Effects of Physical Activity on Obesity

Recent estimates indicate that 25% of children in the United States are overweight and 11% are obese. Obesity in children is a risk factor for several diseases (eg, diabetes, hypertension, elevated blood cholesterol). When diagnosed at an early age, the overall impact of the disease is more deleterious than adult-onset diseases because of the likelihood of complications over the course of a lifetime. Obesity has a negative impact on longevity, and the current population of children may live less-healthy and potentially shorter lives than their parents. Adults who were obese as children have increased morbidity and mortality irrespective of their adult weight. Comorbidities are discussed in more detail by other authors elsewhere in this issue.

Obesity also has been linked to poorer cognitive performance and academic achievement. One potential factor contributing to poor academic performance is discrimination of obese students by teachers and peers. Obesity is highly stigmatized and has social consequences, such as weight bias and weight-based teasing. Evidence also shows long-term consequences, such as social and economic discrimination. The role of weight bias in the lives of children and adults is receiving more attention by researchers and the media, but much needs to be done to increase public awareness and examine stigma-reducing strategies.

Physical inactivity is strongly related to the increased prevalence of childhood obesity in the United States. Physical activity plays a role in obesity because it alters the balance between caloric intake and expenditure. Energy intake in excess of energy expenditure results in a positive energy balance and weight gain, whereas expenditure in excess of intake results in weight loss. Physical activity is the largest modifiable component of energy expenditure, accounting for 15% to 30% of total daily energy expenditure. The mechanisms leading to the increased prevalence of overweight and obesity in children and adolescents are not well understood, but lifestyle changes leading to increased sedentary time and decreased physical activity undoubtedly play a role.

Obesity is a complex issue, with contributions from biologic, social, behavioral, environmental, and economic factors. Interventions designed to reduce obesity in children are most effective when they are part of a comprehensive strategy to address dietary and physical activity change, including social support and environmental change. Effective prevention programs likely involve strategies that affect multiple settings and address both energy intake and expenditure. A meta-analysis of obesity prevention studies for children and adolescents showed that the most successful programs for preventing weight gain targeted a variety of health behaviors rather than only weight control. However, current prevention programs are only minimally effective, highlighting the need for more research to identify components of successful interventions for preventing and treating overweight and obesity.

Prevention may be a more critical target in the obesity epidemic because the effects of behavioral interventions for weight loss in overweight and obese children and adolescents are small to moderate. Behavioral family-based physical activity and diet interventions have been successful in achieving weight loss, as have combined dietary, behavioral, and physical activity interventions. Children may benefit from early interventions to diminish obesity, because lifestyle behaviors are learned at an early age and children may have a greater capacity to change their behavior than adults. However, much research is needed to examine interactions between determinants of weight management and how interventions might target these complex behaviors.

Health Benefits

The level of evidence supporting the benefits of physical activity on musculoskeletal health, components of cardiovascular health, and adiposity in overweight youth is strong. Exercise has been shown to reduce triglyceride and insulin levels in overweight children. Furthermore, adequate evidence suggests that physical activity benefits lipid and lipoprotein level, blood pressure, and adiposity in normal-weight youth. Increasing physical activity during childhood has been associated with an increased life expectancy and decreased risk of cardiovascular disease. Developing chronic diseases as a child is problematic because the diseases persist into adulthood, making promotion of physical activity an important disease prevention strategy.

Physical activity is positively associated with aerobic fitness in children, and aerobic fitness has been linked to the risk for chronic diseases and metabolic syndrome. Metabolic syndrome is a group of risk factors that occur concurrently and increase the odds of coronary artery disease, stroke, and type 2 diabetes. In children, these factors include abdominal obesity and the presence of two or more other health risks: elevated triglycerides, low high-density lipoprotein cholesterol, high blood pressure, and increased plasma glucose. In a cross-sectional analysis of fitness and metabolic syndrome, the number of metabolic syndrome risk factors in elementary school children increased across body mass index groups, with the normal-weight, high-fit group possessing the lowest number of risk factors, and the overweight, unfit group possessing the highest number of risk factors. A high fitness level was associated with a lower number of metabolic syndrome risk factors in overweight children than in low-fit, overweight children. Increasing a child’s fitness level could be a viable approach to reducing the risk of obesity-related comorbidities.

An increase in sedentary, indoor activities has contributed to the rise in childhood chronic diseases and obesity. These lifestyle changes have led to concern in the public health community because this “nature-deficit disorder” is depriving children of opportunities to experience the outdoors and engage in physical activity. Time spent outdoors is related to increased physical activity. A longitudinal study of 10- to 12-year-old children showed that each additional hour spent outdoors was associated with an increase in physical activity of 27 minutes a week, and the prevalence of overweight at follow-up was 27% to 41% lower among those spending more time outdoors at baseline. Outdoor activity not only promotes increases in physical activity but also is associated with prevention of other pediatric health issues, such as asthma, myopia, vitamin D deficiency, stress, and attention-deficit hyperactivity disorder. The Child and Nature Initiative was launched to encourage pediatric health care providers to promote time spent outdoors to children and their families. Pediatricians write “prescriptions” for the amount of physical activity recommended by health organizations; however, the physical activity is to be taken outdoors. The prescription includes ideas such as going to a park or playground, riding a bike, and going bird watching.

