Table 1 The definition of some variables in the 2012 Zhejiang Youth Risk Behavior Survey

 Breakfast (Daily)

During the past 30 days, in an average week, how many days did you eat breakfast? with five answer options, “0 day”, “1–2 days”, “3–4 days”, “5–6 days”, “7 days”. The answers were further classified into three groups, “Never” (0 day), “sometimes” (1–2 days, 3–4 days, and 5–6 days), and “daily” (7 days).

 Fruits (≥2 times/d)

During the past 30 days, how many times did you eat fruits every day? (0 time, <1 time, 1 time, 2 times, 3 times, 4 times, ≥ 5 times)

 Vegetables (≥2 times/d)

During the past 30 days, how many times did you eat vegetables every day? (0 time, <1 time, 1 time, 2 times, 3 times, 4 times, ≥ 5 times)

 Milk (≥3 d/wk)

During the past 30 days, how many days did you drink milk every week? (0 day, <1 day, 1–2 days, 3–4 days, 5–7 days)

 Soft drinks (≥1 times/d)

During the past 7 days, how many times did you drink soft drinks, such as Coke, Pepsi, or Sprite? (0 time, 1–3 times, 4–6 times, once a day, twice a day, three times a day, ≥ 4 times a day)

 Fast food (≥2 d/wk)

During the past 7 days, how many days did you eat fast food, such as hamburger, hotdogs, or potato chips, etc.? (0 day, 1 day, 2 days, 3 days, 4 days, 5 days, 6 days, 7 days)

 Moderate physical activity(≥2d/wk)

During the past 7 days, how many days did you participate in at least 60 min of any kind of physical activity that increased your heart rate and made you breathe hard, such as kicking shuttlecock, fast bicycling, and doing housework, etc.? (0 day, 1 day, 2 days, 3 days, 4 days, 5 days, 6 days, 7 days)

 Muscle strengthening activity (≥2 d/wk)

During the past 7 days, how many days did you do exercises to strengthen or tone your muscles, such as push-ups, sit-ups, or weight lifting, etc.? (0 day, 1 day, 2 days, 3 days, 4 days, 5 days, 6 days, 7 days)

 Attend physical education classes (≥2 d/wk)

In an average week when you are in school, on how many days do you go to physical education classes? (0 day, 1 day, 2 days, 3 days, 4 days, 5 days, 6 days, 7 days)

 Watch TV (≥2 h/d)

On an average school day, how many hours do you watch TV? (0 h, <1 h, 1–2 h, 2–3 h, 3–4 h, 4–5 h, ≥5 h)

 Use computer (≥2 h/d)

During the past 7 days, in an average day, on how many hours did you spend using a computer for playing games, surfing, e-mailing, or watching movies, etc.? (open question)

 Sleep duration (≥8 h/d)

During the past 30 days, how many hours did you spend on sleeping every day? (open question)

 Drink alcohol (yes)

During the past 30 days, on how many days did you have at least one drink of alcohol? (0 days, 1–2 days, 3–5 days, 6–9 days, 10–19 days, 20–29 days, 30 days)”. Participants who answered had drunk alcohol at least 1 day during the past 30 days were considered to have the alcohol drinking behavior