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Juvenile Obesity, Physical Activity, and Lifestyle Changes

Cornerstones for Prevention and Management

Oded Bar-Or, MD

THE PHYSICIAN AND SPORTSMEDICINE - VOL 28 - NO. 11 - November 2000


In Brief: Because many obese children and adolescents become obese adults, the recent rapid increase in juvenile obesity poses a major public health challenge. Obese children and youth are often more sedentary than their nonobese peers, but a low level of physical activity has not yet been proven as a cause of obesity. Nevertheless, enhanced physical activity is a cornerstone in a multidisciplinary approach to preventing and treating juvenile obesity. Giving exercise recommendations focused for obese youth is crucial. For example, patients may do best with aquatic exercise and in groups of obese peers. Cutting down on sedentary behaviors, like watching TV, can reap long-term benefits.

The prevalence of juvenile obesity has risen in the last decade to exceed 20% in the United States, (1,2), and the condition is particularly prevalent among Hispanic and African-American girls. While juvenile obesity is associated with various risk factors for adult chronic diseases, (3,4), the more immediate morbidity, as perceived by the obese youth themselves, is psychosocial (5). Obese adolescents often suffer from low self-image, (6), psychiatric comorbidities, (7), and discrimination by their peers (8). About 50% of obese children 6 years or older are likely to become obese adults, compared with 10% of children who are not obese (9). About 70% of obese adolescents are likely to become obese adults. This risk increases to 80% when one parent is obese.

Because of the relationship between adulthood obesity and conditions such as coronary artery disease, diabetes mellitus, and hypertension, juvenile obesity has become a major public health challenge. This is true not only in technologically advanced countries such as the United States and Canada, but also in less affluent societies (10). Prevention and management of juvenile obesity is therefore of utmost clinical and public health importance. Attempts to instill healthy lifestyle habits during early years will likely be more successful than during adult years (11,12). Evidence indicates that low physical activity and energy expenditure levels are etiologic factors in juvenile obesity, and that enhanced physical activity is efficacious as part of the overall management of juvenile obesity.

Are Obese Youth Insufficiently Active?

Causes for the increasing prevalence of juvenile obesity are not clear. Some epidemiologic observations, however, suggest that a decrease in daily energy expenditure without a concomitant decrease in total energy and fat intake may be the underlying factor (11,13).

Various cross-sectional and longitudinal studies reviewed in Bar-Or and Baranowski (14), and Gutin and Humphries (15), have addressed the possible relationship between adiposity and physical activity in children and youth (16-19). Some, but not all, (20-23), have shown that obese children and adolescents are less physically active than their nonobese peers.

Of particular interest is the strong relationship between the prevalence of juvenile obesity and the extent of TV watching (18,24,25). One study, (24), based on a nationwide sample of US children and adolescents, reported that each additional hour of TV viewing per week increased the risk for obesity by 2%. The likelihood of recovering from obesity after 4 years of being obese is inversely related to the extent of TV viewing (25). Although TV viewing is a sedentary activity, there is surprisingly little (26), or no (27), relationship between daily physical activity and the amount of time children watch TV. An important effect of TV watching is that children snack excessively, possibly because of the large number of food-related commercials (28).

It is tempting to assume that the association between obesity and a low activity level reflects low total energy expenditure in sedentary individuals; however, the relationship between daily energy expenditure and the habitual activity level of children is quite complex. Measured using the doubly labeled water technique—considered the "gold standard" for measuring energy expenditure in free-living individuals—the daily energy expenditure of obese children is similar to, or even higher than, that of nonobese children when calculated in absolute energy units (eg, kilocalories per 24 hours) (11,29). This may reflect the high energy cost of moving a heavy mass (30).

One approach to account for differences in body mass is to divide total daily energy expenditure by resting energy expenditure. The resulting ratio is called the physical activity level (PAL). A study (31), of 1.5- to 4.5-year-old children found an inverse relationship between adiposity and PAL, which suggests that obesity in children is accompanied by a low energy expenditure; however, in a study (29), of 12- to 18-year-old girls and boys, there was no relationship between adiposity level and PAL.

Another approach to express the energy expenditure that results mostly from activity is to subtract basal or resting energy expenditure from total energy expenditure. In one study, (29), adiposity of 12- to 18-year-olds was inversely related to their activity energy expenditure. This, however, was not found among 5- to 7-year-olds (32).

One possible reason for the lack of clear association between juvenile obesity and energy expenditure is that the positive energy balance that is required to become obese can be very small. For example, to gain 4 kg of fat over 1 year, the excess daily energy intake over energy needs is less than 80 kcal. In a 50-kg child, this is equivalent to one slice of bread or 8 to 10 minutes of playing basketball. Such a minor energy imbalance can hardly be detected by available analytical techniques.

It should also be pointed out that the studies cited above did not observe subjects while they were becoming obese; they merely reflect a snapshot of individuals who already are obese. Attempts to determine whether low energy expenditure during the first few months of life precedes the development of obesity have yielded equivocal results. In one study, (33), the development of obesity at age 1 year was associated with low daily energy expenditure in the first weeks of life. This was not confirmed in subsequent studies (34-36).

In conclusion, while it seems that obese children and youth are usually less physically active than their leaner peers, there is inadequate evidence that energy expenditure is lower in obese individuals. Nor is it clear whether a low level of energy expenditure in infancy is a precursor of childhood obesity.

Enhanced Physical Activity

Enhanced physical activity (EPA) is only one element in the management of juvenile obesity. Other cornerstones of management are dietary changes and behavior modification. No surgical techniques or pharmacologic interventions have been found safe and efficacious for obese children or adolescents (12).

