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U.S. Teens in Our World

Understanding the Health of U.S. Youth in Comparison to Youth in Other Countries

Table of Contents | Executive Summary | Introduction | Chapter 1 | Chapter 2 | Chapter 3 | Chapter 4 | Chapter 5 | Chapter 6 | What Was Left Out | Summary

CHAPTER TWO: FITNESS

In this chapter:
Fitness | Physical Activity | Leisure-Time Activities | Nutrition | What Didn't We Know | References

FITNESS

Fitness underlies much of the overall health and well-being described in the previous chapter, affecting both current and future physical and psychological health. Physical activity, nutrition, and lifestyle all contribute to adolescents' daily functioning.

Adolescence is a critical period for the onset of obesity and for obesity-associated illnesses in later life.1,2 During adolescence, overweight youth may face discrimination, rejection, and low self-esteem, affecting their social relationships, school experiences, psychological well-being, and future aspirations.3,4 The U.S. Surgeon General's Call to Action to Prevent and Decrease Overweight and Obesity addresses the epidemic of overweight and obesity among U.S. youth.5 A number of factors contribute to this epidemic, including increases in the number of calories consumed, low levels of physical activity, and high levels of sedentary behavior.

Promoting healthful physical activity is an important way to combat this epidemic while establishing habits that can be sustained into adulthood. Scientific research over the last 50 years, primarily in adults, has shown that moderate physical activity is associated with a number of physical and mental health benefits, while a lack of physical activity has negative consequences. In addition to reducing the risk for being overweight or obese, regular, moderate physical activity also reduces the risk of coronary heart disease, cerebrovascular disease, hypertension, type II diabetes, osteoporosis, and deterioration of functional capacity.6 Among mental health benefits, exercise reduces the risk of anxiety and can improve self-esteem.7,8 In addition to its preventive benefits, physical activity is also recommended as a treatment for clinical depression and type II diabetes.6,7,9 In adults, low levels of physical activity are associated with high risk of diabetes as well as high rates of cardiovascular deaths and deaths from all causes.9

Like adults, adolescents achieve considerable advantages from regular physical activity. As noted in Physical Activity and Health: A Report of the Surgeon General,10 increased physical activity levels in children and adolescents can reduce their coronary heart disease risk factors, such as lowered blood pressure, reducing obesity and helping control type I diabetes. Moreover, physical activity provides physical benefits during childhood and adolescence, such as increased bone mass, and has positive effects on psychological well-being.10,11 Sedentary adolescents have higher resting blood pressure than physically active students, while physically active adolescents have a more favorable blood lipid profile, in terms of their cholesterol and triglycerides, than their sedentary peers.13 The Surgeon General's Call to Action5 stresses the need to reduce sedentary behaviors, such as watching TV, which are also associated with increased consumption of snack foods and soft drinks among adolescents.14 Besides these dietary associations, time spent watching TV detracts from time available for active behaviors. The American Academy of Pediatrics recommends limitation of television viewing to one to two hours per day.15

Adolescents' eating habits not only affect their risks for overweight but also may hamper their ability to grow normally and to conduct their daily activities.16 An increase in the velocity of growth (“a growth spurt”) associated with hormonal, cognitive, and emotional changes of adolescents, and proper nutrition is necessary for this growth. Adolescents' intake of saturated fat, total fat, sodium, and soft drinks, with their relatively low intake of fruits, vegetables, fiber, and calcium-containing foods, may be increasing their future risk of cardiovascular disease, cancer, and osteoporosis.17 Among adolescents, low levels of fiber and high levels of saturated fat, total fat, and sodium may be related to low intake of fruits and vegetables.18 Of all the food groups, the fruits and vegetables groups are the ones that adolescents are least likely to eat in sufficient quantities.19

Even temporary eating habits during adolescence can have a long-term impact on future risk for osteoporosis, since skeletal maturity is achieved during the late stage of pubertal development.20 Bone growth during adolescence accounts for about 45 percent of total attained peak bone mass.21 A decline in milk consumption during adolescence appears to be related to the increase in soft drink consumption by youth, with teens drinking twice as much soft drinks as milk, a reversal of patterns found during the early 1980s.22,23 Replacement of milk with soft drinks may affect bone health through several mechanisms, including reductions in calcium, bone resorption (breakdown) from phosphoric acid in soft drinks, and possible increased calcium excretion from caffeinated drinks.24,25

Other nutritional issues include the relationship of dietary intake and eating patterns on academic achievement; in particular, breakfast consumption and iron intake. Iron deficiency anemia is a nutritional concern for adolescent girls, which can result in fatigue, reduced attention span, and impaired intellectual performance.26

Dietary behavior is also a factor in other weight-related disorders such as anorexia nervosa, bulimia nervosa, anorexia/bulimic behaviors, adverse dieting behaviors and binge eating disorders. These disorders tend to peak in prevalence and severity during adolescence: 10 to 20 percent of girls show anorexic and/or bulimic behaviors, and the more severe forms of these disorders are classified as mental illnesses. Girls are more likely than boys to report dieting behaviors, often because of concern with self-image. While some diets may be healthful, some, such as fasting, are very unhealthy. The Surgeon General's Call to Action stresses the importance of addressing weight and physical activity from the perspective of good health rather than from concern with body image.

