The body of knowledge on the relationship between physical activity and health in children and youth has been growing steadily since the 1950s, and the development of this research field has been particularly rapid over the last two decades (1). Much of the early research was focused on physical fitness and its relationship to growth and development in young persons (2). However, more recently, the emphasis has shifted to the effects of physical activity on risk factors for noncommunicable diseases that typically do not manifest until adulthood. These include cardiometabolic diseases, such as coronary heart disease and type 2 diabetes, and bone health outcomes, including osteoporosis and bone fractures (3). With the marked increase in the prevalence of overweight and obesity in US children, many recent studies have examined the impact of physical activity on adiposity and weight status in young persons (4).
The 2008 Physical Activity Guidelines Advisory Committee Report included an examination of the relationship between physical activity and health in children and adolescents. Key conclusions of that review were that, in school-age children and youth, higher levels of physical activity are associated with better status on indicators of cardiorespiratory and muscular fitness, body composition, cardiometabolic risk, and bone health (5). Those conclusions informed a physical activity guideline which indicated that children and adolescents should accumulate 60 min or more of at least moderate-intensity physical activity daily and that, within that hour of activity, vigorous-intensity physical activity and muscle-strengthening and bone-strengthening activities should be included at least 3 d·wk−1 (6). Notably, this guideline was applied only to youth in the 6- to 18-yr age range. No guideline was included for children younger than 6 yr, because the body of knowledge on physical activity and health in early childhood was very limited.
During the period between 2008 and 2018, a substantial volume of research was undertaken on the relationship between physical activity and health in children of preschool age (7). Further, during that period, physical activity guidelines for children younger than 6 yr were developed by public health agencies in some other countries, and physical activity guidelines for children attending childcare centers were released by the Institute of Medicine in the United States (8). Accordingly, the Youth Subcommittee of the 2018 Physical Activity Guidelines Advisory Committee opted to consider the evidence related to relationships between physical activity and selected health outcomes in children younger than 6 yr. The purpose of this article is to present the findings of a systematic review of the scientific literature addressing this issue. Specific health outcomes considered in the review were body weight and adiposity, bone health, cognition, and cardiometabolic risk factors.
The methods used to conduct systematic reviews for the 2018 Physical Activity Guidelines Advisory Committee Scientific Report have been described in detail elsewhere (9). An initial search limited to systematic reviews, meta-analyses, pooled-analyses, and high-quality reports was conducted. That search yielded too few articles, so the search was repeated to identify relevant original research articles. Accordingly, for this review, a systematic search was conducted to identify randomized controlled trials and prospective cohort studies that assessed the association between any type of physical activity and health outcomes, including adiposity and weight status, bone health, and cardiometabolic health in children younger than 6 yr. The searches were conducted in electronic databases (PubMed®, CINAHL, and Cochrane) and were supplemented by asking subcommittee members, all experts in the area, to provide additional articles identified through their expertise/familiarity with the literature.
Articles published in English from data base inception until February 2017 were included in the Committee Report, and the search was extended to March 2018 for this article. Search terms included age-appropriate physical activity, active play and sedentary behavior terms combined with outcome-specific terms. The full search strategy is available at https://health.gov/paguidelines/second-edition/report/supplementary_material/pdf/Youth_Q1_Under6_Evidence_Portfolio.pdf. The identified articles were independently screened by two reviewers. The full-text of relevant articles was reviewed to include those that met the inclusion criteria. Inclusion/exclusion criteria are presented in Supplemental Material (see Table, Supplemental Digital Content 1, Inclusion/Exclusion Criteria, http://links.lww.com/MSS/B532). Two abstractors independently abstracted data and conducted a quality or risk of bias assessment using the USDA Nutrition Evidence Library Bias Assessment Tool for original research (9,10) and the AMSTAR ExBP for systematic reviews (11). Discrepancies in article selection or data abstractions were resolved by discussion or a third reviewer if needed. The protocol for this review was registered with the PROSPERO database (registration ID CRD42018092740). A summary of the bias assessment of the original research articles included in this review is available in the supplemental material [see Tables, Supplemental Digital Content 2, Nutrition Evidence Library Bias Assessment Tool: Original Research, http://links.lww.com/MSS/B533; and Supplemental Digital Content 3, AMSTAR ExBP: SR/MA, http://links.lww.com/MSS/B534].
