We found that more than 20 % of adolescents reported having no parental rules on eating, and the prevalence of unhealthy eating habits varied between 18 % for skipping breakfast during weekends to 75 % for low consumption of vegetables. A lack of parental rule-setting on eating was strongly associated with unhealthy eating habits among boys and girls. Several sociodemographic characteristics such as gender, family affluence, family structure, urban context and education of parents were related to unhealthy eating habits of adolescents and to lack of parental rules on eating.
More than 20 % of the adolescents perceived no parental rules on eating. Previous evidence focused rather on the association of the presence of parental rules and unhealthy eating habits; literature dealing with the absence of parental rules on eating is scarce. Our further findings suggest that unhealthy eating habits, such as skipping breakfast, low fruit and vegetable consumption, frequent sweets, soft drinks and energy drinks consumption, were very frequent among adolescents. Compared with the findings of the previous HBSC survey in 2009, the prevalence of fruit and vegetable consumption and breakfast skipping did not change importantly, and the prevalence of sweets and soft drinks consumption decreased slightly . The consumption of energy drinks among adolescents was found to be considerably higher than existing findings from Europe suggest . The high prevalence of unhealthy eating habits among adolescents may reflect the limited effectiveness of preventive strategies aimed at improving their eating habits. Generally, these strategies take place in the school environment, which may be the reason of their limited effectiveness. In line with this, Lindsay et al.  highlight the success of interventions within a variety of settings, including schools, health services and the family setting, and emphasize the critical role of parents in these interventions.
Adolescents who perceived a lack of parental rules on eating were at higher risk to eat unhealthily. Those who reported an absence of one of three examined parental rules were also at risk of unhealthy eating, but the chances were lower. Our findings confirm previous evidence about the connection between parental rule-setting on eating and unhealthy behaviour of adolescents [11–14, 16]. In addition, the present findings suggest that when the number of parental rules was reduced, the prevalence of unhealthy eating behaviours increased. Taking into account that parents have been shown to have a crucial role in shaping their children’s dietary practices [8, 9], our findings indicate this to be a possible factor contributing to unhealthy eating habits among adolescents. Given the cross-sectional design of present study, the findings do not imply a causal path. An alternative explanation could be that having parental rules are an expression of more general family food and eating practices which may determine both parental rules and eating habits of adolescents. This is definitely of interest for further research.
Gender was found to have an important association with eating habits of adolescents. Although boys reported a lack of parental rules on eating less often, they were more likely to eat unhealthily than girls. This can point at gender differences regarding the degree of obedience of adolescents to these rules – boys may perceive the existence of the rule but may not follow it. On the other side, given the vulnerability of boys to eat unhealthy , parental rules on eating may be stricter in boys than in girls.
Our findings indicate that several sociodemographic characteristics such as family affluence, family structure, urban context and educational level of parents were associated with parental rule setting on eating and also with eating habits of adolescents. Adolescents reporting low family affluence, incomplete family, low parental education, living in a rural area and a lower socioeconomic status were at higher risk of unhealthy eating habits which is in line with previous findings [3, 24]. We explored this relationship using several indicators related to socioeconomic status (such as education of parents) to ascertain the validity of the results.
Strengths and limitations
The major strengths of our study are its large sample and representativeness for Slovak adolescents and its high response rate. In addition, the measures of food consumption frequency used in the present study have been well-validated  and extensively used in HBSC surveys. We studied a wide range of unhealthy eating habits in the context of parental rules on eating. However, some limitations should be also noted. Firstly, we used a cross-sectional design; thus, no final causal conclusion can be drawn. Second, our data were based on self-reports of adolescents, which are based on the subjective perception of the adolescent including his or her disobedience to rules and may also be influenced by social desirability. Moreover, the measures of parental rules were based on subjective perception which can reflect rather the obedience degree of the adolescents to these rules. However considerable proportion of adolescents do not follow the rules which their parents apply based on their reports, so obedience bias this measurement only marginal. Thirdly, analyses provided in this study targeted the main relationship between parental rule-setting on eating and unhealthy eating of adolescents; we did not address the role of biological and psycho-social factors which can influence this relationship.
The frequent lack of parental rule-setting on eating and the high prevalence of unhealthy eating habits among adolescents indicate the need for interventions carried out in family settings. The findings of the present study on the connection between a lack of parental rule-setting on eating and unhealthy eating habits of adolescents suggest that reinforcing parental rule-setting on eating may improve adolescent eating habits especially among boys and among adolescents with low socioeconomic status.
The associations of a lack of parental rule-setting on eating with unhealthy eating habits of adolescents requires further study to disentangle causality and the mechanisms behind the connection between parental rule-setting and adolescent behaviour. Longitudinal and experimental study designs are needed for this. Moreover objective measures of eating habits (such as anthropometric measures, body composition or body fat) should be explored in future research of family context of unhealthy eating habits of adolescents.