We conducted this study to understand the association between regular breakfast intake and metabolic outcomes by sex, work status, and age group stratification. The breakfast consumption pattern was consistent with previous studies. Of the total participants, 56.4% consumed breakfast regularly. Similar patterns were observed in a Japanese study using Saitama Health Promotion Corporation data, 57.5% (21), and a previous Korean study using KNHANES 2017 data, 57.9% (22).
The results of this study showed that irregular breakfast intake (<5 times per week) was closely linked to higher risk of increasing number of abnormal metabolic outcomes, especially in young men in the working population than regular breakfast intake. 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 outcomes (23). Furthermore, our results corresponds with that of a review article, which reported that daily breakfast consumers were less likely to have cardiovascular disease risk factors, including elevated serum LDL-C levels, low serum HDL-C levels, and elevated blood pressure (24).
As shown in Table 1 and Table 2, regular breakfast eaters had more abnormal metabolic outcomes. After stratification by age, sex, and working status, young male workers and middle-aged female workers had significant differences. Table 3 shows that regular breakfast eaters among young male workers tended to have a smaller number of metabolic abnormalities, while regular breakfast eaters in the middle-aged female worker group had a larger number of metabolic abnormalities. However, after adjusting for covariates, the significance disappeared only in the middle-aged female worker group.
According to a previous study, 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 (25). Our results are consistent with those of the aforementioned study, and 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 (26). One study reported lower BMI and appearance-related satisfaction levels among young Korean university female students compared with European and American students (27). This could increase the risk of eating disorders in young women, which might have affected the results.
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 (28). However, the definition of breakfast skipping in that study was different from that of our study. In that 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, 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 is known to cause a reduction in the basal metabolic rate (BMR) via compensatory metabolism (29). The reduction in the BMR leads to the consumption of excess calories, ultimately leading to weight gain.
The time of meal consumption 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 (30). Another study found that breakfast skipping was associated with higher hemoglobin A1c values, which indicate poorer glycemic control (31). A longitudinal study showed that breakfast skippers had high levels of fasting insulin (32). 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 plays a crucial role in the biosynthesis of cholesterol and lipids. Through these mechanisms, breakfast skipping might lead to increased fasting glucose levels, increased blood pressure, high levels of serum TG, and low levels of HDL-C.
This study observed a more significant relationship between breakfast skipping and abnormal metabolic outcomes in men in the working group 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 working conditions (33). Another study reported a significantly increased risk of metabolic syndrome in working men compared with working women (34). The results of this study further support the idea of the working male population being vulnerable to metabolic diseases.
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 a 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.
This study identified the relationship between breakfast skipping and the number of metabolic abnormalities 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, which had not been used in previous studies. Work status is an important factor that affects daily metabolism. The different strengths 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. This study used a self-reported questionnaire for breakfast consumption because the use of a self-reported questionnaire is common in breakfast consumption studies, and its reliability has been clinically verified in highly cited and qualified studies (3, 6). Moreover, the proportion of regular breakfast eaters in this study was similar to that of previous studies (21, 22). This shows the repeatability of the questionnaire. The large sample size in our study could also reduce the effect of the error. 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, caution must be exercised to establish a causal relationship. A longitudinal interventional study is needed to definitively unveil the exact mechanism. Third, although we stratified participants based on work status, we did not examine 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. In addition, due to the lack of data on dietary quality, the quality and quantity of nutrients could not be analyzed in this study. The quality and quantity of nutrients in relation to breakfast skipping need to be clarified in future studies.
Although breakfast is considered the most important meal of the day, the percentage of regular breakfast eaters among young adults was only 37.92%. This 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 work), breakfast habituation, and the risk of metabolic outcomes.