A total of 1257 studies were identified through the systematic searches (Fig. 1). After screening titles and abstracts, 1166 studies were excluded and 91 reviewed in full. Of these, 19 studies met the full inclusion criteria. An additional eight studies were identified by the authors based on their knowledge in the area. Twenty-seven studies were included in this review until the release of the 2018 Physical Activity Guidelines Advisory Committee Scientific Report. One additional original research article and three systematic reviews were found when the search was updated for the purpose of this article.
Body weight and adiposity
In considering the evidence regarding the relationship between physical activity and body weight and/or adiposity in children younger than 6 yr, the committee identified and reviewed 15 studies (12–26). The study designs, methods, and findings of these studies are summarized in Table 1. All of the studies included in this review used prospective, longitudinal study designs. However, methods for measurement of physical activity were highly variable. Also, the studies were quite variable in terms of children’s age range, years of follow-up, measurement of weight-related outcomes, and analytic procedures. Notwithstanding these differences, the studies were consistent in reporting that higher levels of physical activity were associated with lower levels of weight and/or adiposity in younger children. Twelve of the 15 studies found negative associations between physical activity and weight and/or adiposity (12–16,18,20–25). Although these studies were consistent in observing benefit with higher amounts of physical activity, limitations in study design and variability in methodologies across the studies precluded identifying a particular dose of physical activity that was needed to provide benefits.
The literature search provided eight articles, with two additional articles added by committee members. These 10 articles represented four studies, two of which had prospective longitudinal study designs and two of which were randomized controlled trials (27–36). The study designs, methods, and findings of these studies are summarized in Table 2. Three of the four studies focused on preschool children (baseline ages, 3 to 5 yr) (27–35) and one study focused on infants (36). The dose of physical activity was defined and measured differently among the studies and included recreational gymnastics participation (months) (28–30), device-measured daily activity (min) (31–34), and bone-strengthening physical activity (sessions) (27,35,36). All studies used state-of-the-art imaging (dual-energy x-ray absorptiometry [DXA] and peripheral quantitative computed tomography) to measure bone outcomes and appropriate statistical modeling to control for growth. All studies examining children ages 3 to 5 yr showed statistically significant stronger bone in the more active children. The benefit differences were greater than expected via measurement error and large enough (almost always >3%) to indicate meaningful biological improvements. However, similar to the evidence for body weight and adiposity, the differences in physical activity measures prevented the assignment of a specific dose of physical activity needed for bone health benefits.
Very few studies have examined the association between physical activity and indicators of cardiometabolic health in children younger than 6 yr. The literature search resulted in the identification of three prospective cohort studies that included outcomes related to serum lipid and lipoprotein levels, respiratory symptoms, and blood pressure (13,37,38). One study reported that physical activity appeared to have an indirect association with blood lipids and lipoproteins in 3- to 4-yr-old children, through its relationship with lower levels of body fatness and higher levels of fitness (13), whereas another study reported an inverse association between physical activity and diastolic blood pressure in 5- to 7-yr-old children (38). A final study reported that physical activity at 2 yr of age was not related to respiratory symptoms, such as wheezing or shortness of breath at 3 to 4 yr of age (37). On the basis of the results from these available studies, the committee determined that there was insufficient evidence available to determine the effects of physical activity on cardiometabolic risk factors.
The committee reviewed the scientific literature examining the relationship between physical activity and cognition in children younger than 6 yr. This review was supported by a search of the literature that was independent of the search described above. That review process, and the committee’s related conclusions, are described in detail in another article in this supplement (39). Two systematic reviews of the literature on physical activity and cognitive outcomes in preschool-age children met the criteria for inclusion (40,41). One of those systematic reviews considered seven observational and experimental studies, and the authors reported that six of the seven studies found that a higher amount of physical activity was associated with a beneficial effect on at least one cognitive outcome (40). The second systematic review reported that five of six randomized controlled trials found positive effects of selected indicators of cognitive development in 4- to 6-yr-old children (41). The existing studies and the cited systematic reviews point to possible beneficial effects of physical activity on cognitive outcomes in young children, but there is a clear need for more studies with rigorous research protocols.
Few studies of physical activity and health in children younger than 6 yr have been designed in a manner that allows examination of dose–response relationships. Given the absence of this information in the extant literature, there is a clear need to design experimental trials and prospective cohort studies to answer the question of whether a dose–response relationship exists for physical activity and health during this early period of the lifespan, and if so, what is the nature of that relationship. Such information is important toward not only understanding how physical activity influences health but also toward generating knowledge and support to best provide opportunities for intervention to support public health.
