During adolescence a number of health-compromising behaviours emerge. When entering adolescence, children are normally spontaneously physically active, and there is hardly any use of tobacco, alcohol or other addictive substances. When leaving adolescence, a substantial proportion of adolescents are physically inactive, have started smoking, and some have started using illegal addictive substances. The sexual debut usually takes place during adolescence, and being sexually active without adequate protection against unwanted pregnancies and sexually transmitted diseases, including HIV/AIDS, represents a serious threat to health and wellbeing. According to a report from the World Health Organization international study on Health Behaviour in School-Aged Children (HBSC), the proportion of smokers increases during early adolescence (Currie et al., 2004). At age 11 the average proportion of smokers (smoking daily or weekly) across all samples (35 countries) is 2%, at age 13 it is 8%, and at age 15 it is 24%. The differences between boys and girls for all countries combined (mainly European countries plus Canada and the United States) were negligible. Corresponding figures for weekly alcohol consumption are 5, 12 and 29%. More boys than girls used alcohol weekly at age 15 (34 and 24% respectively). Prochaska et al. (2001) have developed a screening instrument which defines ‘moderate-to-vigorous physical activity’ (MVPA). Their definition was applied to data from the HBSC study. The proportion of young people meeting the MVPA guidelines on physical activity was (across all samples) 38% at age 11 and 29% at age 15; in other words there is a marked decrease with age that most likely continues across the remaining years of adolescence as well as into early adulthood (Stephens et al., 1984). Food habits were also covered by the HBSC survey. The proportion of adolescents who eat fruit daily decreases from 38% among 11 year olds to 29% among the 15 year olds (Currie et al., 2004). Thuen et al. (1992) have shown that use of safety equipment (seat belts, bicycle helmets, reflectors, life jackets) drops dramatically during early adolescence, and the proportion involved in behaviour associated with elevated risks of accidents and injuries increases. It must be kept in mind, however, that a majority of young people never become regular smokers, heavy drinkers or drug addicts, and a substantial proportion of young adults remain physically active and continue eating healthy food throughout and after the adolescent years. During adolescence the basis for a lifelong healthenhancing lifestyle may be established. The effects of health-compromising behaviours during adolescence can be short-term as well as long-term. Drink driving increases the risk of dramatic and fatal accidents, and represents a major short-term threat to young people’s health and lives. Daily smoking may lead to coronary heart disease and lung cancer, but these effects usually become visible only after many years of exposure. The importance of promoting healthy lifestyles among adolescents therefore to some extent depends on the stability of such behaviours. The higher the stability, the more important it is to promote healthy lifestyles at a young age. Jessor et al. (1991) have studied the stability of problem behaviours from adolescence to adulthood, and conclude that there is considerable stability and continuity. They claim that ‘the adolescent is parent of the young adult’. Although few research projects have focused on the stability and change of physical activity from childhood to adolescence, there is one study which concludes that the level of physical activity in childhood and adolescence to some extent predicts the level of physical activity later in life (Anderssen et al., 1996). Other studies of longitudinal tracking of behaviours (physical activity, food preference and smoking behaviour), have provided convincing evidence that behaviours established during early adolescence do predict behaviours measured during late adolescence and beyond (Klepp, 1993; Kelder et al., 1994; Telama et al., 1997). Substantial tracking has also been found for body mass index over an 18 years’ age span (from 15 to 33 years) (Kvaavik et al., 2003). The promotion of healthy lifestyles among young people is obviously important, not only because of its short-term impact on health and wellbeing, but also because of its consequences for healthrelated behaviours later in life.
