Nutritional Epidemiology
Socioeconomic Gradients in Body Weight of German Children
Reverse Direction between the Ages of 2 and 6 Years
1Kristina Langna¨se, Mareike Mast, Sandra Danielzik, Carina Spethmann
and Manfred J. Mu¨ller
2Institut fu¨r Humanerna¨hrung und Lebensmittelkunde, Christian-Albrechts-Universita¨t zu Kiel, 24105 Kiel, Germany
ABSTRACT In Western societies, there is an inverse socioeconomic status (SES) gradient in overweight, but its onset is unknown. It was the aim of this study to assess the onset of the SES gradient in overweight children from birth to age 5–7 y. This was a cross-sectional and longitudinal study of 1326 children [668 boys and 658 girls; median age, 6.3 y; interquartile range (IQR), 0.4 y] in Kiel, northwest Germany. We assessed parental education as an indicator of SES, parental body mass index (BMI) (fathers: median age, 37.0 y; IQR: 8.0 y; mothers: median age, 34.0 y; IQR, 6.5 y), BMI of mothers before pregnancy, and breastfeeding practices. We found a positive SES gradient in birthweight, which was further enhanced by parental overweight. SES gradients in nutritional status disappeared at the age of 1 and 2 y. In contrast, in 5- to 7-y-old children, there was an inverse SES gradient in BMI and overweight, which was further enhanced by parental overweight. We concluded that (i) SES gradients in nutritional state show considerable changes during the first years of life, (ii) the inverse SES gradient in overweight becomes manifest between years 2 and 6 of life and (iii) parental BMI enhances SES gradients in birthweight and BMI at 5–7 y of age. J. Nutr. 133: 789 –796, 2003.
KEY WORDS: ● childhood obesity ● birthweight ● socioeconomic state ● prevention ● nutritional state
There is a worldwide increase in the prevalence of over-weight and obesity (1). Socioeconomic status (SES)3has an
influence on the manifestation of overweight and obesity. An inverse SES gradient in overweight and obesity is obvious in women (2). Among men and children, the relationship is less consistent (2). We have shown recently that social differences in being overweight are already manifest in a group of 5- to 7-y-old children in northwest Germany (3). In these children, the mean prevalence of overweight is 18.5%. There is an inverse SES gradient, and the highest fat mass is observed in children from a low socioeconomic class (3). The odds ratios of being overweight reach 3.1 in boys and 2.3 in girls, respec-tively (low versus high social class) (3). Parental overweight exacerbates the relationship between SES and the prevalence of being overweight, with 37.5% (low socioeconomic class) and 22.9% (high socioeconomic class) of children of over-weight parents being overover-weight (3).
It has been proposed that differences in body mass index (BMI) by socioeconomic class are formed at least in part during early childhood (4). However, the time of manifesta-tion of SES gradients in overweight is unknown. Contrary to the inverse SES gradient in overweight of 5- to 7-y-old
chil-dren, others have found a positive SES gradient in birthweight (5,6). There is a positive linear relationship between increas-ing SES and the proportion of infants weighincreas-ing⬎3,500 g (5), and low birthweight occurs more frequently with socioeco-nomic deprivation (6). There is only preliminary evidence that SES-induced differences in birthweight persist during the first years of life (7). These data suggest that the development of growth and weight is more pronounced in people of higher SES compared with those of middle or lower SES (7). This idea is in line with data from twin studies showing a significant tracking of body size from birth to late adolescence where BMI could be predicted from birthweight and parental BMI (8). Following this idea one would expect a further stabilization of the positive SES gradient in later years that is contrary to the above-mentioned findings of an inverse SES gradient in over-weight in 5- to 7-y-old children and adults (2,3). We therefore analyzed the data of the Kiel Obesity Prevention Study (KOPS) cohort (9,10) to assess the onset of the SES gradient in overweight in children from birth to age 5–7 y. In addition we analyzed the effects of SES, parental BMI, birthweight, breastfeeding, and BMI at 1 and 2 y of life on BMI of 5- to 7-y-old children.