Several studies (14,15,37-42), have compared EPA vs no intervention, or EPA plus diet vs diet alone. Even though cross-sectional studies found that young athletes are usually leaner than nonathletes, training studies (14,37), of nonobese individuals do not show a reduction in body fatness. In contrast, EPA in obese individuals does cause a decrease in body fatness, albeit minimal (38,39).

Low-calorie diets, however, are more effective in reducing body fatness. The main reason for such a difference is that the negative energy balance achieved through EPA is considerably smaller than that induced by dieting. For example, a typical 30- to 40-minute aerobic exercise session for a 10- to 11-year-old is equivalent to no more than 200 to 250 kcal (40). In contrast, the reduction in energy intake that can be achieved through a low-calorie diet sometimes exceeds 1,000 kcal/day.

A potentially detrimental effect of a low-calorie and, especially, very-low-calorie diet is growth stunting, (43,44), which does not occur with exercise. While the exact prevalence of growth stunting and the dose-response relationship between growth stunting and hypocaloric diets is not known, it is usually safe to decrease the existing intake of a school-age child by 200 to 300 kcal/day.

Another problem with very-low-calorie diets is the loss of fat-free mass (45,46). Even if transitory, a loss of fat-free mass in a growing individual is inadvisable. An important potential benefit of EPA is that, when added to a very-low-calorie diet (under close physician supervision), it would decrease or prevent the loss of fat-free mass through the anabolic effect of exercise. This approach was attempted when markedly obese children and adolescents were given a daily aerobic program together with a protein-sparing modified fast (47). While total body weight and body fat decreased, there was no decrease in fat-free mass during the 10-week intervention. This study did not include a control group of diet only. More research is needed in this important area. One promising issue to investigate is the possible effect of resistance training on the preservation of muscle mass.

Effects of EPA, other than on body composition, include a reduction in blood pressure, increased sensitivity to insulin, improved lipoprotein profile, and an increase in physical fitness (15,41). There is some evidence that structured exercise interventions increase the obese child's spontaneous activity and overall energy expenditure (40,48). So far, there is little or no evidence that EPA programs can affect abdominal or visceral fat content (15,42).

A Practical Program for Juvenile Obesity

Is there an optimal EPA program? No studies have systematically compared the efficacy of various exercise dosages; however, practical experience in my clinic, backed by some research findings, has shown that the following elements can be effective:

  • Activity must involve large muscle groups to induce large energy expenditure. Examples include walking, cycling, swimming, dancing, cross-country skiing, skating, basketball, and soccer. By performing such activities for 30 to 45 minutes, 10- to 11-year-old obese children burned 200 to 250 kcal (40). This amount will vary according to the body weight of the child and the intensity of exercise.
  • It is the total energy expenditure, rather than the intensity of the activity, that matters. For example, walking 1 mile will have an almost identical effect to that of running 1 mile. At the start of a program, the intensity and duration of the activities should be low and gradually increase as the program progresses.
  • Activity must be fun, and the child should enjoy it. A play-like, recreational atmosphere is particularly important for children in the first decade of life. Compared with structured prescriptions, "lifestyle" activities yield more compliance during the intervention and a greater adherence once the structured element of the program has concluded (49).
  • Children are more active outdoors than indoors. Temptations for inactive pursuits (eg, TV watching, computer games), as well as for snacking, are greater indoors than outdoors.
  • Water-based activities are often more suitable for obese patients than are land-based activities. The advantages of aquatic activities are threefold: (1) Because of their high fat content, obese individuals are more buoyant than their leaner peers, (2) subcutaneous fat is an excellent thermal insulator, which gives obese people an advantage in cool water, and (3) during water-based activities, most of the body is submerged. This provides a psychological advantage over land-based activities in which the body shape of the obese child is exposed.
  • Obese children and, particularly, adolescents feel less inhibited when they exercise in the company of other obese patients, rather than exercising with nonobese people.
  • Reduction of sedentary pursuits, such as TV viewing, will help divert the child to more active pursuits.

Reducing Sedentary Behavior

Reduction of sedentary pursuits in favor of more physical activities merits special attention. It appears that behavior modification in which children are rewarded for reducing their TV-watching time yields better weight control than regimented exercise prescriptions (50). Furthermore, the long-term effect of this approach remains long after the program concludes. The efficacy of this approach has also been shown in a school environment. In a recent randomized, controlled trial, (51), third-and fourth-graders who reduced their TV, videotape, and video game time over a 7-month period had a statistically significant reduction in body adiposity.

Neither diet nor EPA, even when combined, induces a long-lasting effect beyond the duration of the program (11,12,14). This issue is of immense practical and theoretical importance, because most obese patients who respond well while on a weight-control program lose ground rapidly once the program ends. The apparent solution is to incorporate behavior modification as an integral part of the intervention (12). Such an approach, particularly if it includes the child and a parent, (8,52), seems most effective. One study (8), has shown adherence up to 10 years after the end of the program.

Prevention Notes

Obese children and youth tend to be sedentary. Increasing their activity, particularly when combined with improved nutritional habits and behavior modification, is an important element in the management of juvenile obesity. The best management of juvenile obesity is prevention. While we do not yet know how to optimize prevention, it is likely that efforts should be done within the family unit, at school, and at the community level. More research is needed to elucidate optimal activity prescriptions for different age-groups, as well as the interaction between enhanced activity and other therapeutic modalities.

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Dr Bar-Or is a professor of pediatrics and director of the Children's Exercise & Nutrition Centre, Dept of Pediatrics, McMaster University and Hamilton Health Science Corporation in Hamilton, Ontario. Address correspondence to Oded Bar-Or, MD, Children's Exercise and Nutrition Centre, Chedoke Hospital Division, Evel Bldg 4, Sanatorium Rd, Hamilton, Ontario L8N 3Z5; e-mail to: [email protected]


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