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Associations among dietary intake, physical activity, and cardiovascular disease (CVD) risk factors are inconsistent among male and female youth, possibly from lack of adjustment for pubertal status. The purpose of this report is to describe the associations of CVD risk factors among youth, adjusted for sexual maturation.
Data analyzed in 2007 from a sumsample of 556 children aged 8, 11, and 14 years in Project HeartBeat!, 1991–1993, provide cross-sectional patterns of CVD risk factors by age and gender, adjusting for sexual maturation, within dietary fat and physical activity categories.
Girls consuming moderate- to high-fat diets were significantly less physically active than those consuming low-fat diets. Boys and girls consuming high-fat diets had higher saturated fat and cholesterol intakes than children in low-fat categories. Boys had no significant differences in physical activity, blood pressure, waist circumference, or plasma cholesterol levels across fat categories. Girls’ plasma cholesterol levels showed no significant differences across fat categories. Dietary intake did not differ across moderate-to-vigorous physical activity (MVPA) categories within gender. There were no differences in BMI by fat or MVPA categories for either gender. Girls’ waist circumference differed significantly by fat category, and systolic blood pressure differed significantly across fat and MVPA categories. Boys’ fifth-phase diastolic blood pressure was significantly different across MVPA categories.
Girls consuming atherogenic diets were significantly less physically active than those with low fat intakes, whereas boys consuming high-fat diets did not show differences in physical activity measures. With the prevalence of overweight rising among youth, the impact of atherogenic diets and sedentary lifestyles on CVD risk factors is of concern to public health professionals.
Although cardiovascular disease (CVD) is typically not diagnosed until adulthood, atheromatous lesions have been found in children and young adults.1–3 The presence of atherosclerosis in youth has been associated with CVD risk factors such as obesity, abnormal plasma lipoprotein levels, elevated blood pressure, and insulin resistance.4–11 The positive effect of high-fat diets on plasma lipids in children has been shown to be similar to that observed in adults.7–12 Longitudinal studies have further indicated that measures of BMI, total cholesterol, low-density lipoprotein cholesterol (LDL-C), high-density lipoprotein cholesterol (HDL-C), and blood pressure taken during childhood and adolescence track into adulthood and predict adult values.13–24 Because of the rising prevalence of overweight among youth, future rates of adult CVD could greatly increase.11,25–27
Sedentary behavior such as computer use and TV viewing has been associated with increased risk of CVD in adults28 and children.29 Reports show a direct relationship between daily physical activity and HDL-C among adults.30 Changes in physical activity from adolescence to adulthood, but not during adolescence, have been related to CVD risk factors in adulthood.31,32 Intervention and longitudinal studies involving children and adolescents have shown that healthy diet and physical activity behaviors together are related to decreases in body fat,28,33–36 total cholesterol level,12,36 total cholesterol-to-HDL-C ratio,12,30 and insulin resistance.33,34
Inconsistent associations among youth diet, physical activity, and CVD risk factors have been reported,37–40 possibly because CVD risk factors are associated with onset of puberty, and valid adjustments for pubertal stage have not been considered. Onset of puberty may vary by as much as 4 years within gender.41,42 Sexual maturation and sex hormones are strongly associated with plasma total cholesterol and lipoprotein cholesterol concentrations during adolescence.39 As sexual maturation progresses, plasma total cholesterol and LDL-C decrease, while HDL-C and weight increase.40 The decreases are greater in boys than in girls. Sexual maturation has also been reported to affect the association between physical activity and systolic blood pressure (SBP) and diastolic blood pressure (DBP).43
To stem potential increases in adult CVD incidence resulting from the rising prevalence of youth overweight, risk factor relationships need to be better understood for designing interventions. Project HeartBeat!, a longitudinal study of youth, allows for the evaluation of cross-sectional patterns of CVD risk factors by age and gender groups, adjusting for sexual maturation, within categories of dietary fat intake and physical activity.
Details of the Project HeartBeat! rationale, design, and data collection have been described previously.44 Participants were recruited from schools in The Woodlands and Conroe TX. Informed consent was obtained from the parent or guardian, and child assent or consent was obtained. The protocol was approved by the Committee for Protection of Human Subjects of the University of Texas Health Science Center at Houston and the IRB of Baylor College of Medicine. A total of 678 children (49.1% girls, 79.9% nonblack) aged 8, 11, and 14 years at baseline were enrolled in three cohorts. Data from the baseline examination of 556 children (277 boys and 279 girls) between October 1991 and July 1993 are included in these 2007 analyses. Quality-control measures were used to exclude children who were missing a food-frequency questionnaire (FFQ) (n=90); those with more than six FFQ foods incomplete; those with more than eight FFQ extreme portion sizes (n=12); those with intakes ≥5000 kcal per day (n=10); those missing physical activity measures (n=30); those missing gender or cohort (n=2) or sexual maturation values (n=67); and those missing serum lipids (blood-draw refusals) (n=67). Some were excluded for more than one reason.
