Football as a Health Promotion Strategy
A systematic review of football intervention studies
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Background: Football training can be a primary prevention strategy to reach people who otherwise would not be physically active. This systematic review summarizes the evidence on the health effects of controlled recreational football training as an intervention in children, adolescents, adults and the elderly.
Methods: A systematic review (PROSPERO record CRD42018083665) of the literature was carried out in MEDLINE, Cochrane, Scopus, and SPORTDiscus databases to identify randomized and non-randomized intervention studies in which healthy individuals of any age participated in controlled football training and were investigated for health outcomes related to prevention of obesity, hypertension, diabetes mellitus, and cardiovascular disease.
Results: The studies included—14 randomized and three non-randomized intervention studies—have sample sizes too small for reliable statistical analysis and bear a considerable risk of systematic bias. The evidence of positive effects of playing football is limited to short-term loss of body fat and improvement in aerobic fitness. For all other health outcomes, no conclusive results were found.
Conclusion: A considerable number of intervention studies reporting on football-based intervention programs have been published, and there is a widespread assumption that such programs have positive health effects. However, this systematic review shows that the empirical evidence is insufficient to permit such a conclusion.
Insufficient physical activity as well as an unhealthy diet and lifestyle account for a substantial population attributable fraction of preventable diseases such as cardiovascular disease, diabetes mellitus, hypertension, and obesity (1). Football-based exercise has been discussed as a potential health promotion strategy, especially to reach people who otherwise would not be physically active (2, 3). Building on the worldwide popularity of football, health professionals could recommend football training as a primary intervention to reduce sedentary behavior. Given the complex movements performed, playing football may improve endurance capacity and have positive effects on blood pressure, heart rate, and body weight. Thus, a positive influence on cardiovascular and metabolic health can be expected (4). Moreover, the fact that football is a team sport creates considerable motivational potential for engaging inactive people. Therefore, it has been emphasized in the literature that football-based interventions in sedentary people can achieve both primary preventive effects and far-reaching improvements in health (3, 5). A number of reviews have come to the conclusion that playing football may represent a helpful intervention to treat or prevent disease (6, 7). The present systematic review investigates the extent to which football-based interventions can contribute to the general health of the population. In addition to existing publications, this review assesses the quality of the studies included and their susceptibility to systematic bias.
The aim of this publication is to collate the empirical evidence on the health effects of recreational football training as an intervention in healthy people of any age with regard to the prevention of obesity, hypertension, diabetes mellitus, and cardiovascular disease.
The protocol of this systematic review was preregistered in the International Prospective Register of Systematic Reviews (PROSPERO) (8). The review covered studies on healthy populations of any age (excluding amateur and professional athletes). For inclusion, interventions had to be based on recreational football training that lasted at least 4 weeks with a minimum of two 1-hour-long training sessions per week. In the control groups, either no intervention, other sports-based interventions, or non-exercise interventions took place. All outcomes related to preventable diseases with major morbidity on a population level were taken into consideration. Various database systems (MEDLINE, Cochrane, Scopus and SPORTDiscus) were systematically searched for literature on football/soccer interventions (eTable 1). Studies published between 1 January 1997 and 31 December 2017 were considered. The articles identified underwent independent screening of titles and abstracts by two reviewers (ME and HA). Next, the reviewers independently assessed full texts on the basis of defined exclusion criteria (eTable 2). Included publications referring to the same study populations were combined and considered as one study. The data extracted according to a defined list of variables were collected in a dedicated database. Full details of the methods used can be found in the review protocol (8).
Study quality was assessed using the EPHPP (Effective Public Health Practice Project) Quality Assessment Tool (9). The Cochrane risk of bias tool was used to assess the susceptibility of studies to systematic bias (10). Data synthesis was performed descriptively. The data of the included studies were systematically summarized and results were aggregated by disease domain. The described intervention effects, i.e., change of group means from baseline to the last follow-up in the football intervention group compared with the least active control group, were assessed. Results were assessed as “inconclusive” if the before/after differences in the intervention group were small compared with baseline imbalances between groups or compared with the placebo effect in the control group. Due to the high risk of systematic bias affecting the majority of studies, it was determined, in accordance with the Cochrane handbook (10), that the available evidence does not allow for appropriate quantitative aggregation in a meta-analysis.
A total of 9052 records were identified in systematic searches. After elimination of duplicates, 5953 titles and abstracts were screened. The full texts of 261 publications were assessed against the eligibility criteria, and 229 publications were excluded for the reasons listed in Figure 1. The 32 publications included for analysis originated from 17 different studies (14 randomized controlled trials [4, 11–23] and 3 non-randomized intervention studies [24–26]). Some authors repeatedly published results on the same study populations; for instance, a randomized controlled trial (RCT) on 65 sedentary women randomized to three groups (football, running and an inactive control group) was published in four separate articles (11, 27–29) (eTable 3).
