Int. J. Envron. Res. Publc Health 2014, 11, 3387-3406; do:10.3390/jerph110303387 Artcle Internatonal Journal of Envronmental Research and Publc Health ISSN 1660-4601 www.mdp.com/journal/jerph Socoeconomc Inequaltes n Adult Obesty Prevalence n South Afrca: A Decomposton Analyss Olufune Alaba 1 and Lumbwe Chola 2, * OPEN ACCESS 1 2 Health Economcs Unt, School of Publc Health and Famly Medcne, Unversty of Cape Town, Cape Town 7925, South Afrca; E-Mal: Olufune.Alaba@uct.ac.za PRICELESS SA, MRC/Wts Rural Publc Health and Health Transtons Research Unt (Agncourt), School of Publc Health, Unversty of the Wtwatersrand, Johannesburg 2050, South Afrca * Author to whom correspondence should be addressed; E-Mal: lumbwechola@hotmal.com; Tel.: +27-713-364-261; Fax: +27-117-172-084. Receved: 23 December 2013; n revsed form: 10 March 2014 / Accepted: 11 March 2014 / Publshed: 21 March 2014 Abstract: In recent years, there has been a dramatc ncrease n obesty n low and mddle ncome countres. However, there s lmted research n these countres showng the prevalence and determnants of obesty. In ths study, we examne the socoeconomc nequaltes n obesty among South Afrcan adults. We use natonally representatve data from the South Afrca Natonal Income Dynamc Survey of 2008 to: (1) construct an asset ndex usng multple correspondence analyses (MCA) as a proxy for socoeconomc status; (2) estmate concentraton ndces (CI) to measure socoeconomc nequaltes n obesty; and (3) perform a decomposton analyss to determne the factors that contrbute to socoeconomc related nequaltes. Consstent wth other studes, we fnd that women are more obese than men. The fndngs show that obesty nequaltes exst n South Afrca. Rch men are more lely to be obese than ther poorer counterparts wth a concentraton ndex of 0.27. Women on the other hand have smlar obesty patterns, regardless of socoeconomc status wth CI of 0.07. The results of the decomposton analyss suggest that asset ndex contrbutes postvely and hghly to soco-economc nequalty n obesty among females; physcal exercse contrbutes negatvely to the soco-economc nequalty. In the case of males, educatonal attanment and asset ndex contrbuted more to soco-economc nequaltes n obesty.
Int. J. Envron. Res. Publc Health 2014, 11 3388 Our fndngs suggest that focusng on economcally well-off men and all women across socoeconomc status s one way to address the obesty problem n South Afrca. Keywords: obesty; South Afrca; nequalty 1. Introducton In the last two to three decades, obesty and overweght prevalence have ncreased globally. It s estmated that there are over one bllon overweght and close to half a bllon obese adults n the World [1]. Obesty has hstorcally been a problem of developed countres, but the last few years have seen an ncreased epdemc n mddle ncome countres such as South Afrca (SA). Close to 30% of populatons n mddle ncome countres are ether overweght or obese [1]. South Afrca s prevalence s one of the hghest n sub-saharan Afrca. The 2002 South Afrca Demographc and Health Survey (SADHS) reported that 29% of men and 56% of women were ether overweght or obese [2]. Smlar estmates are also reported n recent cross-sectonal studes conducted n varous settngs n the country [3,4]. The ncreased prevalence of obesty s a major publc health concern n SA, as t has short and long-term negatve health consequences that may mpact negatvely on the country s resources [5]. Obesty s a rs factor for health condtons such as cardovascular dseases and some cancers [6 9], thus the ncrease n obesty has seen a correspondng ncrease n the burden of non-communcable dseases n SA and other low and mddle ncome countres [10,11]. Managng obesty s, therefore, ey to reducng the burden of non-communcable dseases and mprovng the health of populatons. Recently, the South Afrcan Natonal Department of Health released ts 5-year strategc plan for the preventon and control of non-communcable dseases (NCDs), for 2013 2017 [12]. The strategc plan places emphass on the need to reduce obesty and other related rs factors n order to control non-communcable dseases. However, to successfully reduce the obesty rate requres nowledge on not only the prevalence, but also ts determnants and dstrbuton n the populaton. There s, therefore, need to dentfy populatons that are prone to obesty n order to establsh the factors nfluencng the epdemc. However, nformaton on the epdemology of obesty n SA s scant [13]. Whle some studes hghlght the age and gender-specfc prevalence and dstrbuton of obesty n SA [3,4], there are few studes that assess ts soco-economc determnants [3,14]. Studes conducted n many hgh ncome countres show a soco-economc gradent n obesty. The prevalence of obesty s shown to be manly concentrated n people of low socoeconomc status [15,16], probably because socoeconomc status nfluences energy ntae and expendture, and thus body fat [17]. Obesty s also found to be dsproportonately hgh n economcally dsadvantaged women. However, the degree of ncome-related nequaltes s declnng n both men and women [18]. Hajzadeh et al. [19] used the concentraton ndex (CI) to show ncome-related nequaltes n obesty rs n Canada. The study found that obesty was concentrated n rch men and economcally dsadvantaged women, and that the degree of socoeconomc related nequaltes was reducng over tme. Ljungvall and Gerdtham [20] studed the trend n ncome-related nequaltes n obesty n Sweden, and showed that obesty nequaltes were concentrated among the rch.
