Obesity in childhood and adolescence can lead to obesity in adulthood, and it is well known that obesity causes several diseases, such as hypertension, dyslipidemia, diabetes, cardiovascular disease and colon cancer [1, 2]. In particular, this issue emerged as a social problem that exacerbates the burden of future medical problems and costs [3]. One study estimated the social and economic cost of adolescent obesity in Korea was 1,363,800 million Won [4]. Obesity prevalence in aged 2–18 children and adolescent increased from 5.8% in 1995 to 9.7% in 2007 in Korea (6.1% to 11.3%, and 5.5% to 8.0% in boys and girls, respectively) [5].

Childhood and adolescent obesity is affected by home and society which he or she belongs to and genetic factors. One study reported that childhood and adolescent obesity is more related to parents’ socio-economic position (SEP) [6]. As parents’ occupation is an important factor determining the household’s socio-economic position, children’s obesity can be associated with their parent’s job.

With the rapid industrialization for decades in Korea, the number of married women who has occupation is getting higher. Since 2001, the employment rate of married women have been maintained above 50% [7]. In the past, even if a woman before marriage participated in economic activities, she had to quit the job after marriage due to having a baby, housework and parenting at home. But, in recent years, many married women with children are working outside. However, married working women still do a lot of household chores such as taking care of their children, making foods and cleaning. Married woman who has children works both in her workplace and home [8, 9]. With regard to the association between mothers’ work and their children’s obesity, many researches were performed in developed countries [10–13]. There is a study that mothers’ working long hours have positively related to child’s obesity [14]. This result explained that working mothers spend less time with their children at home and they do not have enough time to prepare good and slow foods and to guide child’s recreational activities [14, 15].

There was no study showing the relevance of the mothers’ work hours and children’s obesity in Korea. In addition, most studies dealing with this issue did not consider children’s age and gender. Therefore, we intended to find out the association between mother’s working hours and children’s obesity according to children’s gender and age, using the data of the Korea National Health and Nutrition Examination Survey (KNHANES).

In this study, children’s overweight (including obesity) increased among girls whose mothers worked longer and boys whose mothers worked shorter. Researchers in North America and Europe showed that longer working hours of mothers increased BMI of their children [11, 12, 17–19]. The reason for such results was analyzed as the following; long working hours of mothers made lacking of time for taking care of their children. So, their children spend more time to watch TV, do less physical activity and eat unbalanced diet [18]. Compared with non-working or part time working mothers, full time working mothers are likely to have their children with overweight and obesity [20].

In our study, we found three interesting results. First, obese and overweight boys whose mothers work under 40 hours per week were more prevalent among 13–18 years old boys. These patterns are shown similarly in other age and gender groups (not significant statistically). Employment rate of Korean married women was very low if they had children under 6 [7, 9, 17]. Raising small children interrupted mother’s occupations, while mother’s employment rate increased when their children became school-aged or adolescent for their children’s education cost. However, married women whose career was disconnected are usually hired as part-time or non-regular employees. It makes them working under 40 hours in a week [17, 21]. It is possible to assume that social economic position of mothers who work under 40 hours can be relatively low in Korea. In Korean studies, married women who worked as part-time or non-regular employment were much more prevalent than unmarried women or married men. Such women were relatively low-educated and service-sales workers. The women in the case of highly educated -college or more- engaged in education service, mostly [9, 22, 23]. Part-time working married women in service-sales were hired in order to respond to changes in seasonal workload and worked low level of difficulty and simple tasks, while part-time working married women in education service were hired in order to cut down labor costs or substitute regular worker. One study reported their average contract periods were 6.8 months. So, in any cases, part-time working married women were employed as day laborer or temporary workers and their jobs were unstable and paid low wages [23]. Low SEP of mothers can be associated with their children’s obesity. In developed countries, childhood and adolescents of high SEP groups usually consume more vegetables and fruits, which are low calories, than low SEP group [24]. Additionally, children of low income households do eating higher cholesterol intake, binge eating, and more TV watching [25, 26]. Also, SEP of household is associated with physical activity of adolescence. High parental education was related with increased physical activity of adolescents and decreased TV watching time [27, 28].

The second interesting result was decreasing obesity and overweight of 6–12 year-old boys whose mothers worked over 60 hours, differed from other age and girls. Working over 60 hours a week is very long time. Mothers may go to work very early before their children finish breakfast or return home at very late time after pasting supper time. Young boys’ meals can be neglected under not taking care of mothers for a long time. Adolescents can solve meals without mothers’ help, and young girls can take care of themselves relatively compared to boys of same ages. So, we thought that young boys eat very irregular meals or skip meals. But, more researches about this issue will be needed.

Finally, in 6–12 year-old girls whose mothers work 49 hours or more per week, prevalence of obesity and overweight was much higher. Girls are usually more influenced by their mothers than boys. So, girls whose mothers spend less time with them possibly do less physical activity, and watch TV longer. However, boys are usually outgoing and do more physical activities without mothers’ control, when their mothers work longer. And adolescent girls, compared to adolescent boys, usually do not participated in moderate to high intensity exercise and spend more time watching TV and become less physical activity as grade increases [29, 30]. This can be an explanation for this difference according to gender.

The risk for obesity increases in the children and adolescents with using computer and watching TV longer, short sleeping time, and fewer intakes of fruit [31–33]. Working mothers spend less time for cooking, eating with their children, playing, and having leisure time with their children [15, 18, 34]. It is possible for them to supervise the daily activities of children less [14]. Full-time working mothers who work harder usually may buy instant foods, processed foods, and high calorie food for their families. Furthermore, they rarely serve fruits and vegetables, while they eat high caloric foods [35–37]. In many working mothers, whose working hours are long or who do shift work, it is more frequent that family members tend to eat out and often skip meals and overeat later [38, 39]. In the UK, 8–13 year-old children whose mothers are full-time employed spend more time on watching TV than the same aged children whose mothers are non-employed or part-time employed [40].

In this study, we examined the relevance of children’s obesity and mothers’ working hours according to developmental stages - childhood and adolescence - and gender. Through using these stratifications, we intended to find the different effects of mothers’ working hours on their children’s obesity according to gender and age. So, we found the gender difference in this association. This study also used reliable and representative data of Korea. This is strength of our study unlike other studies dealing with this issue.

This study has some limitations. First, we did not consider fathers’ factors such as occupation and BMI. As we know, father’s occupation can represent socioeconomic position of his family. However, information about fathers with growing children in the KNHANES IV and V data is somewhat limited, and especially in single mothers’ households. So, we excluded fathers’ variables - BMI and employment. We considered household incomes instead of fathers’ occupation. Second, we used cross-sectional and secondary data for analysis. We did not consider mothers’ working period. Results may vary depending on child’s developmental stage when the mother has a job. A study showed that mothers’ full-time employment around age 11 affected probability of overweight at age 16 [41]. And, another study reported that the duration of mothers’ employment was associated with higher BMI of their children cumulatively [42]. Additionally, there was no information about mother’s work schedules – whether on weekdays or weekends. There will be a difference in children’s lives depending on whether their mothers work on weekend.

Future studies should consider the working period of mothers, calorie intake, routine diet, physical activity, time of using computer, watching TV, study time of children and adolescents and food selection at home in order to clarify the cause of children’s obesity according to the mothers’ working hours.