It is already known that physical activity is an essential component of combating overweight and obesity. The positive effects of physical activity on physical and mental health have been demonstrated in numerous studies and reviewsearch ^[1][2][3][1,2,3]. Nonetheless, in 2016 the WHO identified that 28% of the adults (>18 years) worldwide are not physically active enough. This means that these adults are physically active for less than 150 min per week at a moderate intensity or less than 75 min at a vigorous intensity [4]. Additionally, the rate of overweight and obese individuals increased sharply in recent decades and has developed into a leading public health problem. Being overweight causes cardiovascular diseases, diabetes, musculoskeletal disorders, and different types of cancer [5]. According to the WHO, the prevalence of obesity has almost tripled since 1975. In 2016, 1.9 billion people aged 18 years and older were overweight, of which 650 million individuals were considered obese. Accordingly, in 2016, 39% of the adult population worldwide was overweight and 13% was classified as obese [6]. Depending on the geographical region, the classification of overweight and obese differs; for more details see ^[6][7][6,7]. In addition to the most commonly used method for measuring overweight and obesity, the anthropometric method based on BMI classification, two different approaches can be used to assess overweight and obesity. Indirect measurements of BMI include waist circumference, waist-to-hip ratio, or body fat percentage based on skinfold thickness, whereas direct measurements of body fat percentage include dual-energy X-ray absorptiometry [8]. Even though existing evidence shows that higher education levels seem to be associated with a lower probability of being overweight or obese [9], the prevalence among university students is substantially high [10]. In the ACHA-National College Health Assessment (ACHA-NCHA) III by the American College Health Association (ACHA), 38.1% of undergraduate students were classified as overweight or obese [11]. Peltzer et al. investigated the prevalence of overweight and obesity among 15.746 university students from 22 different countries and concluded that 22% were overweight and obese [10]. Additionally, when compared to the general population, weight gain is five times higher among university students [12]. Especially in the first year of college, university students tend to gain weight, which is referred to as the “Freshman 15” phenomenon. Freshman 15 is the myth that college students gain 15 pounds (6.8 kg) during their first year of university [13]. Vadeboncoeur et al. conducted a meta-analysis of 22 studies on this topic and concluded that 60.9% of students gained on average 7.5 pounds (3.38 kg) of weight during their first year of college [14]. This meta-analysis failed to confirm the myth of the Freshman 15, but weight gain in the first year of university was significant. These numbers are alarming given that university students tend to engage in unhealthy habits such as insufficient physical activity. The ACHA-NCHA survey, for example, demonstrated that less than half of the US college students (42.1%) met the guidelines for active adults in spring 2021, meaning being physically active on at least two days a week at a moderate or vigorous intensity and demanding all major muscle groups [11]. Similar figures have already been observed by Irwin in a resviearchw of 35.747 students, stating that half of the students were insufficiently physically active and, consequently, at higher risk for health problems [15]. Furthermore, Keating et al. found in their meta-analysis that around 30 to 50% of college students were not physically active enough to reach any health benefits [16]. Due to the abovementioned reasons, university as a setting should be allocated a significant role in public health promotion.

