Study Finds ‘Protective’ Weight Just As Diet Resolutions Kick In

Whoopsie. Just as all those New Year’s dieting resolutions are kicking in, along comes the latest salvo in the confusing obesity-mortality wars. “Oh, my,” I said this morning when I heard Harvard nutrition expert Walt Willett call this latest study a bunch of “rubbish” on NPR. “They’re playing rough.”

At issue is this simple question: Does being overweight make you die sooner, or might it actually be protective? But of course, very little is simple in the world of nutrition studies, or when links between Body Mass Index and health are concerned.  Here’s the conclusion from the study, that in effect, very obese people tend to die earlier, but more mildly overweight people may tend to live a bit longer:

Relative to normal weight, both obesity (all grades) and grades 2 and 3 obesity were associated with significantly higher all-cause mortality. Grade 1 obesity overall was not associated with higher mortality, and overweight was associated with significantly lower all-cause mortality. The use of predefined standard BMI groupings can facilitate between-study comparisons.

And here’s Walt Willett on NPR today:

“This study is really a pile of rubbish and no one should waste their time reading it,” he says.

Willett says it’s not helpful to look simply at how peoples body mass indexes, or BMIs, influence their risk of death — as this paper did without knowing something about people’s health or fitness. Some people are thin because they’re ill, so of course they’re at higher risk of dying. The study doesn’t tease this apart.

Also, he says the analysis doesn’t address the bigger, more important issues of quality of life. If an overweight person does live longer — is he or she living with chronic diseases?

If you’re hard-core, read the study yourself here, The New York Times report on it here or the report that explains more of the methodological issues here, and let us know what you think in the comments below.

My personal takeaway: The study’s lead author, Katherine M. Flegal of the CDC, strikes me as brave to take on accepted wisdom about ideal weight. But Walt Willett is a towering figure in nutrition research. So I think I’m going to use the ambiguities raised by this study and Flegal’s earlier work to reinforce the resolution I wanted to make anyway: This year, I’ll try to eat better but not make myself crazy about it.

Please follow our community rules when engaging in comment discussion on this site.
  • Tom

    The mortality scale on the lower end of the BMI range includes frail unhealthy, diseased people which make the higher BMI range look like it has a lower mortality. But if normal is defined as 22-25, the attention-getting headlines disappear.

    “In addition, the NHLBI’s classification of normal as a BMI between 18.5 and 25 kg/m 2 obscures the fact that people with a BMI between 18.5 and 22 kg/m 2 have been found to have higher mortality than those with a BMI between 22 and 25 kg/m 2. Lumping them together raises the mortality rate for the normal-weight group, which could explain why their observed mortality is similar to grade 1 obesity.”

    • commonhealth

      Thank you, Tom! That does seem to be the key…

    • CarolynS

      Actually Tom is not correct here. There are good quality analyses that show overweight with lower mortality than normal weight even if normal is defined as a BMI of 22-25. One reason is tht there really aren’t so many people with BMI 18.5 up to 21.9 that they make a big difference one way or the other.

  • Reasonable?

    The key is to think of the relationship between BMI and health as a “U” shaped curve.

    Increased risks for badness are are at both ends.

    That said, the problem is that “BMI” is bad markers because its too blunt.
    Think of it as a fuzzy “U” where don’t know exactly where the sweet spot is.

    A BMI between 25 and 30 is considered “overweight”, but the number doesn’t tell you whether extra pounds are due to fat or muscle.

    If fact, if you saw a fit person with a BMI of 27 or 28, you probably would not think to call them overweight at all.

    Body composition makes a huge difference, but it’s hidden in a BMI oriented study.

    Unfortunately body compositon (density) is much harder to measure (it requiring califbrated water or gas displacement for accurate measurement).

    • commonhealth

      Great analysis as always, Evan — given the flaws of BMI, I’m thinking I want to make this my year of the “Unquantified Self” — Carey