METHODS
Data from the KOPS between 1996 and 1998 were analyzed (9,10). Within this time period school physicians of the city of Kiel (northwest Germany, 248,000 inhabitants) examined a total of 6927 children age 5–7 y. From this population 2561 (37%) children were recruited. The aims of KOPS were explained to all parents and
1This study was supported by grants from the Deutsche Forschungsgemein-schaft (DFG Mu¨ 714 5–2), WirtForschungsgemein-schaftliche Vereinigung Zucker, Bonn, Danone Stiftung, Mu¨nschen, PreCon Bickenbach.
2To whom correspondence should be addressed. E-mail: mmu¨[email protected].
3Abbreviations used: BMI, body mass index; KOPS, Kiel Obesity Prevention Study; SDS, standard deviation score; SES, socioeconomic status.
0022-3166/03 $3.00 © 2003 American Society for Nutritional Sciences.
Manuscript received 24 July 2002. Initial review completed 27 August 2002. Revision accepted 20 November 2002.
children. Parents and children participated voluntarily, and written consent was obtained from all parents. The local ethical committee of the Christian-Albrechts-University Kiel approved the study protocol. Nutritional status of the children was measured, and sociodemo-graphic and lifestyle factors of the children and families were assessed using a questionnaire. For this study 1326 complete data sets on nutritional status of children and their parents as well as social status were available. This subpopulation was 51.7% of the total KOPS population recruited between 1996 and 1998. Anthropometric data (age, gender, body weight and height) suggests that our study popu-lation was representative of the total popupopu-lation of 5- to 7-y-old children in Kiel (11).
Nutritional status. Birthweight, height and weight of the chil-dren at 1 and 2 y were recorded from certificates of preventive medical check-ups. At the age of 5–7 y body weight was measured to the nearest 0.1 kg on a calibrated balance-beam scale with subjects wearing underclothes. Height at 5–7 y was assessed to the nearest 5 mm, and BMI [weight (kg)/height (m2)] was calculated. The
inves-tigators were instructed every year, and the quality of measurements was regularly controlled. The between- and within-observer impreci-sion variance was tested every year and was⬍2.5%. Parental BMI was calculated from self-reported body weight and height. The BMI of mothers before pregnancy was taken from measured values docu-mented on the children’s birth certificates. High birthweights were considered as ⱖ4,000 g corresponding to a period of 38 – 42 wk of pregnancy. Prevalence of overweight in children was defined as equal or above the gender- and age-specific 90th percentiles of BMI
ac-cording to German reference data (12). In parents, overweight was defined as BMI ⱖ25 kg/m2 (1). Parents were grouped as no
over-weight parent, as one overover-weight parent or as two overover-weight parents.
Socioeconomic status. Data on SES were taken from our ques-tionnaire. The highest school education achieved by either mother or father was used to define social class. Parental education was divided into “low,” “middle” and “high.” “Low” was assigned to those who attended school for 9 y, “middle” to those who left school after 10 y and “high” to those who achieved an advanced level of education (13 y). In a previous study this classification was compared with other SES indicators (e.g., size of family, occupational grade of parents) (12). All SES indicators were moderately cross-correlated, but the choice of the indicator did not affect between-group differences. Education was found to be suitable to reflect SES (11).
