Overview of included studies
Country of origin. Data from the 45 studies included in this review originate from the United Kingdom (n = 11), Germany (n = 7), United States (n = 7), Australia (n = 6), Italy (n = 4), France (n = 2), The Netherlands (n = 2), Belgium (n = 1), Canada (n = 1), Republic of Ireland (n = 1), Spain (n = 1), Sweden (n = 1), and Switzerland (n = 1).
Adiposity. Adiposity was determined using BMI in 41 studies. Two studies each used relative weight (# 11, 13) and weight-for-height (# 1, 23). At least one measure of body fat distribution was examined in association with SES in six studies; five used skinfolds (# 10, 13, 41, 43, 44), two also included waist girth and/or waist-to-hip ratio (# 13, 44), and one used fat mass (# 45). Adiposity was analyzed as a categorical variable in 35 studies and a continuous variable in 10 studies. Twelve different growth references were used across the studies for classifying weight status, but the International Obesity TaskForce (9) classification of overweight and obesity was used in 16 studies making it the most consistent definition.
SES indicators. A single SES indicator was reported in 31 studies; the remaining 14 studies reported two (n = 11), three (n = 2), or four (n = 1) indicators. Overall, 63 analyses of associations between an SES indicator and adiposity were identified from the 45 studies included in this review. The majority of SES indicators were measured at the household level including: parental education (n = 26), parental occupation (n = 14), indices of family income (n = 11), and composite SES measures (n = 5). Neighborhood level SES indicators were used in seven analyses.
Bivariate association between SES and adiposity
All SES indicators. A summary of the type of SES-adiposity association in the 45 studies included in this review is shown in Table 1. Complex-inverse associations were found in nine studies (# 11, 14, 15, 23, 26, 28, 30, 32, 38) but for simplicity of presentation herein, this type of association will be described as an inverse association. Inverse (including complex-inverse) SES-adiposity associations were found in 19 studies, there was no association in 12 studies, and in 14 studies the association varied by subgroup with both inverse and no associations. No positive SES-adiposity associations were seen in any of the bivariate analyses in the studies included in this review. In one study, a statistically significant complex-positive association emerged between SES and obesity, but not SES and overweight, in girls but not boys, and only after adjusting for ethnicity and age (# 29). In 9 of the 14 studies (# 4, 6, 11, 14, 19, 20, 34, 38, 40) that included more than one SES indicator, the association was inverse for all indicators. In each of these studies, a measure of parental education featured as one of the SES indicators. In one study there was no association for either indicator (# 10) and in the other four studies (# 7, 9, 12, 35) the type of association varied by SES indicator/level of adiposity/ethnicity, with a mixture of negative and no association.
Table 1. . Summary of the bivariate SES-adiposity association stratified by SES indicator type in 45 studies that satisfied the criteria for this review
|SES indicator type||Inverse association (statistically significant)||Complex-inverse association (statistically significant)||No association (not statistically significant)|
|Education||2 (parental)||38 (maternal)||10 (maternal and paternal)|
| ||4 (maternal)|| ||17 (maternal: overweight and obesity combined, both age groups)|
| ||6 (maternal)|| ||35a (parental: white boys, black boys and girls)|
| ||8 (parental)|| ||43a (parental: boys)|
| ||11 (maternal and paternal)|| ||45 (maternal: boys)|
| ||12a (maternal)|| || |
| ||14a (maternal)|| || |
| ||16 (parental)|| || |
| ||18 (parental)|| || |
| ||19a (maternal and paternal)|| || |
| ||20 (parental)|| || |
| ||34 (maternal and paternal)|| || |
| ||38 (paternal)|| || |
| ||39 (parental)|| || |
| ||40a (maternal and paternal)|| || |
| ||17 (maternal: obesity, 10–12 year olds)|| || |
| ||35a (white girls)|| || |
| ||43a (parental: girls)|| || |
| ||45 (maternal: girls)|| || |
|Occupation||3 (parental)||11(paternal)||7 (parental)|
| ||4 (paternal)||15 (parental)||13 (parental)|
