Prevalence and Incidence of Childhood Overweight
reports the number of participants with BMI data by age, and shows the prevalence and incidence (new onset) of overweight in black and white girls at each age. Overweight was more prevalent in black girls than white girls (odds ratio = 4.9, 95% CI: 1.6-8.2; p<0.0001). Rates of overweight differed significantly by age (p<0.0001), tending to increase as girls grew older, with the increase in age stronger in black girls than white girls, although the interaction term was not significant (age-by-race interaction, p=0.06).
Prevalence and Annual Incidence of Overweight, by Age and Race *
The percent of new onset cases (i.e., incidence) of childhood overweight ranged from 2-5% through age 12, after which the annual incidence was generally lower, approximately 1-2%. The estimated instantaneous risk (hazard) of experiencing overweight was greater during ages 9-12 than it was above age 12 (hazard ratio = 1.6, 95% CI: 1.1-2.3, p=0.03). For black girls, the risk of experiencing overweight onset at any given time was about 1.5 times greater than that for white girls (hazard ratio = 1.5, 95% CI: 1.2-2.0, p=0.003).
To examine the possibility that changes in incidence across age were an artifact of using a BMI-based definition of overweight, which includes a lean body mass component, an analogous analysis was conducted, with overweight defined as being above the age-specific 95th percentile of sum of skinfolds for the NGHS sample. The results were very similar: the incidence of overweight during ages 9-12 was 1.7 times greater (95% CI: 1.2-2.5, p=0.008) than it was after age 12.
Childhood Overweight and Obesity in Young Adulthood
shows the percent of girls who were obese in young adulthood (NGHS Wave II, ages 21-23) by their overweight status at each age from 9 to 18, inclusive. Compared with girls who were not overweight, those who were overweight during ages 9-18 were much more likely to be obese as young adults (p’s<0.0001).
Percent (number) of girls who were overweight in young adulthood (age 21-23) among those who were overweight and those who not overweight in childhood, by age.
Childhood Overweight and Other Adiposity Indicators
shows the association between childhood overweight and three indicators of body fatness by age. For all indicators, the mean values for overweight girls were greater than the mean values for non-overweight girls (p’s<0.0001), and the differences between the measures for overweight and non-overweight girls tended to increase as girls grew older (overweight-by-age interaction, p’s<0.0001). All measures increased with age (p’s<0.0001). For percent body fat and waist circumferences, means were greater for black girls (p’s < 0.0001) by approximately 5.7% (95% CI: 4.4-7.1) and 2.8% (95% CI: 1.9-3.7), respectively, but there was no racial difference for sum of skinfolds (p=0.09). There were significant race-by-overweight interactions for percent body fat and sum of skinfolds (p’s < 0.0003) but not waist circumference (p = 0.81). For percent body fat, the difference between overweight and non-overweight girls was greater among white girls (percent difference = 14.6%, 95% CI: 12.6-16.5) than among black girls (percent difference = 11.2%, 95% CI: 9.4-13.0), although the opposite was true for sum of skinfolds (percent difference = 39.5%, 95% CI: 35.8-43.2, and 48.4%, 95% CI: 43.3-53.7, for white and black girls, respectively).
Mean (SD) of percent body fat, sum of skinfolds, and waist circumference, by age and overweight (OW = overweight girls, ~OW = girls who were not overweight, D = difference calculated as mean for OW minus mean for ~OW).
Based on results of mixed models technique, which is analogous to partial R2
in ordinary multiple regression,40-41
overweight was most strongly associated with sum of skinfolds and waist circumference, and less strongly (albeit significantly) associated with percent body fat, even after adjusting for study site, age and race.
Childhood overweight and outcomes
shows the percent of girls with high levels of blood pressure and lipids (low levels for HDL), by age and overweight.
Percent of overweight (OW) and non-overweight (~OW) girls with unhealthful high or low levels of cardiovascular disease (CVD) risk factors
Compared with non-overweight girls, overweight girls were more likely to exhibit elevated SBP and elevated DBP (p=0.01). Rates of elevated SBP and DBP also changed with age, but these changes depended on overweight. Among overweight girls, rates of elevated SBP increased between ages 9-12 (p=0.03) and decreased thereafter (p<0.0001), although rates of elevated SBP did not change with age among non-overweight girls. Among non-overweight girls only, rates of elevated DBP decreased during early adolescence (ages 9-12, p=0.01); among both overweight and non-overweight girls, rates of elevated DBP tended to decrease after age 12 (p=0.005). Neither measure of elevated blood pressure varied by race (p’s > 0.08).
Compared with non-overweight girls, overweight girls were more likely to exhibit decreased HDL (p<0.0001) and elevated triglycerides (p = 0.002). Overweight was not significantly associated with elevated LDL or elevated total cholesterol (p’s > 0.06).
Among both overweight and non-overweight girls, there were significant decreases in rates of unhealthful total cholesterol (TC), LDL and HDL during ages 9-12 (p’s<0.05); further, among non-overweight girls, unhealthful triglycerides (TG) increased between ages 9-12 (p=0.001). Overweight and non-overweight girls showed different age trends after age 12: among overweight girls, rates of unhealthful TC and LDL (p’s<0.002) tended to increase, although among non-overweight girls, there was an increase after age 12 for low HDL, but a decrease for elevated TG (p’s <0.05). Non-overweight black girls were more likely to exhibit elevated TC than were non-overweight white girls (OR for race = 1.6, 95% CI: 1.1-2.4, p=0.02), whereas overweight black girls were less likely to exhibit elevated TC (OR for race = 0.6, 95% CI: 0.1-1.0, p=0.03). Regardless of overweight, black girls were less likely than white girls to exhibit unhealthful HDL and TG (for HDL, OR for race = 0.6, 95%CI: 0.6-0.7; for TG, OR for race = 0.5, 95% CI 0.4-0.6, p’s<0.0001). There were no racial differences in LDL (p’s>0.31).
Pubertal maturation has been associated with lipids.42-43
If maturation explains some of the variation in unhealthful lipid levels, above and beyond that due to age, maturation may impact the estimated association of lipids with overweight. Therefore, as a secondary analysis, the association of overweight with unhealthful lipid levels was re-estimated in models controlling for maturation as well as age (maturation stage was represented as pre-pubertal, pubertal, post-menarchal < 2 years, or post-menarchal ≥2 years). Although the estimates were generally similar (), adding maturation tended to result in tighter confidence intervals. One consequence was that in the model controlling for maturation, unhealthful LDL was significantly associated with overweight (p=0.01).