Statistical analysis. The statistical analysis was performed with StatView 4.0 (Abacus Concepts). Results were presented as median and interquartile range. The nonparametric Mann-Whitney Utest was used to determine gender differences in nutritional status. The nonparametric Kruskal-Wallis test was performed to analyze nutri-tional status of the children at birth, 1 y, 2 y and 5–7 y, with respect to different SES of the children as well as different parental nutri-tional status. After significant differences between groups were iden-tified, a Bonferroni correction was used to identify the location of the differences. The2tests compared prevalence of high birthweight,
breastfeeding and overweight among groups. To examine changes in nutritional status between birth and the age of 5–7 y,SDscores [SDS
as taken from data obtained within a German reference population of TABLE 1
Characteristics of the 1,326 study children (age 5–7 y) and their parents1
Group/characteristic All children (n⫽1,326) Boys (n⫽668) Girls (n⫽658)
Children Age,y 6.3 (6.0–6.5) 6.3 (6.0–6.5) 6.3 (6.0–6.5) Birthweight,g 3,490 (3,123–3,760) 3,550 (3,150–3,820)a 3,440 (3,100–3,690) BMI,kg/m2 1 y 17.1 (16.2–18.0) 17.3 (16.4–18.2)a 16.9 (15.9–17.8) 2 y 16.3 (15.5–17.3) 16.5 (15.7–17.6)a 16.1 (15.2–17.0) 5–7 y 15.6 (14.6–16.5) 15.6 (14.8–16.5) 15.5 (14.5–16.5) Prevalence,% High birthweight2 14.3 17.0 11.7 Overweight3at 1 y 16.2 16.6b 15.9 Overweight3at 2 y 13.7 18.0b 9.5 Overweight3at 5–7 y 10.5 10.9 9.8 Breastfeeding 0 mo 12.9 14.9b 11.4 ⱕ6 mo 47.6 49.3 46.2 ⬎6 mo 39.5 35.8 42.4 Parents Age,y Father 37.0 (33.0–41.0) 38.0 (34.0–42.0)a 37.0 (33.0–40.0) Mother 34.0 (31.0–37.5) 35.0 (31.0–38.0) 34.0 (31.0–37.0) Current BMI,kg/m2 Father 24.7 (23.2–26.9) 24.5 (23.1–26.5)a 24.9 (23.4–27.4) Mother 22.5 (20.7–25.3) 22.5 (20.6–24.6) 22.4 (20.8–25.5) Prevalence of overweight,4% Father 45.2 41.4b 49.1 Mother 25.9 23.2 28.6
BMI of mother before pregnancy,kg/m2 21.5 (20.0–23.7) 21.5 (20.0–23.5) 21.6 (20.0–23.9)
Prevalence of overweight before pregnancy,% 16.3 15.3 17.4
Change5in BMI of mother,% 3.4 (0.0–9.1) 3.3 (0.0–8.6) 3.5 (0.0–9.5)
Parental education level,%
Low 23.7 23.3 24.1
Middle 29.4 27.1 31.7
High 46.9 49.6 44.2
1Values are median (IQR).aDifferent from girls, Mann-WhitneyUtest,P⬍0.05.bDifferent from girls,2test,P⬍0.05. BMI, body mass index; IQR, interquartile range.
2Birthweightⱖ4,000 g.
3Equal to or greater than sex- and age-specific 90th percentile of BMI according to Kromeyer-Hauschild et al. (11).
4BMIⱖ25.0 kg/m2.
children (11) using the least-mean-square method based on Cole 13)] were calculated for BMI at birth and at the age of 5–7 y. Medians of BMI were stratified according to SES, parental nutritional status and breastfeeding. Within each group, differences between SDS at birth and SDS at the age of 5–7 y were tested by the Wilcoxon signed rank test. Quartiles of birthweight and BMI at 5–7 y were calculated. Differences among groups were analyzed by the nonparametric Kruskal-Wallis test. The level of significance was set atP⬍0.05.
RESULTS
Characterization of the study population. The study pop-ulation is characterized in Table 1. There were significant gender differences in birthweight as well as the nutritional status of infants and prepubertal children. Boys had higher birthweight and BMI at the age of 1 and 2 y. Compared with girls, boys were more often overweight at the age of 1 as well as 2 y. Boys were also more frequently not breastfed. In addition, their fathers were older, had a lower BMI and were less often overweight compared with fathers of girls. There were no differences in the actual or prepregnancy weight of mothers of boys and girls. Comparing the nutritional status of mothers before pregnancy with their current nutritional status when their children were 6 y old, 88.8% of mothers who were overweight before pregnancy remained overweight. The cor-responding data for normal weight and underweight mothers
were 83% and 49.4%, respectively. Only 11.2% of normal-weight mothers became overnormal-weight during the study period. Of the children, 46.9% were of high SES, whereas 53.1% came from families with middle or low parental education. Sex was not associated with parental education.