The studies on physical activity and health in children younger than 6 yr have rarely been designed in a manner that provided for examination of the potential modifying effects of demographic characteristics, such as sex, age, race/ethnicity, weight status, and socioeconomic status. Although studies included participants across a range of demographic characteristics, studies tended to control for potential confounders (e.g., sex, body size, lifestyle) but typically did not conduct stratified analyses to examine effect modification. Given the known differences in physical activity and health outcomes by demographic characteristics in older ages, it is important to understand the extent to which the health effects of physical activity may differ across demographic subgroups across the lifespan. Such information would provide additional understanding of whether the dose of physical activity needed to produce health benefits varies across population subgroups.
The overall conclusion of the systematic literature review presented in this article was that strong evidence demonstrates that higher amounts of physical activity are associated with more favorable indicators of bone health and with better weight status in children ages 3 to 6 yr. However, there was insufficient evidence to show a relationship between physical activity and indicators of cardiometabolic health in children younger than 6 yr. Further, for all health outcomes studied in this age group, evidence was insufficient to determine dose–response relationships and to determine whether the relationships between physical activity and health were moderated by factors, such as age, sex, race/ethnicity, or socioeconomic status. Relatively few studies have addressed the impact of physical activity on health in very young children, and there are a limited number of systematic reviews of this topic. Timmons et al. (7) reviewed the relevant literature for children in the 0- to 4-yr age range, and studies published up to May 2011 were included. Their conclusions were generally consistent with those of the present review. Although noting widely varying qualities of evidence, they concluded that, among preschoolers, higher levels of physical activity were associated with a number of positive health outcomes, including adiposity and indicators of cardiometabolic health.
More recently, systematic reviews have been undertaken to inform the development of the Canadian 24-Hour Movement Guidelines for the Early Years (42). The results of the review of the association between physical activity and health indicators indicated that intervention studies improved motor and cognitive development, and psychosocial and cardiometabolic health, whereas evidence from observational studies showed that physical activity was associated with favorable motor development, fitness, and bone and skeletal health (43). The Carson et al. review identified 96 studies in children 5 yr and younger compared with 25 studies we identified for the current review. However, we applied stricter inclusion criteria which, among other factors, excluded cross-sectional observational studies and studies which delivered parental or group-level interventions. These methodological differences may explain the somewhat different conclusions reached by the two reviews. Nonetheless, the conclusion of both reviews is that physical activity is positively associated with health indicators in preschool age children.
It is well documented that rates of overweight and obesity have increased dramatically in all segments of the US population, and this includes children younger than 6 yr (44). As a result of this trend, prevention of childhood obesity has become an important public health priority in the United States and other economically developed nations (45). In this context, the findings of the current systematic review are particularly important. It was concluded that there is strong evidence that higher amounts of physical activity are associated with better weight- and adiposity-related outcomes in 3- to 5-yr-old children. Several important factors were considered by the authors in arriving at that conclusion. First, rigorous standards were applied in selecting studies for inclusion in the review. Second, all studies included in this review applied prospective, observational research designs, which, in the view of the authors, is the best available method for studying the relationship between physical activity and weight/adiposity outcomes. In theory, experimental studies would be important, but there are concerns about the feasibility of treatments that would involve long-term, controlled exposures to modified physical activity in children younger than 6 yr. Third, most of the studies included in this review used objective, device-based measures of physical activity. Fourth, beneficial effects of higher amounts of physical activity were very consistently reported. Thirteen of the 15 studies included in this review found that more physically active children tended to gain less weight and/or fat mass than their less physically active counterparts. Other systematic reviews have drawn similar conclusions (4), although most have focused primarily on older children.
Although the authors found that the available evidence supports the conclusion that physical activity provides important benefits for weight-related outcomes in preschool age children, it is acknowledged that the existing research literature on this topic has important limitations. Because the number of currently available studies is modest, more studies with device-based measures of physical activity, well-validated measures of adiposity, and multiyear follow-up periods are needed. Further, future studies should carefully assess factors that might confound the relationship between physical activity and weight-related outcomes. These include diet and sleep behaviors. In addition, studies with large and diverse samples of children will be needed to determine whether or not the physical activity–weight/adiposity association is moderated by demographic factors and to describe dose–response relationships. Future studies will be needed to address these limitations. Nonetheless, it is the position of the authors that currently available evidence indicates that promotion of physical activity should be a major aim of public health efforts to prevent childhood obesity.
