The association between BMI and health-related physical fitness among Chinese college students: a cross-sectional study

College is a transitional period from adolescence to adulthood, and is also a crucial period for the development of healthy lifestyles and the formation of healthy behaviors [1]. In recent years, there was a significant decline in physical activity among college students [2]. As reported by Hallal et al. [2], 31.1% of adults aged 15 years or older are not physically active all over the world. One study performed by Wang et al. [3] in Chinese adolescents found that the prevalence of physical activity time less than 1 hour per day was high in students aged 9–22 years. The highest prevalence of physical activity time less than 1 hour per day was 82.5% for 18 years old male students and 89.8% for 21 years old females, respectively. Maintaining physical activity has an important public health implication for the prevention of chronic diseases [4]. Physical activity has been found to be negatively associated with multiple diseases, such as cancer [5], obesity [6], diabetes [7], coronary artery disease [8] and depression [9] etc. The decrease in physical activity could lead to decreased physical fitness. Health-related physical fitness is also influenced by many other factors, such as total body fat and socioeconomic status. There are a few studies that discussed the association between body mass index (BMI) and several components of physical fitness in children [10] and in adolescents [11]. Studies on college students are relatively scarce in China. Existing studies reporting on the levels of physical fitness among college students used relatively few fitness tests as a reflection of physical fitness, which could not comprehensively evaluate the levels of physical fitness. Lu et al. [12] analyzed the 50-m test as an index of physical fitness. 50-m run test just reflected the speed and explosive strength of students. Hao et al. [13] used BMI, vital capacity index, sidestep test and standing long jump to investigate the gender differences in physical fitness. The purpose of this study is to analyze the levels of different physical fitness components in medical school freshmen, and to further evaluate the association between BMI and health-related physical fitness.

The data were from a national survey on the physical fitness conducted among medical college freshmen from the years 2014 to 2016 in Anhui province, China. A sample aged 15–25 years was included to complete the physical fitness tests. All the participants provided written informed consent. For participants under the age of 16, both participants and their parents (or guardians) gave their informed consent. This study was approved by the Ethics Committee of Bengbu medical college (Ref No: 2018–050).

Physical fitness among college students is an important task of sport in schools, and is also one of the components of school physical education. Physical fitness aims to promote students to actively participate in physical exercise, and develop the habit of regular physical exercise, and improve self-health ability and physical health level.

Physical fitness has shown to be an important issue from a public health perspective [15]. The level of physical fitness was associated with health-related outcomes, including obesity, cardiovascular disease, skeletal health and mental health. With the development of society, the popularity and application of electronic products are becoming more and more widespread in the information era. There was a significant decline in physical activity among college students. A significant increasing trend of overweight and obesity was observed in the current cross-sectional study. The prevalence of overweight increased from 15.7 to 18.3% for males and from 7.2 to 12.3% for females during the years 2014 to 2016. For obesity in the same period, the prevalence of obesity increased from 5.2 to 6.7% for males and from 1.6 to 2.8% for females. Based on the data in the year 2014 from the national student physical fitness survey by the Ministry of Education of the People’s Republic of China once every 5 years, the obesity prevalence of students aged 19–22 years in both genders was slightly increased [16]. Although genetic factor plays an important role in obesity, the environmental and lifestyle factors such as physical activity and nutrition patterns are also considered to be of major importance [17]. Studies suggested that this increase could be attributed to rapid change of dietary and physical activity patterns [18].

The findings of the current study showed that overweight and obesity were more prevalent among male than female college students, while normal weight and underweight were more common among females, which was consistent with other studies [13, 19]. This gender difference is a common phenomenon in China. First, it could be explained by the differences in lifestyle. Males were more susceptible to unhealthy lifestyles, such as overeating and drinking. Second, females paid more attention to their body size and image, which made them tend to participate in activities to maintain their weight [20]. Besides, most females desired to be slim by dieting [21]. However, there was no significant gender difference in the United States [22]. On the contrary, overweight and obesity were found to increase only in females in South Africa [23].

Our findings suggested that overweight and obese students showed a higher performance in vital capacity compared with underweight and normal weight students. However, the abnormal weight status showed a bad performance of vital capacity weight index, which was in agreement with the study by Peng et al. [24].

Overweight and obese students achieved poorer performance in sit and reach, standing long jump, pull-up, sit-up, and endurance running compared with normal weight, which was consistent with the study by Mak et al. [25]. Obesity students might be less likely to take part in physical activity because of fear of poor performance and stigmatization [26]. However, Mak et al. [25] pointed out that the results should be interpreted with caution as overweight or obese subjects needed to use more energy to lift a greater body mass. What’s more, when adjusting for fat mass, the relations between overweight and obesity status and deficit in weight-bearing fitness tests were either attenuated or even reversed [15]. Underweight students were found to have higher performance in endurance running and bent-leg sit-up, but this was observed only in females in the present study.

Our results suggested that the relation between BMI and physical fitness was non-linear, which could be characterized by an inverted J-shape association. Polynomial regression and spline regression were clearly superior to the linear regression model, which was similar with Nikolakaros’s population-based study in Finnish healthy young men [27]. In our results, BMI explained around 17.34% of physical fitness variation in males by spline regression, 15.08% of physical fitness variation in females. Normal weight college students generally had better physical fitness than underweight, overweight and obese students, especially in males.

There are some limitations to the study that should be considered. Firstly, the sample could not truly represent the entire college students in China, as above 90% participants of the study were from Anhui province, which is located in east of China. However, the large sample size is one of the greatest strengths of this study. Secondly, the information on income level of their parents and residence (rural and urban, Southern and northern) were not obtained. As reported by Bohr et al. [28], girl students of lower socioeconomic status were associated with lower scores on the FITNESSGRAM assessments and less likely to achieve Healthy Fitness Zone status than those with higher socioeconomic status. Thirdly, the current study was a cross-sectional study. We could not establish a cause-and-effect relation but only identified association between BMI and physical fitness index.

This study provided the evidence on the prevalence and trends of weight status and evaluated the relation between BMI and health-related physical fitness. Future prospective, longitudinal cohort studies to identify the causal relations and potential mechanism in a good manner are required.