The more hours young adults spend watching television each day, the greater the likelihood that they'll have a higher body mass index and bigger waist circumference, a 15-year analysis by the University of Pittsburgh Graduate School of Public Health revealed.

The association did not hold in later years, indicating that young adulthood is an important time to intervene and promote less television viewing, according to the research published online in the journal SAGE Open.

"We were quite surprised to find that television viewing was associated with subsequent obesity for young adults, but not for the middle-aged," said lead author Anthony Fabio, Ph.D., M.P.H., assistant professor of epidemiology at Pitt Public Health. "This suggests that middle-aged adults may differ from young adults in how they respond to the influence of TV viewing."

Dr. Fabio and his colleagues analyzed data from 3,269 adults recruited from Birmingham, Ala., Chicago, Minneapolis, and Oakland, Calif., who participated in the Coronary Artery Risk Development in Young Adults (CARDIA) Study. For 15 years starting in 1990, the participants reported their television viewing habits and had their waist circumference measured and their body mass index (a measure of weight and height that can indicate obesity) calculated every five years.

The more time participants spent watching television when they were approximately 30 years old, the more likely they were to be obese five years later, compared to their peers who spent less time in front of the television. The team did not have data on younger ages.

Dr. Fabio and his team suspect many potential reasons for the association, including that young adults may be more likely to snack during television viewing and consume unhealthy food due to their greater susceptibility to the seduction of junk food advertising on television. In support of that hypothesis, the CARDIA study also found that participants were more likely to eat healthier foods as they aged.

The analysis found that 23 percent of the men and 20.6 percent of the women participating in the study watched four or more hours of television daily. Within that group of heavy TV watchers, 35.9 percent were black, and 8.6 percent were white; and 40.8 percent had a high school education or less, vs. 17.4 percent with an education beyond high school.

A lower family income and higher rates of smoking and drinking also were associated with more time spent watching television.

"Television viewing and obesity are both highly prevalent in many populations around the world," said Dr. Fabio. "This means that even small reductions in television viewing could lead to vast public health improvements. Reducing sedentary time should be a healthy lifestyle guideline heavily promoted to the public. Our study indicates that the biggest bang for the buck would be in targeting young adults for interventions to reduce television viewing. Healthy lifestyle behaviors should start at early ages."