Although few studies have focused on physical activity and bone health in preschool children, the results of the existing studies indicated that young children who engaged in bone-strengthening activities or in high levels of total physical activity have stronger bones. This conclusion is supported by observational evidence that the age of independent walking in toddlers is associated with greater lower-limb bone strength (46,47) and experiments that show mechanical loads create positive adaptations in the bones of young animals (48,49). The evidence related to relationships between physical activity and bone health in children younger than 6 yr when combined with the strong evidence that impact and muscle forces due to physical activity cause positive bone adaptations in older children and adolescents (50) indicate the important role of physical activity for ensuring strong and healthy bones throughout the growing years.
There is a paucity of information on the relationship between physical activity and cardiometabolic risk factors in children younger than 6 yr. In general, most preschool age children have a healthy cardiometabolic profile. Although the primordial prevention of cardiovascular disease is a lifelong endeavor, children do not typically begin to develop adverse cardiometabolic health outcomes until after being exposed to poor lifestyle behaviors for several years. With the exception of overweight and obesity, most available studies did not recruit children with elevated cardiometabolic risk factors. Therefore, there is a pressing need for studies among children with elevated levels of risk factors, in addition to the identification of novel cardiometabolic health markers that are sensitive to lifestyle changes, such as increased physical activity.
The study of cognition sits within the broader field of brain health, which is a broad term conceptualized as the optimal or maximal functioning of behavioral and biological measures of the brain, including subjective experiences that arise from brain function (e.g., attention, mood). Brain health can be measured using biological markers of the brain (e.g., structural brain morphology) or via subjective manifestations of brain function, including mood and anxiety, perceptions of quality of life, cognitive function (e.g., attention and memory), and sleep. Relative to children younger than 6 yr, little is known regarding the relationship of physical activity to cognition and brain health. The available, preliminary evidence points to a beneficial association of physical activity to cognitive and academic outcomes, which should not be surprising given that findings in studies of older children and adults populations is much further along, and has evidenced benefits to brain structure and function, and a variety of cognitive outcomes. Regardless, further research is necessary to extend these effects to children younger than 6 yr, and to understand the nature of physical activity effects on cognition in this age group.
Children and youth—6 to 17 yr
The systematic review described above was focused on children younger than 6 yr. Though not described in detail in this article, the committee also reviewed systematic reviews and meta-analyses addressing the relationships between physical activity, sedentary behavior, and health outcomes in school-age children and youth (ages 6 to 17 yr; see detailed search description in the committee’s report (50)). The findings for 6- to 17-yr-olds are consistent with, but go beyond, the findings for preschool-age children (3 to 6 yr). Similar to 3- to 6-yr-olds, higher amounts of physical activity were found to be associated with better indicators of bone health and with reduced risk for excessive increases in weight and adiposity among older children (50). Accordingly, for those two important health outcomes, the committee concluded that physical activity provides important benefits for young persons across the entire 3 to 17 yr age range. However, for several other health outcomes, beneficial effects of physical activity were found for older children but not documentable for children younger than 6 yr. These included indicators of cardiometabolic health, cardiorespiratory fitness, muscular fitness, cognition and risk of depression (39,50). The body of knowledge on physical activity and health is much more robust for school-age children than for children younger than 6 yr. Therefore, additional research will be needed to determine whether or not all the benefits of physical activity that have been documented for older children also accrue to those younger than 6 yr.
Strengths, limitations, and delimitations
The strengths of the review include a well-designed and transparent search and review process. In addition, most of the studies of adiposity or weight status used device-measured physical activity. All the studies of bone health used state-of-the-art bone imaging procedures. The primary limitation is that relatively little research has been conducted on the relationship between physical activity and health in children younger than 6 yr. The existing volume and quality of research is sufficient to conclude that a beneficial relationship exists for bone health and weight status, but provides insufficient information about dose–response or any potential effect modification by age, sex, or race/ethnicity.
In the context of developing physical activity guidelines for dissemination to the public and professional groups, it is highly desirable to identify a specific amount of physical activity, or range of amounts of activity, that is known to be associated with important health outcomes. Hence, the authors’ finding that the existing research is not sufficient to inform conclusions about dose–response relationships is particularly limiting. It was concluded that higher amounts of physical activity are associated with better outcomes for weight/adiposity and bone health than lower amounts of activity. However, the available research did not point to a specific dose of activity that was needed to produce these benefits. It is recognized that some authoritative groups have provided public health guidelines on physical activity for children younger than 6 yr (51–53). These guidelines have recommended that young children engage in three or more hours of total physical activity (light, moderate, and vigorous intensity), a level that corresponds approximately to the median for device-based measurement of physical activity in 3- to 5-yr-old children (8).