Between 1980 and 2008, the prevalence of obesity nearly doubled [6]. Overweight and obesity is a current topic in all different age groups. To avoid an even more overweight and obese population, it is important to act now. There are already many approaches to increase physical activity and reduce BMI. Positive effects of such interventions have been confirmed in adults and children ^{[2][17][18][19][20][21][22][23]}[2,48,49,50,51,52,53,54]. Long-term findings on physical activity were highlighted by Reiner et al. [2] in their resviearchw, which included overweight and obesity in adolescents and adults: Over five years, weight change was examined concerning physical activity. It was found that those who reduced their daily physical activity gained considerable weight. Individuals who maintained their activity did not gain weight and, in turn, individuals who increased activity lost weight. Hankinson et al. ^[18][49] studied the change in BMI of adults older than 20 years with high habitual activity over 20 years. They found a smaller increase in BMI, waist circumference, and weight per year compared to subjects with low habitual activity. Furthermore, maintaining a higher level of activity in adulthood was able to reduce the subjects’ weight gain over their lifetime. Morano et al. ^[22][53] also found improved physical activity with a simultaneous decrease in BMI in children. The resviearchw by Brown et al. ^[19][50] showed that children aged 6 to 12 years and adolescents up to 18 years have a lower risk of obesity from physical activity alone. By adding physical activity, they could show a low to moderate reduction in BMI from 6 to 18 years of age ^[19][50]. In summary, the results of these studies and reviewsearch suggest that higher levels of physical activity lead to lower weight gain or reduce the BMI of the subjects. This csystematic review confirms that physical activity can help to reduce BMI in obese and overweight students. Although not all studies sof this systematic review show significant results, it can be concluded that physical activity can improve BMI in overweight and obese students in tertiary institutions. Accordingly, the systematic researchview can be integrated into the current state of research, as 11 out of 16 studies were able to demonstrate a significant reduction in BMI among overweight and obese students and only one study was unable to reduce the BMI of the participants through physical activity. Nonetheless, the prevalence of physical inactivity has risen sharply in recent years ^[24][55], even in early childhood and adolescence ^[23][54]. As already mentioned, there are numerous interventions designed to counteract the risk factors of physical inactivity, such as excessive body fat percentage, overweight and obesity, unhealthy lipid profile, high blood pressure, etc., through physical activity. All these risk factors could result in cardiovascular diseases, strokes, etc., or even in an increased morbidity ^[24][25][55,56]. Since physical inactivity is especially prevalent among university students, interventions designed to mitigate these risk factors are necessary. Interventions and reviewsearch have already demonstrated that the above-mentioned risk factors could be significantly reduced through physical activity both in children and adults ^{[19][20][22][23]}[50,51,53,54]. In addition to BMI, the physical-activity-based interventions in thave is systematic review have shown further improvements in other risk factors of physical inactivity. In line with the current literature, this csystematic review could therefore demonstrate that physical activity is useful in reducing or even eliminating the risk factors associated with physical inactivity. Relaxation techniques should be used as an additional tool to implement the approach of comprehensive health promotion. Moore and Cunningham ^[26][57] stated in their systematic resviearchw that exposure to higher stress was associated with poorer nutrition and, additionally, higher body weight. Conversely, a higher level of stress could be an obstacle to weight loss. In addition, Stults-Kolehmainen and Sinha ^[27][58] were able to demonstrate in their systematic resviearchw that objective stress (e.g., life events), as well as subjective stress (e.g., distress), impair efforts to be physically active. On the other hand, physical activity is known to be stress-buffering, as regular physical activity can buffer the negative consequences of stress on health, which is postulated by the stress-buffering hypothesis ^[28][29][59,60]. Given that the target population of this systematic review was university students who are particularly exposed to high levels of stress ^[30][31][61,62], relaxation strategies should be considered for a comprehensive treatment of the increasing prevalence of overweight and obesity and physical inactivity. SAt the level of the individual studies used for this review, some limitations can be identified. Particularly, the insufficient and homogeneous sample size is frequently cited, which may impair the informative value of the results and make generalization to the general population difficult. Another limitation arises from the inadequate control over the intensity in some studies. Therefore, all studies which did not control the intensity with objective indicators (e.g., heart rate) impair generalizability in terms of highest effectiveness according to the intensity. This issue could be additionally exacerbated by the varying adherence to the training program. Even though eleven of the included studies showed significant improvements after the intervention, it should be considered that the intervention periods of these studies lasted only up to four months. Therefore, it is not possible to determine any long-term effects of the conducted interventions. Concerning statistical analysis, only one of the included studies reported effect sizes in addition to the significance level ^[32][44]. Since p-values always refer to the sample size in contrast to effect sizes ^[33][47], effect sizes should be reported more frequently to facilitate detecting patterns across studies and to infer the effectiveness of the included studies. Even though effect sizes could be computed with Psychometrica for the remaining studies, they