REVIEW

Obesity and asthma

PAEDIATRIC RESPIRATORY REVIEWS (2006) 7, 223–228

Susan Chinn*

King’s College London, UK

KEYWORDS

asthma;

incidence;

obesity;

prevalence

Summary There is a large literature on the possible association between obesity andasthma or asthma-like symptoms. However, no previous review has been confined tostudies in children and adolescents. Studies vary in definitions of overweight or obesityand in whether outcomes were parent-reported symptoms or doctor-diagnosed asthma;there is no consistency in the findings. Three studies provide evidence for a greaterincidence of diagnosed asthma in those obese at baseline compared to those not obese.The evidence for a stronger association in girls than in boys is weak. No study in aWestern society has found objective evidence for the association, and results might bedue to increased reporting of symptoms in obese children or to diagnostic bias.Intervention studies to prevent or reduce obesity could provide data to support or

refute the association. � 2006 Elsevier Ltd. All rights reserved.

INTRODUCTION

Many epidemiological studies suggest an associationbetween obesity and asthma or asthma-like symptoms inadults and children. There is some evidence that theassociation in children is of recent origin, whereas that inadults is more long standing. Hence, the underlyingmechanisms might differ and a review concentrating onstudies in children is warranted. Most of the evidence for anassociation comes from cross-sectional population surveysand, to a lesser extent, from case-control or cohort studies;these will be summarised. But first, definitions of over-weight and obesity in children require some explanation.

DEFINITIONS OF OVERWEIGHT ANDOBESITY IN CHILDREN

Overweight in adults is defined as a body mass index (BMI;weight in kilograms divided by the square of height inmetres) of 25 to 30 kg/m2, and obesity as 30 kg/m2 or

* Division of Asthma, Allergy and Lung Biology, 5th floorCapital House, 42 Weston Street, London SE1 3QD, UK.Tel.: +44 20 7848 6607; Fax: +44 20 7848 6605.

E-mail address: sue.chinn@kcl.ac.uk.

1526-0542/$ – see front matter � 2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.prrv.2006.04.007

more. Although BMI is not perfectly correlated with directmeasures of fatness, it is not feasible to use direct measuresin most population studies. The above definitions havebeen accepted since the early 1980s.

There are two problems in children. One is that BMIchanges with age and in mean value.1 Hence the cut-offpoints need to be age specific and the BMI must betransformed into a z-score (i.e. standard deviation score)if it is to be used as a continuous variable when data fromchildren of more than one age are combined. In studies inthe USA, the 95th centile of the US Centers for DiseaseControl (CDC) reference curves for BMI has usually beenused as the definition of overweight/obese,2 with childrenat or above the 85th but below the 95th centile classified asat risk. In 2000, the International Obesity Task Forceproposed ‘international’ definitions of overweight andobesity, based on the centiles corresponding to BMI 25and 30 at age 18 in six countries.1 Although the averagingacross countries has been criticised,3 these definitions havethe merit of being less arbitrary than fixed centiles and havecontinuity with the adult definitions. Definitions based onthe adult categories also discourage ‘drift’. As the preva-lence of obesity has increased over time in most popula-tions, the 85th and 95th centiles have also increased. If the

224 S. CHINN

reference population is kept fixed this is not a problem butthis might not be universally understood. Studies have beenincluded in this review regardless of cut-off points used.

The second problem is that, in children – even more sothan in adults – BMI is only a proxy for fatness4 and thedegree of correlation changes with age. Skinfold thicknesseshave been used in a few studies but there is no agreed cut-off and most researchers have used BMI.

CROSS-SECTIONAL STUDIES OFASTHMA OR ASTHMA-LIKESYMPTOMS

In the first extensive review, published in 2001, Tantisiraand Weiss did not comment separately on children andadults.5 In 2003, Chinn summarised six studies in which asubstantial proportion of participants were aged under 12years.6–13 Each study provided some evidence for a positiveassociation. Results from the National Health and Exam-ination Survey (NHANES) III, carried out from 1988 to1994, were reported in two papers,10,13 showing strongevidence for an association between asthma and age-standardised BMI. By contrast, NHANES II (which wasconducted from 1976 to 1980) found no statistically sig-nificant trend in asthma with BMI, although the trend forwheeze was significant.9 Results from the 1994 UK NationalStudy of Health and Growth (NSHG) showed strongevidence of an association with asthma in white boysand girls.11 This was in contrast to earlier findings from1977,7 and suggests that the association arose after theearly 1980s.14 Another UK study found little evidence of anassociation in data collected in 1965 and 1969.8 Chinnconcluded that the association in children was of recentorigin but found few studies in adults to provide corre-sponding information.6 One unpublished study suggestedthat the association was present in adults by the 1980s;since then, the Swedish Conscript Study of 17-year-oldmales has reported an association as early as 1952–1961.15

