Science You Can Use: New study says breastfeeding is not associated with obesity risk. Let’s take a closer look.

Baby breastfeedingHave you heard the news that a recent study found that breastfeeding is not associated with lowered risk of childhood obesity?

But you may also have heard that breastfeeding protects against obesity.  In fact, the Surgeon General writes that not breastfeeding is associated with a 32% increased risk of obesity. And a World Health Organization analysis also found evidence that breastfeeding lowers the risk of childhood obesity.

So what’s the truth?  We thought it was worth taking a closer look at this most recent study.

This finding is one in a series of research questions tackled by the PROBIT studies, conducted in Belarus.  In this large and long term project, 17,000 participants were enrolled in 1996 and 1997, and have been tracked in the years since.  The lead author of these studies is Dr. Michael Kramer of McGill University.

This series of studies is notable because of its size and also because it achieves something very difficult in breastfeeding research – a randomized controlled design.  This is challenging because, given what we know about the health and other effects of breastfeeding, it just wouldn’t be ethical to tell women not to breastfeed their children in order to compare them to breastfed children.

So this research does what some consider to be the next best thing (we’ll get to how it falls short in a moment).  It created comparison groups by controlling the kind of care they got in the hospital.  They randomized mothers to either the standard hospital care or Baby Friendly Hospital care, and as expected the exclusive breastfeeding rates in the Baby Friendly group were much higher than in the standard care group.  This allowed for a comparison of health outcomes over time, while excluding other factors.

So these 17,000 mother/child pairs have been assessed for a variety of health outcomes over time.  The PROBIT project has previously found:

Reductions in gastrointestinal episodes and rashes, higher verbal IQ scores, and longer exclusive breastfeeding rates for subsequent children but no statistically significant differences in the child’s body mass index, blood pressure, or dental health.

So now on to the most recent publication, which focused on the adiposity (fatness) and insulin-like growth factor of the PROBIT subjects, measured when the children were 11.5 years old.

The authors’ conclusions:

Among healthy term infants in Belarus, an intervention that succeeded in improving the duration and exclusivity of breastfeeding did not prevent overweight or obesity, nor did it affect IGF-I levels at age 11.5 years.

And an editorial note which follows:

Breastfeeding has many advantages but population strategies to increase the duration and exclusivity of breastfeeding are unlikely to curb the obesity epidemic.

Now let’s look at some of the issues raised about the study design.  For this I owe a debt to Drs. Melissa Bartick and Nathan Nickel, who wrote an excellent analysis for the Academy of Breastfeeding Medicine blog.  Their critique boils down to these points:

1) Exclusively formula fed infants were excluded from this study, eliminating a key comparison with breastfed and exclusively breastfed children.  Remember the statement from the Surgeon General above that not breastfeeding is associated with a 32% increased risk of childhood obesity?  There were no “not breastfeeding” kids in this study, so it’s not possible to compare exclusively breastfed children to exclusively formula fed children, which may have shown the most significant difference.  In other words, this study only compared babies who received ALL breastmilk to those that received SOME (mix of formula/breastfeeding) which is very different than comparing NO breastfeeding to ALL breastfeeding.  The difference in outcome was most likely to be seen if exclusive breastfeeding were compared to exclusive formula feeding.

2) These studies were not set up to be able to pick up small to moderate differences in obesity rates, only “extremely large ones,” because the rates of breastfeeding were quite low, even in the intervention group. Prior research has established a dose dependent relationship between breastfeeding and obesity, with effects generally seen with exclusive breastfeeding for six months.  So the fact that the percentage of exclusive breastfeeding in both groups at that point was very small is a problem.

Then there is the question of industry ties, which I won’t get into but will send you to this older blog post and you can make up your own minds.

Now let’s get into the politics of how this research has been used.  Back in 2009, The Atlantic published a critique of breastfeeding called “The Case Against Breastfeeding.”  It cited the PROBIT studies as evidence that the connections between breastfeeding and health outcomes is “thin.” Then a few months later an article in the The Times (U.K.) made a similar claim, reporting that Kramer’s view was that “much of the evidence used to persuade mothers to breastfeed was either wrong or out of date.”

To the author’s credit, he objected vociferously in the press about how his research was being “grossly misrepresented.”  Said to be “spitting tacks” about it, he commented:

There really isn’t any controversy about which mode of feeding is more beneficial for the baby and the mother, but when you read the article in The Times it sounds like there is.” Furthermore, he points out: “I’m not aware of any studies that have observed any health benefits of formula feeding. That’s important, and any mother weighing the benefits of breastfeeding vs formula feeding needs to know that.

What to make of all this?  At a minimum it’s a good reminder that it’s important to look beyond – sometimes far beyond – the headlines when you hear about breastfeeding research in the news.

3 thoughts on “Science You Can Use: New study says breastfeeding is not associated with obesity risk. Let’s take a closer look.

  1. The reason PROBIT found better outcomes with IQ and less diarrhea is presumably because these outcomes were so large as to be picked up by a study such as PROBIT. The design of PROBIT is very robust and is not flawed. However, it is only sensitive enough to pick up very large health outcomes. If PROBIT finds a difference in an outcome, we can be pretty sure that that difference is real. But if it doesn’t find a difference, that does not mean there isn’t one. It only means that any actual difference in health outcomes is is moderate to small at best.


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