did not ask the study participants. Participants were not asked to report the obesity status of immediate family members. Also, the nurses' mothers were not asked why they ceased breastfeeding. Perhaps there are differences in those who stopped after one week versus those who breastfed more than 6 months, such as what type of birth attendant the mother had (ob/gyn, family practice physician, or midwife), where the birth was at (home, hospital, or somewhere else), whether the mother had complications (c-section, mastitis, illness in baby, illness in mother), or other reasons (told to stop by doctor, didn't like it, hurt, took too much time, didn't get needed help).

Next up: how they asked what they asked. First, they had the nurses rate, based on pictures, what their body shape was. I don't know about the rest of you, but I know I have a warped image of what I look like (thanks to our American culture), and most women I know do as well. I'm sure I would choose a different image for myself than my husband or mother would choose for me.

And to ask me what shape I was at age 5 or 10?

I don't even remember anything from when I was 5 years old, and all I remember about being 10 was that my Grandpa died (or was that when I was 9? Or 11?). The authors themselves admit that the figures are "imperfect".

Moreover, "body fatness" based on appearance has NOTHING to do with BMI. Someone could be heavy in pounds due to muscles and therefore have a high BMI, but not be a "9" from the figures. And, few participants said they were a level 5 at ages 5 or 10. In other words, none of them wanted to state that they were "fat".

Second, the way they asked the duration of breastfeeding does not make any sense. They did not even use the same intervals for 'ever breastfed' and 'exclusively breastfed'. And can you tell me how a person could possibly be in the "exclusively breastfed" category when the duration of breastfeeding was <1 week?

What else did that baby eat for the rest of its infancy?

The authors state "women who were breastfed for more than 9 months had a risk of becoming overweight or obese similar to that of women who were breastfed for less than 1 week or exclusively bottle-fed"; however, these were not the exclusively breastfed babies- just the "ever" breastfed. This means that the baby could have been nursed once a day and had 11 formula feedings.

And finally, their results were "null", meaning no effect. Normally those studies are rarely even published, because who wants to read an article about a lack of causation? We all want to read a cause-and-effect article (smoking causes lung cancer, pollution causes asthma, hormones do (do not, do, do not...) cause heart attacks (breast cancer, osteoperosis...).

For future activity: someone needs to look into the funding sources for the authors and their departments. I wonder, perchance, if any of the funding comes from anyone such as my friends a few blocks from here, at the big factory that makes baby formula?

Labels: Stats and Studies

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