| ||9 (parental)|| ||29a,b (parental)|
| ||11 (paternal)|| ||36 (parental)|
| ||24a (parental: girls)|| ||41 (parental)|
| ||25(paternal: boys)|| ||42 (parental)|
| || || ||24a (parental: boys)|
| || || ||25 (paternal: girls)|
|Indices of family income||5a||14a||12a|
| ||20|| ||27|
| ||34|| ||33a|
| ||7(obesity)|| ||7 (overweight)|
| ||9 (overweight and obesity combined)|| ||9 (obesity)|
| ||21a (white 8–16 year olds)|| ||21a (white 2–7 year olds, all black and Mexican|
| ||35a (white girls)|| ||American 2–16 year olds)|
| || || ||35a (white boys, black boys and girls)|
|Composite measure||6||28a (girls)||31a (study of girls only)|
| ||19a||32 (boys)||28a (boys: overweight)|
| ||28a (boys: at risk of overweight)|| ||32 (girls)|
|Neighborhood||44 (girls: BMI, waist girth, WHR)||23||1a|
| || ||26||22|
| || ||30a (boys)||37|
| || || ||30a (girls)|
| || || ||44 (boys: all adiposity measures, girls: skinfolds)|
Parental education. Parental education (parental (n = 8), maternal (n = 12), or paternal (n = 6)) was used as the SES marker in 20 studies, with 26 analyses presented. Inverse associations between parental education and adiposity were found in 15 of the 20 studies and in one study there was no association (# 10). In four studies the association was mixed, in two the association was inverse for girls but not boys (# 43, 45), in one study the association was inverse for white girls but there was no association for boys or black girls (# 35), and in the other, there was no association with combined overweight and obesity but there was an inverse association for obesity in 10–12 year olds (# 17).
Parental occupation. A total of 14 analyses of the association between parental (n = 10), maternal (n = 1) or paternal (n = 3) occupation and adiposity was presented across 13 studies. An inverse association between parental (including maternal and paternal) occupation and adiposity was found in 5 of the 13 studies and in 6 studies there was no association. A varied association was found in two studies, with an inverse association for one gender (girls (# 24), boys (# 25)) but not the other.
Indices of family income. Associations between indices of family income and adiposity were presented in 11 studies. An inverse association was found in 4 of the 11 studies, in 3 studies there was no association, and the association varied in 4 studies (# 7, 9, 21, 35). In two studies from the United Kingdom, the association varied according to the level of adiposity (International Obesity TaskForce definition), with income inversely associated with combined overweight and obesity but not obesity alone (# 9), and with obesity but not overweight (# 7). In two studies from the United States, the SES-adiposity association varied by ethnicity, and age or gender. In one of these studies, income was inversely associated with adiposity in white boys and girls aged 8–16 years but there was no association for white boys and girls aged 2–7 years, nor for black or Mexican-American children of any age (# 21). In the second study, income was inversely associated with adiposity in white girls but there was no association for boys or black girls, as seen for parental education in the same study (# 35).
Composite measure of SES. Five studies presented associations between a composite measure of SES and adiposity. An inverse association was found in two studies and there was no association in one study that included only girls. In the remaining two studies the association varied by subgroup; SES was inversely associated with adiposity in boys but not girls (# 32), and overweight in both boys and girls but “at risk of overweight” only in girls (# 28).
Neighborhood SES. The association between neighborhood SES and adiposity was presented in seven studies. An inverse association was found in two of them and there was no association in three. In the other two studies, the association varied according to the type of adiposity measure or gender. In one of these studies the association was inverse for boys but not girls (# 30) and in the other neighborhood SES was inversely associated with BMI, waist girth, and waist-to-hip ratio but not skinfolds in girls, and was not associated with any adiposity measure in boys (# 44).
