The Relation of Childhood BMI to Adult Adiposity: The Bogalusa Heart Study
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《小儿科》
Division of Nutrition and Physical Activity, Centers for Disease Control and Prevention, Atlanta, Georgia
Tulane Center for Cardiovascular Health, Tulane University School of Public Health and Tropical Medicine, New Orleans, Louisiana
ABSTRACT
Objective. Although many studies have found that childhood levels of body mass index (BMI; kg/m2) are associated with adult levels, it has been reported that childhood BMI is not associated with adult adiposity. We further examined these longitudinal associations.
Design. Cohort study based on examinations between 1973 and 1996.
Setting. Bogalusa, Louisiana.
Participants. Children (2610; ages 2-17 years old) who were followed to ages 18 to 37 years; the mean follow-up was 17.6 years.
Main Outcome Measures. BMI-for-age and triceps skinfold thickness (SF) were measured in childhood. Subscapular and triceps SFs were measured among adults, and the mean SF was used as an adiposity index. Adult obesity was defined as a BMI 30 kg/m2 and adult overfat as a mean SF in the upper (gender-specific) quartile.
Results. Childhood levels of both BMI and triceps SF were associated with adult levels of BMI and adiposity. The magnitude of these longitudinal associations increased with childhood age, but the BMI levels of even the youngest (ages 2-5 years) children were moderately associated (r = 0.33-0.41) with adult adiposity. Overweight (BMI-for-age 95th centile) 2- to 5-year-olds were >4 times as likely to become overfat adults (15 of 23 [65%]), as were children with a BMI < 50th centile (30 of 201 [15%]). Even after accounting for the triceps SF of children, BMI-for-age provided additional information on adult adiposity.
Conclusions. Childhood BMI is associated with adult adiposity, but it is possible that the magnitude of this association depends on the relative fatness of children.
Key Words: body mass index obesity adult adiposity Bogalusa Heart Study longitudinal study skinfolds
Abbreviations: BMI, body mass index SF, skinfold thickness
Despite the limitations of weight-height indices,1,2 they are widely used as indicators of adiposity. Several investigators have found that childhood body mass index (BMI; kg/m2) is moderately correlated with body fatness as estimated by dual-energy x-ray absorptiometry3–6 and is predictive of adult BMI (reviewed in refs 7–10). A recent report from the Newcastle Thousand Families Study,11 however, found that childhood (age 9 years) BMI was associated with adult (age 50 years) BMI but not with adult body fatness as determined by bioelectrical impedance analysis. These contrasting associations with adult levels of BMI and body fatness have led several investigators11–13 to conclude that (1) childhood BMI is not a good predictor of adiposity, and (2) the tracking (persistence of relative rankings) of BMI from childhood to adulthood reflects continuities in "body build" rather than adiposity.
The purpose of the current study is to compare the accuracy of childhood levels of BMI and triceps skinfold thickness (SF) in predicting adult adiposity. Among the 2610 subjects in this cohort, the mean age at baseline was 10 years (range: 2.5–17 years), the mean length of follow-up was 17.6 years, and adult ages ranged from 18 to 37 years.
METHODS
Sample
Bogalusa is a semirural community in Washington Parish, Louisiana, and is 70 miles north of New Orleans. Seven cross-sectional studies of schoolchildren, along with 1 study of 2- to 5-year-olds, were conducted in Ward 4 of Washington Parish between 1973–1974 and 1992–1994.14 In addition, 4 studies of adults (ages 18–37 years) who had been previously examined as children were conducted between 1982 and 1996. Protocols were approved by appropriate institutional review boards, and informed consent was obtained from all participants.
Longitudinal analyses could be conducted because most subjects participated in >1 cross-sectional examination. For example, a 6-year-old examined in 1976 could have been reexamined in any of the 6 studies conducted between 1978 and 1996. Although specialized techniques are required to analyze repeated, longitudinal data, the current analyses are stratified by childhood age so that each subject contributes only 1 observation to an age category. To maximize the length of follow-up, we used data from the first childhood examination and the final adult examination.
Measurements of weight, height, and triceps SF were available from 11 411 children who were examined between 1973 and 1994. Of these participants, 2905 (37%) of 7923 age-eligible children were actually reexamined in adulthood. (A 7-year-old examined in 1988, for example, could not have been reexamined as an adult.) Participation varied by gender: 31% of boys vs 42% of girls were reexamined in adulthood.