The characteristics of the included studies are described in detail in Table 1. The study populations comprised:
- Healthy women (one study)
- Sedentary women with hypertension (two studies)
- Sedentary men with hypertension (six studies)
- Homeless men (one study)
- Sedentary adults of all sexes (one study)
- Sedentary adolescents (one study)
- Healthy children (one study)
- Overweight children (four studies)
The football-based interventions were similar across studies: A short warm-up period was followed by small-sided games (eTable 4). The training sessions lasted approximately 60 minutes and were conducted 2–3 times per week. The duration of the intervention varied from 8 to 68 weeks. None of the studies continued with long-term follow-up of health outcomes after the end of the intervention phase.
Study quality and risk of systematic bias
Quality evaluation according to the EPHPP checklist classified 8 of the 17 studies (47%) as weak (4, 14, 15, 17, 18, 24–26) and the remaining 9 studies (53%) as moderate (11–13, 16, 19–23). None were of high quality.
Moreover, the studies showed a substantial risk of systematic bias. In particular, there was a high or unclear risk of attrition bias, selection bias, and performance bias (Figure 2). In most cases reporting bias could not be assessed because the study protocols had not been published.
None of the included studies was judged as having a low risk of bias. This considerably restricts the credibility of the study results and therefore the validity of the scientific evidence derived from them (eTables 5, 6).
Sixteen studies reported results on changes in either body mass index (BMI) or body fat mass (Table 2). Four studies found a positive, body fat mass-reducing effect of 1 to 4 kg in the football group during intervention periods of 12 to 68 weeks (12, 15, 16, 23). Seven studies showed small positive effects that were judged inconclusive because of considerable baseline imbalances among the study groups (11, 14, 17–20, 24); the same applied to one study that showed a small negative effect (26).
Similarly, most studies presented inconclusive results for the effect on BMI (11, 12, 15, 17, 19, 24–26). Four studies found no effect of football on BMI (4, 13, 14, 22), one study a small BMI-reducing effect (20), and one study a clinically relevant reduction of 2.7 kg/m² (16).
None of the studies analyzed the incidence or severity of clinical hypertension. Twelve studies documented systolic blood pressure (SBP). For normotensive study participants, seven of eight studies reported no or only inconclusive effects on SBP (11, 14, 17, 21, 22, 24, 25); in one study SBP was lowered by 6 mm Hg (17). Three of four studies on hypertensive populations documented a positive effect of football interventions on lowering SBP (15, 18, 23). A distinct decrease was also observed in many control groups (regression to the mean).
None of the publications reported long-term follow-up of the incidence of type 2 diabetes mellitus in healthy study populations. Eight studies analyzed blood glucose levels as an endpoint and found no effect on glucose levels or the results of oral glucose tolerance tests (11, 12, 14, 15, 17, 20, 23, 24). Studies conducted in previously-diagnosed type 2 diabetes patients were not included in this review, because the focus was primary prevention.
None of the included studies used the incidence of cardiovascular disease as an endpoint. The cardiovascular outcomes reported were only short-term adaptations, such as heart rate, time to exhaustion, transmitral flow, and right ventricular function.
The American Heart Association (AHA) recently recommended measurement of aerobic fitness in terms of VO2max as a standard clinical test (30). Eleven studies presented data on VO2max levels. Seven of these found an improvement corresponding to around 1 metabolic equivalent of task (MET) (11, 12, 14, 18, 19, 23, 24); one study found an increase of 3.1 MET (16); and the other three studies reported inconclusive results (4, 17, 20).
Since hypercholesterolemia is a known risk factor for cardiovascular disease, we also evaluated data from 10 studies on cholesterol and triglyceride levels. Four studies found no change (12, 14, 17, 18), and the other six studies presented inconclusive results (11, 15, 19, 20, 23, 24).
None of the reviewed studies reported health outcomes beyond the predefined domains. In seven studies bone density was measured, but without documentation of the clinical relevance with regard to the risk of osteoporosis (11, 12, 14, 15, 17, 23, 24). The described changes in bone density in the intervention group were generally very small compared with the differences between the groups at baseline.
Adverse events were reported in 12 studies. Most of these involved injuries of the locomotor apparatus leading to premature dropout from the intervention group. Such severe injuries occurred up to three times per study (eTable 5). Complete documentation of adverse events was found in only one study (13).
The 32 publications eligible for this systematic review presented data from 17 different studies. We found a number of multiple publications on the same study results, and many studies were conducted by the same research groups. We first assessed study quality and risk of bias, in order to appropriately assess the evidence whether football-based interventions have an effect on health outcomes related to the incidence of preventable diseases, namely obesity, hypertension, diabetes mellitus, and cardiovascular disease. We found considerable weaknesses of the published studies, especially poor study quality, incomplete follow-up, inadequate randomization, high risk of performance bias, and small sample size. The results suggest positive health effects such as short-term loss of body fat and improvement of aerobic fitness. Four of 12 studies found a reduction in body fat mass of 1 to 4 kg during the intervention. Eight of 11 studies reported an increase in VO2max levels of between 0.3 and 3.1 MET.