Int. J. Envron. Res. Publc Health 2014, 11 3389 The authors recommended that reducng ncome nequaltes would be the most effectve strategy to reduce obesty nequaltes. Costa-Font and Gl [21] also found evdence of socoeconomc nequaltes n obesty n Span. Usng a decomposton analyss, they showed that educaton and demographc factors were some of the man contrbutors to nequaltes n obesty. Nolaou and Nolaou [18] examned the ncome-related nequaltes n obesty n 10 European countres, usng the European Communty Household Panel dataset. The study found that ncome-related nequaltes n Europe were more concentrated n economcally dsadvantaged women. In low and mddle ncome countres, obesty has been shown to be manly prevalent among populatons of hgh socoeconomc poston, whch are ncreasngly adoptng Western lfestyles and dets [22]. It can be shown that as economes transton from low to mddle and hgh ncome, obesty trends shft from beng concentrated among socety s well off to the poor [23,24]. We would expect the obesty trends n SA and other mddle ncome countres to follow the Western pattern, but ths s not certan. It s, therefore, mportant to montor the trends n obesty and the related socoeconomc gradent. A thorough understandng of trends n the relatonshp between socoeconomc status and overweght and obesty prevalence wll provde useful nsghts for developng effectve overweght nterventon programmes and polces. We dd not fnd any publshed lterature on the formal measurement of the socoeconomc gradent of obesty n SA. Ths study thus ams to fll ths gap. In ths study, we examne the socoeconomc nequaltes n obesty prevalence n South Afrcan adults. We use the CI to examne the relatonshp between socoeconomc status and obesty. The CI, an approach for studyng nequalty used n economcs, has proved useful to understandng the socoeconomc gradent of obesty, as well as other health factors [15,20,25]. The CI s a measure of assocaton whch ndcates the degree to whch a factor, such as obesty, vares wth a measure of household resources. The CI s most useful because t not only provdes a sngle ndex of wealth related nequalty n obesty, but can also be used n decomposton analyss to examne the underlyng causes of the wealth related nequalty. Thus n ths study, we also perform a decomposton analyss, n order to determne the factors drvng socoeconomc nequaltes n obesty. 2. Methods 2.1. Data Our nvestgaton s based on the adult questonnare of the frst wave of the South Afrcan Natonal Income Dynamcs Study (SA-NIDS) verson 5.0. The SA-NIDS s a natonally representatve survey undertaen n 2008 by the South Afrcan Labour and Development Research Unt (SALDRU) based at the Unversty of Cape Town (UCT). The survey s a panel study that documents the dynamc structure of household members and changes n ther ncomes, expendtures, assets, access to servces, educaton, health and other dmensons of well-beng. A household questonnare, together wth an adult questonnare, was admnstered to every household member aged 15 years and older. The survey conssted of 16,800 adults. In our study, the unt of analyss s an ndvdual, classfed as an adult above 18 years. After cleanng the data and excludng ndvduals less than 18 years, pregnant women and those wth mssng body mass ndex (BMI), our sample reduced to
Int. J. Envron. Res. Publc Health 2014, 11 3390 11,326 ndvduals; the analyss was further dsaggregated by sex wth 4,983 males and 6,343 females. A detaled report on the SA-NIDS methodology s provded elsewhere [26]. 2.2. Measures 2.2.1. Health Indcators Obesty the heghts and body weght of adults were taen durng the SA-NIDS survey. These were used n our analyss to compute the body mass ndex (BMI). Body mass s defned as a person s weght n logrammes dvded by the square of heght n meters. Obesty s defned as BMI greater or equal to 30 g/m 2 [27]. In our analyss, obesty was measured as a bnary varable wth (1) obese and (0) not obese. 2.2.2. Measurement of Inequalty There are varous measures of nequalty, but as suggested by Wagstaff et al. [28], any nequalty measure s expected to meet three mnmal requrements: (1) the measure should reflect the experences of the whole dstrbuton of the populaton rather than the extremes of the socal class; (2) the measure should be senstve to changes n the dstrbuton of the populaton across socoeconomc groups; and (3) the measure should exhbt socoeconomc features to nequaltes n health. The CI s among the few measures that satsfy ths crtera [28]. In lght of ts usefulness not only to quantfyng health nequaltes, but also to decomposng the contrbuton of varous factors to health nequaltes, we used the CI to examne socoeconomc related nequalty n the dstrbuton of obesty across the adult populaton of SA. The CI s used to examne relatve nequalty n health and ts values le between negatve one ( 1) and postve one (+1). Negatve values mply that the health measure used (obesty n ths