The Role of Physical Activity in Cognitive Development

Movement itself is a critical component of cognitive development. As a child moves, the interaction between sensorimotor integration and the environment plays an important role in the development of cognitive abilities. The child’s actions become coupled with perceptions of the environmental consequences, which influences further actions. Small variations in how children practice these actions may influence how they learn associations between physical actions and their effects. For this reason, physical activity games that are unpredictable and require problem solving may provide conditions that foster the emergence and development of cognitive abilities.

This action-perception coupling that occurs during movement plays a role in the development of children’s motor skills. Children with greater motor proficiency may find it easier to engage in physical activity. Positive associations between motor proficiency in preschoolers and time spent in moderate-to-vigorous physical activity (MVPA), and inverse associations between motor proficiency and sedentary time have been observed. Similar associations between motor skills and physical activity level have been shown in older children (aged 8–10 years). Targeting improvement in motor skills could be an important way to increase physical activity in children with poor motor skills.

Encouraging unstructured play is another way to increase physical activity levels in children, and could be a strategy to reduce the obesity epidemic. Currently, a trend has been seen away from free play to more structured activities in schools and at home because of family structure and increased focus on academics and enrichment activities. Unstructured play is important for healthy brain development; children learn to engage and interact with the world through play. During play, children gain experience in working in groups, negotiating, conflict resolution, and self-advocacy. It is important for teachers and parents to promote free play because it contributes to the cognitive, physical, social, and emotional development of a child and provides an opportunity for physical activity.

A considerable amount of research has examined the role of physical activity in cognitive function during childhood and adolescent development. A meta-analysis conducted by Sibley and Etnier suggested that physical activity is related to cognitive function during development. A positive association was found between physical activity and cognitive function, including perceptual skills, intelligence quotient, academic achievement, verbal tests, mathematics tests, developmental level, and academic readiness in school-aged children (aged 4–18 years). The results of this meta-analysis highlighted the importance of promoting physical activity in children and adolescents for the purpose of optimizing cognitive development.

The Impact of Physical Activity on Academic and Cognitive Performance

Increasing evidence supports the notion that children who engage in more physical activity have better academic performance. Several large-scale correlational studies have shown associations among measures of physical activity, fitness, and academic achievement. Experimental studies have also supported the role of physical activity in improving standardized achievement scores, reading scores, and grades. Other experimental and correlational studies have shown no effect of physical activity on school achievement. However, the results of some of these studies must be interpreted cautiously because of lack of random assignment and potentially biased measures of achievement.

Recent studies have also shown the positive relation between cardiovascular fitness and cognition in preadolescent children using several measures, such as reaction time and response accuracy, brain function, and brain structure. Evidence from a randomized controlled trial also shows that sedentary children who become physically active experience enhanced cognitive abilities. Children who were randomized to a 13-week exercise program performed better on the “planning” scale of the Cognitive Assessment System, a standardized assessment of cognitive processes. The results suggest that physical activity interventions may have selective effects on children’s cognition. Increasing physical activity in children may produce the greatest improvements in complex mental processing, known as executive function , which includes the ability to achieve goal-directed behavior, self-monitoring, and self-control.

Schools provide a unique opportunity for young people to engage in physical activity, because most children in the United States spend most of their day there. Unfortunately, many school districts have removed physical activity opportunities from the school day despite increasing literature showing the impact of physical activity on cognitive and academic performance. Studies that examined physical activity during physical education classes, at recess, and in the classroom were recently reviewed in a publication by the Centers for Disease Control and Prevention (CDC). Much of the research in this area has examined the influence of physical education classes on academic achievement. Of the 14 studies of physical education interventions, 11 reported one or more positive associations between physical education and academic performance; the remaining 3 showed no significant associations.

Other studies that were included in the CDC review examined the effects of classroom-based physical activity in the form of physically active academic lessons or short activity breaks between regular academic lessons. Eight of the nine reviewed studies reported a positive significant association between classroom-based physical activity and indicators of academic achievement. The results of a 3-year randomized control trial by Donnelly and colleagues were not included in the CDC review. This study examined the impact of physically active academic lessons on academic achievement and BMI. Physically active academic lessons of moderate intensity improved performance on a standardized test of academic achievement and slowed the rate of BMI gains in students with the greatest exposure to the intervention. Physically active academic instruction does not compete for time allocated for academic instruction and provides an inexpensive, easily implemented, and sustainable approach that may allow elementary schools to meet the competing demands of improving student health while also improving academic achievement.