Introduction

University students are subjected to multiple pressures such as from family, academics, employment, and so on. These stressors add to students’ mental health burden, which can lead to more serious psychological problems in this group. A Norwegian survey study used the Hopkins Symptom Checklist to assess psychological symptoms among university students. The results showed that during the COVID-19 pandemic, psychological symptoms among university students (41% men, 62% women) and the risk of suicide increased dramatically [1]. A survey of American university students reported a significant increase in depressive symptoms among women, and physical activity did not seem to prevent the worsening of these symptoms. Another study reported that German university students had a 31% prevalence of depression; this trend continued to increase during the COVID-19 pandemic [2]. A survey of Ugandan university students showed that the prevalence of depression, anxiety, and psychological symptoms was 80.7%, 98.4%, and 77.9% respectively [3]. A university survey in mainland China found that the prevalence of depression and anxiety was significantly higher among university students during the COVID-19 pandemic, negatively impacting psychological symptoms [4]. Other studies have reported a prevalence of psychological symptoms among Chinese university students of 16.22%, with this prevalence among male students (14.75%) being lower than that in female students (17.35%) [5]. It has also been demonstrated that university students with higher levels of physical activity have a lower risk of developing psychological symptoms compared with their counterparts who have low physical activity levels, and there is a negative correlation between the two [6]. Thus, the issue of mental health among university students has received extensive attention from scholars as this continues to affect the academic and personal lives of students. Research shows that many factors affect the mental health of university students, including physical activity, exercise habits, dietary behaviors, and sleep, among others [7, 8]. Relevant research is needed on those factors influencing university students’ psychological symptoms to understand the effect among the factors influencing mental health in this population.

Low levels of physical activity have a negative impact on the physical and psychological health of university students, which have become among the leading problems in this group. The World Health Organization and many countries have issued their own physical activity guidelines, most of which recommend that adolescents and young adults perform at least 60 min of moderate-to-vigorous-intensity physical activity (MVPA) every day, but few groups meet this standard [9, 10]. A Korean survey found a small difference in total physical activity between adolescents observed before and during the COVID-19 pandemic (≥ 60-minute MVPA ≥ 4 days/week: 20.8% vs. 19.9%) [11]. Overall, governments and schools should encourage adolescents to be physically active and improve physical activity via monitoring, publicity, and the promotion of healthy lifestyles. The study stated that there is a clear need to strengthen monitoring of physical activity and health and to improve the implementation and evaluation of policies, particularly in the aftermath of the COVID-19 pandemic when many indicators and associated problems are likely to have worsened [12]. The results of surveys among adolescents in countries such as Portugal and Lebanon also reveal a widespread lack of physical activity [13, 14]. The China 2022 Report Card also showed that Chinese adolescents have low levels of physical activity, with most levels below the recommended guidelines, although total physical activity ratings have improved since recent revisions. The prevalence of sedentary behavior in this population is also high. Therefore, interventions and policy measures should be adopted to encourage adolescents to increase physical activity levels and reduce sedentary behavior [15]. The evidence shows that the problem of physical inactivity is particularly pronounced in adolescents, and the occurrence of physical inactivity has a direct impact on the development of psychological symptoms. A national survey of Norwegian university students showed a negative dose–response correlation between physical activity and the development of psychological symptoms, with a nearly three-fold increase in depression among less physically active women compared with those who exercised almost every day. The study also recommended that active participation in physical activity be promoted to facilitate better mental health development [16]. Another study confirmed that there is a positive association between sedentary behavior and the development of mental disorders, as well as between physical activity and mental health, and that effective exercise interventions have a positive impact on the mental health of university students [17].