In a recent meta-analysis Milanović et al. summarize the evidence on the basis of 31 publications (31). They come to the conclusion that football has “multiple positive effects on health-related physical fitness” including blood pressure, body fat, and cholesterol levels. They claim that football represents “an effective broad-spectrum non-pharmacological treatment of lifestyle diseases, such as hypertension and metabolic syndrome” (31). Our systematic review supports these results only with regard to body fat, where four studies reported a positive effect. With regard to blood pressure, we found either no effect or inconclusive results for normotensive populations. In three of four studies with hypertensive populations, a small positive effect of football interventions on systolic blood pressure was observed. As for cholesterol levels, six studies had inconclusive results and four studies showed no improvement. Moreover, we noted a low reliability of the laboratory tests.
Milanović and colleagues conclude a “very likely extremely largely beneficial” effect of football on systolic blood pressure in women (31). Our review of the literature indicates, that the evidence for such a statement is limited, given that this conclusion is based on three studies with a total sample of 48 women completing a football intervention (12, 14, 15). The authors may have overinterpreted the statistical effect size of the football intervention. Thus, the mean reduction of 4.2 mm Hg in systolic blood pressure is certainly not an “extremely large” effect, particularly in comparison with the efficacy of dietary approaches, which achieve a mean decrease of 6.7 mm Hg (32). Taking into account the multiple inclusion of identical study populations, the clinical relevance of effect sizes, and the susceptibility of the results to systematic bias, we come to more cautious conclusions than Milanović et al.
The limitations of the reviewed studies are mainly related to (eBox):
- Small sample size
- Inadequate randomization
- Incomplete outcome data due to attrition
- Selective reporting of results
- Lack of blinding of study personnel
None of the studies took adequate measures to compensate the effects of participant dropouts before the end of the study. It would have been adequate to carry out an intention-to-treat analysis (ITT) in order to guard against the assumption that participants who ended the intervention phase prematurely had the same benefits as those who completed the protocol. The few studies that have used ITT, however, did not collect outcome data for participants who discontinued the intervention. Instead, the last-observation-carried-forward method was used, which assumes that the improvements at the time of discontinuation are still present at conclusion of the study.
To lower the risk of bias in this domain in the future, established methods to deal with missing outcome data in RCTs should be used (33). Further, it is essential to increase the sample size in order to avoid false-positive results and to increase the statistical power. The majority of the studies we analyzed had fewer than 25 participants with complete data in the intervention group. The study protocols were mostly not published, and in many publications data were selectively reported, increasing the risk of reporting bias. Moreover, all of the studies we investigated used several different outcome measures without adequate adjustment for multiple testing. The risk of chance findings in the reviewed publications is therefore high (34).
Another point of concern is the choice of short-term endpoints rather than long-term observation of morbidity and mortality. To demonstrate that football training can have positive effects for a sedentary population, it would be essential to evaluate long-term and morbidity-related outcomes. Furthermore, careful discussion of study results with regard to the clinical relevance of the effects is needed.
This review shows that the studies conducted to date have not conclusively demonstrated a health-promoting effect of football training as a prevention strategy. It is possible that health benefits exist in reality and were obscured by limitations in study design and small sample sizes. There is currently no evidence based on high-quality intervention studies. Our review suggests that the results of football intervention studies published to date are prone to extensive bias.
Significance and limitations
This systematic review is the first to focus on explicitly health-related outcomes in the assessment of the efficacy of football-based intervention studies with special consideration of study quality and susceptibility to systematic bias. Given that we did not contact the authors, our assessment may be based on incomplete information. We restricted our review to intervention studies and RCTs. The possible effects on the health of amateur football players can only be evaluated through long-term observational studies and were not considered in this review. It was not possible to conduct a meta-analysis of the data, because the susceptibility of the included studies to systematic bias prevented valid quantitative data synthesis.
Many intervention studies have been published, and there is a widespread belief that research has proved the positive primary preventive effects of football training with regard to health (15). The empirical evidence for this assumption is, however, inadequate. Ultimately, only a short-term reduction in body fat mass and a small improvement of aerobic fitness can be concluded on the basis of this review (Box). There are no morbidity-related data on the health-promoting effect of playing football. Long-term intervention studies with larger sample sizes and higher study quality are needed to strengthen the evidence.
We are grateful to Sandra Weinmann for her help in translating
the article into German.
Conflict of interest statement
The authors declare that no conflict of interest exists.
Manuscript received on 9 April 2019,
revised version accepted on 30 July 2019.
Prof. Dr. med. habil. Henning Adamek
Medizinische Klinik 2
Am Gesundheitspark 11
51375 Leverkusen, Germany
Cite this as:
Eberl M, Tanaka LF, Klug SJ, Adamek HE:
Football as a health promotion strategy—a systematic review of
football intervention studies. Dtsch Arztebl Int 2019; 116: 721–8.
For eReferences please refer to:
Department of Medicine 2, Leverkusen Hospital: Prof. Dr. med. habil. Henning Adamek
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