We excluded 295 subjects who were followed for <10 years, resulting in a study cohort of 2610 subjects. Thirty-two percent of the cohort was black.
Anthropometry
Schoolchildren were examined while wearing underpants, an examination gown, and socks; adults wore street clothes (excluding sweaters, jackets, belts, and shoes). Weight was measured to the nearest 0.1 kg by using a balance-beam scale, and height was measured to the nearest 0.1 cm with a manual height board.14,15 National US data (1963–1994) were used to convert weight, height, and BMI into gender- and age-specific z scores and centiles.16,17 These z scores and centiles represent BMI levels among the participating children relative to those in national studies; overweight is defined as BMI-for-age 95th centile.18,19
The triceps SF and subscapular SF were measured 3 times with Lange skinfold calipers, and the mean SF of these 2 sites is used as a measure of overall adiposity among adults. Because the subscapular SF was not measured in the initial examinations, the triceps SF is used as an adiposity index among children.
Adults with a BMI 30 kg/m2 were considered to be obese, whereas those with a mean SF in the upper gender-specific quartile (men: 21 mm; women: 30.3 mm) were considered to be "overfat." In adulthood, 464 (18%) of the 2610 subjects were both obese and overfat, 139 (5%) were obese but not overfat, and 153 (6%) were overfat but not obese.
Statistical Analyses
We examined baseline and follow-up levels of various characteristics among boys (n = 1115) and girls (n = 1495); the mean length of follow-up was 17.6 years (range: 10–24 years). The relations of childhood levels of BMI and triceps SF to adult levels of BMI and mean SF were compared in gender- and age-stratified analyses by (1) using Spearman (rank) correlation coefficients and (2) examining the proportion (positive predictive value) of children in various BMI-for-age categories who became obese (BMI 30 kg/m2) or overfat (upper quartile of mean SF) adults.
Linear regression was used also to assess the relation of childhood (ages 2–9 years) levels of BMI and triceps SF to the mean SF in adulthood; these models included race, gender, childhood age, adult age, and various interaction terms as covariates. These analyses also assessed whether childhood levels of BMI and triceps SF provided independent information on adult adiposity. Predicted levels of mean SF are displayed graphically.20
RESULTS
Thirty-seven percent of all eligible children were reexamined in adulthood (Table 1), and compared with children lost to follow-up, the study cohort was slightly older (10.0 vs 8.9 years, boys) and participated in earlier (1975 vs 1978, boys) examinations. However, mean levels of weight-for-age, height-for-age, and BMI-for-age were close to 0 in both groups. After adjustment for age and examination year, the only significant difference between the 2 groups was a slightly higher prevalence of overweight (BMI 95th centile) among boys in the study cohort (7% vs 6%; P = .02).
Of the 1115 boys and 1495 girls in the study cohort, 76% were initially examined in 1973–1974, and 94% participated in either the 1988–1991 or 1995–1996 screening of adults. Ages at baseline ranged from 2 to 17 years, and the mean age at follow-up was 27.5 years (range: 18–37 years). At follow-up, 23% were obese (BMI 30 kg/m2), and 25% were overfat (mean SF in the upper quartile).
Childhood levels of both BMI-for-age and triceps SF were significantly associated with adult levels of BMI and mean SF, with the overall correlations ranging from r = 0.44 to 0.64 (Table 2). Although the strongest associations were seen between BMI levels in childhood and adulthood, childhood BMI was also associated with adult levels of mean SF in all age groups including 2- to 5-year-olds (boys: r = 0.41; girls: r = 0.33). Furthermore, childhood BMI was almost as strongly associated with adult mean SF, as was childhood triceps SF; correlations among 6- to 8-year-old boys, for example, were r = 0.48 (BMI-for-age) and r = 0.50 (triceps SF). In general, the magnitudes of the correlations were stronger among boys than among girls and stronger among older (9–17 years) children than younger children. Between the ages of 2 and 5 years and 9 and 11 years, the association between BMI-for-age and adult mean SF increased from r = 0.41 to 0.56 among boys and from r = 0.33 to 0.51 among girls.