Influence of parental education on nutritional status. Stratifying nutritional variables according to parental educa-tion showed between group differences in age and BMI at the age of 5–7 y (Table 2). Children with low parental education were older and had a higher BMI than children with high parental education. There were significant social gradients in the prevalence of high birthweight and breastfeeding (positive gradients for parental education) as well as the prevalence of overweight at the age of 5–7 y (inverse gradient for parental education). No parental education gradient was seen for BMI of children at the age of 1 and 2 y. Parental education was also related to parental BMI (negative gradient). Means for parental BMI and the prevalence of overweight were lowest in parents with high education and highest in parents with low education. There was an inverse parental education gradient in the preva-lence of overweight in mothers before pregnancy as well as in their weight gain (current BMI minus BMI before pregnancy).
Influence of parental overweight on nutritional variables. There was a close association between parents’ and children’s
TABLE 2
Characteristics of 1,326 study children (age 5–7 y) and their parents stratified according to parental education level1
Parental education level
Low (n⫽309) Middle (n⫽391) High (n⫽626)
Children Age,ay 6.3 (6.1–6.6)b 6.3 (6.0–6.5) 6.2 (6.0–6.5) Birthweight,g 3,430 (3,080–3,750) 3,500 (3,100–3,770) 3,500 (3,188–3,760) BMI,kg/m2 1 y 16.9 (16.0–18.1) 17.2 (16.3–18.0) 17.1 (16.2–18.0) 2 y 16.3 (15.5–17.2) 16.4 (15.6–17.3) 16.3 (15.4–17.2) 5–7 ya 15.7 (14.7–17.2)b 15.6 (14.6–16.6) 15.5 (14.6–16.4) Prevalence,% High birthweight2 14.0 13.6 15.3 Overweight3at 1 y 16.7 15.3 16.1 Overweight3at 2 y 14.5 13.5 13.2 Overweight3at 5–7 yc 16.3 10.5 7.2 Breastfeedingc 0 mo 25.1 14.8 6.0 ⱕ6 mo 51.0 48.0 44.9 ⬎6 mo 23.9 37.2 49.1 Parents Age,y Fathera 36.0 (32.0–39.0)b 36.0 (33.0–40.0)b 39.0 (35.0–42.0) Mothera 32.0 (29.0–36.0)bd 33.0 (31.0–36.0)b 36.0 (33.0–38.0) Current BMI,kg/m2 Fathera 25.7 (23.9–27.8)bd 24.9 (23.5–27.3)b 24.2 (22.7–26.1) Mothera 23.4 (21.3–27.0)bd 23.1 (21.1–25.8)b 21.9 (20.3–23.8) Prevalence of overweight,4% Fatherc 56.8 48.3 37.3 Motherc 36.9 32.0 16.8
BMI of mother before pregnancy,akg/m2 22.0 (20.2–25.4)bd 22.1 (20.3–24.2)b 21.1 (19.8–22.8)
Prevalence of overweight before pregnancy,c% 26.8 20.2 8.7
Change5in BMI of mother,a% 5.2 (0.0–13.2) 3.9 (0.0–9.2) 2.8 (0.0–7.1)
1Values are median (IQR).aDifferent among groups, Kruskal-Wallis test,P⬍0.05.bDifferent from high parental education level, Bonferroni post hoc test,P⬍0.05.cDifferent among groups,2test,P⬍0.05.dDifferent from middle parental education level, Bonferroni post hoc test,P⬍0.05.
BMI, body mass index; IQR, interquartile range.
2Birthweightⱖ4,000 g.
3Equal to or greater than sex- and age-specific 90th percentile of BMI according to Kromeyer-Hauschild et al. (11).
4BMIⱖ25.0 kg/m2.