paper) s more concentrated among the poorer socoeconomc populaton. Postve values mply that obesty s more concentrated among the rcher populaton. Larger absolute values of CI ndcate wder nequaltes n obesty. A CI value of zero sgnfes that obesty s equally dstrbuted across socoeconomc status [29,30]. The CI can be defned smply as twce the covarance between the health varable ( y : obesty) of ndvdual and the ranng of the socoeconomc status, r, dvded by the mean of the health varable ( ): 2 CI cov( y, r ) (1) As suggested by Kawan et al. [31], the CI can also be computed easly by what s called the convenent regresson approach. Ths method of calculaton has the advantage of not only yeldng an estmate of CI, but also generatng standard errors from whch statstcal nferences can be produced. Ths can be wrtten as follows: 2 2 ( y / ) r (2) r
Int. J. Envron. Res. Publc Health 2014, 11 3391 2 where r s the varance of the ran (r), whle the other varables remans as defned n Equaton (1), ^ s an estmated concentraton ndex, and s the stochastc error term. Ths CI yelds what s called an unstandardzed concentraton ndex. Standardzaton s an adjustment technque that s used to account for dfferences n populaton demographc structures, to produce a refned descrpton of the relatonshp between health and socoeconomc status [30], and also to facltate comparsons for dfferent populatons, sub-populatons or overtme. Standardzaton s best utlzed under the assumpton that demographc varables such as sex and age are correlated wth ether the health measure, socoeconomc status, or both [30]. For nstance, older people tend to have hgher levels of body weght [32,33] and women have much more perpheral body fats n the legs and hps than men [21]. There are two ways of standardzng: the drect and ndrect methods [30]. The ndrect standardzaton method s preferred and commonly used over drect standardzaton because of ts greater accuracy when dealng wth ndvdual-level data [30]. The ndrect standardzaton method s used n ths paper. Although the estmate of CI can be ndrectly standardzed by subtractng the nfluence of all standardzng varables from the unstandardzed CI [30], an equvalent approach of obtanng an ndrectly standardzed CI s to nclude the standardzng varables drectly nto the convenent regresson, ether for full or partal correlatons of the health varable, wth the standardzng varables. In the case of the former, only the standardzng varables, whch can also be seen as confoundng varables are ncluded n the regresson whle n the latter case, other non-confoundng varables are ncluded n order to estmate the correlaton of the confoundng varables condtonal on those other varables [30]. Ths procedure was used to standardze obesty n ths paper. s The ndrect standardzed health varable (n ths case obesty) y^ s attaned by a smple regresson on actual health varable ( y ) of ndvdual as follows: y j x j zj (3) j where x j are the confoundng varables for whch we want to age-standardze ndvdual (age s the only standardzng varable used n our study, due to the fact that there was a splt n the analyss: females and males);, and are the parameter vectors, the z are the non-confoundng varables for whch we do not want to standardze, but to control for, n order to estmate partal correlatons wth the confoundng varables, and s the error term. To standardze for full correlatons, non-confoundng varables, z are excluded from the regresson. The z varables n our study nclude educaton, employment status, race, asset ndex, martal status, and behavoural ndcators: exercse and smong. The Ordnary Least Squares (OLS) parameter estmates from Equaton (3) are then used to obtan the predcted values of the health ndcators as expressed below: ^ y ^ ^ ^ j x j z (4) j The estmate of the ndrectly standardzed obesty ( s y^ ) s the dfference between actual health measure ( y ) and x-expected health ( ), plus the sample mean ( y ) as expressed n Equaton (4): y^
Int. J. Envron. Res. Publc Health 2014, 11 3392 ^ s y ^ X y y y (5) We can therefore nfer that the dstrbuton of s y^ across socoeconomc groups s the dstrbuton of obesty n the female and male populatons that would be expected, rrespectve of dfferences n the dstrbuton of age. Gven that our health measure was bnary (1,0), the bounds of the concentraton ndex are not 1 and 1 but depend on the mean of the varable [34,35]. A normalzaton process that ensures that the CI s quantfed n the range 1 to 1, for any gven mean of the health measure (obesty) s therefore recommended, by multplyng the calculated concentraton ndex by (1/1 μ) [34]. Recently, there has been a debate regardng the approprate normalzaton process between Wagstaff [34,35] and Erreygers (ehe Erreygers ndex (E c ) can be stated as (4μ/b a) CI, where, a and b are upper and lower lmts of the health varable, CI s the standard concentraton ndex, and μ s the mean of the health varable) [36,37]. Kjellson and Gerdtham [38] gve a detaled dscusson on the dfferences between nequalty ndces and state that the normalzaton choce should be based on the researcher s value judgment. Also of note s that Erreygers ndex does not change the drecton of nequalty, but only the magntude. Erreygers ndex can also be obtaned by scalng what s obtaned from Wagstaff s normalzaton by the factor (4μ/1 μ) [39]. We therefore, decded to apply the normalzaton process proposed by Wagstaff (we obtaned smlar results usng both Wagstaff and Erreygers normalzaton ndex). 