Body Mass Index and Mortality

I just added the following to the VeganHealth.org article on disease markers of vegans, and thought it might be of some interest to readers:

Recent research has shown that a BMI of 22.5 to 25.0 is associated with the lowest mortality rate. It has been known for some time that a lower BMI has been associated with an increased risk of death, but that was thought to be due mostly to smoking-related diseases. A 2009 meta-analysis of 900,000 people found that even in those who never smoked, there is a slight increase in mortality below a BMI of 22.5. (1)

The excess mortality below 22.5 has not been explained. One theory is that the excess mortality might be due to lower fat-free mass, which would most likely be lower muscle mass (though could also technically be bones, or even some organs). (1, 2) Studies on BMI and mortality to date have not differentiated between fat and fat-free body mass.

1. Prospective Studies Collaboration, Whitlock G, Lewington S, Sherliker P, Clarke R, Emberson J, Halsey J, Qizilbash N, Collins R, Peto R. Body-mass index and cause-specific mortality in 900 000 adults: collaborative analyses of 57 prospective studies. Lancet. 2009 Mar 28;373(9669):1083-96.

2. Wändell PE, Carlsson AC, Theobald H. The association between BMI value and long-term mortality. Int J Obes (Lond). 2009 May;33(5):577-82.

4 Responses to “Body Mass Index and Mortality”

  1. Kristin Says:

    It would be interesting to see what the lower BMI subjects died of. Perhaps they are more active people in general and tend to get into more accidents? Bike accidents, ski accidents, etc.??

  2. Robyn Says:

    I looked at the summary of the second study listed. The study by Wandell, Carlsson, & Theobald ends with the following conclusion:

    “Underweight was associated with higher mortality among men, but not when adjusting for covariates, whereas underweight was associated with lower mortality among women when adjusting for smoking.”

    If I read this right, then underweight is associated with a lower death rate for women.

    Is that how you read it as well?

    Isn’t there research out there on mortality and reduction of calories? How does that match with this research?

  3. Jack Norris RD Says:

    Yes, that is how I read it. Remember that this study was only one finding, while the other study is a meta-analysis, so I would give the Prospective Studies Collaboration more weight. But that is very interesting.

    > Isn’t there research out there on mortality and reduction of calories?

    I’m not aware of any studies specifically looking at lower calorie intake and mortality in humans. I did a quick search at PubMed and didn’t find anything. I’m guessing that the researchers are using BMI as a proxy for caloric intake, although I can see how that might not be a good assumption. Studies of vegetarian mortality have included measurements of caloric intake, but I don’t recall one of them ever using the caloric intake of the individuals as the variable for measuring mortality rates.

  4. Anne Says:

    http://www.nature.com/oby/journal/vaop/ncurrent/abs/oby2009191a.html
    “A significant increased risk of mortality over the 12 years of follow-up was observed for underweight (BMI <18.5; relative risk (RR) = 1.73, P 35; RR = 1.36, P <0.05). Overweight (BMI 25 to <30) was associated with a significantly decreased risk of death (RR = 0.83, P 0.05). Our results are similar to those from other recent studies, confirming that underweight and obesity class II+ are clear risk factors for mortality, and showing that when compared to the acceptable BMI category, overweight appears to be protective against mortality. Obesity class I was not associated with an increased risk of mortality.”

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