Researchers here argue that the common study methodology of assessing weight only at a single point in time greatly underestimates the increased mortality rate produced by choosing to become overweight, or worse, obese. The trajectory of weight over time is a significant factor, and being overweight at any time in life increases risk even if that weight is lost later. The longer an individual is overweight, the more damage is done:
Researchers have found that prior studies of the link between obesity and mortality are flawed because they rely on one-time measures of body mass index (BMI) that obscure the health impacts of weight change over time. The new study maintains that most obesity research, which gauges weight at only a single point in time, has underestimated the effects of excess weight on mortality. Studies that fail to distinguish between people who never exceeded normal weight and people of normal weight who were formerly overweight or obese are misleading because they neglect the enduring effects of past obesity and fail to account for the fact that weight loss is often associated with illness. When such a distinction is made, the study finds, adverse health effects grow larger in weight categories above the normal range, and no protective effect of being overweight is observed.
Researchers tested a model that gauged obesity status through individuals' reporting of their lifetime maximum weight, rather than just a "snapshot" survey weight. They found that the death rate for people who were previously overweight, but reported normal weight at the time of survey was 27 percent higher than the rate for people whose weight never exceeded that category. The researchers used data from the large-scale National Health and Nutrition Examination Survey, linking data available from 1988 to 1994 and 1999 to 2010 to death certificate records through 2011. The survey asked respondents to recall their maximum lifetime weight, as well as recording their weight at the time of the survey. Of those in the normal-weight category at the time of the survey, 39 percent had transitioned into that category from higher-weight categories.
The study used statistical criteria to compare the performance of various models, including some that included data on weight histories and others that did not. The researchers found that weight at the time of the survey was a poor predictor of mortality, compared to models using data on lifetime maximum weight. "The disparity in predictive power between these models is related to exceptionally high mortality among those who have lost weight, with the normal-weight category being particularly susceptible to distortions arising from weight loss. These distortions make overweight and obesity appear less harmful by obscuring the benefits of remaining never obese."