One in 3 US children is obese (1) and therefore vulnerable to obesity’s immediate (2,3) and long-term (4) negative health consequences. There has been a marked proliferation of school interventions in recent years (5,6) targeting many elements of the school setting, including the academic curriculum, the physical and social environment, food and physical activity policies, and governance structures (6,7). Qualitative and meta-analytic reviews of this expansive literature suggest that school-based interventions have at least a moderate positive effect on obesity prevention in children (8–11).

Much less attention has been directed toward out-of-school time than in-school time. Yet most students are out of school for 185 to 190 days per year (12). It is important to consider what happens during extended periods of out-of-school time, particularly the summer break, when children and adolescents do not have access to the school lunch program and regular physical activity during recess and physical education. Although an academic “summer achievement slide” has been documented, particularly in groups of people with low socioeconomic status (SES) (13,14), work to date has not considered whether parallel disparities in weight status exist as well.

Several studies (15–21) have examined whether weight gain in children occurs at a faster or more variable rate during summer vacation compared with the school year. Although a recent review considered this body of literature (22), the study was not conducted from a health disparities perspective. The objective of our study was to compile and synthesize research on seasonal patterns of weight gain, with an emphasis on racial/ethnic disparities, to provide greater insight into the role of schools in the prevention of child obesity, the need for summer interventions to control weight gain in children, and the possible need for such programs to focus on certain demographic groups.

Of the 7 studies reviewed, 6 identified accelerated weight gain among school-aged children during the summer vacation compared with the school year for at least a portion of the study population (15–19,21). In the general population, a large nationally representative longitudinal study found evidence of accelerated summer weight gain among kindergarten and 1st-grade children (17), and a 5-year longitudinal study in Texas found evidence of accelerated summer weight gain among elementary students (18).

Findings varied among study populations and by race/ethnicity. Whereas the studies conducted among American Indians (19,20) and in Japan (21) did not observe accelerated summer weight gain compared with the school year for their overall study populations, a large nationally representative longitudinal study in the United States observed pronounced effects of summer vacation on weight gain among black and Hispanic children (17). In contrast, a smaller longitudinal study in Texas did not observe differences by race/ethnicity (18).

We found fairly strong support for accelerated summer weight gain among children who were already overweight or obese; 6 of 7 studies demonstrated this trend across diverse study populations (15–19,21). Both longitudinal studies observed a pronounced effect of summer on weight gain among overweight students (17,18). Similarly, accelerated summer weight gain was observed in 2 studies focusing on overweight and obese children participating in weight-control trials (15,16). In 2 studies, accelerated summer weight gain was limited to children who were overweight at baseline (19,21).

The studies reviewed suggested many potential mechanisms for accelerated summer weight gain. Such mechanisms included decreased physical activity (15,19), increased sedentary behaviors (16–18,21), increased access to unhealthy snacks (17,18,21), unstructured schedules and boredom (15–17), less self-monitoring (15–17), irregular sleep patterns (16,17), and less access to healthier meals through school breakfast and lunch (16,18) during the summer relative to the school year. These mechanisms are purely speculative, however: none of the studies tested these potential mechanisms. Few studies have examined school-year versus summer-time differences in children’s diet, physical activity, and screen-related behaviors.

Overall, findings suggest that school-aged children may gain weight at a faster rate during the summer compared with the school year and that accelerated rates are most pronounced among those at greatest risk of obesity (ie, overweight children and potentially racial/ethnic minority children). Reasons for faster rates of summer weight gain among overweight and racial/ethnic minority children are unclear. The most likely mechanism is SES. Racial/ethnic minority and overweight children are disproportionately represented in low-SES groups. Children from low-SES families may have less access to out-of-school activities such as summer camps (25), which may protect against summer weight gain. Children who attend summer camps engage in greater levels of physical activity during the summer (26), and children who engage in organized activities during the summer are at lower risk for obesity than children who do not engage in organized activities (27).