Dietary intake was assessed using an interviewer-administered, quantified 137-item FFQ validated for this population. The FFQ food list was developed from 3-day food records from approximately 150 children in Grades 5–12 from the targeted communities.45 Procedures for creation of the FFQ are similar to those for other reports,46,47 with foods contributing ≥0.1% to nutrient intake included on the FFQ food list.
Interviews using FFQs were conducted at the child’s home or in the field center within 3 weeks of the baseline examination. Children aged 8 and 11 years were accompanied by a parent or an adult involved in food preparation for the 35- to 40-minute interview; those aged 14 years were interviewed independently. Children reported the frequency and quantity of consumption of foods for the week preceding the interview, estimating portion size using two-dimensional and household food models. Food- and brand-specific serving sizes (e.g., slices of bread or cans of soft drinks) were also used. The Food Frequency Data Entry and Analysis Program 1.0 and FIAS 2.0 were used for data entry and nutrient analyses utilizing nutrient and gram weight information from the U.S. Department of Agriculture Nutrient Database for Individual Intake Surveys.48
Physical activity and TV viewing were assessed using a 24-hour recall, interviewer-administered questionnaire adapted from a 7-day recall instrument for use with pre-adolescent children that has been validated among 5th-grade children.49 Parents assisted children aged <10 years. Using a segmented-day approach, children recalled the active and sedentary behaviors in which they participated during the preceding 24 hours. Most interviews (97%) were scheduled to allow recall of weekday physical activity and TV viewing. Only 15% of children interviewed on Monday afternoons recalled a weekend activity from the preceding 24 hours.
For each physical activity, participants estimated three time segments: (1) total time (e.g., 2-hour baseball game); (2) time spent truly participating (e.g., 1 hour participating in the baseball game); and (3) time in vigorously intense activity (activity that caused hard breathing or sweating; e.g., 10 minutes of the baseball game). Physical activities were grouped into categories of moderate and vigorous physical activity (MPA; VPA) using published estimates of physical activity energy expenditure in MET equivalents (ratio of work metabolic rate to a standard resting metabolic rate).49,50 A measure of minutes per day was calculated for each of the child’s physical activities by category. As part of the questionnaire, participants recalled time spent (minutes/day) viewing TV during the preceding 24 hours.
Children’s sexual maturity stages were assessed by research nurses at the field center using the method of Reynolds and Wines51,52 later popularized by Tanner.53 Tanner staging classifies the sexual development of children and adolescents using a 5-point scale from 1 (pre-pubertal) to 5 (complete sexual maturation), based on the development of genitalia and pubic hair for boys and breasts and pubic hair for girls. Tanner female breast and male genitalia stages (without consideration of pubic hair stages) were used to adjust for sexual maturation in analyses.
Blood pressure was measured during two appointments within 2 weeks of the baseline examination using mercury sphygmomanometers and detailed procedures.44 Participants were seated for at least 5 minutes followed by two sets of blood pressure measurements. The mean of the readings was used for SBP; fourth-phase DBP (DBP4); and fifth-phase DBP (DBP5).
Height and weight were measured with children in light clothes without shoes. Height was measured using a wall-mounted stadiometer and recorded to the nearest 0.1 cm. Weight was measured to the nearest 0.1 kg with a beam-balance scale calibrated daily. Participants’ BMI was calculated by standard formula (kg/m2). Waist circumference was measured to the nearest 0.1 cm at the level of the greatest frontal extension of the abdomen between the bottom of the rib cage and the top of the iliac crest. Bioelectrical impedance assessments were obtained following standardized procedures using an RJL Systems bioelectric impedance analyzer BIA 101-A measuring reactance and resistance.24 Percent body fat was calculated by gender-specific formulas54–57 using resistance, anthropometry, and skinfold measurements among the various formulas for bio-impedance. Skinfold measurements were taken for triceps, subscapular, midaxillary, abdominal, distal thigh, and lateral calf areas with calipers, following standardized procedures, each recorded in triplicate and analyzed as the mean value for the measures.24,54–58
Blood samples, after an 8- to 12-hour fast, were drawn by a phlebotomist on the morning of each examination at the participant’s home, as described elsewhere.44 Plasma lipid concentrations were determined in plasma samples using standardized methods in the Lipid Research Laboratory of the Baylor College of Medicine in Houston TX.