We then examined the ability of BMI to predict which children would be obese (BMI 30 kg/m2) or overfat (upper quartile of mean SF) in adulthood (Table 3). The positive predictive value for adult obesity and overfat consistently increased as childhood BMI-for-age increased, and this was evident among even the youngest children. Among 2- to 5-year-olds girls, for example, 15% (18 of 121) of those with a childhood BMI 50th centile became overfat adults, whereas 53% of overweight (BMI 95th centile) girls became overfat adults. Among overweight children, positive predictive values were generally higher for adult obesity than for adult overfat; of the 51 overweight 9- to 11-year-old boys, for example, 76% were obese in adulthood, and 67% were overfat. With the exception of estimates among 2- to 5-year-old boys, which were based on small numbers, the positive predictive values of childhood overweight for adult obesity generally increased with age. In contrast, the positive predictive values for adult overfat showed little variation with age, and similar estimates were seen among 6- to 8-year-olds (70–84%) and 15- to 17-year-olds (65–81%). Additional analyses (data not shown) indicated that changes in (age-adjusted) levels of BMI and triceps SF during childhood showed similar associations with adult overfat.
To examine further the importance of BMI levels among the 2- to 9-year-olds (n = 1362), various regression models were constructed to determine if levels of BMI-for-age and triceps SF were independently associated with adult mean SF. (Models also included race, gender, childhood age, adult age, and various interaction terms.) Although the inclusion of both BMI-for-age and triceps SF in a model only slightly improved the prediction of adult mean SF as compared with models containing only 1 of these variables (multiple R2 = 0.44 vs 0.39 and 0.42), both childhood characteristics were independently associated (P < .001) with adult adiposity.
These independent effects are shown in Fig 1 for boys (left) and girls (right). Predicted levels of adult mean SF (y-axis) are shown among white adults according to childhood levels of BMI-for-age (x-axis) and the 10th, 50th, and 90th centiles of triceps SF. For example, among boys with a triceps SF of 10 mm (median value), estimated adult levels of mean SF increased from 14 to 17 mm as the BMI z score increased from 0 to 1.645 (95th centile of BMI). Similarly, among boys with a BMI z score of 1.0, the predicted adult mean SF increased by 5 mm as triceps SF increased from 10 to 17 mm. The observed associations with adult mean SF did not significantly differ by race or gender, but adult levels were higher among women (Fig 1 right) than men and among black women than white women (data not shown).
DISCUSSION
Our results show that childhood levels of both BMI and triceps SF are associated with adult adiposity, as assessed 18 years later by the mean thickness of the triceps and subscapular SFs. Although the magnitude of the correlations between childhood and adult levels increased with the age at which the childhood measurement was obtained, the BMIs of even 2- to 5-year-olds were moderately associated (r = 0.33–0.41) with adult adiposity. Furthermore, overweight 2- to 5-year-old boys and girls were >4 times as likely to become overfat adults (15 of 23 [65%]) as were those who had a BMI < 50th centile (30 of 201 [15%]). Childhood triceps SF showed a slightly stronger association with adult adiposity than did childhood BMI, but both characteristics provided independent information on adult adiposity.
Several investigators have examined either the tracking of BMI from childhood to adulthood or the accuracy of childhood BMI in predicting adult obesity. Although there are exceptions,21 the larger studies consistently found that as compared with relatively thin children, fatter children are more likely to become obese adults.22–27 Although most investigators have examined the tracking of weight-height indices, similar (but slightly weaker) longitudinal associations have been observed for various SFs.22,24,28,29 For example, among 9- to 10-year-olds who were followed for 18 years, the tracking of BMI ranged from r = 0.60 (females) to r = 0.73 (males), whereas longitudinal correlations for triceps SF ranged from r = 0.50 to 0.58.24 Other investigators have also reported correlations that range from r = 0.4 to 0.5 for the triceps SF29 and biceps SF22 measured at ages 6 to 9 years and 18 to 25 years. These estimates agree well with the associations we observed between childhood triceps SF and adult mean SF (r = 0.38–0.59) over 17.6 (mean) years. We also found that the tracking of triceps SF was weakest among 2- to 5-year- olds, and SFs measured among 1- to 5-year-olds have shown28 little association (r = 0.14) with levels measured 20 years later.
A report from the Newcastle Thousand Families Study11 found that although BMI levels at ages 9 years and 50 years were correlated (r = 0.24), childhood BMI was not associated with adult adiposity (r = 0.10; P > .05) as assessed by bioelectrical impedance analysis. These contrasting associations have led several authors to suggest that childhood BMI is a poor indicator of adiposity, and that the tracking of BMI reflects the persistence of "body build" rather than adiposity.11–13 Cross-sectional studies, however, have found that childhood BMI is associated with body fatness as assessed by dual-energy x-ray absorptiometry.3–6 In addition, a previous study30 reported that increases in BMI percentile rankings between the ages of 3 and 9 years among girls was associated with adult adiposity as determined by hydrostatic weighing. Therefore, it is unlikely that childhood BMI is merely an indicator of body build.
