Health Department director-general Pornthep Siriwanarangsan said on Thursday that parents should pay closer attention to their children's health, particularly during the school holidays.
He said a research study had found that less than half of high school pupils exercise three days a week, while the popularity of foods high in fat, flour and sugar continues to increase among young people.
According to the study, most young people spend their free time watching TV and playing computer games. The research found 24.3% of pupils spent more than two hours a day sitting in front of the TV.
It also found that 51.8% of high school students exercise less than three days a week, but consume the same amount of food as those who exercise more, a contributing factor in rising obesity.
Three out of four teenagers with excess body fat run a high risk of being obese as adults and are more susceptible to heart disease, diabetes, high-blood pressure and other diseases, the study said.
"Parents should provide foods from all five major food groups - grains, fruits, vegetables, dairy, and protein - to their children at least once a day. They should avoid sweet, salty and fatty foods and consume fruits and vegetables of different colours," Dr Pornthep said.
He added that children should drink two glasses of milk a day, adding that overweight children should drink non-fat milk and reduce their intake of foods high in flour, fat and sugar such as pastries, pizzas, cakes and cookies.
He also said consumption of soft drinks should be limited and children should exercise at least five days a week, for 30 minutes each session.