can only be seen as approximate values. At the review level, it is important to note that the heterogeneity of the included studies in terms of sample size, gender, type and duration of physical activity, duration of the intervention, and BMI of subjects aggravated to identify patterns across the studies. As the included interventions were conducted in countries with different cultural backgrounds, a cross-cultural effect could be suspected. However, considering the low number of studies and the wide range of existing cultural practices, beliefs, and behaviors, the findings of this review cannot be generalized. They can only serve as a guide for further investigation of effective strategies to address overweight and obesity for a given cultural setting. Additionally, only 31% of the included studies were of strong quality, thus generalization should be taken with caution. Since there was no possibility to translate studies that were in a language other than English or German, the search was limited to the English language from the beginning. In addition, limited access to the full text of nine potentially suitable studies made it difficult to select appropriate studies. Due to these organizational limitations, there is a possibility that other relevant research from the literature could not be included in this review. The choice of BMI as the dependent variable of the intervention proved to be the biggest limitation and bias. Frequently, studies had to be excluded because they reported on body characteristics other than BMI, such as body fat levels. As the relationship between BMI and body fat mass is nonlinear, individuals having the same BMI may have different body compositions (e.g., varying body fat percentages or muscle mass) ^[34][63]. Thus, with regard to the effectiveness of physical activity, the main concern is whether a reduction in BMI is associated with a reduced fat mass at all, or conversely, especially for novice exercisers, whether an increase in muscle mass does not initially translate into an increased BMI. The present resviearchw was deliberately limited to the aspect of physical activity. However, since a promising feature of the intervention studies by Brown et al. ^[19][50] and Martin et al. ^[23][54] that examined children and adolescents is the implementation of a program combining physical activity and dietary changes, future reviewsearch should additionally include the aspect of nutrition. In addition, there is a need to further investigate the effectiveness of aerobic exercises in overweight and obese university students, as well as identifying which types of aerobic exercises are most effective. More intervention studies tailored to the distinct cultural backgrounds in this field are needed to detect cross-cultural patterns. As adherence can play an important role in the success of an intervention, it is important to develop interventions in which participants are more likely to participate regularly. In this regard, it might be useful to differentiate different levels of adherence to gain more comprehensive data, which helps to identify patterns of low and high adherence. This, in turn, would serve as a basis for removing barriers for participants in future interventions and for developing recommendations for practitioners. Since another study demonstrated significant BMI reductions by combining pre-meal water intake with HIIT training sessions, the role of water consumption in combination with exercising represents a promising approach to weight loss that needs further investigation, especially regarding different types of exercises. Furthermore, as the BMI is prone to bias, it should be reconsidered as an appropriate variable to capture the change in body characteristics of a physical activity intervention for future research eviews. For this reason, it is important that future research adopts a comprehensive health perspective by examining other health-relevant indicators. Additionally, future research should ensure larger sample sizes as well as representativeness of the sample to facilitate generalization. Since Bothmer and Fridlund ^[35][64] concluded that male students are less interested in health enhancing activities, comparative analyses by gender can also help to better understand gender-specific needs of overweight and obese university students. In order to design tailored interventions and programs, studies should ensure an equal gender distribution. To be able to assess long-term effects, longer intervention periods need to be conducted. Combining different training programs for future research might also be a promising approach for reaching better effects. Similarly, as university students are especially exposed to high levels of stress which, in turn, is known to impede weight loss, relaxation strategies should be considered in future interventions. Web-based approaches, using, for example, messaging or informative websites, and technical devices, such as activity trackers, are promising approaches to foster physical activity. They can not only facilitate sampling and flexibility as an organizational aspect, but also help to supervise the intensity of physical activity during the implementation. To be able to set optimal stimuli for participants, more research and interventions with a study design based on a theoretical approach are needed to identify key concepts and to be able to better understand the mechanisms of change behind the intervention effects. In general, more intervention studies of physical activity in overweight and obese university students are needed to gain insight into optimal types of interventions, their frequency, and their duration to explore more contemporary and sustainable opportunities. As the highest BMI reductions were achieved in interventions conducting aerobic exercises, it might be important for practitioners to focus on exercises promoting aerobic energy expenditure to achieve BMI improvements. Even though this needs to be approved by further research, this method can still serve as a guide. Given that one study demonstrated that high adherence was crucial for significant BMI reduction, methods to ensure consistent participation might be critical for the effectiveness of an intervention and should be considered by practitioners.