Public health vs. individual health

I spent some time yesterday catching up on the scientific brouhaha surrounding Dr. Walter Willett’s dissing of the CDC epidemiologist, Katherine Flegal,* who earlier this year published the results of a huge meta-analysis in The Journal of the American Medical Association suggesting that being a bit overweight is not necessarily detrimental to health. It’s the latest in a growing body of research on the so-called “obesity paradox,” which is not quite accurate, since these studies do suggest that actually being obese results in worse health outcomes than being “normal” weight…on a population level, anyway.
What Dr. Willett, chair of the nutrition department of the Harvard School of Public Health, doesn’t like about research like Dr. Flegal’s (he called it “a pile of rubbish”), is that it might confuse people and make them think that they shouldn’t lose weight or avoid gaining weight. In a nutshell, he doesn’t want the facts to get in the way of a good public health message. 
The trouble with this is twofold.
First, Dr. Flegal’s research has its acknowledged limitations, one of which is that it’s very difficult to know why the people who live longer really live longer, and why the people who die younger really die younger. Is it because of what they weigh, or because of one or more other factors that just happen to coexist with their body weight? Smoking status, healthfulness of diet and level of physical activity are a few things that jump out at me. Further, are the people who live longer spending those extra years healthy, or struggling with chronic disease?
Second, even though I’m getting my graduate degree in public health and I fully believe that public health initiatives are vitally important, you can’t always take health observations made on a population level and then say that every individual should act accordingly. Limiting sodium is a great example: Some people totally need to, but other people are doing just fine as they are.
The significance of body weight (as measured by body mass index) and health shifts as we change focus from the population to the individual. Just as we don’t do a research study on one individual and try to extrapolate the results to an entire population, when considering the health of an individual we must consider not what epidemiology tells us, but what that person’s current health and habits, past health and habits and family health history tells us. Body mass index is only one possible measure of health, and it’s not the best measure. 
If you take two women of the same age, same height and same weight, can you really say that they are equally healthy (or not healthy)? Of course not. One woman could eat healthfully most of the time, go for a walk every day, and have four grandparents who lived to be over 100. The other woman could exist on fast food, sit at a desk all day and on the couch all evening, and have four grandparents who died of heart attacks or cancer by the age of 60. Or, you could go really crazy, and have the healthy eater and exerciser have grandparents who died young, while the woman with the less healthy lifestyle won the genetic luck of the draw. Do you see what I’m getting at?
A few weeks ago, Nature published an article and an editorial about the whole Willett-Flegal dust-up. I particularly enjoyed this bit from the editorial:

The political mantra on public-health advice is clear: don’t send mixed messages. The media and those who get their information from the media prefer things in black and white: red wine is good for you; chocolate is bad for you. But, of course, science does not deal in black and white, hence the common criticism that scientists cannot make up their minds. One week, one group argues that extreme exercise is positive for health; the next week, a different set of researchers says the opposite.

Sigh. This is too true. I believe in science, but few truths are absolute, and what we know to be “true” today can change tomorrow as we continue to learn new things and advance the boundaries of our knowledge. And the media often likes to pounce on research results and make absolute proclamations, even if the science is preliminary.
My bottom line remains that there is no one path to health and no single perfect ratio of weight-to-height. Because we are not all cut with the same cookie cutter!
[*Ironically, Dr. Flegal is credited with being the person to notice and call attention to the fact that the weight of the nation was increasing rapidly back in the late 1980s.]