Trends in Risk Factors for Cardiovascular Disease Among Children and Adolescents in the United States

METHODS

RESULTS

There were 12 265 subjects 2 to 17 years of age who attended the mobile examination center in NHANES III and 3611 such subjects in NHANES 1999–2000. The correlation coefficients for correlations between percentiles of body mass index and risk factors for cardiovascular disease were similar for the 2 surveys (Table 2). The strongest correlations were noted between percentiles of body mass index and systolic blood pressure and between percentiles of body mass index and triglyceride concentrations. The correlation coefficients between percentiles of body mass index and total cholesterol, glucose, and glycosylated hemoglobin concentrations were small.

Among children and adolescents 2 to 17 years of age, the mean waist circumference increased by 2 cm (Table 3). Although the mean waist circumference did increase for most age groups, many of the increases were not statistically significant. Among children and adolescents 8 to 17 years of age, the mean systolic blood pressure increased by 2.2 mm Hg. Increases were notable primarily among children 8 to 10 years of age. The mean concentrations of total cholesterol and high-density lipoprotein cholesterol among youths 4 to 17 years of age were almost the same in the 2 surveys, as were the mean concentrations of low-density lipoprotein cholesterol among youths 12 to 17 years of age. However, the mean concentration of triglycerides among youths 12 to 17 years of age in NHANES 1999–2000 was almost 10% lower than the mean in NHANES III. The mean concentration of glucose decreased by 3% but the mean concentration of glycosylated hemoglobin changed little. Additional results for male and female participants are presented in Tables 4 and 5.

Increases in waist circumference were particularly evident among black and Mexican American male subjects 12 to 17 years of age and female subjects 12 to 17 years of age (Table 6). With the exception of white male subjects 12 to 17 years of age, increases in mean systolic blood pressure occurred for each gender, racial/ethnic, and age group. Few changes in mean concentrations of total cholesterol and high-density lipoprotein cholesterol occurred. Concentrations of total cholesterol and high-density lipoprotein cholesterol decreased by ∼5% among black male subjects 12 to 17 years of age. Among participants 12 to 17 years of age, decreases in concentrations of low-density lipoprotein cholesterol and triglycerides were noted for most gender and racial/ethnic groups, although only the decrease in low-density lipoprotein cholesterol concentrations among white female subjects was significant; the decrease in triglyceride concentrations was of borderline significance. Except for black male subjects, mean glucose concentrations were lower in NHANES 1999-2000 than in the previous survey. However, the decreases reached statistical significance only for white male subjects and Mexican American female subjects.

DISCUSSION

Our study offers the most current and comprehensive examination of trends in risk factors for cardiovascular disease among children and adolescents in the United States. Despite the significant increases in the prevalence of obesity among youths in the United States, we found few effects on the risk factor profiles for cardiovascular disease. We had anticipated that the increase in obesity would have resulted in increases in systolic blood pressure and concentrations of low-density lipoprotein cholesterol, triglycerides, and glucose and in a decrease in concentrations of high-density lipoprotein cholesterol. Although a significant increase in the mean systolic blood pressure did occur, the mean concentrations of total cholesterol, high-density lipoprotein cholesterol, low-density lipoprotein cholesterol, and glycosylated hemoglobin were relatively unchanged. Unexpectedly, the mean concentrations of triglycerides and glucose decreased.

Other analyses of NHANES III and NHANES 1999–2000 data showed similar findings among adults. For example, obesity prevalence, mean waist circumference, and hypertension prevalence increased significantly.^14–16 Small decreases in the mean concentration of total cholesterol occurred.¹⁷ However, the prevalence of diabetes mellitus was relatively unchanged.¹⁸ Mean concentrations of glycosylated hemoglobin also remained stable.¹⁹ Trends in other lipid concentrations were unclear.

Few trend studies of physiologic or biochemical risk factors for cardiovascular disease among youths in the United States have been conducted. Among children 7 to 9 years of age who participated in the Bogalusa Heart Study, high-density lipoprotein cholesterol concentrations increased by 6 to 13 mg/dL, systolic blood pressure decreased by 4 to 6 mm Hg among white boys, white girls, and black girls, diastolic blood pressure decreased by 1 to 2 mm Hg among white boys and girls but increased by 1 to 2 mm Hg among black children, and triglyceride concentrations increased by 8 to 36 mg/dL between 1981 and 1991.⁵ Among children and adolescents 12 to 17 years of age who participated in several national surveys, mean cholesterol concentrations decreased by 7 mg/dL between 1966–1970 and 1988–1994.⁶ Among children 10 to 14 years of age from Minneapolis, Minnesota, the age- and ethnicity-adjusted mean systolic blood pressure was 1.5 mm Hg higher among male subjects and 0.7 mm Hg higher among female subjects and the mean diastolic blood pressure was 1.7 mm Hg lower among male subjects and 2.0 mm Hg lower among female subjects in 1996 than in 1986.⁷ In the Princeton School Study, Ohio students in third and fifth grades exhibited increases in mean body mass index, total cholesterol concentrations, systolic blood pressure (2.4–4.9 mm Hg), and diastolic blood pressure (0.3–3.1 mm Hg) between 1973–1975 and 1989–1990.⁸ In contrast to all except 1 of the aforementioned studies, our analyses provide nationally representative information on trends in risk factors for cardiovascular disease during a critical period when the prevalence of overweight among children and adolescents increased greatly.

