Long working hours can have negative health consequences (1), and research has indicated that long working hours may lead to an heightened risk of hypertension (2), diabetes (3), cardiovascular disease (4, 5), obesity (6), even depression and suicidal tendencies (7). Therefore, it is crucial to plan work hours reasonably. The statutory limit on weekly work hours is less than 48 hours in most European nations (8), and about half of these nations have set a 40-hour workweek cap. Nevertheless, approximately one-third of the global labor force still works more than 48 hours per week.

It is beneficial for health to engage in exercise during free time, and it is also essential for sustaining and enhancing physical strength and work performance (9, 10). One might think that physically demanding work has an advantageous impact on health (11). However, the physical demands of work may actually be detrimental. Reports suggest that jobs with high physical demands are linked to greater levels of disability, reduced body function, and decreased in muscle power (12–14).The differences in physical activity patterns between work and leisure time might be a crucial factor in clarifying this occurrence (15). Furthermore, intense manual labor could contribute to a lack of exercise during leisure time (16).

Non-alcoholic fatty liver disease (NAFLD) is a prevalent metabolic condition with an increasing global incidence and has become a significant factor in chronic liver disease in many parts of the world (17, 18). As people’s understanding of the disease deepens, it has been discovered that NAFLD is actually a disease related to metabolic dysfunction. It was renamed metabolic dysfunction-associated fatty liver disease (MAFLD) in 2020 and subsequently renamed metabolic dysfunction-associated steatotic liver disease (MASLD) in 2023, and a new category, metabolic dysfunction and alcohol-associated liver disease (MetALD), was proposed (19–21). The natural course of NAFLD includes a wide range of pathological conditions, from simple steatosis to steatohepatitis (NASH), as well as with varying levels of fibrosis and cirrhosis (22). Many factors are associated with NAFLD, including obesity, diabetes, hypertension, and dyslipidemia (23, 24). The main cause of death in patients with NAFLD is cardiovascular disease (25). NAFLD is becoming a more significant public health challenge (17), making prevention crucially important.

Both long working hours and heavy physical labor may impact people’s health, therefore we aim to analyze the connection between working hours, working type, and NAFLD. Previous studies have shown that working long hours is strongly linked to NAFLD (26), but they did not indicate whether the association is linear or a dose-response. Additionally, the Korean data may not be representative of the situation in other regions, as it only represents a subset of the Asian population. This study aims to expand the population to include the United States to understand whether the link between long working hours and NAFLD exists in this context. Additionally, no one has explicitly studied the relationship between working type and NAFLD. We will classify occupations based on work intensity to investigate whether different types of occupations are associated with NAFLD.

In this study, we found that long working hours significantly increased the risk of NAFLD even after adjusting for confounding factors. Previous research indicated that long working hours raise the likelihood of NAFLD among Koreans (26). In our study, we expanded the population to the United States and found that working hours continued to show a significant relationship with NAFLD even after adjusting for confounding variables, which is consistent with the findings in Koreans. The risk in the US population (before correcting for confounding variables) was relatively higher (OR: 1.57, 95%CI: 1.21-2.05) compared to Korea. This may be attributed to the higher prevalence of NAFLD in North America (31.2%) compared to East Asia (29.7%) (31). Using unconstrained cubic splines based on the Korean study, we demonstrated a linear relationship between working hours and NAFLD (P _nonlinear = 0.162) (26). A Chinese study showed that prolonged and high-frequency night shift work increases the risk of NAFLD in male steel workers by 27% (32). Furthermore, the 2024 study by Robert Maidstone et al. on biobanks in the UK demonstrates that long-term night shift work elevates the risk of fatty degeneration by 8% (33). These findings are consistent with our research. Night shift work has been shown to be associated with various liver diseases. The study by Wang Feng et al. indicates that night shift work among Chinese workers is positively correlated with liver function abnormalities (34). Similarly, a study involving the Korean population found that night shift work is positively correlated with NAFLD among young female workers with poor sleep quality (35). Moreover, prolonged night shift work increases the risk of dyslipidemia and liver and kidney function abnormalities among nurses (36). Since there is no data on night shifts in the NHANES database, we used weekly working hours to investigate its association with NAFLD. These studies consistently indicate that poor work patterns can lead to the occurrence of various liver diseases.