BMI. Using parental BMI as a continuous variable, there was a positive association between BMImothers and BMIchildren (r
⫽ 0.289;P ⬍ 0.001). After categorization (BMImothers ⬍25 versus⬎25 kg/m2),rwas 0.154 for normal weight and 0.201
for overweight and obese mothers, respectively (each P
⬍ 0.001). Table 3 presents nutritional variables of children and parents stratified according to parental nutritional status. Birthweight, prevalence of high birthweight and BMI at the age of 5–7 y significantly differed between groups of normal and overweight parents. Children of normal weight and un-derweight parents had lower birthweights compared with the other children. The prevalence of high birthweight was lowest in children of normal-weight parents. By contrast, the highest values were seen in children of two overweight parents. There was a positive gradient between the prevalence of parental overweight and the BMI of children at the age of 5–7 y. Children with two overweight parents had the highest BMI and were more frequently overweight compared with the other groups (Table 3). No significant between-group differences were observed in the prevalence of breastfeeding.
Influence of breastfeeding on nutritional variables. With the children stratified according to breastfeeding practices, there were no between-group differences in age, birthweight, BMI or prevalence of overweight at 1, 2 and 5–7 y (Table 4). In a comparison of breastfed with nonbreastfed infants in the latter group, mothers had higher current as well as
prepreg-nancy BMI. The prevalence of breastfeeding increased with parental education.
Effect of parental education and parental overweight on birthweight and nutritional status of infants and children.
Figure 1shows the effect of parental education and parental BMI on birthweight and BMI at the age of 1, 2 and 5–7 y. There was a positive gradient between parental education and birthweight (Fig. 1a). This gradient was enhanced by parental overweight. At 1 and 2 y there was no effect of either SES or parental overweight on children’s BMI (Fig. 1b and 1c). At the age of 5–7 y, there was an inverse parental education gradient in BMI that was further enhanced by parental over-weight (Fig. 1d). Thus, children of two overover-weight parents with low education had the highest mean BMI.
Longitudinal relationships between birthweight and BMI at the age of 5–7 y showed that children in low and high parental education groups have different weight changes. In Table 5, SDS-BMI scores are used to illustrate changes over time. Overall, SDS-BMI decreased but there was no significant decrease for (i) children in groups with low parental educa-tion, (ii) those with two overweight parents and (iii) non-breastfed children. Of children with high birthweight and low as well as middle parental education, 33% tended to remain within a high quartile at the age of 5–7 y. When compared with high parental education, about twice as many children of TABLE 3
Characteristics of 1,326 study children (age 5–7 y) and their parents stratified according to parental nutritional status1
Parental nutritional status No overweight2parent
(n⫽644)
One overweight parent (n⫽492)
Two overweight parents (n⫽190) Children Age,y 6.3 (6.0–6.5)b 6.3 (6.0–6.5) 6.2 (6.0–6.5) Birthweight,ag 3,440 (3,100–3,720) 3,500 (3,180–3,765)b 3,550 (3,105–3,925)b BMI,kg/m2 1 y 17.0 (16.2–18.0) 17.1 (16.2–18.0) 17.0 (16.2–18.0) 2 y 16.3 (15.6–17.3) 16.3 (15.4–17.2) 16.2 (15.3–17.3) 5–7 ya 15.3 (14.5–16.2) 15.5 (14.6–16.6)b 16.4 (15.5–17.6)bc Prevalence,% High birthweight2d 10.6 16.7 21.7 Overweight3at 1 y 15.2 17.4 16.9 Overweight3at 2 y 14.5 13.3 10.5 Overweight3at 5–7 yd 4.5 13.2 22.4 Breastfeeding 0 mo 10.8 12.4 18.1 ⱕ6 mo 47.9 46.9 50.0 ⬎6 mo 41.3 40.7 31.9 Parents Age,y Father 37.0 (33.0–41.0)b 38.0 (34.0–42.0) 36.0 (33.0–41.0) Mother 34.0 (31.0–37.0) 34.0 (32.0–38.0) 34.0 (31.0–37.0)
BMI of mother before pregnancy,akg/m2 22.8 (19.6–22.0) 21.9 (20.1–24.2)b 25.6 (23.5–28.2)bc
Prevalence of overweight before pregnancy,d% 3.0 18.1 58.7
Change,4in BMI of mother,a% 1.8 (0.0–6.6) 4.2 (0.0–10.3)b 9.7 (2.6–18.1)b
Parental education level,d%
Low 18.5 24.0 37.9
Middle 26.7 30.3 36.8
High 54.8 45.7 25.3
1Values are median (IQR).aDifferent between groups, Kruskal-Wallis test,P⬍0.05.bDifferent from group with no parent overweight, Bonferroni post hoc test,P⬍0.05.cDifferent from group with one parent overweight, Bonferroni post hoc test,P⬍0.05.dDifferent between groups,2test, P⬍0.05. BMI, body mass index; IQR, interquartile range.