Descriptive measures are provided of dietary intake and physical activity with CVD risk factors by gender and age. Categories of total fat intake (% kcal) were created based on dietary intake recommendation levels for total fat intake at the time of data collection.59–61 The category of low fat intake was ≤30% kcal per day, the moderate category was 31%–38% kcal, and the high category was ≥39% kcal. Moderate-to-vigorous physical activity (MVPA; minutes/day) categories were created using tertiles of the population distribution, resulting in categories of low active (0–30 minutes per day); moderate active (31–89 minutes per day); and high active (≥90 minutes per day). The CVD risk factor differences were assessed using ANCOVA adjusted for age (continuous variable) and sexual maturation (categoric variable) within gender, across categories of fat intake and MVPA. Predicted Ms and SEs were calculated from the covariance procedure for variables in the fat and MVPA categories. A p-value of 0.05 was used as the criterion for all statistical testing, and no correction was made for repeated testing. Within-gender comparisons provide descriptive information of the relationships of CVD risk factors at baseline, setting the context for longitudinal analyses.
Descriptive information regarding age, height, weight, and sexual maturation is in Table 1. Mean daily energy intakes ranged from 2026 kcal for girls aged 8 years to 2796 kcal for boys aged 14 years. Mean protein, carbohydrate, total fat, and saturated fat intakes as a percentage of energy were almost identical across age and gender groups. Total fat averaged 34%–35% kcal, and saturated fat about 13% kcal, across age and gender groups. Mean intakes of cholesterol and fiber were similar within gender groups; boys aged 14 years had the highest absolute values for cholesterol (310 mg) and fiber (17 g). Mean calcium intakes ranged from 1020 mg among girls aged 8 years to 1438 mg among boys aged 14 years.
On average, those aged 11 years reported watching more TV than other age groups, ~2 hours per day for girls and 1.75 hours per day for boys (Table 1). Interestingly, the boys aged 11 years also reported the highest amount of MPA, 33 minutes. Girls’ mean MPA, 11 minutes, was the same for all age groups. Girls and boys aged 8 years reported the highest number of minutes of VPA. For MVPA, boys aged 11 years and girls aged 8 years had the highest reported amounts, 94 and 70 minutes, respectively.
Mean values for diet, physical activity, and CVD risk factors by fat categories and gender, adjusted for age and sexual maturation, are in Table 2. Seventeen percent consumed high-fat diets; means were 42% kcal for boys and 43% kcal for girls. Girls in the high-fat category had significantly higher protein (% kcal), saturated fat (% kcal), and cholesterol intakes, and lower carbohydrate (% kcal) intakes, than girls consuming diets low in fat. Boys in the high-fat category had significantly lower carbohydrate (% kcal) and higher saturated fat (% kcal) and cholesterol intakes than boys in the lower-fat categories. Mean calcium intake was significantly greater among boys with low versus high fat intakes.
Boys and girls in the high-fat category had higher mean minutes of TV viewing than those in the low-fat category (Table 2). Girls had significant differences in minutes of VPA and MVPA across fat categories; those in the low-fat category had the highest mean minutes. There were significant differences in SBP and waist circumference of girls across fat categories; the lowest values being in the moderate fat category.
Time spent on TV viewing was significantly greater in the low-MVPA category (Table 3). There were no significant gender differences in dietary intake by MVPA. Girls had a significant difference in SBP across MVPA categories. Boys had a significant difference in DBP5 across MVPA categories.
Differences in VPA, MVPA, SBP, waist circumference, and DBP5 were evident within gender groups when children were grouped by fat intake and MVPA, after adjustment for sexual maturation and age. Children with high fat intakes consumed more saturated fat and cholesterol, and girls in this category spent significantly less time engaged in MVPA and VPA than those eating low-fat diets. Boys had no significant differences in physical activity measures, blood pressure, BMI, waist circumference, or plasma cholesterol levels across fat intake categories. Girls also had no significant differences in BMI and plasma cholesterol across fat categories. Because dietary risk factors for CVD such as high intake of fat and cholesterol62–64 have the possibility of continuing into adulthood and having a negative effect on morbidity and mortality,65,66 children with high fat intakes are a concern.
Dietary intake as a percentage of energy was similar across age groups, consistent with previous research.45 Dietary intake was similar to that of youth (aged 12–19 years) in the 1994–1996, 1998 USDA Continuing Survey of Food Intakes by Individuals (CSFII).67 The mean cholesterol intake of boys in the high-fat category, and the mean intake of boys aged 12–19 years in the CSFII, exceeded the American Heart Association recommendation of <300 mg per day.68 Fiber intake was similar to, and calcium intake greater than, the intake of CSFII youths. Project HeartBeat! children’s dietary intake was typical of youth at the study time, supporting extrapolation of these results to American youth.
