Associations of body composition with physical activity, nutritional intake status, and chronotype among female university students in Japan

Maintaining muscle mass and strength in middle-aged individuals, minimizing the decline in older age, and increasing the peak values achieved in adolescence and young adulthood have been noted to delay or prevent future sarcopenia [1, 2]. Furthermore, although exercise interventions in frail older adults play an important role in improving muscle strength and physical function, their effect on increasing muscle mass is likely limited [3]. Therefore, increasing the peak muscle mass, particularly in early adulthood, may be important for the prevention of sarcopenia. Consequently, identifying the factors associated with muscle mass acquisition in young adults is of great importance.

Among young adults, university students undergo significant lifestyle changes that are thought to affect their weight [4] and body composition [5]. Further, individuals have a time orientation, known as the chronotype, and the sharp maximum of lateness in the chronotype (i.e., becoming an evening type (ET)) occurs around the age of 20 years [6]. Recent studies have also reported an association between chronotype and body physique in university students. For instance, a study of 384 students (150 males and 234 females) from the United States found a significant increase in body mass index (BMI) with age and lateness of chronotype and later chronotypes gaining weight faster than early chronotypes [7]. Another study with a larger sample size also concluded that university students with ET were more likely to be affected by overweight/obesity [8].

In addition to BMI, several recent studies have reported associations of chronotype with body composition and lifestyle behaviors. A study of 387 female students in Saudi Arabia found no significant main effects of chronotype on body composition, nutrient intake status, and energy expenditure [9]. However, the study participants had a higher mean body fat percentage (%BF) of 36.6% and a lower mean muscle mass (skeletal muscle index; SMI of 5.7 kg/m²) [9], which is the almost same as the cut-off value for the diagnosis of sarcopenia in older people [2, 10]. Therefore, this study examined a student population with extremely low muscle mass and high body fat. In contrast, a study of 327 German students (58% female) found that a later chronotype was associated with higher visceral fat mass and lower muscle mass [11]. The %BF of the participants in this study was 16.2% in males and 26.5% in females, calculated from BMI and fat mass index, which falls within the normal range. Although this study suggested that this association may be related to physical activity, the nutritional status of the participants was unknown [11]. Therefore, research is needed to determine the relationship between lifestyle behaviors, including nutritional intake status, with chronotype and body composition in the general student population.

As described above, identifying the factors associated with high muscle mass in university students is an important consideration for preventing future sarcopenia. Muscle mass in the diagnosis of sarcopenia is assessed by an index calculated from the sum of the appendicular muscle mass (AMM) [2, 10], commonly referred to as SMI. It is worth noting that individuals of Asian ethnicity may have lower fat-free mass than those of other ethnicities [12]. In particular, a high proportion of young Japanese women are underweight [13, 14] and have body composition concerns, such as normal-weight obesity [15, 16], which are associated with low fat-free mass. Therefore, studies on SMI in Japanese female university students have been reported in recent years from the perspective of future prevention of low SMI [17–20]. Previous studies showed that nutritional status such as low total energy intake (TEI) and/or protein intake (PI) has been associated with SMI in female university students [17, 20]. Low current physical activity level (PAL) was also found to be associated with low SMI, regardless of previous exercise participation [19]. Further, a previous study has indicated that university students in Japan have a shorter sleep duration compared to those in other countries [21]. This may be due to socially required early wake-up and bedtime schedules, particularly among young people who have a later chronotype, which can lead to health problems caused by the mismatch between biological and social time [22]. Therefore, it is important to investigate the factors associated with SMI among Japanese female students, considering chronotype aspects. The present study examined the relationship between lifestyle factors (PAL and dietary intake) and chronotype in relation to body composition, including the SMI in Japanese female university students.

The measurements for each group are presented in Table 1 (right). Participant characteristics including age, height, weight, and BMI did not differ significantly between the groups. Regarding body composition, the %BF of ETs was significantly lower (27.3 ± 5.5%) than that of non-ETs (29.9 ± 4.8%). Furthermore, the AMM and SMI of non-ETs (17.0 ± 2.5 kg and 6.8 ± 0.9 kg/m²) were significantly higher than those of ETs (15.8 ± 2.1 kg and 6.4 ± 0.6 kg/m²). However, the differences were small (d = 0.21 and 0.22). These results indicate that ET individuals had higher body fat and lower muscle mass than those with higher MEQ scores, although no differences in body physique were found, and the measured differences were negligible.

