We conducted the current study to understand the association between regular breakfast intake and metabolic outcomes with sex, work status, and age group stratification. The results of this study showed that irregular breakfast intake (< 5 times per week) was closely linked to an increased risk of abnormal metabolic outcomes, especially in young men in the working population.
These results are consistent with a previous study that used the KNHANES data and showed that breakfast consumption patterns were associated with a risk of metabolic syndrome (20). Furthermore, our results are compatible with a review article, which reported that daily breakfast eaters were less likely to have Cardiovascular disease risk factors, including elevated serum LDL-C levels, low serum HDL-C levels, and elevated blood pressure (21).
There was no significant association between breakfast skipping and abnormal metabolic outcomes in women. A Japanese longitudinal cohort study on factory employees showed that the average frequency of breakfast skipping was not associated with BMI and waist circumference in women (22). Our results are consistent with those of the aforementioned study. There are several explanations for this result. Postmenopausal status is known to be associated with abnormal metabolic outcomes. In middle-aged women, postmenopausal status has been reported to affect the outcome (23). One study reported lower BMI and appearance-related satisfaction among young Korean university women students compared with European and American students (24). This could increase the risk of eating disorders in young women, which might affect the result.
Another cross-sectional study using the KNHANES data reported a different result and stated that the risk of abnormal metabolic outcomes increased in both men and women (25). However, the definition of breakfast skipping in the study was different from that of our study. A breakfast skipper was defined as a subject who had skipped breakfast 1 day or 2 days before the survey. This definition has a limitation in the overall representation of breakfast consumption. From 2013, the KNHANES changed the question regarding breakfast consumption habit from “Did you skip breakfast before 1 day or 2 days?” to “What is the average frequency of breakfast consumption per week for the past 1 year?” We considered the latter question in this study.
Herein, we have proposed several mechanisms to explain the association between abnormal metabolic outcomes and breakfast skipping. Breakfast is the very first meal of the day, which kick-starts the daily metabolism of the human body. Energy consumption will be lower than the energy requirement if breakfast is skipped before going to work. Food deprivation state is known to cause reduction in the basal metabolic rate (BMR) via compensatory metabolism (26). The reduction in the BMR leads to the consumption of excess calories, ultimately leading to weight gain.
The time of consumption of a meal affects the postprandial increase in energy expenditure and blood glucose levels. A randomized repeated-measures study showed that skipping breakfast was compensated by consuming big meals at lunch. In addition, the study found that breakfast skipping increased the overall 24 hour average blood glucose levels (27). Another study found that breakfast skipping was associated with higher Hemoglobin A1c values, which indicate poorer glycemic control (28). A longitudinal study showed that breakfast skippers had high levels of fasting insulin (29). Poor glycemic control is associated with high levels of glucose, insulin resistance, and high levels of fasting insulin. Insulin is known to stimulate hydroxy-methyl-glutaryl Co-A reductase activity, which has a crucial role in the biosynthesis of cholesterol and lipids. Through these mechanisms, skipping breakfast might lead to raised fasting glucose, increased blood pressure, high levels of serum TG, and low levels of HDL-C.
The current study observed a more significant relationship between breakfast skipping and abnormal metabolic outcomes in working men than in women in all other groups. A previous study indicated that men in the working group, compared with women in the same group, had a higher risk of metabolic syndrome associated with the working condition (30). Another study reported a significantly increased risk of metabolic syndrome in working men compared with working women (31). Results of the current study further support the idea of the working male population being vulnerable to metabolic syndrome.
To the best of our knowledge, our study is the first and largest sample-sized study to explore the association between abnormal metabolic outcomes and breakfast skipping in the Korean population. Only few studies have investigated the effect of work status on the association between breakfast skipping and abnormal metabolic outcomes. Our research indicated that the detrimental effect of breakfast skipping was evident in the working Korean male population, especially in young adults. Educating young male workers regarding the benefits of eating breakfast could be a great way to prevent further metabolic diseases.
The present study identified the relationship between breakfast skipping and the risk of metabolic syndrome and proposed a novel hypothesis to explain the variable strength of association according to the stratifications. We considered stratifications, such as age and work status, that had not been used in previous studies. Work status is an important factor that affects daily metabolism. The different strength of association according to work status implies that daily activity or stress levels might be an effect modifier of the association between breakfast skipping and abnormal metabolic outcomes.
Our study has several limitations. First, we used a self-administered questionnaire to acquire information about breakfast consumption, which could involve a recall bias. However, even highly cited interventional or randomized controlled trials (3) have considered self-reporting of consumption habits by the participants. Thus, it may be considered that recall bias could be present in our study, as well as in other studies. Moreover, the large sample size in our study could have reduced the effect of the bias because the expected bias might be similar among all individuals. In addition, our questionnaire was designed to include the 1-year average frequency to appropriately reflect the long-term dietary habits of the participants. Second, considering the cross-sectional design of our study, more caution must be exercised to establish a causal relationship. A longitudinal interventional study is needed to definitively unveil the exact mechanism. Thirdly, although we stratified participants based on work status, we did not examine the specific working conditions such as shift work, long working hours, manual work, and clerical work. Further analysis based on working conditions is required to determine whether breakfast skipping is an important risk factor for abnormal metabolic outcomes in the working population.
Finally, since the energy requirement for work was not quantified in this study, we could not directly compare the morning energy expenditure between the working and non-working populations. Further detailed studies are required to reveal the relationship between early morning working, breakfast skipping, and the risk of abnormal metabolic outcomes.
Although breakfast is considered the most important meal of the day, the percentage of regular breakfast eaters among young adults was only 37.92%. The trend is in progress, accelerating the risk of metabolic outcomes among young adults. The risk is accentuated in the working population of young men, and further studies are required to clarify the association between specific working conditions (working hours or shift working), breakfast habituation, and the risk of metabolic outcomes.