Children in the low-MVPA category had higher mean minutes of TV viewing than those in other categories, with girls being significantly different. This pattern of TV viewing rather than physical activity is occurring across the nation and can lead to obesity, high blood pressure, and increased risk of CVD.65,69,70 Although computer and video game use was minimal at the time of the study, 2–4 hours per day are now spent on this activity71,72; thus the impact of TV viewing instead of physical activity is enhanced in today’s environment. Watching TV has been associated with the consumption of high-fat, sugar, and energy foods and with low intakes of fruits and vegetables, habits that can increase weight gain.73–77 Older boys had less VPA than those aged 8 years. Results, however, do not show significant differences in BMI among children stratified by fat categories or by MVPA categories, after adjustment for age and sexual maturation. Variability in the children’s BMI is not explained by differences in fat intake or MVPA minutes. These cross-sectional data are not adequate to assess the risk for future CVD, but because these negative behaviors may track into adulthood, their persistence is worrisome.
Girls’ SBP differed significantly within fat and MVPA categories; the highest SBP was among those with low fat intake and high MVPA. Among boys, DBP5 differed significantly by MVPA category, with the highest value in the moderate category. These results are contrary to inverse associations between youth physical activity and blood pressure shown in other studies,78–83 and to direct associations among age, sexual maturation, and blood pressure,84 which may be stronger than the decrease in blood pressure seen with physical activity. The current results may be explained by small numbers in categories, the use of a 24-hour measure of physical activity, and the multiple comparisons evaluated. These results may be consistent with results85 showing that boys experienced a sixfold increase and girls a threefold increase in SBP during pubertal growth. Participants may not have engaged in amounts of regular MVPA adequate to produce a blood pressure decrease. Another explanation may be that cross-sectional associations of diet and physical activity with CVD risk factors have been inconsistent86–90 because precise estimation is difficult. However, differences in blood pressure measures across these categories are small in absolute value and may not be clinically significant; thus, they are interpreted cautiously.
The effect of using a distributional approach to creating MVPA categories was assessed by comparing categories of MVPA using physical activity recommendation levels of 0–29 minutes, 30–59 minutes, and ≥60 minutes. Results were the same; except for girls, a significant difference in plasma cholesterol was seen across MVPA categories (data not shown).
There are several possible limitations to interpretation. First, stratifying the data into categories resulted in small cell sizes, producing less-stable estimates. However, because these data are descriptive and include measures of variability, they do offer informative profiles of the variable relationships. Second, unlike trends from longitudinal analyses, cross-sectional trends cannot show cause and effect. Third, FFQs have been shown to overestimate children’s intakes; nevertheless, the FFQ is reported as a good group measure for ranking youth intakes.86–88 The FFQ data are appropriate to explore patterns of intake and relationships with CVD risk factors. Fourth, collection of physical activity data may be compromised for children aged <10 years because of a lack of cognitive ability.89,90 In addition, the 24-hour recall measure of physical activity may not have captured the true variation in physical activity, causing a bias in these analyses. Although some SDs of the physical activity measures are large, the similarity in variability across age groups, and not just among the younger youths, increases the validity of these results. Fifth, racial/ethnic variations were not included in these analyses because of limited numbers; the population was 20.1% black and 79.9% nonblack.
The use of a validated physical activity assessment tool for young children was a strength; however, the instrument has not been validated among children aged 12–18 years. Total physical activity minutes are underestimated because the measure of MVPA in this study includes only minutes of physical activity at this level of intensity, and total physical activity minutes were not part of the physical activity measure. Further, age- and gender-specific percentiles were not used to make BMI comparisons of boys and girls. This issue was addressed by stratifying data by gender with adjustment for age and sexual maturation.
Girls who consumed atherogenic diets were significantly less physically active than those with low fat intakes, whereas boys consuming high-fat diets did not show differences in physical activity measures. There were no BMI differences by fat and MVPA categories, yet SBP, DBP5, and waist circumference did differ significantly (not in the same way for both genders) after adjustment for sexual maturation. With the prevalence of youth overweight rising, the impact of atherogenic diets and sedentary lifestyles on CVD risk factors is of concern to public health professionals.91,92 These gender-specific findings of the relationships among diet, physical activity, and CVD risk factors, adjusted for sexual maturation, provide a benchmark for investigating the trajectories of risk factors during children’s growth.
The authors thank Stephanie A. Carter for assistance with manuscript preparation. The authors gratefully acknowledge the contribution of each Project HeartBeat! participant and family. The researchers recognize the support of the Conroe Independent School District and the generous support of The Woodlands Corporation. Funding for the study was from the National Heart, Lung, and Blood Institute through Cooperative Agreement U01-HL-41166 and by the CDC through the Southwest Center for Prevention Research (U48/CCU609653).
The findings and conclusions in this report are those of the authors and do not necessarily represent the views of the CDC.
No financial disclosures were reported by the authors of this paper.
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