It is possible that the negative findings of the Newcastle Thousand Families Study are due to the relative thinness of children born in the United Kingdom in 1947. As compared with 9-year-olds in the current study, who were born between 1963 and 1976, similarly-aged children in Newcastle, on average, weighed 9 lb less and were 2 inches shorter,31 resulting in lower BMI levels. Furthermore, 16% of 9-year-olds in the current study had a BMI > 95th centile in Newcastle (boys: 20.5 kg/m2; girls: 18.5 kg/m2) (C. M. Wright, MD, written communication, 2003). As the accuracy of BMI as an index of body fatness increases with the degree of adiposity,32–34 it is possible that differences in childhood BMI levels between the 2 studies could account for the contrasting associations with adult adiposity. The variability in BMI levels among thinner children largely reflects differences in fat-free mass,34 and if this were the case in Newcastle, childhood BMI levels would not be expected to be associated with adult adiposity. In contrast, the BMI levels of Bogalusa children may reflect differences in fat mass more accurately.
The negative findings of the Newcastle study may also reflect the limitations of bioelectrical impedance analysis. Although levels of various cardiovascular disease risk factors showed fairly similar cross-sectional associations with BMI and body fatness among adults in the Newcastle study, some investigators35–37 have found bioelectrical impedance estimates of body fat to be less accurate than those based on SFs. In addition, bioelectrical impedance estimates can be influenced by the possible association between electrical conductivity and the amount of fat mass,35 the choice of the calibration equation,38 and various clinical characteristics.39
Although we found that most overweight (BMI 95th centile) children were obese or overfat in adulthood, with positive predictive values ranging from 53% to 90%, only 20% of obese or overfat adults had been overweight children. This estimate is comparable to those reported in other studies,11,23,28,40 but it should be realized that estimates of sensitivity are greatly influenced by the prevalences and classifications of childhood overweight and adult obesity. For example, use of the 85th centile rather than the 95th centile as the childhood BMI cut point resulted in sensitivities of 42% (overfat) and 51% (obese). Furthermore, of the 114 adults with a BMI 40 kg/m2, 46% had a childhood BMI 95th centile and 70% had a childhood BMI 85th centile. Although there are certainly additional factors that operate in early adulthood to promote obesity, a child with a high BMI-for-age is much more likely to become an obese adult than is a relatively thin child.
Several limitations of the current study should be considered when interpreting our results. Although only 37% of age-eligible children were reexamined as adults, their baseline levels of weight, height, and BMI were fairly similar to those among children who were not reexamined. In addition, we were able to follow children only through early adulthood (maximum age: 37 years), and it is likely that a longer follow-up would have reduced the magnitudes of the various associations. Although there are few studies on the tracking of childhood BMI beyond the age of 40 years, BMI levels at ages 30 and 50 years are highly correlated (r = 0.7-0.8).21 In addition, the current study did not investigate the associations with adult risk factors for coronary heart disease, but previous studies11,41 have shown that the most important correlate is the adult BMI level. Childhood BMI, however, may be important in the development of atherosclerosis. We have found, for example, relatively high levels of the carotid artery intima-media thickness among obese adults who had been overweight children but not among obese adults who had been normal-weight children.42
Our findings indicate that childhood BMI is associated with adiposity in adulthood and that overweight children have a greatly increased risk for becoming overfat adults. Our findings also provide a partial explanation for the variability observed in previous studies of BMI tracking from childhood to adulthood. Because the accuracy of childhood BMI as an indicator of body fatness increases with the degree of adiposity, the degree of tracking would be expected to be highest in populations with a high prevalence of overweight.
ACKNOWLEDGMENTS
This work was supported by National Institutes of Health grants HL-38844 (National Heart, Blood, and Lung Institute), HD-043820 (National Institute of Child Health and Human Development), and AG-16592 (National Institute on Aging), funds from the Centers for Disease Control and Prevention, and the Robert W. Woodruff Foundation.
We thank the investigators of the Newcastle Thousand Families Study for helping us compare body mass index levels between children in Bogalusa and Newcastle.