The many health benefits of soy product consumption have been confirmed in research studies. A mixed-effects model of 10,901 older adults in Zhejiang, China showed that the corrected odds ratio (OR) and 95% confidence interval (CI) for depression in those with a high frequency of soy product consumption (4–7 days per week) was 0.46 (0.39–0.54), compared with those who did not consume soy products frequently. This finding suggests that frequent soy product consumption is associated with a lower risk of depression [18]. A study of adolescents also reported an association between soy product consumption and adolescent physical health and that consuming soy products affects adolescent development [19]. Other studies have confirmed the association between soy product consumption and adolescent psychological symptoms [20] and mental health [21]. Soy isoflavones may also have beneficial behavioral effects. Specific gut microbes may convert soy isoflavones into more beneficial forms, such as equol derivatives, which may improve neurobehavioral outcomes [22]. There is a close association between soy product consumption and the occurrence of cardiovascular and cerebrovascular diseases, with higher soy product consumption having a positive role in their prevention [23]. In a prospective cohort study among a population with no history of cardiovascular disease, participants who consumed soy ≥ 4 days per week had significantly lower mortality from acute myocardial infarction than those who never or rarely consumed soy [24]. Additionally, a study showed an association between greater soy product consumption and a lower risk or prevalence of cognitive impairment [25]. However, the results are not consistent with studies in Chinese older adults showing a negative association between high consumption of soy products such as tofu and cognitive function. The effect of tofu on cognitive function remains controversial as it has not been found in Western populations [26].

A synthesis of previous research reveals that many studies have been conducted on MVPA and psychological symptoms in university students, with an overall consensus that there is a negative correlation between MVPA and the occurrence of psychological symptoms [27, 28]. Further research has been conducted on soy product consumption and psychological symptoms in adolescents, with a reported positive influence on the development of mental health [29]. However, there are some limitations in these previous studies. On the one hand, most studies have examined MVPA and soy product consumption in relation to psychological symptoms, but few studies have examined the interaction of MVPA, soy product consumption, and psychological symptoms. On the other hand, covariates were rarely controlled during previous studies. Many factors affect adolescents’ psychological symptoms. Thus, when analyzing the effect of a certain factor on psychological symptoms in university students, the effect of additional related variables should be controlled. Previous studies have mainly focused on adults and elderly people, with fewer studies conducted among younger people, especially university students. To clarify the underlying reasons for psychological problems among university students, targeted research is needed to help promote the development of mental health in this population. To this end, we investigated MVPA, soy product consumption, psychological symptoms, and related variables among 7267 university students in China. In this study, we aimed to analyze the association among MVPA, soy product consumption, and psychological symptoms to provide a reference for the promotion and development of psychological symptoms among university students. This study also informs and contributes to the promotion of mental health and effective interventions at the university.

Materials and methods

Participants

In this study, participants were sampled using stratified whole cluster sampling, and the sampling process was divided into three stages. First, based on the geographic regions of China and taking into account regional differences, Dalian, Chizhou, Suzhou, Urumqi, and Shenzhen were selected as study cities. Second, two universities were selected in each city for the survey. Third, in each school, five classes were randomly selected in whole clusters from the first to the fourth year of university. Eligible university students in each class comprised the survey respondents in this study. The inclusion criteria were as follows: Chinese university students who were enrolled in school, age 19–22 years, no serious physical or psychological illnesses, and volunteered to participate in this study. Finally, a total of 8079 university students from 200 classes were enrolled. At the end of the survey, questionnaires with missing relevant key demographic information, invalid or ambiguous questionnaires, questionnaires with a response rate of less than 90 per cent, and data with calculated values outside the normal range were excluded. A total of 812 questionnaires were excluded, and 7267 valid questionnaires were retained, 3145 from male students and 4122 from female students. Written informed consent was obtained from the students before the survey. This study was approved by the Jiangxi Science and Technology Normal University (22,154,564). The specific sampling process of university students in this study is shown in Fig. 1.

Fig. 1
figure 1

Selection process of university students in China

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Soy product consumption

In this study, participants’ soy product consumption was surveyed using a questionnaire. The survey mainly queried participants’ soy product consumption in the past 7 days. In this study, soy products included tofu, dried beans, bean paste, soy snacks, soy milk, and other soy products. Participants were categorized into three groups according to soy product consumption in the past 7 days: ≤2 times, 3–5 times, and ≥ 5 times, respectively [30].

Physical activity

Participants’ physical activity was assessed using the International Physical Activity Scale Short Form [30]. The scale has been widely adopted in countries worldwide, including in China, and in studies among university students [31, 32]. The scale comprises seven main entries and focuses on participants’ engagement in low-, moderate-, and high-intensity physical activity as well as sedentary behaviors over the past 7 days. The survey queries the frequency and duration of each included physical activity, including walking, moderate-intensity outdoor chores, high-intensity outdoor chores, and bicycling, among others. The duration of physical activity over the past 7 days was calculated based on participants’ responses. Walking was defined as low-intensity physical activity and other physical activity behaviors were defined as MVPA for the purpose of this study. According to national physical activity recommendations, adults should perform at least 60 min of MVPA per day [33,34,35]. In this study, MVPA was categorized into three groups according to age and sex: <30 min/d, 30–60 min/d, and > 60 min/d.

