Cause specific mortality, social position, and obesity among women who had never smoked: 28 year cohort studyBMJ 2011; 342 doi: https://doi.org/10.1136/bmj.d3785 (Published 28 June 2011) Cite this as: BMJ 2011;342:d3785
All rapid responses
This unique study, which included detailed information on
occupational class, measured height and weight, and an impressive 28 years
of follow-up on cardiovascular disease (CVD), cancer, and all-cause
mortality, found that women who had never smoked and were not obese had
the lowest mortality rates, regardless of their social position. While
the authors effectively addressed two important confounders of the
relationship between body mass index (BMI) and mortality (smoking behavior
and occupational class), they failed to account for another confounder of
equal or greater importance: cardiorespiratory fitness (hearafter
Poor fitness (as determined objectively from a maximal treadmill
test) is a well-established independent mortality predictor(1,2). Higher
levels of fitness greatly attenuate the mortality risks associated with
overweight and obesity (3). Two recent observational studies of over
14,000 women from the Cooper Clinic found that obese women who were fit
had no higher all-cause or cancer mortality risk compared with the women
who were normal-weight and fit(4,5). These results accord with two
previous landmark studies that examined all-cause and CVD mortality in
over 25,000 men from the Cooper Clinic (6,7). Taken together, these
findings strongly suggest that using BMI to predict mortality may be
misleading unless fitness is also considered. Failure to recognize this
limitation can lead to faulty conclusions and perpetuate overstatement of
the hazards of obesity, especially by the news media. Even more to the
point, the headline of a recent article covering the study in question
reads, "Obesity a Major Cause of Early Death in Women: Study" (Health Day,
30 June 2011).
Other studies on BMI and health outcomes claim to use "validated
physical activity questionnaires" as proxies for fitness. However, a
recent systematic review of the validation reports found that the
correlations between self-reported physical activity and a gold standard
are typically in the 0.3-0.5 range, and thus the self-reports account for
~10-25% of the variance in the exposure (8).
Important research questions about physical activity, fitness,
obesity and mortality remain unanswered. What are the separate and
combined influences of physical activity and fitness on mortality risk?
Are the benefits of fitness independent of weight loss? How do these
variables interact with age, sex, ethnicity, and health status? Future
studies must obtain simultaneous objective measures of fitness, physical
activity and adiposity in order to address these important research
Despite their commendable attempts to assess the influence of social
position in the absence of current or past smoking behavior, Hart and
colleagues did not provide clear evidence that obesity is independently
linked to premature death. If the relationship between BMI and mortality
is to have any meaning, investigators must include simultaneous, objective
measures of fitness or physical activity.
1. Blair SN, Kohl HW III, Paffenbarger RS, et al. Physical fitness
and all-cause mortality: a prospective study of healthy men and women.
2. Myers J, Prakash M, Froelicher V, Do D, Partington S, Atwood JE.
Exercise capacity and mortality among men referred for exercise testing. N
Engl J Med 2002;14:793-801.
3. LaMonte MJ, Blair SN. Physical activity, cardiorespiratory
fitness, and adiposity: contributions to disease risk. Curr Opin Nutr
Metab Care 2006;9:540-6.
4. Farrell SW, Fitzgerald SJ, McAuley PA, Barlow, CE.
Cardiorespiratory fitness, adiposity, and all-cause mortality in women.
Med Sci Sports Exerc 2010;42:2006-12.
5. Farrell SW, Finley CE, McAuley PA, Frierson GM. Cardiorespiratory
Fitness, Different Measures of Adiposity, and Total Cancer Mortality in
Women. Obesity (Silver Spring). 2011 Feb 3. [Epub ahead of print]
6. Lee CD, Blair SN, Jackson AS. Cardiorespiratory fitness, body
composition, and all-cause and cardiovascular disease mortality in men. Am
J Clin Nutr 1999;69:373-80.
7. Wei M, Kampert JB, Barlow CE, Nichaman MZ, Gibbons LW,
Paffenbarger RS, et al. Jr, Blair SN. Relationship between low
cardiorespiratory fitness and mortality in normal-weight, overweight, and
obese men. JAMA 1999;282:1547-53.
8. Van Poppel MNM, Chiapaw MJM, Mokkink LB, van Mechelen W, Terwee
CB, et al. Physical activity questionnaires for adults: A systematic
review of measurement properties. Sports Medicine 2010;40:565-600.
Competing interests: No competing interests