2BMIⱖ25.0 kg/m2. 3Birthweightⱖ4,000 g.
4Equal to or greater than sex- and age-specific 90th percentile of BMI according to Kromeyer-Hauschild et al. (11). 5BMI before pregnancy minus current BMI.
low SES families and normal birthweight increased their BMI disproportionately within the observation period.
Determinants of BMI at 5–7 y of age. Using the BMI of 5- to 7-y-old children as a dependent variable in a multivariate regression analysis, a total of 11.3% of the variance could be explained by BMImother (standard regression coefficient,
0.232), BMIfather (0.111), birthweight (0.135) and parental education (0.065), whereas breastfeeding was not significant. We also tried to explore the possible three-way interaction among birthweight, nutritional status and education of parents predicting BMI of 5- to 7-y-old children. Main effects were seen for birthweight (P⬍0.0001), parental BMI (P⬍0.0001) and education (P⬍ 0.05). By contrast, there were no signif-icant interactions for birthweight ⫻ parental BMI, birth-weight ⫻ parental education, parental education⫻ parental BMI and parental education⫻ parental BMI⫻ birthweight.
DISCUSSION
There are considerable and increasing data on SES gradi-ents in health (14). As Michael Marmot stated, “Within societies, health and ill-health follow a social gradient: lower socio-economic position, worse health” (15). Most of the SES gradients in health status have their onset in adulthood. How-ever, some authors have proposed that differences in BMI by
social class are formed at least in part during early childhood (4). The major finding of this study was that SES gradients in nutritional status become manifest early in life. There was an inverse SES gradient in the BMI of 5- to 7-y-old children (Fig. 1, Table 2). This gradient was enhanced by parental over-weight (Fig. 1). There also were inverse SES gradients in parental overweight (Table 2). By contrast, there was a posi-tive SES gradient in birthweight (Table 2, Fig. 1). This gra-dient was also enhanced by parental overweight (Fig. 1). In addition, there were no associations between BMI of 1- and 2-y-old children and SES (Fig. 1). It was shown that (i) the association between SES and children’s nutritional status changes during the first 6 y of life and (ii) the inverse SES gradient in overweight becomes manifest between the second and the sixth years of life.
Our findings cannot provide an explanation for the associ-ation between SES and body weight in children. SES may influence health status (social causation), but health status may also contribute to SES (social drift or selection) (14). It is tempting to speculate that early manifestations of health risks and diseases are likely to contribute to social drift and thus will have a long-term effect on these children. In children SES differences in overweight may contribute to subsequent SES gradients in health status in adulthood. However, SES by itself TABLE 4
Characteristics of 1,326 study children (age 5–7 y) and their parents stratified according to breastfeeding1
Duration of breastfeeding 0 mo (n⫽208) ⱕ6 mo (n⫽605) ⬎6 mo (n⫽513) Children Age,y 6.2 (6.0–6.5) 6.3 (6.0–6.5) 6.3 (6.0–6.5) Birthweight,g 3,390 (2,985–3,718) 3,470 (3,148–3,782) 3,450 (3,188–3,730) BMI,kg/m2 1 ya 17.3 (16.6–18.2) 16.9 (16.1–17.9) 17.1 (16.3–18.0) 2 y 16.4 (15.6–17.4) 16.2 (15.4–17.2) 16.3 (15.4–17.2) 5–7 y 15.7 (14.8–16.9) 15.6 (14.6–16.7) 15.6 (14.5–16.5) Prevalence,% High birthweight2 11.9 15.2 13.4 Overweight3at 1 yb 22.1 13.5 18.5 Overweight at 2 y 15.4 13.0 13.9 Overweight at 5–7 yb 17.8 10.2 8.8 Parents Age,y Fathera 36.0 (32.0–39.0)c 36.0 (33.0–40.0)c 38.0 (35.0–42.0) Mothera 33.0 (30.0–36.0)cd 34.0 (31.0–37.0)c 35.0 (33.0–38.0) Current BMI,kg/m2 Father 24.9 (23.4–27.8) 24.7 (23.3–26.6) 24.7 (23.1–26.9) Mothera 23.1 (21.1–27.6)cd 22.7 (20.8–25.4)c 22.0 (20.4–24.3) Prevalence of overweight,4% Father 49.2 45.3 44.0 Motherb 40.3 26.8 21.4
