For those of you who've followed me from Blogger, you know I write about my concerns regarding use of BMI to measure an individual's health...a LOT. However, a new study just released this month confirms what I've been writing about - notions of an "unhealthy weight" may be seriously off-kilter. This study shows that people with a BMI of under 35 are not at any higher risk of death than those of "normal weight" (to give you an idea, the cutoff means a woman at 5'4" weighing 205lbs or a man at 5'10" weighing 240lbs.)
Just so you know, this study isn't telling us anything new. In fact, it's not really a study, but a compilation of studies from all over the world - but apparently this is a new way to look at old numbers. What's more interesting is that there did not appear to be any attempt to measure the mortality rate of those whose BMI fell under 20 (less than 18 is considered underweight,) which I consider to be more revealing about our cultural values than about health research.
All that being said, these studies don't really tell us much. Using #3 on our science-news-assessing-tool, we can see that these studies show a correlation between a higher BMI and a risk of mortality (or lower BMI and less risk, except for the BMI numbers they chose not to count.) Keep in mind that the word "risk" here is also a bit misleading - it sounds like the researchers are clairvoyant, but basically it just means that during the study period, more people in the "high-risk" groups died than in the "low-risk" groups. Because this study focused solely on BMI, so we have no way of knowing if other factors caused the increase.
My theory is this: healthy behaviors - which may also result in weight loss - are the key to better health. If you are overweight and following healthy behaviors - eating per dietary guidelines, moving per physical activity guidelines - I am guessing your risk level is probably not different from a skinny person. In the past, I've discussed studies that show this to be true. Conversely, there are factors that can reduce healthy behavior that themselves pose danger - for instance, those facing depression may struggle to eat healthy food and exercise, but their risk of mortality is probably unrelated to their BMI.
Why, then, do scientists stay so focused on BMI? While anti-fat bias is definitely an issue culturally, I don't think it's at play here. Scientists use BMI because it is offers a constant that can be measured - a point of reference for assessing the health of a large population of people, even if individual results may offer an inaccurate picture of health.
Katherine Flegal, a research scientist for the CDC's National Center for Health Statistics and lead on this particular study, was in large part responsible for the discovery of the "obesity epidemic." She found an 8% increase in BMI over the US population between 1988-1991 when that number had been stable for many years; a significant jump. Dr. Flegal does note that some studies show associations between a higher BMI and cardiovascular-related deaths, but that the overweight category has less risk otherwise. This may be why being moderately overweight may correlate with a lower risk of death (a fact that is frequently glossed over in health news, which sadly does often show an anti-fat bias.) Dr. Flegal comments:
"Journalists often try to elicit some sort of comment about how this is so important, or this is what I predict, or why this is happening. There are a lot of questions that come up all the time and I really don't know the answer to those questions. It's certainly a temptation to say, "Well, it's probably caused by TV watching" or something like that. But we really don't know. I don't.
So one difficult thing is not going beyond the data, trying to not over-interpret what we have. This is just a factual report of data on the US population. This is what it is. It's not saying anything else."