Additional considerations for interpreting findings
Gender. In this review, 19 studies (Appendices 1b and 2b) presented data on the association between SES and adiposity stratified by gender and 1 study included only girls (# 31). In 10 studies the associations were consistent for boys and girls. In seven studies there was an inverse association for one gender (girls (# 24, 43, 44, 45), boys (# 25, 30, 32)), but not the other. In the remaining studies, there were inconsistent SES-adiposity associations between genders according to ethnicity (# 35) and level of adiposity (# 28). Overall, the SES-adiposity association was consistent for boys and girls in more than half of the reviewed studies that presented results stratified by gender. In the remaining studies there appeared to be no gender differences in the proportion of inverse associations.
Age. The studies could be broadly classified by the age range of participants: children (5–11 years, n = 18), adolescents (12–18 years, n = 9), and both children and adolescents (n = 18). In 10 of the 18 studies of children (# 6, 8, 11, 14, 15, 16, 18, 19, 20, 38), SES was inversely associated with adiposity, no association was found in 4 studies (# 1, 36, 37, 42), and the association varied by subgroup in 4 studies (# 7, 12, 17, 43). In five of the nine studies of adolescents there was an inconsistent SES-adiposity association between the genders (# 24, 25, 32, 35, 45) (as described in the gender section), an inverse association was shown in one (# 5) and in three studies there was no association (# 22, 31, 41). In the remaining 18 studies that included both children and adolescents, an inverse association was found in 8 (# 2, 3, 4, 23, 26, 34, 39, 40), there was no association in 5 (# 10, 13, 27, 29, 33), and the association varied by subgroup in 5 (# 9, 21, 28, 30, 44). It appears that inverse SES-adiposity associations are more common in studies of children compared with adolescents.
Ethnicity. The treatment of ethnicity in the studies included in this review can be classified in three categories: 12 studies included white children or children with a European ancestry (>95% of sample) or the data analysis was restricted to these groups, 15 included a variety of ethnic groups (>5% of sample) in the data analyses, and in 18 studies the ethnic composition of the sample was not specified. The studies including only white/European ancestry children originated from the United Kingdom (n = 5), Germany (n = 4), Italy (n = 1), and The Netherlands (n = 2). In eight of these studies, including those from Germany, Italy, The Netherlands, and one from the United Kingdom (# 26), SES was inversely associated with adiposity. In the remaining four studies from the United Kingdom, there was either no association (# 36, 42) or the association varied according to the level of adiposity (# 7, 9).
The 15 studies with a variety of ethnic groups originated from the United States (n = 7), United Kingdom (n = 4), Australia (n = 2), Belgium (n = 1), and Germany (n = 1). In five studies, including those from Australia, Belgium, Germany, and one from the United States (# 14), SES was inversely associated with adiposity. There was no association in four studies, one from the United States (# 1) and three from the United Kingdom (# 29, 31, 33), although as already reported, a complex-positive association emerged for obesity in girls in one of these studies after adjusting for age and ethnicity (# 29). In six studies the association varied by subgroup with differences between genders in two (United States (# 43), United Kingdom (# 30)), type of SES indicator in one (United States (# 12)), the level of adiposity in one (United States (# 28)), and in the other two US studies that stratified results by ethnicity, an inverse SES-adiposity association was shown only for white girls but not boys and black children (# 35) and white girls and boys aged 8–16 years but not younger white children or children from other ethnic groups of any age (# 21). Because the majority of studies did not specify the ethnicity of the sample, and in those that did, few stratified results by ethnicity, it was difficult to determine its influence on the association between SES and adiposity. However, in the studies that presented results separately by ethnicity and gender, there appeared to be no association between SES and adiposity in black children, while inverse associations were more common for white girls.
Sample size. We estimated that a sample size of 1,000 or more participants would be required to detect a twofold risk (odds ratio (OR) = 2.0) of adiposity in one SES group compared to another in a study with three, similarly sized SES groups. A sample size of 1,000 would provide 95% power, and a sample size of 500 (for gender-stratified analyses) would provide 75% power. Thirty-three studies included in this review had a sample size of at least 1,000, and in these, the proportion of inverse, no association, and mixed (inverse/none) associations was 52, 18, and 30%, respectively.