2.2.3. Explanng Inequalty: Decomposton of the Concentraton Index The CI can be decomposed nto the contrbutons of explanatory factors usng regresson analyss, thereby allowng for an analyss of the contrbuton of each ndvdual factor to the measured degree of soco-economc nequaltes n health [40]. However, the method reles on the lnearty of the underlyng regresson model, and n cases where the health outcome s bnary n nature, whose deal specfcaton s nonlnear, a decson on whether to approxmate the model by a lnear specfcaton usng lnear probablty model (LPM) or to approxmate the decomposton of the CI [41] s needed. We chose to approxmate the model by applyng a lnear probablty model on the health outcome because the approxmaton of the decomposton of the CI generates a non-unque result [30], whch captures the lnear assocaton between the health varable and the covarates. Ths lnearty n parameters s a useful property for the decomposton of the nequalty ndex [30,40,41]. Moreover, prevous studes that measure nequalty and decomposton of nequalty show that whle estmatons produced by lnear probablty models may be less robust and precse than those generated by non-lnear models, they both yeld smlar results and the estmated parameters of LPM are generally consstent [30,42]. Therefore, gven a lnear addtve regresson model of ndvdual health y expressed as: y x, the CI can be wrtten as follows to nclude the elastctes and nequaltes of the varous determnants [30,40]: CI ( x / ) CI GCI / (6)
Int. J. Envron. Res. Publc Health 2014, 11 3393 where s the mean of the health varable ( y : obesty n ths case), the ndex refers to the regressors ncluded n the obesty equaton; s the coeffcent for each of the health determnants from equaton 6, x _ s the mean of each of the regressors, ndex for the error term ( ) [30]. The component. x s defned as elastcty ( GCI s the generalzed concentraton ) of y wth respect to x [30] and measures the mpact of each covarate on obesty and CI s the concentraton ndex for each of the ndvdual regressors; t measures the degree of unequal dstrbuton across socoeconomc status. We appled the Wagstaff normalzaton not only to the concentraton ndex but also to the decomposton. 2.2.4. Constructon of the Lvng Standard Measure: Asset Index Varous ways of measurng lvng standards are used n lterature ncome, expendture and wealth ndex. In developng countres, the use of the measure of ncome and consumpton are prone to varous bas ncludng recall bas [43], varaton of ncome from season to season and reluctance to dvulge nformaton [44]. It should be noted that socoeconomc nequaltes n a health varable may be senstve to the choce of welfare ndcator, dependng on the health varable beng examned. Wagstaff and Watanbe [45] found that the concentraton ndces for health care utlzaton were more senstve to the choce of welfare ndcator but not to the health outcome (chld malnutrton). Also of note s the fact that the CI reflects the relatonshp between the health varable and lvng standards ran and not the varance of the lvng standards measures [30]. In ths paper, the household assets ndex [46,47] s the man socoeconomc ndcator used (there are other ndcators of socoeconomc status le educaton, employment status, composte socoeconomc status among others, ndcatng postons n the socety or as defned by Oaes and Ross [48]: dfferental access (realzed and potental) to desred resources ). However, the measurement stll lacs conceptual clarty, and the use depends on the queston beng ased by the researcher. In ths paper, gven that we are analysng a concentraton ndex, a contnuous varable s needed as a measure of wellbeng). The asset ndex was constructed usng multple correspondence analyss (MCA) [49], a preferred technque for categorcal varables [50] expressed as follows: a F 1 c 1 (7) where a s the value of the asset ndex for the th observaton, c s the value of the th dummy/categorcal varable (for = 1,.K) descrbng the set of sx household assets and lvng condtons consdered n the analyss, whch ncluded type of dwellng, access to water, access to tolet, rado, coong gas and possesson of a cellular phone. None of these varables contaned negatve values. F 1 s the asset ndex weght of the frst component of the analyss. The resultng asset scores generated were used to ran the populaton from lowest to the hghest, and to ran and dvde the populaton nto wealth quntles (from 1-lowest to 5-hghest). Although, there are varous methods of constructng asset ndces such as prncpal components analyss (PCA) [46] and factor analyss [51], MCA was chosen for ths analyss because t s better