Psychological symptoms

Psychological symptoms among participants in this study were investigated using the multidimensional sub-health questionnaire of adolescents [36]. The scale has been widely used in surveys and research on adolescent mental health in China, with good reliability and validity [37]. The scale comprises 15 items, with questions such as “I often feel nervous,” “I always feel that people are against me,” and “I am often annoyed by small things.” The questionnaire is mainly used to investigate respondents’ mental health status in the previous 3 months. Response options are categorized as (1) lasted more than 3 months, (2) lasted more than 2 months, (3) lasted more than 1 month, (4) lasted more than 2 weeks, (5) lasted more than 1 week, and (6) none or lasted less than 1 week. Scores ranged from 1 to 6 points, with 1 point for options 1–3 and 0 points for options 4–6. Each response was single choice. The total scale score was 0–15 points. Participants with a total questionnaire score of ≥ 7 were considered to have the presence of psychological problems. The three dimensions included emotional symptoms, behavioral symptoms, and social adaptation difficulties, with a score of ≥ 4, ≥1, or ≥ 2, respectively, indicating the presence of psychological problems in that dimension.

Covariates

The covariates in this study comprised basic demographic information such as age, urban/rural residence, and whether the respondent was an only child. Basic demographic information included the participants’ region, school, major, and class. Covariates also included father’s education, mother’s education, socioeconomic status (SES), body mass index (BMI), screen time, and sleep quality. Father’s education and mother’s education were categorized into three levels: elementary school and below, middle or high school, and college or above. SES was based on a questionnaire used in several studies and was classified according to parental occupation, family income, and other socioeconomic characteristics of respondents [38]. SES scores were divided into three classes according to percentile: low (< P30), medium (P30–70), and high (> P70). BMI was calculated as weight (kg) divided by height (m2), which were measured using the instruments and methods required in the China National Survey of Students’ Physical Fitness and Health. Height was accurate to the nearest 0.1 cm and weight to the nearest 0.1 kg. BMI was divided into four groups: underweight, normal weight, overweight, and obese, according to standards of the World Health Organization. Video screen time was queried to assess the time spent using computers, cell phones, and tablets in the previous 7 days. Based on the results of previous research, screen time was divided into ≤ 2 h/d and > 2 h/d. Sleep quality was investigated using the Pittsburgh Sleep Quality Index (PSQI), which consists of seven items and is scored from 0 to 21. Participants were categorized into three groups of sleep quality according to PSQI score: good (≤ 5 points), moderate (6–7 points), and poor (> 7 points) [28].

Statistical analyses

The data in this study were analyzed after stratification by sex. We analyzed the basic information, covariates, and psychological symptoms of university students by sex. We then conducted a comparison of the psychological symptoms and detection rate of each dimension among Chinese university students according to different frequencies of MVPA and soy product consumption, using the chi-square test. The associations between frequencies of MVPA and soy product consumption and psychological symptoms among university students were assessed using binary logistic regression analysis. The presence of psychological symptoms was analyzed as the dependent variable and MVPA and soy product consumption as the independent variables. We conducted analysis to determine the interaction of MVPA and soy product consumption with psychological symptoms. The statistical analysis in this study was conducted using a generalized linear model. Age, urban/rural residence, being an only child, father’s education, mother’s education, SES, BMI, screen time, and sleep quality were controlled as covariates. Model 1 was the crude model. Model 2 was adjusted for age, urban/rural residence, whether the respondent was an only child, father’s education level, mother’s education level, and SES, based on Model 1. Model 3 was adjusted for BMI, screen time, and sleep quality, based on Model 2.

The statistical analysis in this study was carried out using IBM SPSS 25.0 (IBM Corp., Armonk, NY, USA). Images were created using Graph Pad Prism 8.0 (GraphPad Software, San Diego, CA, USA). Differences were considered statistically significant with P < 0.05.