FOOTNOTES
Accepted Jun 14, 2004.
No conflict of interest declared.
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Tulane Center for Cardiovascular Health, Tulane University School of Public Health and Tropical Medicine, New Orleans, Louisiana
ABSTRACT
Objective. Although many studies have found that childhood levels of body mass index (BMI; kg/m2) are associated with adult levels, it has been reported that childhood BMI is not associated with adult adiposity. We further examined these longitudinal associations.
Design. Cohort study based on examinations between 1973 and 1996.
Setting. Bogalusa, Louisiana.
Participants. Children (2610; ages 2-17 years old) who were followed to ages 18 to 37 years; the mean follow-up was 17.6 years.
Main Outcome Measures. BMI-for-age and triceps skinfold thickness (SF) were measured in childhood. Subscapular and triceps SFs were measured among adults, and the mean SF was used as an adiposity index. Adult obesity was defined as a BMI 30 kg/m2 and adult overfat as a mean SF in the upper (gender-specific) quartile.
Results. Childhood levels of both BMI and triceps SF were associated with adult levels of BMI and adiposity. The magnitude of these longitudinal associations increased with childhood age, but the BMI levels of even the youngest (ages 2-5 years) children were moderately associated (r = 0.33-0.41) with adult adiposity. Overweight (BMI-for-age 95th centile) 2- to 5-year-olds were >4 times as likely to become overfat adults (15 of 23 [65%]), as were children with a BMI < 50th centile (30 of 201 [15%]). Even after accounting for the triceps SF of children, BMI-for-age provided additional information on adult adiposity.
Conclusions. Childhood BMI is associated with adult adiposity, but it is possible that the magnitude of this association depends on the relative fatness of children.
Key Words: body mass index obesity adult adiposity Bogalusa Heart Study longitudinal study skinfolds
Abbreviations: BMI, body mass index SF, skinfold thickness
Despite the limitations of weight-height indices,1,2 they are widely used as indicators of adiposity. Several investigators have found that childhood body mass index (BMI; kg/m2) is moderately correlated with body fatness as estimated by dual-energy x-ray absorptiometry3–6 and is predictive of adult BMI (reviewed in refs 7–10). A recent report from the Newcastle Thousand Families Study,11 however, found that childhood (age 9 years) BMI was associated with adult (age 50 years) BMI but not with adult body fatness as determined by bioelectrical impedance analysis. These contrasting associations with adult levels of BMI and body fatness have led several investigators11–13 to conclude that (1) childhood BMI is not a good predictor of adiposity, and (2) the tracking (persistence of relative rankings) of BMI from childhood to adulthood reflects continuities in "body build" rather than adiposity.
The purpose of the current study is to compare the accuracy of childhood levels of BMI and triceps skinfold thickness (SF) in predicting adult adiposity. Among the 2610 subjects in this cohort, the mean age at baseline was 10 years (range: 2.5–17 years), the mean length of follow-up was 17.6 years, and adult ages ranged from 18 to 37 years.
METHODS
Sample
Bogalusa is a semirural community in Washington Parish, Louisiana, and is 70 miles north of New Orleans. Seven cross-sectional studies of schoolchildren, along with 1 study of 2- to 5-year-olds, were conducted in Ward 4 of Washington Parish between 1973–1974 and 1992–1994.14 In addition, 4 studies of adults (ages 18–37 years) who had been previously examined as children were conducted between 1982 and 1996. Protocols were approved by appropriate institutional review boards, and informed consent was obtained from all participants.
Longitudinal analyses could be conducted because most subjects participated in >1 cross-sectional examination. For example, a 6-year-old examined in 1976 could have been reexamined in any of the 6 studies conducted between 1978 and 1996. Although specialized techniques are required to analyze repeated, longitudinal data, the current analyses are stratified by childhood age so that each subject contributes only 1 observation to an age category. To maximize the length of follow-up, we used data from the first childhood examination and the final adult examination.
Measurements of weight, height, and triceps SF were available from 11 411 children who were examined between 1973 and 1994. Of these participants, 2905 (37%) of 7923 age-eligible children were actually reexamined in adulthood. (A 7-year-old examined in 1988, for example, could not have been reexamined as an adult.) Participation varied by gender: 31% of boys vs 42% of girls were reexamined in adulthood.
We excluded 295 subjects who were followed for <10 years, resulting in a study cohort of 2610 subjects. Thirty-two percent of the cohort was black.