This pattern parallels the well-documented summer achievement slide in which students’ reading and math scores decline during the summer, particularly in low-SES groups (13,14). As with the summer achievement slide, accelerated summer weight gain among low-SES children may exacerbate disparities. While many interventions have been developed to address declining math and reading skills during the summer (28,29), we are unaware of any programs targeting summer weight gain, particularly among low-SES groups. Although Baranowski and colleagues recently reviewed this body of literature (22), their conclusions focused on differences between overweight and obese children and healthy-weight children and emphasized seasonal differences in physical activity; they did not fully consider the implications by race/ethnicity and health disparities.

Our review has several implications. Children are engaged in a range of settings during the summer, including the family, community programs, and summer camps. These settings should be considered as possible solutions for unhealthy patterns of weight gain during the summer, when children do not have the regular school-time physical activity opportunities of recess and physical education. Greater access to summer camps and parks and recreation programming, improved food and physical activity policies for summer camps, and increased access to community physical activity resources through the development of joint-use agreements are also potential solutions. Increased use of summer food programs, which provide meals outside of the school year, should also be considered as a potential mechanism for improving eating habits among populations that have unhealthy summer weight gain.

Our study has several limitations. Because of the small number of studies identified, any conclusions drawn from this review are tentative. Relatively broad inclusion criteria were set to identify as many studies as possible. This review did not exclude studies from countries other than the United States. However, cultural or structural differences between countries may prohibit direct comparisons. For example, the summer break in Japan is only approximately 40 days (21), which is shorter than breaks in many school districts in the United States, potentially mitigating any detrimental effects of the summer break on weight gain.

Conclusions from this review are also limited by methodological limitations of the studies identified. Three of the 7 studies reviewed were interventions (15,16,20), and 2 studies were school-based interventions targeting obesity and its behavioral precursors. Although such studies are meaningful and can inform our understanding of summer weight gain, they do not provide a clean test of the association between summer and weight gain. Data from school-based interventions effectively test differences in weight gain between the school year (plus intervention) and the summer (without intervention). This design confounds the absence of school with the absence of an intervention. As noted by von Hippel et al, season also confounds any assessment of differences in weight gain during the school year and summer (17). Other limiting factors include 1) the use of clinical populations (16) which limits the generalizability of findings; 2) the use of data compiled between 1972 and 1998 (for example, Kobayashi and Kobayashi [21]), a period of shifting growth patterns and accelerated weight gain among children worldwide; 3) the absence of studies testing mechanisms of summer weight gain; and 4) differences in the extent to which studies accounted for variations in children’s height and weight that reflect expected growth.

Finally, a limitation in making comparisons across studies is the use of various measures of change in weight status. Although BMI z scores may be preferable for comparing weight status on a single occasion, BMI or percentage-change in BMI are preferred for measuring and interpreting changes (23,24). For longitudinal studies of child obesity, change in BMI is recommended as a measure over change in BMI z score because z scores are derived from charts developed by the Centers for Disease Control and Prevention using cross-sectional data and may not reflect actual growth patterns of children (23). In particular, the study by Moreno et al, which identified no significant differences by race/ethnicity (18), and the study by Zhang et al, which identified no significant differences at all (20), were based on the use of BMI z scores.

The studies reviewed suggested that rates of weight gain accelerate during the summer compared with the school year, particularly among racial/ethnic minority and overweight children. Because of the implications for preventing childhood obesity and reducing health disparities, additional research on this topic is needed. To determine whether the findings of the studies in this review reflect a real pattern, additional longitudinal (nonintervention) research is needed to assess in-school versus out-of-school differences in student BMI and differences by race/ethnicity, weight status, and SES. Such research would not only inform clinicians, policy makers, educators, and family members seeking to promote healthy childhood growth but would also help to determine whether summer-time resources should be universally applied or targeted to certain groups to most effectively reduce health disparities.