Two further cross-sectional studies of symptoms orasthma in children or adolescents under 17 years werereferenced by Ford16–18 and another seven have also beenidentified.19–25 One of these was a re-analysis of NHANES IIIdata,22 although with a slightly different age range to theprevious reports.10,13 Of the remaining eight studies, threefound a strong relation of current asthma with BMI withadjustment for confounding factors.17,19,21 Wickens et al.reported a strong relation of current asthma medication withBMI.25 Three found no significant relation,18,20,24 but Sulitet al. found a weak relation that persisted after additionaladjustment for sleep-disordered breathing.23 Most research-ers also analysed recent wheeze as an outcome, with threestudies reporting a strong association,17,19,25 one a weakassociation23 and one a weak association only in girls and norelation in boys.18 Each statistically significant association wasof increasing asthma or wheeze with greater BMI but authors

differed as to whether BMI was dichotomised, analysed ascontinuous or divided into quartile or quintile groups with atest for trend performed.

Wickens et al. found a stronger relation of symptomswith BMI in 2000 than in 1989,25 although the differencewas not statistically significant. They concluded that adiagnostic shift from wheezy bronchitis to asthma couldexplain a lack of association in early studies.

CASE-CONTROL STUDIES OFASTHMA

One of the three studies of asthmatic children compared tocontrols reported in earlier reviews showed a strongrelation to obesity,26 a second showed very little differencebetween cases and controls27 and the third showed arelation of borderline significance,28 as did two recentlypublished case-control studies.29,30

HETEROGENEITY OF CROSS-SECTIONAL AND CASE-CONTROLSTUDIES

Ford considered that the cross-sectional and case-controlstudies in children and adolescents showed less consistentresults than those in adults.16 Potential sources of variationin results include study size, period when the study wascarried out, age range of the participants and cut-off pointto define obesity. Significant odds ratios for associationsbetween obesity and asthma ranged from 1.4 to 3.4. Smallstudies lack power to detect odds ratios of this magnitudebut not all studies that found no association were small insize. The Canadian National Longitudinal Survey of Chil-dren and Youth included 5596 boys and 5501 girls aged 4–11 years,24 finding little difference in current asthmabetween those in the highest BMI category and those inthe lowest in either sex and evidently no trend. To et al.grouped all children above the 85th centile of the sampledata ‘to ensure sufficient numbers within each category andto preserve the generally accepted definition of obesity asBMI � 85th percentile in population-based research’.24

The corresponding BMI values were not given and the85th centile is no longer ‘generally accepted’. It is possiblethat this definition of obesity encompasses children nor-mally classified as overweight or at risk of overweight. Thismight be the reason for the lack of association but –alternatively – the association might not be present inCanadian children. Results from the Canadian NationalPopulation Health Survey found no statistically significantrelation of asthma to BMI unadjusted for age in adolescentsand young adults aged 12–24 years,31 although in femalesthere was an association in adults aged 25 years and older.

No pattern in the association with age range is apparent.The tendency of recent studies to show an association andfor those carried out before 1980 to show little relationbetween obesity and asthma, was noted above. The Six

OBESITY AND ASTHMA 225

Cities Study, carried out in the US from 1974 to 1979, wasan exception to this, finding a strong relation of asthma andwheeze to BMI in children aged 7–14 years, although BMIwas not age adjusted.17