Int. J. Envron. Res. Publc Health 2014, 11 3394 suted when there s a combnaton of bnary and categorcal varables, t avods prevous computatons of squeezng categorcal assets nto bnary as needed n the framewor of PCA [50,52]. 2.2.5. Other Varables Apart from socoeconomc status measured by asset ndex, a range of varables, whch studes have shown to nfluence BMI and also the socoeconomc gradent n obesty rates, were ncluded n estmatng the CI and the regresson model used for the decomposton analyss [15,20,21]. These nclude: Educaton, measured n years of schoolng and categorzed as 1-no educaton, 2-prmary wth 1 7 years, 3-secondary wth 8 12 years and 4-tertary, wth 13+ years of schoolng; Employment status, a bnary varable wth 1-employed and 0-unemployed, the unemployed conssts of all those who are not n formal or nformal employment; Martal status, was coded 1-never marred 2-marred/cohabtng and 3-dvorced/wdowed/separated; and Area of resdence, a bnary varable wth 1-rural and 0-urban. Race was ncluded as a categorcal varable wth 1- Afrcan, 2-coloured, 3-Asan and 4-Whtes. The South Afrcan populaton s made up of 79.8% Afrcan, 8.9% Coloured, 2.6% Indan/Asan and 8.7% Whte people, accordng to the 2013 md-year populaton estmates [53]. Lfestyle factors, ncludng det, physcal actvty, the use of tobacco and alcohol, have been dentfed as rs factors for obesty [19]. We ncluded two lfestyle factors namely physcal exercse, a dummy varable (1 f an ndvdual exercses at least once a wee) and smong (1 f an ndvdual smoes, 0 otherwse). Snce obesty s essentally an mbalance between calore ntae and expendture, the ncluson of nformaton on physcal actvty s mportant. Although there s hgh probablty that the ncluson of the lfestyle factors could be endogenous, we strongly beleve n the mportance of these factors n the decomposton analyss not only because of the demonstrated assocaton of these factors wth obesty and obesty nequalty n the lterature [54] but also because they can be classfed as polcy- relevant varables n the context of the measurement of socoeconomc related nequalty n health or healthcare (Gravelle estmated a drectly standardzed partal concentraton ndex, and suggests the use of three types of varables n the regresson equaton used for the decomposton analyss, namely: ncome, need standardzng varables and other possble polcy-relevant varables [55]). Consequently, our estmated regresson model can be vewed as a reduced form demand model for obesty and no causal nterpretaton s mpled. 2.3. Data Analyss Management of data and analyss were done n Stata 12 (Stata Corp. Inc., College Staton, Texas, USA) and ADePT verson 5.0, whch was developed by the World Ban and specfcally desgned for analyzng nequalty n health outcomes and related research [56]. Ch-squared (χ 2 ) sgnfcance tests were used to assess dfferences between quntles n the dstrbuton of obesty for men and women, and OLS regresson was appled n the decomposton analyss. The SA-NIDS was a multstage samplng procedure, thus all estmates too nto account the samplng weghts and adjustment was made for clusterng and stratfcaton of the survey data.
Int. J. Envron. Res. Publc Health 2014, 11 3395 3. Results 3.1. Descrptve Statstcs Table 1 shows the soco-demographc characterstcs of the study sample by gender. There were 11,326 adults ncluded n the sample, of whch 56% were female. The age-standardzed obesty was 35% among females and 12% among males, and approxmately 25% of the total populaton was obese. Table 1. Soco-demographc characterstcs, health and lfestyle measures. Varables Male 4,983 (44%) Female 6,343 (56%) Total 11,326 Unstandardzed obesty (age-standardzed * ) 11% (12%) 36% (35%) 24% (25%) Age n years (standard devaton) 37 (13.9) 39 (16.1) 38 (15.2) Martal status Marred 46% 44% 45% Wdowed 5% 15% 10% Never marred 49% 41% 45% Educaton No school 7% 11% 9% Prmary 21% 21% 21% Secondary 60% 58% 59% Tertary 12% 10% 11% Employment status Employed 60% 38% 43% Unemployed 40% 62% 52% Resdence Urban 64% 59% 61% Rural 36% 41% 39% Race Afrcan 81% 80% 80% Coloured 7% 8% 8% Asan 2% 2% 2% Whte 10% 10% 10% Lfestyle factors Physcal exercse 40% 20% 28% Smong 40% 9% 23% Note: * Ths s the ndrectly standardzed obesty, for age-only. The mean age was around 38 years (SD = 15.2). The majorty of respondents had secondary (59%), followed by prmary level (21%) and only a few had tertary (11%) educaton. The proporton of unemployed persons was 52%. About 36% of the females and 60% of the males were employed. The proporton of marred (45%) and never marred (45%) were equally dstrbuted n the populaton, and about 11% were wdowed. The majorty of the populaton lved n urban areas (61%).
