Widespread use of tablets and smartphones means that kids and teens today log more screen time than previous generations. As a result, they have become more sedentary. Kids’ inactive lifestyles during childhood and adolescence are not just bad for them in the short term; it can significantly affect their health later on, a study shows.

In fact, the amount of time a child spends watching TV during childhood and adolescence are better predictors of their body mass index, or BMI, and fitness levels in adulthood than their current viewing habits, according to a set of long-term findings from the Dunedin Multidisciplinary Health and Development Study in New Zealand by researchers from the University of Otago.

“Changing the beginning changes the whole story.”

Few long-term studies have looked at how TV viewing time in children and adolescents affects their later risk for metabolic syndrome, a group of risk factors for diabetes mellitus, cardiovascular disease and stroke that can lead to an early death, but researchers from the Dunedin study recently found that more TV viewing time at the ages of five and 15 increased participants’ risk of metabolic syndrome at age 45.

The results of the current analysis suggest there may be a window, or vulnerable period, during childhood when excessive screen time has the greatest effect on kids’ health when they're adults. As the researchers write, “These findings lend support to the World Health Organization recommendation that children and adolescents should limit their recreational screen time.”

The Dunedin study researchers followed more than 1,000 participants who were born in New Zealand in 1972 and 1973, long before the Internet. Parents reported how much time their five-, seven-, nine- and 11-year-old children spent watching television on weekdays. At the ages of 13 and 15, teens reported how much time they spent watching TV. They did so again at 32.

To calculate body mass index, height and weight measurements were taken when participants were five years old. The BMI of 938 of the 997 surviving members of the original study was calculated again at forty years later, at age 45.

Participants also had their blood pressure and waist circumference measured at age 45, and took an exercise test on a stationary bike to measure their fitness level. Non-fasting blood samples were taken to measure levels of high-density lipoproteins (HDL), triglycerides and glycated hemoglobin — HbA1C.

More TV viewing time at five and 15 increased participants’ risk of metabolic syndrome at age 45, suggesting that there may be a vulnerable period during childhood when excessive screen time has the greatest effect on one’s health as an adult.

Metabolic syndrome was defined for study purposes as having any three of the following risk factors: HbA1C levels greater than or equal to 5.7 percent; waist circumference greater than or equal to 102 cm for men and 88 cm for women; triglyceride levels greater than or equal to 200 mg/dL; HDL levels less than 40 mg dL for men and 50 mg/dL for women; and blood pressure greater than or equal to 130/80 mmHg (or medication for hypertension).

The findings highlight the need to consider media use over the entire lifespan, said Pooja Tandon, author of a related editorial. She emphasized that interventions are needed at the family, school, community and policy levels to change more sedentary childhoods that come with too much screen time. Tandon, a pediatrician and principal investigator at the Seattle Children’s Research Institute, put it this way: “Changing the beginning changes the whole story.”

The study and related commentary are published in Pediatrics.