Results

This study was conducted among 7267 university students in China, among whom 3145 (43.28%) were male students and 4122 (56.72%) were female students. Participants’ mean age was 20.16 ± 1.03 years. The proportion of university students with MVPA < 30 min/d, 30–60 min/d, and > 60 min/d was 76.1%, 19.3%, and 4.6%, respectively. The proportion of male students with MVPA > 60 min/d was higher (7.9%) than that of female students (2.1%). The proportion of respondents with soy product consumption ≤ 2 times/wk, 3–5 times/wk, and ≥ 5 times/wk was 25.8%, 42.4%, and 31.7%, respectively. The detection rate of psychological symptoms among university students was 17.9%, with female students (18.9%) having a higher rate than male students (16.6%). The rates of emotional symptoms, behavioral symptoms, and social adaptation difficulties were 19.2%, 19.3%, and 15.9%, respectively. The results by sex are shown in Table 1.

Table 1 Basic information, covariates, MVPA, soy product consumption, and psychological symptoms in Chinese university students
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The study results showed statistically significant differences in the detection rates of emotional symptoms, behavioral symptoms, social adaptation difficulties, and psychological symptoms among university students with different levels of MVPA (χ2 values 56.659, 18.403, 13.847, 54.079; P < 0.01). Significant differences were also found according to the consumption of soy products for emotional symptoms, behavioral symptoms, social adaptation difficulties, and psychological symptoms (χ2 values 56.661, 65.493, 53.454, 65.551, P < 0.001). A comparison of psychological symptoms and dimensions by sex is shown in Table 2.

Table 2 Comparison of psychological symptoms among university students according to frequency of MVPA and soybean product consumption
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The results of this study showed that compared with university students who had MVPA > 60 min/d as the reference group, after adjusting for covariates, university students with MVPA < 30 min/d had a significantly higher risk of psychosomatic symptoms (OR = 1.88, 95% CI: 1.31–2.69, P < 0.01). Taking university students who consumed soy products ≥ 5 times/wk as the reference group, those with soy product consumption 3–5 times/wk (OR = 1.21, 95% CI: 1.04–1.41) and ≤ 2 times/wk (OR = 1.68, 95% CI: 1.43–1.98) had a significantly higher risk of developing psychological symptoms (P < 0.05 or 0.001). The results of the analysis by sex are shown in Table 3.

Table 3 Binary logistic regression analysis of psychological symptoms among university students according to MVPA and soy product consumption
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Our results showed that with Chinese university students who had MVPA < 30 min/d and soy product consumption ≤ 2 times/week as the reference group, the lowest risk of psychological symptoms was among university students with MVPA > 60 min/d and soy product consumption ≥ 6 times/week (OR = 0.198, 95% CI: 0.100–0.393, P < 0.001); this group was followed by students with MVPA > 60 min/d and soy product consumption 3–5 times/week (OR = 0.221, 95% CI: 0.102–0.479, P < 0.001). The results of the analysis according to sex are shown in Table 4.

Table 4 Analysis of the association among MVPA, soy product consumption, and psychological symptoms in Chinese university students
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Discussion

As a growing concern, university students’ mental health has been extensively studied. The results of the present study showed that the detection rate of psychological symptoms among Chinese university students was 17.9%, with more female students (18.9%) affected than male students (16.6%). Our result is low compared with those of related studies. The reason for this may be because of differences in the groups investigated, including differences in culture, geographic region, and age. Additionally, the reason for the low detection rate of psychological symptoms among university students may be related to the choice of cities in this study. The sites in the present study were universities in small and medium-sized cities of China. The academic pressure among university students in these cities is somewhat lower than that of students in first-tier cities. The former group also may have more opportunities to participate in various cultural and sports activities, which may be an important reason for our finding of fewer psychological symptoms among university students. Our findings are also lower than a survey of university students in the U.S. 24.9% of university students have various types of psychological problems and this percentage is increasing, negatively affecting the health of U.S. university students [39]. It has been shown that during the New Crown Pneumonia pandemic, because of the strict home quarantine policy for the epidemic, which led to a reduction in outdoor activities and offline socialising time for university students, as many as 25% of Chinese university students experienced mild and above psychological symptoms, which was higher than the results of this study [40]. This is because during the COVID-19 epidemic, physical and social activities were restricted and the outbreak likely created panic among students. It has also been shown that during the COVID-19 period, 41.1% of Chinese university students had anxiety symptoms that called for timely intervention [41]. We found that the proportion of Chinese university students with MVPA < 30 min/d, 30–60 min/d, and > 60 min/d was 76.1%, 19.3%, and 4.6%. Also, the proportion of soy product consumption ≤ 2 times/wk was as high as 25.8%, a relatively high rate.

