ANN ARBOR—It’s about as easy to lose weight as it is to get rich, according to a University of Michigan study that tracked weight changes among a nationally representative sample of more than 10,000 adults over a 13-year period.

According to the study, conducted by the U-M Institute for Social Research, the world’s largest academic survey and research organization, about 51 percent of adult men in the middle ranges of body mass index in 1986—that’s a weight of about 175 pounds at a height 5’10″—were in more or less the same weight category in 1999. Over this 13-year-period, about 27 percent had gained a substantial amount of weight and 21 percent had lost a substantial amount.

For adult women, about 55 percent of those in the middle ranges of body mass index in 1986 (a weight of about 150 pounds at a height of 5’5″) were in about the same weight category in 1999, while about 28 percent had gained a substantial amount of weight and 17 percent had lost a substantial amount.

“Our analysis confirms anecdotal accounts that there is substantial weight mobility—the yo-yo diet effect—over the adult life course,” says Frank Stafford, the U-M researcher who directs the Panel Study of Income Dynamics, funded by the National Science Foundation and the National Institute on Aging. “We compared the extent of changes in body mass index and household wealth mobility over this time period, and found that both are of the same order of magnitude. That’s understandable, since people often gain both weight and net worth as part of the aging process.”

In the study, which was also funded by the National Institute of Child Health and Human Development, Stafford and colleagues Yong-Seong Kim and Katherine McGonagle found that the U.S. population as whole has been experiencing an “obesity drift” rather than an obesity epidemic, as many have claimed.

Weight gains between 1986 and 1999 were pervasive across all age and gender groups studied, for Black as well as white adults, the researchers report. But these gains were slight, with median body mass index rising from 24.3 in 1986 to 25.9 in 1999. According to Stafford, this upward weight drift does not seem to be the result of changes in the age distribution of the U.S. population, or changes in levels of regular exercise.

The researchers also examined weight gains and losses in children, finding that overall, almost one-fifth of children ages 2 to 12 were overweight, with another 14 percent at risk of overweight. About 39 percent of boys fit into these two categories, compared with about 28 percent of girls. Because of the unique, intergenerational nature of the study, the researchers were able to link the body weight of children with that of their parents and grandparents. They found the children’s body mass index was highly correlated not only with their parents’ but also with their grandmother’s.

In comparing the health risks of overweight among Black and white adults, the researchers confirmed recent published reports that underweight is actually riskier than overweight for Black Americans. “We found ‘healthy’ weights are substantially higher for both Black and white Americans than conventional guidelines indicate,” says Stafford. “We don’t know why this is the case, and more study is certainly needed. But we found that over the 13-year period studied, men and women who were somewhat overweight according to federal guidelines had the lowest risk of dying from their mid-30s through their mid-50s.”

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Established in 1948, the Institute for Social Research (ISR) is among the world’s oldest survey research organizations, and a world leader in the development and application of social science methodology. ISR conducts some of the most widely-cited studies in the nation, including the Survey of Consumer Attitudes, the National Election Studies, the Monitoring the Future Study, the Panel Study of Income Dynamics, the Health and Retirement Study, and the National Survey of Black Americans. ISR researchers also collaborate with social scientists in more than 60 nations on the World Values Surveys and other projects, and the Institute has established formal ties with universities in Poland, China, and South Africa. Visit the ISR Web site at www.isr.umich.edu for more information.