BMI of mother before pregnancy,akg/m2 23.1 (20.2–26.0)cd 21.6 (20.0–23.7) 21.4 (20.0–23.1)
Prevalence of overweight before pregnancy,b% 33.1 16.8 12.9
Change5in BMI of mother,% 5.1 (0.0–12.8) 3.7 (0.0–10.0) 2.9 (0.0–7.1)
Parental education level,b%
Low 42.7 23.9 13.3
Middle 35.9 32.0 29.4
High 21.4 44.1 57.3
1Values are median (IQR).aDifferent between groups, Kruskal-Wallis test,P⬍0.05.bDifferent between groups,2test,P⬍0.05.cDifferent from group that was breastfed⬎6 mo, Bonferroni post hoc test,P⬍0.05.dDifferent from group that was breastfedⱕ6 mo, Bonferroni post hoc test,
P⬍0.05. BMI, body mass index; IQR, interquartile range.
2Birthweightⱖ4,000 g.
3Equal to or greater than sex- and age-specific 90th percentile of BMI according to Kromeyer-Hauschild et al. (11).
4BMIⱖ25.0 kg/m2.
also influences health status. Data from longitudinal studies show that early child development and SES are empirically linked to adult health status (16). In regard to overweight and obesity, there was a significant association between BMI in childhood or adolescence and BMI in adulthood (17–19,20). Persistence of obesity into adulthood appeared to rise linearly throughout childhood. In addition, childhood overweight has been shown to be predictive for adult morbidity and mortality (21). Antecedents of adult disease (hypertension, hyperlipid-emia, impaired glucose tolerance) occur with increased fre-quencies in obese children and adolescents (22–26). There are only a few long-term follow-up data spanning the period from childhood to adulthood. These data suggest life-long persis-tence and health consequences of overweight and obesity in many children (21). Because SES contributes to childhood overweight, it also contributes to its short- and long-term health consequences. It is reasonable but presently unknown whether intraindividual SES differences in overweight persist throughout life.
SES differences in adult health status have been in part explained by early education. Our present finding of SES gradients in early-onset overweight suggested that the devel-opment of unhealthy habits occurs before the age of 6 y. In fact, there are SES gradients in food intake, physical activity and inactivity in 5- to 7-y-old children (3,10,27). SES also affects early feeding practices. The prevalence of breastfeeding increased with parental education (Table 4). Our data sup-ported previous results showing a “protective” effect of breast-feeding on childhood overweight, i.e., breastfed infants are less likely to be overweight in childhood (28 –30). Our data also showed that compared with breastfed infants, the prevalence of overweight at the ages of 1 and 5–7 y was significantly increased in children who were not breastfed as infants (Table 4). However, using a multivariate regression analysis with chil-dren’s BMI at 5–7 y as a dependent variable, breastfeeding did not reach significance. In the group of nonbreastfed infants, the mothers’ current and prepregnancy BMI were increased, suggest-ing a possible interaction between different factors. Ussuggest-ing analysis of variance, we found no interactions among SES, parental weight and birthweight (see Results). Pragmatically, all of these data suggested that measures of obesity prevention should start very early in life and should specifically tackle overweight chil-dren from a low socioeconomic class.
