A US study found those who spent more then fives hours a day in front of the television ate more fried and fast food five years later.
The results, published in the International Journal of Behavioural Nutrition and Physical Activity, were blamed on the adverts the youngsters were subjected to.
Dr Daheia Barr-Anderson, who worked with researchers from the University of Minnesota, said, "To our knowledge, this is the first study to examine the association between TV viewing and diet over the transition from adolescence into young adulthood.
"We've shown that TV viewing during adolescence predicts poorer dietary intake patterns five years later."
Dr Barr-Anderson added: "These less than healthy foodstuffs are commonly advertised on television while healthy foods rarely receive the same publicity.
"Although young people may be aware that many foods advertised on television are not healthy, they may choose to ignore or do not fully realise the consequences."
The study was based on more than 2,000 schoolchildren in America.