Several news stories appeared yesterday, based on new research that suggests that almost one third of 9 month old babies are obese (along with 34% of 2 year olds). I haven’t been able to access the original study yet, but according to this news story: “The researchers classified the babies’ weights based on CDC growth charts, which compare a baby’s growth to a standardized growth curve. Kids in the 95th percentile of weight were categorized as “obese,” while kids in the 85th to 95th percentile were counted as “at-risk,” similar to the adult category of “overweight,”.
I see a problem with this. The researchers were using the CDC growth charts when even the CDC itself recommends that the World Health Organization (WHO) growth charts should be used for children ages 0-2. The WHO charts, unlike the CDC charts, are based on the growth of babies under biologically normal conditions (breastfeeding, mother’s who don’t smoke etc). When looking at a child’s growth, it is important to be comparing to the biological norm, not a cultural one (if they had used the WHO charts they likely would have found even higher numbers of “obese” babies!).
The report goes on to say: “The find hints at an unfortunate pattern: Kids who start out heavier end up heavier. Of kids who were normal weight at 9 months old, 75 percent were still normal weight at 2 years. But kids who were at-risk at nine months had only a 50 percent chance of being normal weight at age 2. More than 28 percent of at-risk kids ended up obese by their second birthday.”
Another problem: what is a “normal weight” for a baby? Weight for age growth charts compare a child’s weight to other children of the same age, which really is not very useful. Is your baby healthier than mine because he weighs more or less than mine does? Of course not. All babies are different and someone has to be in the lowest percentile, just like someone has to be in the highest.
The 50th percentile on a growth chart does not equal “normal” weight. It simply means that statistically speaking, 1/2 of all babies of the same age will weigh more, and 1/2 will weigh less. Isolated points on a growth chart have little meaning. What is important is whether or not your not your child is following his/her own curve. If your child falls on the 75th percentile, then he should continue to follow that line on the chart. If your baby falls on the 5th percentile, and is following that curve, then your baby is growing appropriately. A baby on a lower percentile isn’t any more or less healthy than a baby on a higher percentile.
I am curious how the researchers decided that babies who fell in the 95th percentile would be classified as “obese”, and those falling in the 85th-95th would be classified as “at risk”. Obesity in adults is classified according to Body Mass Index (BMI), not weight. It is not clear from the news story which growth charts the researchers were using in the study (weight for age, weight for length or BMI for age). The type of chart used makes a big difference to how the results should be interpreted.
Childhood obesity is certainly a serious issue that needs to be addressed, but I worry that news stories like these will cause parents to worry unnecessarily about their baby’s weight. Not having a proper understanding of how growth charts work and how they should be used, can lead to parents (and even health care providers!) making changes to a baby’s diet that are usually not in the best interest of the baby. Parents being told to supplement breastfeeding with formula because their baby is on the 3rd percentile and therefore “too small” or being told to restrict feedings or give water between feedings because their baby is “too big” are getting bad advice (and yes, I’ve heard both from parents that I’ve worked with!). Growth patterns can vary widely among young children. Some start out very long and lean and slowly put more weight on, and others start out shorter and heavier and gradually grow into their weight. Labeling a baby as “obese” at 9 months old when most babies haven’t even started walking yet seems absurd.
Thoughts?
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My 12 week old baby at his last weigh-in (about 2 weeks ago) was measuring above the 97th percentile. Was my child health nurse worried about it? No. She said that as my son is exclusively breastfed, there is nothing anyone can do about his weight and she certainly wouldn’t be trying to get me to restrict his feeds. The nurse also said that once my son is mobile, his weight will probably drop down as he will be using plenty of energy.
Thank you Dahly for your comment and the additional info you have provided. I certainly agree that parents should not be attempting to prevent growth in babies and young children, and that the emphasis needs to be on promoting biologically normal feeding (breastfeeding rather than formula feeding), and healthy behaviours.
Hello. I am frequently bothered by the concept of an obese 9 month old (2 year old, etc.).
The definition of obesity is an accumulation of adipose tissue that is deleterious to health. In adults, obesity definitions (BMI>30, for example) are based on epidemiological studies relating levels of adipose to serious health outcomes such as cardiovascular disease or death. In infants and children it is not so straightforward, since they don’t have heart attacks, or die from obesity. Instead we are more interested in predicting the future…how does a certain size at 9 months relate to probability of obesity later in life?
Figuring this out in a robust way is much more complicated than many seem to realize. Some issues:
1) Regression to the mean
2) Catch -up growth
3) Most importantly: Children have a developmental tempo that is not equal to age – most parents understand that perfectly healthy kids achieve developmental milestones (including growth) at different ages. Early in life, a difference in a week of “developmental age” can be large, so in fact we might not expect healthy individuals to “track” along the growth chart – unless their specific tempo perfectly matches the average tempo, across all time points.
While I have no doubt that the seeds of obesity are planted, at least in part, in early life – we should be much more focused on the development of behaviors that lead to positive energy imbalance later in life. For example, I’m much more interested in how exclusively breastfeeding vs. formula feeding is related to obesity risk, rather than big baby vs. small baby. Body size is simply too crude a measure to worry about too much. The growth charts are really designed to pick up kids who are failing to thrive and facing serious, immediate health risks. They are not designed to identify “early obesity.”
It is critical to keep in mind that growth promotion is a cornerstone of public health practice. Growth in early life, particularly linear growth (which is reflected in weight) is associated with all kinds of positive outcomes. There is no evidence that parents should be acting to prevent growth – this is what I am most troubled about when the media pick up on this research.
For people wanting to learn more about growth and or “early obesity”, a few names to enter into google scholar: Tim Cole, Cesar Victora, Linda Adair, Michelle Lampl, Bill Dietz.
There is also a report hosted on the national obesity observatory (which I co-authored) about how difficult it is to predict who will become obese based on data from <9months of age, including body size.