Anthropometry
Schoolchildren were examined while wearing underpants, an examination gown, and socks; adults wore street clothes (excluding sweaters, jackets, belts, and shoes). Weight was measured to the nearest 0.1 kg by using a balance-beam scale, and height was measured to the nearest 0.1 cm with a manual height board.14,15 National US data (1963–1994) were used to convert weight, height, and BMI into gender- and age-specific z scores and centiles.16,17 These z scores and centiles represent BMI levels among the participating children relative to those in national studies; overweight is defined as BMI-for-age 95th centile.18,19
The triceps SF and subscapular SF were measured 3 times with Lange skinfold calipers, and the mean SF of these 2 sites is used as a measure of overall adiposity among adults. Because the subscapular SF was not measured in the initial examinations, the triceps SF is used as an adiposity index among children.
Adults with a BMI 30 kg/m2 were considered to be obese, whereas those with a mean SF in the upper gender-specific quartile (men: 21 mm; women: 30.3 mm) were considered to be "overfat." In adulthood, 464 (18%) of the 2610 subjects were both obese and overfat, 139 (5%) were obese but not overfat, and 153 (6%) were overfat but not obese.
Statistical Analyses
We examined baseline and follow-up levels of various characteristics among boys (n = 1115) and girls (n = 1495); the mean length of follow-up was 17.6 years (range: 10–24 years). The relations of childhood levels of BMI and triceps SF to adult levels of BMI and mean SF were compared in gender- and age-stratified analyses by (1) using Spearman (rank) correlation coefficients and (2) examining the proportion (positive predictive value) of children in various BMI-for-age categories who became obese (BMI 30 kg/m2) or overfat (upper quartile of mean SF) adults.
Linear regression was used also to assess the relation of childhood (ages 2–9 years) levels of BMI and triceps SF to the mean SF in adulthood; these models included race, gender, childhood age, adult age, and various interaction terms as covariates. These analyses also assessed whether childhood levels of BMI and triceps SF provided independent information on adult adiposity. Predicted levels of mean SF are displayed graphically.20
RESULTS
Thirty-seven percent of all eligible children were reexamined in adulthood (Table 1), and compared with children lost to follow-up, the study cohort was slightly older (10.0 vs 8.9 years, boys) and participated in earlier (1975 vs 1978, boys) examinations. However, mean levels of weight-for-age, height-for-age, and BMI-for-age were close to 0 in both groups. After adjustment for age and examination year, the only significant difference between the 2 groups was a slightly higher prevalence of overweight (BMI 95th centile) among boys in the study cohort (7% vs 6%; P = .02).
Of the 1115 boys and 1495 girls in the study cohort, 76% were initially examined in 1973–1974, and 94% participated in either the 1988–1991 or 1995–1996 screening of adults. Ages at baseline ranged from 2 to 17 years, and the mean age at follow-up was 27.5 years (range: 18–37 years). At follow-up, 23% were obese (BMI 30 kg/m2), and 25% were overfat (mean SF in the upper quartile).
Childhood levels of both BMI-for-age and triceps SF were significantly associated with adult levels of BMI and mean SF, with the overall correlations ranging from r = 0.44 to 0.64 (Table 2). Although the strongest associations were seen between BMI levels in childhood and adulthood, childhood BMI was also associated with adult levels of mean SF in all age groups including 2- to 5-year-olds (boys: r = 0.41; girls: r = 0.33). Furthermore, childhood BMI was almost as strongly associated with adult mean SF, as was childhood triceps SF; correlations among 6- to 8-year-old boys, for example, were r = 0.48 (BMI-for-age) and r = 0.50 (triceps SF). In general, the magnitudes of the correlations were stronger among boys than among girls and stronger among older (9–17 years) children than younger children. Between the ages of 2 and 5 years and 9 and 11 years, the association between BMI-for-age and adult mean SF increased from r = 0.41 to 0.56 among boys and from r = 0.33 to 0.51 among girls.
