Previous investigations have examined the BMI and mortality relation using body weight prior to follow-up and have generally found that past weight results in a stronger positive association with mortality than more recent weight ( 14– 19), possibly suggesting that past weight mitigates reverse causation. A BMI measured from a prior period is more likely than later BMI to capture weight unaffected by serious chronic illness ( 10). Studies of the BMI and mortality relation which incorporate such populations but which still take steps to account for potential reverse causation would be of particular value.Īn approach that may help account for reverse causation is to examine BMI from a prior period, such as 10 years prior to mortality follow-up. Moreover, some have suggested that findings based on restriction of the analysis to apparently healthy individuals may yield results of uncertain applicability to those with chronic conditions and smokers ( 13). However, those restrictions may not eliminate all deaths attributable to undetected chronic illness at study entry, especially in populations which typically experience high levels of morbidity such as smokers ( 12). To mitigate this type of potential bias, some analyses exclude at baseline all study participants with diagnosed chronic illness and those with recent weight loss ( 1, 11). As it relates to the BMI and mortality association, reverse causation refers to chronic illness present at the time of weight assessment causing both previous weight loss and subsequent death ( 9, 10). Much of the uncertainty underlying the nature of the relation between BMI and mortality is the issue of reverse causation. Rising prevalence of overweight and obesity in the United States ( 7) and other parts of the world ( 8) in recent decades underscores the timeliness of understanding the relation between body mass index (BMI the weight in kilograms divided by the square of height in meters) and mortality. Conflicting results regarding the shape of the relationship have included a positive ( 1), a J-shaped ( 2) or U-shaped ( 3, 4), an inverse ( 5), and a null ( 6) association. The association between body weight and mortality remains controversial despite several decades of research.
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