Parent-prepubertal offspring associations for BMI were in-vestigated in our study, and the risk of childhood obesity significantly increased with the BMI of the parents (Table 3, Results). Using a multivariate analysis with children’s BMI at the age of 5–7 y as the dependent variable, 7.6% of its variance was explained by the BMI of mothers and fathers in a previous study on the KOPS population (31). In the present study this number could be increased to ⬃11% by adding SES, birth-weight and breastfeeding to the analysis. We have previously
FIGURE 1 Median birth weight, BMI at 1 y, BMI at 2 y and BMI at 5–7 y of 1326 children stratified by parental social class and nutritional status. (a) Median birth weight, ANOVA:Psocial class⫽0.061;Pparental BMI
⬍0.01;Psocial class⫻parental BMI⫽0.977; difference between children with no versus one overweight parent, Bonferroni post hoc test: P
⬍0.05. (b) BMI at 1 y, ANOVA: NS. (c) BMI at 2 y, ANOVA: NS. (d) BMI at 5–7 y, ANOVA:Psocial class⬍0.01;Pparental BMI⬍0.0001;Psocial class
⫻parental BMI⫽0.121; differences between children from low versus high SES and among all groups of parental nutritional status, Bonferroni post hoc test:P⬍0.05. BW, birthweight; 0P, no parent overweight; 1P, one parent overweight; 2P, two parents overweight; NS, not significant; SES, socio-economic status.
shown that parental BMI had its strongest effect in the group of overweight children (3). These data suggest a significant but limited familial effect on childhood BMI and the prevalence of childhood obesity. Some authors propose that shared environ-ment exerts only a minor effect on nutritional traits, suggest-ing an impact of biological factors (32). However, family members show similarities in food preferences, eating habits and activity patterns. Thus in addition to genes, behavioral factors and/or gene-environment interactions are likely to add to children’s BMI (33,34). This idea is also illustrated by the finding that inverse SES gradients in BMI are, but SES gradi-ents in health habits are not, affected by parental overweight (3). Because the inverse SES gradients in children’s BMI and the changes in BMI during the first years of life are both affected by parental overweight, our data may be taken as evidence for gene-environment interaction(s).
Our data supported the positive association between SES and birthweight. SES gradients in nutritional state show con-siderable changes during the first years of life. The inverse SES gradient in overweight became manifest between years 2 and 6 of life. Parental BMI enhanced SES gradients in birthweight and BMI at 5–7 y of age.
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TABLE 5
Standard deviation scores for body mass index (SDS-BMI) at birth and in 1,326 study children (age 5–7 y) stratified according to parental education level, parental nutritional status and breastfeeding1
n SDS-BMI at birth SDS-BMI at 5–7 y Pvalue, WRT
All children 1,326 0.22 (⫺0.97 to 0.89) 0.05 (⫺0.61 to 0.55) ⬍0.05
Parental education level
Low 309 0.27 (⫺0.44 to 0.94) 0.05 (⫺0.56 to 0.77) N.S.
Middle 391 0.25 (⫺0.38 to 0.89) 0.05 (⫺0.61 to 0.57) ⬍0.05
High 626 0.19 (⫺0.38 to 0.85) 0.00 (⫺0.63 to 0.46) ⬍0.05
Parental BMI2a
No overweight parent 644 0.13 (⫺0.47 to 0.80) ⫺0.04 (⫺0.70 to 0.37) ⬍0.05
One overweight parent 492 0.27 (⫺0.28 to 0.95)b 0.04 (⫺0.64 to 0.57) ⬍0.05
Two overweight parents 190 0.43 (⫺0.34 to 1.10) 0.35 (0.00 to 1.10) N.S.
Breastfeeding, mo
0 208 0.24 (⫺0.38 to 0.99) 0.05 (⫺0.45 to 0.76) N.S.
ⱕ6 605 0.25 (⫺0.37 to 0.92) 0.05 (⫺0.64 to 0.59) ⬍0.05
⬎6 513 0.23 (⫺0.38 to 0.86) 0.05 (0.64 to 1.19) ⬍0.05
1Values are median (IQR).aDifferent SDS-BMI at birth between groups, Kruskal-Wallis test,P⬍0.05.bDifferent SDS-BMI at birth from group with no overweight parents. IQR, interquartile range; N.S., not significant; WRT, Wilcoxon signed rank test.
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