We then examined the ability of BMI to predict which children would be obese (BMI 30 kg/m2) or overfat (upper quartile of mean SF) in adulthood (Table 3). The positive predictive value for adult obesity and overfat consistently increased as childhood BMI-for-age increased, and this was evident among even the youngest children. Among 2- to 5-year-olds girls, for example, 15% (18 of 121) of those with a childhood BMI 50th centile became overfat adults, whereas 53% of overweight (BMI 95th centile) girls became overfat adults. Among overweight children, positive predictive values were generally higher for adult obesity than for adult overfat; of the 51 overweight 9- to 11-year-old boys, for example, 76% were obese in adulthood, and 67% were overfat. With the exception of estimates among 2- to 5-year-old boys, which were based on small numbers, the positive predictive values of childhood overweight for adult obesity generally increased with age. In contrast, the positive predictive values for adult overfat showed little variation with age, and similar estimates were seen among 6- to 8-year-olds (70–84%) and 15- to 17-year-olds (65–81%). Additional analyses (data not shown) indicated that changes in (age-adjusted) levels of BMI and triceps SF during childhood showed similar associations with adult overfat.
To examine further the importance of BMI levels among the 2- to 9-year-olds (n = 1362), various regression models were constructed to determine if levels of BMI-for-age and triceps SF were independently associated with adult mean SF. (Models also included race, gender, childhood age, adult age, and various interaction terms.) Although the inclusion of both BMI-for-age and triceps SF in a model only slightly improved the prediction of adult mean SF as compared with models containing only 1 of these variables (multiple R2 = 0.44 vs 0.39 and 0.42), both childhood characteristics were independently associated (P < .001) with adult adiposity.
These independent effects are shown in Fig 1 for boys (left) and girls (right). Predicted levels of adult mean SF (y-axis) are shown among white adults according to childhood levels of BMI-for-age (x-axis) and the 10th, 50th, and 90th centiles of triceps SF. For example, among boys with a triceps SF of 10 mm (median value), estimated adult levels of mean SF increased from 14 to 17 mm as the BMI z score increased from 0 to 1.645 (95th centile of BMI). Similarly, among boys with a BMI z score of 1.0, the predicted adult mean SF increased by 5 mm as triceps SF increased from 10 to 17 mm. The observed associations with adult mean SF did not significantly differ by race or gender, but adult levels were higher among women (Fig 1 right) than men and among black women than white women (data not shown).
DISCUSSION
Our results show that childhood levels of both BMI and triceps SF are associated with adult adiposity, as assessed 18 years later by the mean thickness of the triceps and subscapular SFs. Although the magnitude of the correlations between childhood and adult levels increased with the age at which the childhood measurement was obtained, the BMIs of even 2- to 5-year-olds were moderately associated (r = 0.33–0.41) with adult adiposity. Furthermore, overweight 2- to 5-year-old boys and girls were >4 times as likely to become overfat adults (15 of 23 [65%]) as were those who had a BMI < 50th centile (30 of 201 [15%]). Childhood triceps SF showed a slightly stronger association with adult adiposity than did childhood BMI, but both characteristics provided independent information on adult adiposity.
Several investigators have examined either the tracking of BMI from childhood to adulthood or the accuracy of childhood BMI in predicting adult obesity. Although there are exceptions,21 the larger studies consistently found that as compared with relatively thin children, fatter children are more likely to become obese adults.22–27 Although most investigators have examined the tracking of weight-height indices, similar (but slightly weaker) longitudinal associations have been observed for various SFs.22,24,28,29 For example, among 9- to 10-year-olds who were followed for 18 years, the tracking of BMI ranged from r = 0.60 (females) to r = 0.73 (males), whereas longitudinal correlations for triceps SF ranged from r = 0.50 to 0.58.24 Other investigators have also reported correlations that range from r = 0.4 to 0.5 for the triceps SF29 and biceps SF22 measured at ages 6 to 9 years and 18 to 25 years. These estimates agree well with the associations we observed between childhood triceps SF and adult mean SF (r = 0.38–0.59) over 17.6 (mean) years. We also found that the tracking of triceps SF was weakest among 2- to 5-year- olds, and SFs measured among 1- to 5-year-olds have shown28 little association (r = 0.14) with levels measured 20 years later.
A report from the Newcastle Thousand Families Study11 found that although BMI levels at ages 9 years and 50 years were correlated (r = 0.24), childhood BMI was not associated with adult adiposity (r = 0.10; P > .05) as assessed by bioelectrical impedance analysis. These contrasting associations have led several authors to suggest that childhood BMI is a poor indicator of adiposity, and that the tracking of BMI reflects the persistence of "body build" rather than adiposity.11–13 Cross-sectional studies, however, have found that childhood BMI is associated with body fatness as assessed by dual-energy x-ray absorptiometry.3–6 In addition, a previous study30 reported that increases in BMI percentile rankings between the ages of 3 and 9 years among girls was associated with adult adiposity as determined by hydrostatic weighing. Therefore, it is unlikely that childhood BMI is merely an indicator of body build.