With respect to physical activity, the PAL of non-ETs (1.76 ± 0.25) was significantly higher than that of ETs (1.68 ± 0.26), but the effect size was trivial (d = 0.14). This finding indicates that ET individuals are physically less active than those with higher MEQ scores, but the difference is slight.

The daily dietary intake status showed that although the TEI of non-ETs (1858 ± 490 kcal/day) tended to be significantly higher than that of ETs (1708 ± 582 kcal/day), the TEI/TEE did not differ significantly between the groups. Moreover, the FI and CI were not significantly different between the groups. Furthermore, although the PI of non-ETs (62.1 ± 20.5 g/day) was significantly higher than that of ETs (54.9 ± 20.6 g/day), the effect size was trivial (d = 0.16). Thus, the results indicate that although ET individuals had lower energy and protein intakes, the differences were slight.

The distribution of chronotypes was slightly skewed toward lower MEQ scores (Fig. 2). This finding suggests that many of the students in this study had lower MEQ scores (i.e., ETs), as previous research has shown that peak chronotype lateness occurs around the age of 20 [6]. The mean MEQ score for non-ETs was 58.3, which is “neither type” close to MT [28]. The mean SMI for all participants in this study was 6.55 kg/m², which was slightly higher than the mean of 6.4 ± 0.5 kg/m² found in a study of 426 female Japanese university students using the same body composition device [17]. Furthermore, although the correlation was weak, SMI was significantly positively correlated with the MEQ, and the SMI of ETs was significantly lower than that of non-ETs. On the other hand, the %BF results were the opposite, i.e., %BF was significantly negatively correlated with MEQ and was higher in ETs than in non-ETs. Therefore, ET individuals have an imbalance in body composition resulting in less muscle mass and more body fat but show no differences in body mass.

Differences in body composition are likely related to lifestyle behaviors. In previous studies, SMI was significantly associated with TEI in female university students [17] or students with high PAL [20]. However, in the present study, %BF and SMI did not significantly correlate with TEI, whereas AMM was significantly associated with TEI, although the correlation was weak (r = 0.13). The mean TEI in the previous study was low (1562 ± 489 kcal), suggesting that many participants could not consume sufficient nutrients for protein synthesis in accordance with their physical activity and that TEI was related to SMI [17]. The TEI in the present study was 1781 kcal for all participants, and the TEI was slightly lower than the TEE (TEI/TEE was approximately 0.9). Such a difference in energy intake may explain the discrepancy of results from this and previous studies on the association between TEI and SMI.

Regarding dietary intake and chronotype, recent systematic reviews have found that both MTs and ETs had similar energy intakes [30, 31]. In the present study, the TEI of ETs tended to be significantly lower than that of non-ETs, but the effect size was small, and no significant difference in TEI/TEE was found. These findings suggest that the difference in TEI between chronotypes is not large. However, the distribution of nutrients in the three meals may differ between chronotypes, even if the difference in the TEI is small. An ET was more likely than an MT to skip breakfast or consume fewer calories during breakfast while consuming more calories during dinner and snacks in the afternoon or at night [30–32]. Additionally, irregular meal patterns were also found to be associated with chronotype among Japanese university students [33]. Therefore, they may have different daily intake patterns despite having the same daily energy intake. Furthermore, the frequency of breakfast intake among Japanese female university students had shown a significant positive correlation with SMI [17]. In another study, ensuring adequate protein intake for each of the three meals as well as total daily protein intake was important for high muscle mass among Japanese university students [34]. These findings suggest that a more detailed analysis of the dietary intake is required in future studies.

