Original Article

International Journal of Obesity (2007) 31, 652–662. doi:10.1038/sj.ijo.0803457; published online 26 September 2006

Self-selection contributes significantly to the lower adiposity of faster, longer-distanced, male and female walkers

P T Williams1

1Life Sciences Division, Ernest Orlando Lawrence Berkeley National Laboratory, Donner Laboratory, Berkeley, CA, USA

Correspondence: Dr PT Williams, Life Sciences Division, Ernest Orlando Lawrence Berkeley National Laboratory, Donner Laboratory, Berkeley, CA 94720, USA. E-mail: ptwilliams@lbl.gov

Received 26 January 2006; Revised 8 June 2006; Accepted 12 June 2006; Published online 26 September 2006.

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Abstract

Objective:

 

Cross-sectional studies show that active individuals are leaner than their sedentary counterparts. The purpose of this paper is to determine the extent that this is due to self-selection bias, specifically to initially leaner men and women choosing to exercise longer and more intensely.

Methods:

 

Walking quantity (weekly distance) and intensity (speed) were compared to current body mass index (BMI) (BMIcurrent) and BMI at the start of walking (BMIstarting) in 20 353 women and 5174 men who on average had walked regularly for exercise for 7.2 and 10.6 years, respectively.

Results:

 

The relationships of both BMIcurrent and BMIstarting to both distance and intensity were nonlinear (convex). On average, BMIstarting explained >70% of the association between BMIcurrent and intensity, and 40 and 17% of the associations between BMIcurrent and distance in women and men, respectively. Although the declines in BMIcurrent with distance and intensity were greater among fatter individuals than leaner individuals, the portions attributable to BMIstarting remained relatively constant regardless of fatness. Thus, self-selection bias accounted for most of the decline in BMI with walking intensity and smaller, albeit significant, proportions of the decline with distance.

Conclusion:

 

Although walking intensity and walking distance are both strongly associated with greater leanness, intensity is less likely to be causally related to leanness than is distance, and the latter should be encouraged for reversing or preventing weight gain.

Keywords:

walking, body mass index, adiposity, self-selection

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