It is possible that the negative findings of the Newcastle Thousand Families Study are due to the relative thinness of children born in the United Kingdom in 1947. As compared with 9-year-olds in the current study, who were born between 1963 and 1976, similarly-aged children in Newcastle, on average, weighed 9 lb less and were 2 inches shorter,31 resulting in lower BMI levels. Furthermore, 16% of 9-year-olds in the current study had a BMI > 95th centile in Newcastle (boys: 20.5 kg/m2; girls: 18.5 kg/m2) (C. M. Wright, MD, written communication, 2003). As the accuracy of BMI as an index of body fatness increases with the degree of adiposity,32–34 it is possible that differences in childhood BMI levels between the 2 studies could account for the contrasting associations with adult adiposity. The variability in BMI levels among thinner children largely reflects differences in fat-free mass,34 and if this were the case in Newcastle, childhood BMI levels would not be expected to be associated with adult adiposity. In contrast, the BMI levels of Bogalusa children may reflect differences in fat mass more accurately.
The negative findings of the Newcastle study may also reflect the limitations of bioelectrical impedance analysis. Although levels of various cardiovascular disease risk factors showed fairly similar cross-sectional associations with BMI and body fatness among adults in the Newcastle study, some investigators35–37 have found bioelectrical impedance estimates of body fat to be less accurate than those based on SFs. In addition, bioelectrical impedance estimates can be influenced by the possible association between electrical conductivity and the amount of fat mass,35 the choice of the calibration equation,38 and various clinical characteristics.39
Although we found that most overweight (BMI 95th centile) children were obese or overfat in adulthood, with positive predictive values ranging from 53% to 90%, only 20% of obese or overfat adults had been overweight children. This estimate is comparable to those reported in other studies,11,23,28,40 but it should be realized that estimates of sensitivity are greatly influenced by the prevalences and classifications of childhood overweight and adult obesity. For example, use of the 85th centile rather than the 95th centile as the childhood BMI cut point resulted in sensitivities of 42% (overfat) and 51% (obese). Furthermore, of the 114 adults with a BMI 40 kg/m2, 46% had a childhood BMI 95th centile and 70% had a childhood BMI 85th centile. Although there are certainly additional factors that operate in early adulthood to promote obesity, a child with a high BMI-for-age is much more likely to become an obese adult than is a relatively thin child.
Several limitations of the current study should be considered when interpreting our results. Although only 37% of age-eligible children were reexamined as adults, their baseline levels of weight, height, and BMI were fairly similar to those among children who were not reexamined. In addition, we were able to follow children only through early adulthood (maximum age: 37 years), and it is likely that a longer follow-up would have reduced the magnitudes of the various associations. Although there are few studies on the tracking of childhood BMI beyond the age of 40 years, BMI levels at ages 30 and 50 years are highly correlated (r = 0.7-0.8).21 In addition, the current study did not investigate the associations with adult risk factors for coronary heart disease, but previous studies11,41 have shown that the most important correlate is the adult BMI level. Childhood BMI, however, may be important in the development of atherosclerosis. We have found, for example, relatively high levels of the carotid artery intima-media thickness among obese adults who had been overweight children but not among obese adults who had been normal-weight children.42
Our findings indicate that childhood BMI is associated with adiposity in adulthood and that overweight children have a greatly increased risk for becoming overfat adults. Our findings also provide a partial explanation for the variability observed in previous studies of BMI tracking from childhood to adulthood. Because the accuracy of childhood BMI as an indicator of body fatness increases with the degree of adiposity, the degree of tracking would be expected to be highest in populations with a high prevalence of overweight.
ACKNOWLEDGMENTS
This work was supported by National Institutes of Health grants HL-38844 (National Heart, Blood, and Lung Institute), HD-043820 (National Institute of Child Health and Human Development), and AG-16592 (National Institute on Aging), funds from the Centers for Disease Control and Prevention, and the Robert W. Woodruff Foundation.
We thank the investigators of the Newcastle Thousand Families Study for helping us compare body mass index levels between children in Bogalusa and Newcastle.
FOOTNOTES
Accepted Jun 14, 2004.
No conflict of interest declared.
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