Further, it is important to acknowledge that the authors conducted this systematic review within certain delimitations. The charge to the 2018 Physical Activity Guidelines Advisory Committee was to consider new evidence that might inform revision of the 2008 Physical Activity Guidelines for Americans. Children younger than 6 yr were not included in the 2008 guidelines because, at that time, very limited research had been conducted on the health effects of physical activity in that age group. Accordingly, an important goal of the 2018 committee was to determine whether or not the available scientific evidence supported a conclusion that physical activity is related to important health outcomes in children younger than 6 yr. Hence the focus of the review was on studies in which amount of physical activity, of various types, was examined in relationship to one or more physiologic risk factors for development of noncommunicable diseases, such as cardiovascular disease, type 2 diabetes, and osteoporosis. The committee did not consider exposures, such as the behavioral quality of the physical activity exposure (e.g., enjoyment) or outcomes, such as fundamental motor skills. Nonetheless, it is noted that these are important constructs and are worthy of consideration in future comprehensive reviews of physical activity and health in young children.
Recommendations for future research
In reviewing the research evidence on the relationships between physical activity and health outcomes in children younger than 6 yr the committee found many areas in which existing evidence is limited and new studies are needed. Table 3 lists seven research recommendations that, if addressed in future investigations, would address current limitations and markedly expand the body of knowledge on physical activity and health in young people. The rationale for each of these recommendations is provided in the full 2018 Physical Activity Guidelines Advisory Committee Scientific Report (50). In particular, there is a need for studies in large samples using rigorous designs and methodologies. Because the committee’s charge was to address questions and draw conclusions that inform public health guidelines on physical activity, the research recommendations identified by the committee were selected on the basis of their relevance to the guidelines development process. It is acknowledged that much remains to be learned about the effects of physical activity on health-related factors in children and youth in many areas that are not directly relevant to public health guidance.
For children younger than 6 yr, the evidence linking physical activity to health was rated as strong only for two outcomes, weight/adiposity and bone health. Accordingly, there is a great need for research that will bolster our knowledge of other health outcomes, particularly including indicators of cardiometabolic health and cognition. Further, existing research is not adequate to identify clear dose–response relationships or to determine whether or not the health effects of physical activity are influenced by demographic factors such as sex, age, maturational status, race/ethnicity, or socioeconomic status. In addition, the research evidence on physical activity and health is very limited in children younger than 3 yr, and for this age group, methodological studies are needed to identify appropriate measures of physical activity for use in future investigations.
SUMMARY AND CONCLUSIONS
The 2018 Physical Activity Guidelines Advisory Committee reviewed the primary research literature addressing the relationship between physical activity and health outcomes in children younger than 6 yr. It was concluded that there is strong evidence indicating that higher amounts of physical activity are associated with better bone health and with better weight status/reduced risk for increases in weight and adiposity in children age 3 to 6 yr. The evidence was too limited to support conclusions regarding the effects of physical activity on cardiometabolic health and cognition, to delineate dose–response relationships, or to determine the influence of demographic effect modifiers. The evidence is particularly limited for children younger than 3 yr.
The committee also considered the relationships between physical activity and multiple health outcomes in children and youth across developmental stages from birth to adolescence. Most of the available evidence addressed these relationships in school-age youth (ages, 6–17 yr). The conclusions for the older age group were consistent with the findings for children younger than 6 yr in that higher amounts of physical activity were found to be associated with beneficial effects on adiposity and bone health.
The authors thank Janna Borden and Gaye Groover Christmus, MPH of the University of South Carolina and Deborah Galuska, PhD of the Centers for Disease Control and Prevention for their contributions to the development of the article. The authors do not have any conflicts of interest.
This article is being published as an official pronouncement of the American College of Sports Medicine. This pronouncement was reviewed for the American College of Sports Medicine by members-at-large and the Pronouncements Committee. Disclaimer: Care has been taken to confirm the accuracy of the information present and to describe generally accepted practices. However, the authors, editors, and publisher are not responsible for errors or omissions or for any consequences from application of the information in this publication and make no warranty, expressed or implied, with respect to the currency, completeness, or accuracy of the contents of the publication. Application of this information in a particular situation remains the professional responsibility of the practitioner; the clinical treatments described and recommended may not be considered absolute and universal recommendations.
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