After reading Peter’s post from earlier this week on the problems with BMI, I thought I should repost the below article from a couple years ago.
I had an interesting experience at my last physical exam, and I thought it would be worth sharing here on the blog. Before the physical a nurse put me through an eye test, then took my blood pressure, height and weight. She then calculated my BMI, and told me that I was in the “normal range” (it was around 24.5). But, she continued, I was pretty close to the overweight cut-off of 25.0, so I should “watch my weight” moving forward.
I said “un-huh” and sort of laughed inside my head, given that I’m an obesity researcher and we have written the odd post about BMI here on Obesity Panacea. But the more I talked about the episode with other people, the more ridiculous it seemed. Here’s why.
BMI is only one aspect of health
As we have said before, weight ≠ health. BMI is moderately useful at estimating body fat, and therefore health risk (especially at the population level). However, as Peter and I have argued many times, your behaviour matters as much (or more) than your weight when it comes to health. If you are physically active and eat a healthy diet, you’re likely to be relatively healthy whether your BMI is 22 or 32. It’s not that weight doesn’t matter at all, but it’s far from the only thing that matters.
(For more on the relationship between BMI and health, I suggest this excellent post by Peter from earlier this week. Or, for a review paper on the health-benefits of exercise regardless of body weight, click here.)
My weight has been stable for several years. I am (extremely) physically active. I try to limit the amount of time I spend sitting. And thanks to my wife’s positive influence, I eat a reasonably healthy diet (mostly homemade vegetarian food for breakfast and supper, with leftovers and/or pizza for lunch). My metabolic health is also fine although, ironically, I had to specifically ask before I was be told my HDL and triglyceride levels.
Why would you counsel a weight stable person with a BMI in the healthy range about their weight (as opposed to their behaviour) anyway? I’m certainly not the only person to have this experience – here’s what colleague and fellow PLOS blogger Atif Kukaswadia had to say on twitter:
My BMI is around 24.9. My doc told me to “not gain any more weight” for the same reason.
Yet, as our science blogging friend DrugMonkey pointed out on twitter, none of that means that we’re going to be weight stable forever. I’m in my late 20’s, cutting back on my participation in competitive sports, recently married, and nearing the end of grad school. It wouldn’t be at all surprising if someone in my position were to begin putting on a few pounds over the next few years. In that context, the nurse’s advice seems to make perfect sense.
Except for one (very big) oversight.
How do you “watch your weight”, anyway?
This is really the crux of the problem. Weight is an outcome, not a behaviour. When someone tells you to watch your weight, what do they really mean?
Obviously one would assume that the nurse meant that I should be physically active and eat a healthy diet. Except she didn’t say anything about either of those things. She didn’t ask about my level of activity or my diet (although it had been recorded during an earlier visit), nor did she give me any counseling on what a healthy diet should look like.
What if someone in my position were to take the nurse’s advice and begin dieting to reduce their weight or prevent weight gain (despite being weight stable and already healthy)? As our colleague Dr Arya Sharma has argued, trying to lose weight is actually a pretty good way to gain weight over the long term.
as I have said before, all weight loss attempts should be medically indicated and anyone attempting to lose weight needs to be warned that they may in fact be increasing their long term risk of becoming (even more) overweight or obese.
I don’t want to be too harsh on the nurse because she clearly meant well. But a clinical strategy that focuses exclusively on body weight, with no information or counselling related to healthy behaviours, and completely ignoring all context, is almost certainly going to fail (and possibly make things worse than they were at the beginning).
As always, I’m curious to hear what others think. Has anyone had a similar experience? Have a different perspective on the nurse’s advice? You can also check out Dr Freedhoff’s thoughts on the issue over at Weighty Matters.