This chapter shows how U.S. students compared to students from other countries on measures related to fitness, including frequency and intensity of physical activity, amount of television viewing, and dietary habits. The comparisons of physical activity across countries should be interpreted cautiously, because the questionnaires were not administered at the same time during the school year in every country and seasonal differences in opportunities for outdoor activity vary greatly from one country to another.

PHYSICAL ACTIVITY

While most U.S. students exercise on two or more occasions per week, comparatively they rank in the lower half of countries for exercise frequency. And, as is the case with all other countries measured, regular exercise is more common among boys than girls, with 74 percent of boys and 54 percent of girls exercising twice a week or more. For boys, exercise frequency of two times or more per week ranges from 90 percent in Northern Ireland to 60 percent in Greenland. For girls, the most frequent exercisers are in Germany and the Czech Republic (66 percent) and the least frequent are in Greenland (37 percent). For exercise frequency, perhaps the most disturbing trend is the decline among girls as they age. While 65 percent of U.S. 11-year-olds report exercising two times or more per week, that figure declines to 62 percent for 13-year-old girls, and 54 percent for the 15-year-olds, as noted above. Unfortunately, this same downward trend exists among girls in all the countries measured, while there is not a decline among boys.

U.S. students rank in the middle for exercise length, with more than half reporting that they exercise for two hours or more a week. As with exercise frequency, U.S. boys are more likely than girls to exercise two hours or more a week: 67 percent compared to 51 percent. There is also wide variability among countries for exercise length. Eighty-six percent of Austrian boys exercise at least two hours a week compared to 48 percent of boys in Portugal. Among girls, 70 percent of students in Germany report exercising two hours or more per week compared to 25 percent in Portugal.

Graph: "How Often Do You Usually Excercise In Your Free Time So Much That You Get Out Of Breath Or Sweat?"[d]

LEISURE-TIME ACTIVITIES (TELEVISION WATCHING)

Interestingly, the percent of frequent television watchers among U.S. students declines steeply from age 11 to age 15. U.S. students at age 11 are in the top third of countries for frequent television watching, with 34 percent of girls and 36 percent of boys reporting that they watch four or more hours of television per day. At age 13, U.S. students are in the middle range of countries, with 28 percent of girls and 33 percent of boys reporting heavy television viewing. By age 15, as the chart indicates, the U.S. students ranked in the lowest third of countries, with 18 percent of girls and 27 percent of boys watching television for four hours or more per day. Among boys, frequent television watching ranges from 46 percent (Lithuania and Slovakia) to 17 percent (France). Similar patterns hold true for girls.

Graph: "How Many Hours A Day Do You Usually Watch TV?"[d]

NUTRITION

The U.S. ranked among the lowest third of countries, with 58 percent of girls and 53 percent of boys reporting daily fruit consumption. Proportions eating fruit in other countries ranged from about one third of students in Greenland to over 90 percent in Portugal. Across countries more girls than boys ate fruit, with the proportion decreasing with age. U.S. students followed this gender and age pattern.

Students in the U.S. were among the top five countries in the proportion eating fried potatoes daily: 21 percent of U.S. girls and 31 percent of boys. Across countries, more boys than girls ate french fries or fried potatoes every day, with the proportion decreasing with age. Levels were highest in Northern Ireland, Scotland, Israel, England, and the U.S. About 5 percent or fewer students ate fried potatoes in the nine countries with the lowest proportions.

  Graph: "How Often Do You Eat Fruit?"[d]   Graph: "How Often Do You Eat French Fries Or Fried Potatoes?"[d]

U.S. students ranked about in the middle of all countries (11th out of 28 countries) with 51 percent of girls and 53 percent of boys eating sweets or chocolate daily. Across countries, candy consumption decreased with age and gender differences were minimal.

U.S. students rank third for daily consumption of soft drinks with sugar, following only Northern Ireland and Israel, with 66 percent of girls and 72 percent of boys. Across countries, more boys than girls drank soft drinks every day and boys showed a greater increase in this percentage with age. Most other countries have a wider gender gap than in the U.S. In nearly two-thirds of countries, half of all 15-year-old boys drank soft drinks every day, compared to one-quarter of girls.