In terms of univariate analysis, the results of this study showed that university students with MVPA < 30 min/d had a higher risk of psychological symptoms than those with MVPA > 60 min/d (OR = 1.88). This result is consistent with the findings of several studies [5, 42]. A survey of university students in the Unites States showed that those with lower levels of physical activity had a higher risk of developing psychological symptoms; conversely, those with higher levels of physical activity had a lower risk of developing psychological symptoms [43]. Another study confirmed that participants with lower physical activity levels had a greater risk of depression and anxiety [44]. University students with higher levels of physical activity tend to participate in more physical exercise, during which they can release stress and pressure. Effective socialization can also take place during physical exercise, which can help alleviate psychological problems like anxiety. Finally, sufficient physical activity can promote the increase of cerebral blood flow and improve the secretion of brain hormones, which has a positive effect on maintaining a good psychological state. In terms of soy product consumption, our study results showed that compared with university students who consumed soy products ≥ 5 times/wk, those with soy product consumption ≤ 2 times/wk had a higher risk of psychological symptoms (OR = 1.68). Another study confirmed that soy product consumption positively affects mental health, mainly through the action of soy isoflavones, which act on brain hormone secretion, promote the development of cognitive functions, and favor mental health [45].

In this study, associations analysis of MVPA and soy product consumption with psychological symptoms showed that compared with university students who had MVPA < 30 min/d and soy product consumption ≤ 2 times/week, those with MVPA > 60 min/d and soy product consumption ≥ 6 times/week had the lowest risk of psychological symptoms (OR = 0.198), followed by those with MVPA > 60 min/d and soy product consumption 3–5 times/week (OR = 0.221). This suggests that an increase in soy product consumption may have a positive effect on reducing the incidence of psychological symptoms in university students, as well as ensuring a certain amount of MVPA time. Currently, there are few similar studies on this interaction effect. The results of the present study suggest that university students should actively increase their physical activity level and maintain a reasonable and healthy diet, with a focus on consuming soy products to promote better psychological health. A study among university students demonstrated that soy isoflavones that are produced with soy product consumption had a positive effect on cardiovascular and mental health [46, 47]. Moreover, phytoestrogens are naturally occurring substances with estrogenic or anti-estrogenic activity that are found primarily in legumes, especially soybeans and many soy products. Because of their action, phytoestrogens are used as replacement therapy for menopausal disorders and benign prostate hyperplasia, with positive effects on health [48]. In addition, emerging microbiome research confirms that there is an interconnection between the gut microbiome and the brain, and that there is a bidirectional communication system within the gut-microbiome-brain axis, and that soya intake and physical activity positively affect the gut microbiome, thereby effectively promoting healthy brain development and facilitating the regulation and promotion of mental health [49].

This study has several strengths as well as some limitations. First, among the few previous studies on the relationship between soybean product consumption and psychological symptoms, we further analyzed the relationship of MVPA and soybean product consumption interactions with psychological symptoms to provide a reference for the intervention and prevention of psychological symptoms among Chinese university students. Second, the sample size in this study was larger than that of past studies, and we included a greater number of covariates, to better analyze the interaction of MVPA and soy product consumption with psychological symptoms. Also, our study results are representative, to a certain extent. Regarding limitations, this was a cross-sectional study, which can only identify correlations between variables but not causal associations. Second, the measures of physical activity, soy product intake, and psychological symptoms in this study were qualitative and may have been greatly influenced by underlying factors. For example, important factors that determine psychological symptoms among Chinese college students, including academic and employment stress, have not been assessed. These potential confounding factors may lead to uncertainty in the results and should be investigated in future studies. Finally, the assessments in this study were mainly subjective and recall questionnaires, which are not free from some deviation between them and the objective reality, and more objective assessment tools should be used in the future to assess physical activity, soya product consumption, and psychological symptoms.

Conclusion

This study demonstrated the association of MVPA and soy product consumption with psychological symptoms among Chinese university students. Students with MVPA > 60 min/d and soy product consumption ≥ 6 times/week had the lowest risk of developing psychological symptoms, followed by those with MVPA > 60 min/d and soy product consumption 3–5 times/week. This suggests that it is important for university students to maintain a certain level of MVPA for good psychological health. A certain amount of soy products should also be consumed to promote mental health in university students. The results of our study can provide a reference for mental health interventions among university students in the future.