Diet and physical activity are important determinants of the risk factors for cardiovascular disease included in this study. Therefore, changes in these 2 lifestyle factors could have affected temporal trends in the risk factors. Changes in diet and physical activity among children and adolescents were reviewed recently by Eisenmann.⁹ On the basis of data from NHANES studies, mean energy intakes were 6758 kJ among children 2 to 5 years of age, 8553 kJ among children 6 to 11 years of age, and 9726 kJ among youths 12 to 19 years of age in 1971–1974.²⁰ In 1976–1980, these means were 6353 kJ, 8213 kJ, and 9498 kJ, respectively. In 1988–1994, the mean energy intakes were 6498 kJ, 8265 kJ, and 10 127 kJ, respectively. For each age, gender, and racial/ethnic group, the mean energy intake was lower in 1976–1980 then in 1971–1974. The most pronounced increase in energy intake observed in 1988–1994 was for black female subjects 12 to 19 years of age (861 kJ). Decreases occurred in the age-adjusted mean intakes of energy from total fat (from 36–37% to 33–34%) and from saturated fat (from 14% to 12%) during the course of the 3 national surveys. However, data from the 1965 and 1977–1978 Nationwide Food Consumption Surveys and the 1989–1991 and 1994–1996 Continuing Survey of Food Intake by Individuals did not demonstrate increases in energy intake among participants 11 to 18 years of age.²¹ Mean energy intakes were 9.92 MJ in 1965, 8.78 MJ in 1977–1978, 8.77 MJ in 1989–1991, and 9.58 MJ in 1994–1996. Decreases in total fat and saturated fat, expressed as percentages of energy intake, did progressively decrease. The inconsistent findings between the 2 sets of studies indicate the difficulty of monitoring trends in energy intakes.

Information on temporal trends in physical activity levels among children and adolescents is scarce. Data from the Youth Risk Factor Surveillance System showed that, among US students in grades 9 through 12, 65.8% reported engaging in vigorous physical activity in 1993, 63.7% in 1995, 63.8% in 1997, 64.7% in 1999, and 62.6% in 2003.^22–26 Indirect evidence of physical activity levels among high school students can be gleaned from the percentages of students who participated in physical education classes. The percentages of US students in grades 9 through 12 who reported daily attendance in physical education classes were 41.6% in 1991, 34.3% in 1993, 25.4% in 1995, 27.4% in 1997, 29.1% in 1999, and 28.4% in 2003.^22–27 The amount of time spent in sedentary activities such as watching television, playing video games, or using a computer provides another measure of activity levels among children and adolescents. In 1997, parents of children and adolescents 2 to 17 years of age reported that their children watched television an average of 2.1 hours each day.²⁸ By 2000, this value had increased to 2.45 hours.²⁹ In 2000, children and adolescents spent, on average, 382 minutes each day watching television or videotapes, playing video games, using a computer, talking on the telephone, or reading. Therefore, the apparent slight decline in the percentage of high school students who reported participating in vigorous physical activity and the increase in the mean time spent watching television might have contributed to the increases in abdominal obesity and blood pressure that we noted in this study.

The inconsistent trends in the risk factors for cardiovascular disease are difficult to explain. The NHANES surveys are designed to make trend studies possible by using the same methods. There are inevitable differences, however; new machines may be used and reagents may change even if they are obtained from the same manufacturer. Nevertheless, strict quality control measures for laboratory analyses are maintained. Furthermore, the correlation coefficients for correlations between percentiles of body mass index and the risk factors for cardiovascular disease were very similar for the 2 surveys.

The exact lag period for changes in risk factors for cardiovascular disease after weight gain is not known, but weight loss studies clearly show rapid changes in many risk factors in fairly short times, certainly within 1 year. Therefore, any effects of weight increases on risk factors for cardiovascular disease should have manifested themselves during the 8- to 9-year interval between the 2 surveys. Few young participants were receiving glucose-lowering or cholesterol-lowering prescription medications, and any change in the use of prescription medications is unlikely to explain our results.

A few limitations of the study deserve comment. The sample size of NHANES 1999-2000 was inadequate for detailed trend studies according to gender and race/ethnicity. Also, not all variables were measured for all age groups. Future studies with more extensive data may be able to provide information on trends in risk factors for cardiovascular disease in these age groups that we were unable to explore. We are not aware of previously published information on the intraobserver and interobserver variability of assessments of the risk factors for cardiovascular disease included in our analyses. Therefore, the changes or lack thereof between the 2 surveys must be interpreted cautiously.

CONCLUSIONS

The obesity epidemic among US youths appears not to have had serious effects on their metabolic health, at least in terms of trends in various risk factors for cardiovascular disease; only systolic blood pressure was significantly increased. The reasons for this lack of deleterious effects are unclear. Our findings do not argue for complacency, however. Surveillance of trends in obesity and potential effects on risk factors for cardiovascular disease should continue. The high prevalence of obesity among US youths is of immense public health concern, because many obese youths will become obese adults,³⁰ because the origins of many chronic diseases can be traced to childhood, because obesity-associated morbidity is occurring at earlier ages, and because obesity among youths is associated with increased morbidity and death in adulthood. Therefore, efforts to address increasing obesity among children and adolescents must be pursued vigorously.