Int. J. Envron. Res. Publc Health 2014, 11 3396 3.2. Inequaltes n Obesty 3.2.1. Wealth Dstrbuton Fgure 1 shows the dstrbuton of unstandardzed obesty by wealth (as measured by the asset ndex) and gender. The proporton of obesty was about 20% n the lowest quntle, compared to 30% n each of the hghest two. Generally, there was a postve relatonshp between obesty and wealth for both men and women (p < 0.05). Obesty was more pronounced n females, rangng from 28% n quntle 1 to 39% n quntle 5 and 41% n quntle 4. Among males, obesty steadly ncreased wth socoeconomc status from 6% n the lowest quntle to 18% n the hghest quntle. Fgure 1. Dstrbuton of unstandardzed obesty by wealth and gender. 3.2.2. Concentraton Indces Note: The error bars n Fgure 1 represent 95% confdence ntervals. The normalzed concentraton ndces for males and females are gven n Table 2. Included n the table are three ndces: (1) the unstandardzed, (2) ndrectly standardzed for age only and (3) ndrect standardzed for age and other non-confounders. The 3 methods produced dfferent but smlar fgures, however, to be consstent wth our decomposton analyss, all dscussons herewth are based on the thrd ndex age ncludng non-confoundng varables. The ndces were all postve, ndcatng that obesty was more concentrated among the rch. The normalzed concentraton ndex was 0.12 for the entre populaton, 0.09 for females and 0.27 for males. Ths ndcates that there s more nequalty n the dstrbuton of obesty among men compared to women. Though obesty s manly found among the wealthy, the dstrbuton of obesty s not very dfferent between poor and rch women. However, rcher men are more lely to be obese than ther poorer counterparts.
Int. J. Envron. Res. Publc Health 2014, 11 3397 Table 2. Concentraton ndces of adult obesty. Method Female Male Total Standardzaton method CI 95% confdence 95% confdence 95% confdence CI CI nterval nterval nterval Unstandardzed 0.11 (0.06 0.16) 0.28 (0.18 0.38) 0.13 (0.009 0.17) Age standardzed only 0.13 (0.05 0.19) 0.26 (0.17 0.35) 0.13 (0.09 0.17) Age and non-confoundng varables 0.09 (0.03 0.14) 0.27 (0.17 0.36) 0.12 (0.06 0.17) Notes: All CIs are Wagstaff normalzed ndces. CI = Concentraton Index. Non-confoundng varables nclude: Socoeconomc status (asset ndex), martal status, educaton, employment, resdence, race and lfestyle factors. 3.3. Explanng Obesty Inequaltes 3.3.1. Results of the Lnear Regresson Table 3 shows the results of the lnear probablty regresson model. Age and socoeconomc status (measured by the asset ndex) were statstcally sgnfcant and postvely assocated wth obesty among females, whle the postve factor assocated wth obesty n males was hgher educaton. Martal status was sgnfcantly assocated wth obesty. For both genders, the sngle and never marred were less lely to be obese than marred people. Increased physcal actvty was assocated wth low obesty n women. Smong was a sgnfcant negatve factor for obesty for men as well as women. Table 3. Results of the lnear probablty regresson model. Varables Female Male Coeffcents Standard Error Coeffcents Standard Error Age 0.008 0.00 0.003 0.00 Socoeconomc status (asset ndex) 0.051 0.01 0.018 0.01 Employment Status Unemployed (base) Employed 0.050 0.02 0.034 0.01 Educaton No school (base) (base) Prmary 0.094 0.03 0.058 0.02 Secondary 0.098 0.03 0.096 0.03 Tertary 0.058 0.05 0.132 0.04 Martal status Marred (base) (base) Wdowed 0.048 0.03 0.073 0.04 Never marred 0.079 0.02 0.049 0.02 Area of resdence Urban (base) (base) Rural 0.028 0.02 0.005 0.01
Int. J. Envron. Res. Publc Health 2014, 11 3398 Table 3. Cont. Varables Female Male Coeffcents Standard Error Coeffcents Standard Error Afrcan (base) Coloured 0.021 0.04 0.015 0.03 Asa/Inda 0.124 0.07 0.050 0.09 Whte 0.036 0.05 0.070 0.04 Physcal actvty 0.098 0.03 0.036 0.02 Smong 0.107 0.04 0.058 0.01 Intercept 0.024 0.05 0.062 0.05 Observatons 6816 4510 R 2 0.88 0.09 Note: Coeffcents sgnfcantly dfferent from zero (at p < 0.05) are n bold typeface. 3.3.2. Decomposton of Socoeconomc Inequalty n Obesty Table 4 presents the decomposton analyss based on OLS regressons, ndcatng the elastcty, concentraton ndex (Wagstaff normalzed) and contrbuton of each covarate to the overall obesty nequalty. If the contrbuton of a factor s postve, ths can be nterpreted as ceters parbus, the obesty nequalty would be lower f that factor was not present (the opposte for negatve contrbutons). Table 4. Decomposton of concentraton ndces for women and men. Varables Female Male Elastcty CI Contrbuton Elastcty CI Contrbuton Age 0.865 0.003 0.000 (0%) 1.080 0.021 0.023 (8.5%) Socoeconomc status (asset ndex) 0.028 2.614 0.054 (71.4%) 0.045 1.767 0.067 (24.8%) Employment Status Unemployed (base) (base) (base) (base) (base) (base) Employed 0.054 0.124 0.007 (10.0%) 0.184 0.100 0.018 (6.6%) Educaton No school (base) (base) (base) (base) (base) (base) Prmary 0.055 0.179 0.010 ( 14.3%) 0.107 0.247 0.026 ( 9.6%) Secondary 0.159 0.082 0.013 (18.6%) 0.527 0.060 0.032 (11.9%) Tertary 0.017 0.450 0.008 (11.4%) 0.141 0.466 0.066 (24.4%) Martal status Marred (base) (base) (base) (base) (base) (base) Wdowed 0.021 0.026 0.001 (1.4%) 0.030 0.004 0.000 (0.0%) Never marred 0.089 0.059 0.005 (7.1%) 0.219 0.066 0.014 (5.2%) Area of resdence Urban (base) (base) (base) (base) (base) (base) Rural 0.031 0.453 0.014 (20.0%) 0.017 0.447 0.008 (3.0%)