  Graph: "How Often Do You Eat Candy Or Chocolate?"[d]   Graph: "How Often Do You Drink Soft Drinks?"[d]

U.S. students were most likely to be on a diet or to feel that they should be across countries. Girls far outpaced boys, 62 percent to 29 percent. Gender differences were strong in all countries. In nearly all countries, younger students were satisfied with their weight, but by age 15, nearly half of the girls in 16 countries were dieting or felt that they should be on a diet.

Graph: "Are You On A Diet To Lose Weight?"[d]

What Didn't We Know?

Most U.S. students exercise twice a week or more but still rank among the lowest among all countries for frequency. Of those who exercise, U.S. students rank in about the middle for time spent exercising. In all countries, boys exercise more than girls and the proportion of girls who exercise decreases between ages 11 and 15. The HBSC doesn't include all possible activities involving exercise that students may participate in due to the complexity of establishing comparability among country-specific activities and changes in participation rates over the recall time of the survey.

Time spent watching TV decreases with age for U.S. students, a pattern that is not consistent across all countries. The HBSC asked about time spent playing computer games, but not time spent on other computer activities such as using the Internet for other activities and chatting with friends. The proportion of U.S. students who play computer games and time spent playing also tended to decrease with age, again not a consistent pattern across all countries (data not shown).27 We don't know if time spent using the Internet compensates for the decreasing time spent with TV as U.S. students age.

An analysis across all countries in the HBSC shows that the hours spent watching television or playing computer games are correlated with increased consumption of soft drinks, sweets, and potato chips. The correlation is particularly strong between TV hours and soft drinks and sweets for 11- and 13-year-old boys and 11-year-old girls in all countries.27 U.S. research documents increases in portion sizes and use of soft drinks; with students drinking twice as much soft drinks as milk, a reversal of patterns of 20 years ago.19,22 We didn't know that U.S. students are more likely to consume soft drinks and french fries than students in almost all other countries, but the relation between the diet of U.S. students, their physical activity, and obesity is firmly established.5 The HBSC study didn't ask how often U.S. students eat out in comparison to other countries, nor does the HBSC have information on serving sizes. Serving sizes in restaurants and fast-food or carry-out locations have increased in the U.S., contributing to the epidemic of obesity.23,28

The international comparisons of factors related to fitness in this report are suggestive, raising questions not only about exercise, diet, and obesity, but about the relationship of fitness to U.S. students' overall health and well-being as described in the previous chapter. We have been concerned about increases in obesity among children and adolescents since the 1970's.5 As noted in the previous chapter, comparatively high proportions of U.S. students report negative feelings about themselves, feeling low and lonely, with relatively high rankings on physical symptoms such as headache, backache, stomachache, difficulty sleeping, and feeling tired in the morning.

The social and psychological concerns described under health and well-being may be correlated with the higher reports of negative feelings associated with poorer fitness profiles. As noted by Faulkner, Gortmaker, and others, weight status is associated with social relationships, school experience, psychological well-being, and future aspirations with possible economic consequences.3,4 Faulkner notes that obese girls report more adverse social, educational, and psychological correlates; obese and underweight boys noted more adverse social and educational correlates. In the HBSC, U.S. students ranked first in feeling they should be on a diet, with two-thirds of girls feeling this way. This is a higher proportion than are actually overweight in the U.S., reinforcing recommendations by the Surgeon General that weight concerns should focus on health rather than body image.5

These findings suggest that the social and psychological risks associated with not meeting weight and body shape ideals may be embedded in our larger culture, while the analysis by Gortmaker, et al., documents that overweight during adolescence has important social and economic consequences in adulthood for outcomes such as lower educational levels, income, and marriage rates. Of course, the family, neighborhoods, communities and cultural norms also influence these outcomes among students, along with patterns of diet and physical activity.5 U.S. students were not among the HBSC students who exercise most frequently or spend higher proportions of time exercising. Time spent with TV and using computers may be correlated with physical activity, obesity, and diet. As noted by Robinson, children who watch more TV are more likely to eat high-fat food and drink soft drinks.14

Preliminary data analysis from some HBSC countries indicate that U.S. students ages 11, 13 and 15 are significantly more likely to be at risk of overweight and obesity than students in any of the 15 European countries collecting measures on height and weight. This is of particular concern as we have recognized the emergence of type II diabetes as prevalent in U.S. youth.5

The international report of the HBSC reported that over all the countries, students on diets had a higher daily consumption of fruits and vegetables and low-fat milk, and lower consumption of less nutritious foods.27 Along with the substitution of soft drinks for milk among U.S. students, we are concerned about the effects of soft drink consumption on bone mineral density due to increased odds of bone fractures in both physically active and inactive girls who drink carbonated beverages and the longer term risks for osteoporosis.21,24