According to a previous review, the relationship between physical activity and chronotype among university students is inconsistent [35]. In the present study, multiple regression analyses showed that both AMM and SMI were significantly positively associated with MEQ and PAL, and %BF was significantly negatively associated with MEQ and PAL. This suggests that lateness of chronotype and lower physical activity are associated with lower muscle mass and higher body fat. However, the PALs of both groups in the present study were within the normal range of the DRIs-J classification, and the effect size was trivial. A previous study suggested that although weekday physical activity is unlikely to show differences by chronotype, weekend physical activity (i.e., free from academic/social obligations) of ETs is lower than that of MTs [36]. Thus, although only weekday PAL was assessed in this study, the difference in PAL could have been be greater if the total weekly PAL was assessed.

A similar pattern was observed for nutrient intake. The PI was significantly lower in non-ETs in this study, which was not surprising, as the TEI was also significantly lower tend (i.e., lower total food intake). However, no significant differences were observed between the groups in FI and CI, suggesting that these intakes may have been relatively higher in ETs. In previous studies, the intake of lipids, carbohydrates, sodium, and polyunsaturated fatty acids [32] or sucrose and saturated fatty acids [37] was higher in ETs than in MTs, and a later chronotype was associated with a lower overall dietary quality index [38]. Furthermore, this difference was more pronounced on weekends [37]. Therefore, examining not only daily PAL and dietary intake but also their variation over the week would provide more evidence of the relationship between lifestyle and body composition, considering chronotype.

Previous studies have suggested that the association between BMI and chronotype may vary based on factors such as location, age, and gender [9], as well as among different racial/ethnic groups [7]. However, our study with Japanese female students found that body composition was related to chronotype and physical activity, similar to a previous study with German students (which did not investigate nutrient intakes) [11]. It is therefore possible that this relationship may be observed across racial/ethnic groups. However, it is important to note that studies have reported variations in chronotype among different racial/ethnic groups, with a higher prevalence of MT in blacks compared to whites [39]. As the chronotype of the present study population skewed toward the later side, different findings might have been observed in a population with a greater prevalence of MT. In addition, although this study was conducted during the almost autumn season in Japan (late September to early December), chronotype variation has been reported in various seasons [40]. Seasonal differences should therefore be considered in future research.

This study had several limitations. First, because PAL, nutrient intake, and chronotype were assessed using a questionnaire, future research will need to assess these factors objectively. In particular, chronotype must be assessed using clock gene analysis, as a previous study suggested that the intra-week variation in physical activity differed between clock genes and questionnaire-based assessments of chronotype [36]. Secondly, social jetlag (SJL), which refers to the difference in the midpoint of sleep between work/school days and free days, is a topic of interest in chronobiology and health research. In the Japanese population, a late chronotype has been found to be associated with an increase in SJL, and females may be at higher risk of developing adverse health symptoms because they have a higher SJL than males [22]. A previous study found that having electronic devices in the bedroom and using the Internet before going to bed are associated with SJL [41]. Although the participants in the present study attended university during the day, their nocturnal activity remains unknown. As a recent report has shown that an increase in SJL is linked to less physical activity in Japanese female students [42], future research should investigate sleep status and night-time activities as well as chronotype. Finally, as this was a cross-sectional study conducted at three universities with a limited number of participants, future research should examine changes in body composition in relation to changes in chronotypes and lifestyle behaviors. Although the sample size in this study was similar to that in recent studies on the association between chronotype and body composition in university students [7, 9, 11], further research is required to investigate this association at more universities with a larger number of participants.

This study examined the relationships between physical activity, dietary intake, chronotype, and body composition in Japanese female university students. ET individuals had higher body fat and lower AMM and SMI than those with high MEQ scores, but no differences in body mass were found. Although ET individuals had lower PAL, TEI, and PI than those with high MEQ scores, the differences were small. However, multiple regression analyses showed that %BF was negatively associated with MEQ and PAL, whereas AMM and SMI were positively associated with MEQ and PAL. These results suggest that lateness of chronotype and low physical activity are associated with higher body fat and lower muscle mass in female university students. Previous reports have shown that people around the age of 20, such as university students, have peak chronotype lateness [6]. Further, university students undergo lifestyle changes compared to their high school years, which may lead to changes in body weight [4] or composition [5]. Therefore, chronotype-specific interventions (especially for ET individuals) are necessary among young females to help maintain an appropriate body composition.