Int. J. Envron. Res. Publc Health 2014, 11 3399 Table 4. Cont. Varables Female Male Elastcty CI Contrbuton Elastcty CI Contrbuton Blac (base) (base) (base) (base) (base) (base) Coloured 0.005 0.43 0.002 ( 2.9%) 0.010 0.359 0.003 (1.1%) Asan/Indan 0.008 0.66 0.005 ( 7.1%) 0.010 0.501 0.005 (1.9%) Whte 0.10 0.67 0.007 ( 10.0%) 0.061 0.645 0.040 (14.8%) Lfestyle Physcal actvty 0.054 0.370 0.020 ( 28.6%) 0.130 0.102 0.013 ( 4.8%) Smong 0.028 0.290 0.008 ( 11.4%) 0.209 0.023 0.005 (1.9%) Resdual * 0.017 0.029 Notes: CIs are Wagstaff normalzed ndces. CI = Concentraton Index. We also performed the analyss by excludng lfestyle varables as a chec, and the sze and magntude of all varables except socoeconomc status (whch ncreased) dd not change. * Resdual s the part of socoeconomc related nequalty not explaned by the chosen determnants. Low educaton s concentrated among the poor n both males and females. The wdowed and never marred were concentrated n low-ncome groups among females, but only sngles were n ths group among men. Lfestyle factors consstng of physcal exercse and smong had postve ndces for women, mplyng that these actvtes were concentrated among wealthy ndvduals. Among men, smong was more an attrbute of the poor. The hghest elastctes for both men and women were observed wth age. However, age dd not mae a large contrbuton to the overall concentraton ndex. The largest contrbutor to the obesty nequalty for both men and women was socoeconomc status (asset ndex). Socoeconomc status contrbuted to about 71.4% of the nequaltes among women, compared to 24% among men. Educaton was a major contrbutor to the obesty nequalty among men, but not women. Tertary educaton contrbuted to about 24.4% of the nequalty for men. Other notable drvers of nequaltes among men were age (8.5%) and employment (6.6%). Lvng n a rural area (20.3%), physcal actvty ( 28.6%) and smong ( 11.4%) contrbuted sgnfcantly to the obesty nequalty among women, but not men. 4. Dscusson Ths paper provdes evdence of the socoeconomc nequaltes n obesty among South Afrcan adults. To our nowledge, ths s the frst study to formally measure the socoeconomc gradent of obesty n SA usng the concentraton ndex (CI). We use data from the 2008 South Afrcan Natonal Income Dynamcs Study (SA-NIDS) to calculate CIs for men and women, and apply a decomposton analyss to estmate the contrbutors to obesty nequalty. An advantage of CI s that t s more senstve to changes n the socoeconomc dstrbuton. A major lmtaton of CI s that t may only be appled f a strct ranng socoeconomc varable, such as ncome or consumpton s avalable. However, t has been shown that the asset ndex provdes a reasonable proxy for measurng nequaltes n the absence of ncome or consumpton [57]. Includng the behavoral factors n the decomposton may ntroduce bas n the study due to endorgenety. For nstance, physcal exercse s endogenous because causalty cannot be dentfed wth regard to whether people who exercse have a
Int. J. Envron. Res. Publc Health 2014, 11 3400 lower probablty of beng obese or people who are not obese are more lely to exercse [58]. In our analyss, we tested the effect of the lfestyle factors and dd not fnd any sgnfcant dfferences when they were removed from the regresson analyss. Thus we ncluded them on the bass that they are mportant to nformng polcy decsons on obesty preventon [55], and also because they have been shown to be assocated wth obesty n the lterature [59]. The use of a large natonally representatve sample made t possble to examne relatonshps between obesty and ts socoeconomc determnants, and measure obesty nequaltes. The heght and weght scores used to estmate obesty n the SA-NIDS were measured and not self-reported, hence we are confdent that errors n measurement were mnmal. Causal nference on the determnants of obesty s, however, lmted by the cross-sectonal nature of the study. Therefore, a comprehensve longtudnal study s requred to provde a more detaled descrpton of the reasons for changes n weght [20]. The rapd economc growth experenced by many low and mddle ncome countres n the last few years has seen a parallel nutrton transton, wth dets ncreasngly consttutng greater amounts of sugar, fats and refned carbohydrates [60]. Ths has exarcerbated excess weght gans for both men and women. Our study, however, suggests some gender dspartes n the dstrbuton of obesty, showng that obesty s hgher n women than men. Ths s consstent wth fndngs from other studes n SA [3,4], and other developng countres [61], whch show that women tend to be more obese than men. The study shows that unle n many developed countres where the socoeconomc gradent n obesty s concentrated among the poor [15,16], obesty n SA s postvely related to wealth, wth well-off ndvduals more lely to