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) (BMI(current)) and BMI at the start of walking (BMI(starting)) 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 BMI(current) and BMI(starting) to both distance and intensity were nonlinear (convex). On average, BMI(starting) explained >70% of the association between BMI(current) and intensity, and 40 and 17% of the associations between BMI(current) and distance in women and men, respectively. Although the declines in BMI(current) with distance and intensity were greater among fatter individuals than leaner individuals, the portions attributable to BMI(starting) 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.

译文

目的:横断面研究表明,活跃的人比久坐的人更苗条。本文的目的是确定这是由于自我选择偏见所致,特别是针对最初较苗条的男性和女性,他们选择运动时间更长,强度更大。
方法:比较20353名女性和5174名男性的步行量(每周距离)和强度(速度)与当前体重指数(BMI)(BMI(当前))和步行开始时(BMI(开始))的BMI。平均分别定期锻炼7.2年和10.6年。
结果:BMI(电流)和BMI(起始)与距离和强度的关系均为非线性(凸)关系。平均而言,女性的BMI(开始)解释了> 70%的BMI(当前)与强度之间的关联,而男性和女性分别解释了BMI(当前)与距离之间的40%与17%之间的关联。尽管较胖的个体的BMI(当前)随距离和强度的下降幅度更大,而较瘦的个体更显着,但无论肥胖程度,可归因于BMI(起始)的部分都保持相对恒定。因此,随着步行强度的增加,自我选择偏见是BMI下降的主要因素,而随着距离的减小,自选偏差则占较小的(尽管很明显)比例。
结论:虽然步行强度和步行距离都与较高的瘦度密切相关,但强度与倾斜度的因果关系远比距离小,因此应鼓励后者以逆转或防止体重增加。

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