Currently, there is no research indicating that heavy physical labor increases the risk of NAFLD. Existing studies have shown that heavy physical labor raises the likelihood of work absence (37, 38), musculoskeletal diseases (39), hypertension (40), cardiovascular disorders (41, 42), and even the risk of mortality from all causes in men (43). In addition, moderate-intensity aerobic exercise can lead to a 2-4% absolute reduction in liver fat degeneration in adults with MAFLD (44). We considered that heavy physical labor may affect the risk of NAFLD through some underlying mechanisms. To verify this hypothesis, we categorized participants into groups based on work intensity: mental labor, light manual work, moderate manual work, and heavy manual work. We observed that the risk of NAFLD increased with greater work intensity. This indirectly suggests that work-related exercise does not provide equivalent health benefits compared to free-time physical activity (45). When adjusting for age, sex, race, education level, PIR, TC, TG, SBP, DBP, ALT, smoking, and diabetes, only heavy physical labor was notably linked to NAFLD (OR: 1.39, 95%CI: 1.06-1.81). One possible explanation for this phenomenon is that while appropriate physical activity can improve mood, excessive physical activity may not only fail to enhance mood, but can also lead to mood deterioration (46, 47). This deterioration can manifest as sleep disturbances, weight and appetite loss, fatigue, irritability, emotional instability, and even depression. Sleep disturbances (48), emotional instability, and depression (49, 50) have been established as risk factors for NAFLD. Additionally, heavy manual work may contribute to a lack of leisure exercise, which is vital for promoting health and enhancing physical fitness and work capacity (9, 51). Furthermore, heavy physical labor can increase the cardiovascular burden on male construction workers, significantly raising the incidence of NAFLD (52). The specific mechanisms still need to be confirmed through extensive research.

Our subgroup analysis showed that the relationship between working hours and NAFLD was more significant in women and high-income individuals. The distribution of body fat in women changes with hormonal cycles compared to men (53), which results in a relatively higher probability of obesity in women (54). This physiological difference may contribute to an increased risk of NAFLD in women. The connection between working type and NAFLD was more pronounced in men and the elderly. This may be due to the higher proportion of male workers in heavy physical labor, as age increases, the consequences of prolonged heavy physical work become more apparent (55, 56).

Long working hours and heavy physical labor are associated with the development of various diseases. Research indicates that for every 10-hour increase in weekly working hours, the likelihood of sleep deprivation increases by approximately 50%, and the risk of difficulty falling asleep also rises significantly (57). Long working hours can lead to an increased probability of obesity (6), which may be caused by the impact of prolonged work on metabolic response mechanisms (58). Existing evidence indicates that long working hours contribute to coronary heart disease, stroke, hypertension, depression, and other chronic diseases (59). The cumulative effect of these factors greatly increases the risk of developing NAFLD. Therefore, it is crucial to properly arrange working hours and avoid intense physical labor.

However, certain limitations exist. Firstly, because this study employed a cross-sectional design, the observed connection does not inherently indicate a cause-and-effect connection. Secondly, we employed a non-direct method (the HSI assessment tool) instead of imaging studies or pathological assessments to determine NAFLD. However, the HSI has been thoroughly confirmed and can be used to predict the existence and severity of NAFLD in numerous extensive studies (29). Thirdly, since our research only consisted of the demographic in the United States, our findings ought to be corroborated in various racial groups. Ultimately, even though we tried to adjust for several potential risk variables, there could still be unaccounted confounding factors or biases beyond our control, such as participants’ specific sleep habits, daily exercise time, and dietary habits. Finally, we investigated the relationship between working hours and NAFLD; however, the associations of other steatotic liver disease categories, such as MASLD and MetALD, remain unexplored. We hope to further explore the connections between working hours, working type and other liver diseases in future research. Regardless of these limitations, this study offers important strengths, such as a large representative sample from across the nation, standardized exceptional clinical and laboratory data gathering, and thorough details on different confounding influences.

The findings of this cross-sectional study suggest significant connection between long working hours and heavy physical labor with NAFLD. Furthermore, our research indicates that these factors heighten the likelihood of developing NAFLD, which may offer insights into innovative interventions and approaches for reducing the risk of NAFLD.