be obese than the poor. Ths s consstent wth fndngs from other studes n low and mddle ncome countres that show that obesty tends to ncrease wth hgher socoeconomc status [22]. Whle socoeconomc status (measured by the asset ndex) was a major contrbutor to obesty n both men and women, the concentraton ndex of obesty n women was small, ndcatng that there were less nequaltes n the dstrbuton of obesty for women across socoeconomc status. Ths was dfferent for men, where wealther men were sgnfcantly more lely to be obese than ther poorer counterparts. The decomposton analyss demostrates that other measures of socoeconomc poston, ncludng employment and hgher educaton were major contrbutors of nequaltes n obesty for men, but not for women. Lfestyle factors of physcal actvty and smong, and lvng n a rural area had a hgher mpact on obesty nequaltes n women than men. There could be varous reasons for these dspartes n the dstrbuton of obesty nequaltes among women and men, whch are not analysed here, but should be analysed n future studes. Frstly, though the nutrton transton observed n low and mddle ncome countres has contrbuted to excess weght gan n both genders, t has had a hgher mpact on women and wealther men, partcularly on ther physcal actvtes [62]. There could be hgh physcal nactvty related to both employment and unemployment or underemployment. Increased economc actvty n SA has led to hgher employment among men, partcularly n professonal, manageral, or admnstratve wor that promote sedentary behavour. Wardle et al. [63] show that men wth low socoeconomc status are more lely to be employed n wor that s physcally demandng, thus may have a lower rs of obesty than men who are well off. Women on the other hand, are more lely to be caregvers or n wage labour that does not requre vgorous physcal actvty [64]. Further, women as caregvers are manly responsble for preparaton of
Int. J. Envron. Res. Publc Health 2014, 11 3401 food and meals n the home. Wth the prolferaton of oss, supermarets and fastfood restaurants, there s an ncreased accessblty of food [65] whch does not requre expended energy and effort to prepare, contrbutng to reduced physcal actvty. In ths study, we fnd that rural women are less lely to be obese than women lvng n urban areas, and area of resdence was a sgnfcant contrbutor to obesty nequaltes, partcularly among women. Ths could be due to the mpact of the nutrton transton, whch tends to be hgher n urban areas, as well as the fact that the role that rural women play n the home and also ther employment, manly n argraran wor, requres vgorous physcal actvty [66]. Cultural ssues may also play a sgnfcant role n obesty, and ths mght be a reason why the socoeconomc gradent n obesty among women was small. Partcularly among the blac South Afrcan populaton, who are n the majorty, larger body sze s consdered to be a sgn of beauty, prosperty and good health [67]. These vews on body mage may be a major contrbutor to overweght and obesty among women of all socoeconomc bacgrounds, and wealthy men. 5. Conclusons Ths paper provded an analyss of the socoeconomc gradent n obesty n the South Afrcan adult populaton. The study shows that obesty s sgnfcantly hgher n women than men. The results also show that the socoeconomc gradent n obesty favours the rch. However, the gradent s more pronounced n men than women. A regresson-based decomposton of wealth related nequalty n obesty was also performed, showng that whle the man contrbutor s socoeconomc status (measured by asset ndex) for both men and women; other man contrbutors to obesty nequaltes are economc factors of educaton and employment among men, and lfestyle factors of physcal actvty among women. These fndngs have wder mplcatons for polcy and future research on obesty. There s need for more research on the socoeconomc gradent n adult obesty, whch wll help understand the cause of the obesty epdemc and facltate the reducton n health dspartes. The fact that the prevalence of obesty n women s hgh should be of concern, because obese women are lely to gve brth and rase chldren who mght become obese or overweght [68]. Thus focusng on reducng obesty n women could beneft future generatons. Promoton of physcal actvty, partcularly among women s very mportant to the reducton of obesty nequalty. Based on our fndngs, we suggest that polces amed at preventng obesty n women should be populaton based, and target women across the socoeconomc spectrum. There s also need for worplace actvtes that promote physcal actvty, better nutrton and generally mproved lfestyle and lvng. These ssues are acnowledged as mportant to reducng NCDs and ther rs factors, such as obesty, n the SA Department of Health NCDs strategc plan [12]. Acnowledgments The authors wsh to acnowledge the Southern Afrca Labour and Development Research Unt (SALDRU) based at the Unversty of Cape Town, School of Economcs, whch conducted the South Afrcan Natonal Income Dynamcs Study (SA-NIDS). We would also le to than Karen Hofman,
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