Magnitude of bivariate SES-adiposity associations. The magnitude of SES-adiposity associations between extreme SES groups was examined using an exploratory analysis of the ORs for adiposity in the lowest SES group with the highest SES group as the reference group. When the original report presented ORs calculated using the lowest SES group as the reference, these were recalculated. Similarly, when the original report presented results as proportions, these were converted to ORs if sample size data were available. Results from 24 studies (70 analyses) are presented in Table 2 stratified by the total number of SES groups and the definition and level of adiposity. Within a study, separate analyses were sometimes performed for different levels of adiposity, gender, ethnicity, age, and type of SES indicators.
Table 2. . Summary of the OR (95% CI) for adiposity in the lowest SES group when the highest SES group is the reference group in bivariate analysis, listed according to the total number of SES groups in analyses
From these 24 studies, 21 (42 analyses) found a significant inverse SES-adiposity association in at least one analysis. Regardless of the total number of SES groups or the definition or level of adiposity, the OR for adiposity in the lowest compared with the highest SES children, ranged from 1.30 (# 8) to 6.70 (# 24) with a median OR of 2.04. In 11 of these studies, ORs for two levels of adiposity were available (e.g., BMI > 90th percentile and BMI > 97th percentile) and in all of these, ORs were larger for the higher level of adiposity.
There was a non-significant inverse association (OR > 1, 95% confidence interval (CI): <1 to >1) in at least one analysis in 10 studies ((# 7, 15, 17, 21, 24, 26, 27, 29, 30, 31); 20 analyses). The ORs for adiposity in the lowest SES group compared with the highest SES group ranged from 1.04 (#7, 31) to 2.92 (# 15) with a median OR of 1.28. In three studies (# 12, 21, 29; 7 analyses) there was a non-significant positive association (OR < 1, 95% CI: <1 to >1). The OR for adiposity in the lowest SES group compared with the highest SES group ranged from 0.64 (# 12) to 0.97 (# 29) with a median OR of 0.81. In one study a significant positive association (OR < 1, 95% CI <1) emerged for black girls aged 8–16 years, the OR (95% CI) for adiposity in the lowest SES group compared with the highest SES group was 0.60 (0.36–0.99) (# 21), although in the original report there was not a statistically significant difference between SES groups.
Multivariate association between SES and adiposity
Multivariate analyses were reported in 16 of the 45 studies included in this review. In addition to alternate SES indicators, the following covariates were included in one or more of the analyses: age, gender, parental adiposity, parental marital status, parental area of birth, maternal age, maternal employment status, maternal smoking during pregnancy, birth weight, birth length, birth order, infant feeding, size of flat, own bedroom, dietary intake, parental control over intake, physical activity, sedentary activity, family car ownership, behavior problems, cognitive stimulation score, academic grade retention, medication, depressive symptoms, geographical region, parent road safety concern, and school/district factors. Different combinations of covariates were used across different studies making it difficult to draw general conclusions.
Because parental adiposity is often the strongest predictor of adiposity in children (e.g., # 3, 16), we examined the effect of including it as a covariate in the multivariate analyses. Nine of the sixteen studies reporting multivariate analyses included parental adiposity as a covariate (# 3, 6, 12, 16, 19, 34, 40, 43, 45). In six of them, the inverse SES-adiposity association remained for maternal education (# 45), parental education (# 43), overweight but not obesity (# 6, 16), maternal education but not paternal education (# 19, 34) and income (# 34). In three, the inverse association between SES and adiposity disappeared after including parental adiposity as a covariate (# 3, 12, 40). In the seven studies that included other covariates but not parental adiposity, the inverse SES-adiposity association did not change in four studies (# 8, 9, 17, 39), and remained for paternal occupation (# 11), maternal but not paternal education (# 11), parental education but not income (# 20), and paternal occupation but not maternal education (# 4).