There are strong implications of U.S. student patterns of physical activity, obesity, and diet for their overall health and well-being as described in the previous chapter. It may be that the high levels of overweight and poor body image are contributing to the relatively higher levels of negative feelings reported. Our physical activity and diet patterns may be contributing to high reports of feeling low, difficulties sleeping and tiredness in the mornings. Carbohydrate loading from both the types of foods and soft drinks may affect efficient daylong functional status and sleep. In addition, the high levels of caffeine in our soft drinks, along with coffee consumption not measured in the HBSC, may also contribute to both the physical and psychological symptoms of headache and feeling low. Physical activity, diet, health and well-being are also integrally affected by our family and peer relationships, as described in the following chapter.

REFERENCES:

* Note: If you used a link in the text to reach these footnotes, please use the "Back" button on your browser to return to the text you were reading.

  1. Dietz WH. Health consequences of obesity in youth: Childhood predictors of adult disease. Pediatrics 1998;101(suppl):518-525.
  2. Whitaker RC, Wright JA, Pepe MS, Seidel KD, Dietz WH. Predicting obesity in young adulthood from childhood and parental obesity. N Engl J Med 1997;337:869-873.
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  4. Falkner NH, Neumark-Sztainer D, Story M, Jeffery RW, Beuhring T, Resnick MD. Social, educational, and psychological correlates of weight status in adolescents. Obesity Research 2001;9:32-42.
  5. U.S. Department of Health and Human Services. The Surgeon General's call to action to prevent and decrease overweight and obesity. Rockville, MD: U.S. Public Health Service, 2001.
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  10. Centers for Disease Control and Prevention. Physical Activity and Health: A Report of the Surgeon General. Washington, DC: Centers for Disease Control and Prevention, National Center for Chronic Disease Prevention and Health Promotion; President's Council on Physical Fitness and Sports, 1996.
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  14. Robinson T. Reducing children's television viewing to prevent obesity. JAMA 1999;282:1561-1567.
  15. American Academy of Pediatrics, Committee on Communications. Children, adolescents, and television. Pediatrics 1995;96:786-787.
  16. Spear BA. Adolescent growth and development. J Am Diet Assoc 2002;102 (Suppl):S23-29.
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  18. Munoz KA, Krebs-Smith SM, Ballard-Barbash R, Cleveland LE. Food intakes of U.S. children and adolescents compared with recommendations. Pediatrics 1997;100:323-329.
  19. Kennedy E, Goldberg J. What are American children eating? Implications for public policy. Nutr Rev 1995; 53:111-126.
  20. Cadogan J, Eastell R, Jones N, Barker ME. Milk intake and bone mineral acquisition in adolescent girls: Randomised, controlled intervention trial. BMJ 1997;315:1255-1260.
  21. Sentipal JM, Wardlaw GM, Mahan J, Matkovic V. Influence of calcium intake and growth indexes on vertebral bone mineral density in young females. Am J Clin Nutr 1991;54:425-428.
  22. Jacobson MF. Liquid Candy: How soft drinks are harming Americans' health. Washington, DC: Center for Science in the Public Interest, 1997.
  23. Wilson JW, Enns CW, Goldman JD, Tippett KS, Mickle SJ, Cleveland Le, Chail PS. Data Tables: Combined Results from USDA's 1994 and 1995 Continuing Survey of Food Intakes by Individuals and 1994 and 1995 Diet and Health Knowledge Survey. Riverdale, MD: US Dept of Agriculture, Food Survey Research Group, Beltsville Human Nutrition Research Center, 1995;pages 61-78.
  24. Wyshak G. Teenaged girls, carbonated beverage consumption, and bone fractures. Arch Pediatr Adolesc Med 2000;154:610-613.
  25. Neumark-Sztainer D, Moe JK. Weight-related concerns and disorders among adolescents. In: Worthington-Roberts BS, Williams SR, eds. Nutrition throughout the Life Cycle. 4th ed. Boston: McGraw Hill, 2000;pages 288-317.
  26. Politt E. Iron deficiency and cognitive function. Annu Rev Nutr 1993;13:521-537.
  27. Currie C, et al. (eds.) Health and Health Behaviour among Young People (International Report from the HBSC 1997/98 Survey). WHO Policy Series: Health policy for children and adolescents, 2000;pages 80-81. (Available on the HBSC Website: http://www.hbsc.org)
  28. Young LR, Nestle M. The contribution of expanding portion sizes to the US obesity epidemic. Am J Public Health 2002;92:246-249.

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Table of Contents | Executive Summary | Introduction | Chapter 1 | Chapter 2 | Chapter 3 | Chapter 4 | Chapter 5 | Chapter 6 | What Was Left Out | Summary

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