LONGITUDINAL STUDIES OFASTHMA AND OBESITY

There have been far fewer longitudinal studies on obesityand asthma. The NSHG was a series of cross-sectionalstudies of children aged 4–11 years but, because the sameschools were visited at each study occasion, the data alsohad a considerable longitudinal element, with most childrenfollowed up for at least 4 years. For children not recordedto have parent-reported wheeze or asthma at age 5 or 6years, there was a considerably increased risk of asthma orwheeze 4 years later in those obese at baseline comparedto those of normal weight, in both sexes.6,14 Three cohortstudies in the US have analysed data on asthma or symp-toms in relation to obesity or BMI.32–34 The Six CitiesStudy34 found an increasing incidence of parent-reportedand doctor-diagnosed asthma with wheeze with increasingBMI z-score at baseline or annual increment in BMI z-scorein girls aged 6–8 years at baseline; the relationship in boyswas U shaped. The Children’s Health Study, carried out inSouthern California, found increased incidence of reportedphysician-diagnosed asthma in those overweight or obeseat baseline in 1993.33 In the Tucson study, new infrequentwheezing was more common in girls who became over-weight or obese between age 6 and 11 years compared tothose who did not.32 Corresponding results were notreported for boys as prevalence of wheeze at age 11was unrelated to change in BMI. The incidence was notanalysed in relation to baseline obesity.

Estimates of incidence are dependent on the sensitivityof the criterion to exclude prevalent cases at baseline and itis known that bias in relative risks is also related to thesensitivity.35 An alternative analysis, which does not require

Table 1 Net change over 4 years in prevalence of asth6 years at baseline, by obesity status at baseline, in the UK

% atbaseline

% with asthmaattacks atbaseline

%attfol

Boys (n = 3625) 5.8 8Underweight 22.1 5.6 8Normal weight 68.1 5.5 8Overweight 8.5 8.1 10Obese 1.3 8.3 20

Girls (n = 3457) 3.7 5Underweight 28.6 2.9 4Normal weight 63.1 4.1 5Overweight 7.3 3.6 6Obese 1.0 2.9 13

an exclusion criterion, is to analyse net change in prevalencein the whole sample with data on two occasions. Resultsfrom NSHG data, used previously to derive relative risks,6

are shown in Table 1. The baseline prevalence of over-weight and obesity was slightly greater in boys than in girls.The baseline prevalence of underweight was higher in girls(column 2). Baseline prevalence of asthma attacks in the lastyear was greater in boys than in girls (column 3), and therewas a larger net overall increase in boys than in girls. Only1.3% of boys and 1.0% of girls were obese at baseline, soconfidence intervals for the net increases in the prevalenceof asthma attacks in these groups were wide, overlappingzero for boys. Overall, allowing for differences betweenboys and girls in prevalence and change in prevalence, therewas a borderline significant association between net changeand obesity group (P = 0.061) and a greater net increase inchildren obese at baseline compared to those of normalweight (P = 0.007).

Even if the Tucson results show no statistically signifi-cant relation, overall there is convincing evidence for anoverall relation of reported asthma incidence to baselineobesity. The studies rule-out reverse causality as theexplanation, i.e. it is unlikely to be due to asthmatic childrenbeing more sedentary and hence developing overweightor obesity.

CONCOMITANT INCREASES INASTHMA AND OBESITY

Parallel increases in the prevalence of asthma and obesity,together with the reported association, have led to spec-ulation that the increase in obesity might be responsible forpart of the rise in asthma;16 few studies have looked at thisdirectly. The NSHG showed trends in BMI and prevalenceof wheeze and asthma in UK children from the early 1980sto 1994 but the trend in BMI explained almost none of thetrends in asthma or wheeze.14 A similar result was found inNew Zealand from 1989 to 2000,25 and for young men in

ma attack in last 12 months in boys and girls aged 5 orNational Study of Health and Growth 1982 to 1994

with asthmaacks atlow-up

Net change inprevalence (%)

95% confidenceinterval fornet change (%)

.7

.4 2.7 1.1 to 4.4

.4 2.9 1.9 to 3.9

.1 1.9 �1.3 to 5.2

.8 12.5 �1.1 to 26.2

.1

.4 1.4 0.2 to 2.7

.1 1.0 0.1 to 1.9

.3 2.8 �0.2 to 5.8

.9 11.1 0.8 to 21.4

226 S. CHINN

Sweden from 1969 and 1994.15 Inconsistency in the asso-ciation of asthma and obesity over time might explain theNSHG results. Calculation has shown that the association isnot strong enough to explain more than a small percentageof the rise in asthma.36

IS THERE A DIFFERENCE IN THEASSOCIATION BETWEEN BOYS ANDGIRLS?

Weiss and Shore speculated that ‘clear-cut sex differencesin the epidemiology of obesity and its impact suggest thatfemale sex hormones may be contributing to the increasedrisk of asthma in obesity’.37 However, Ford pointed out thatthe evidence for a sex difference was inconsistent andconsidered the issue far from settled.16 Few studies wereconfined to prepubertal children. The study of 5- and 6-year-old children in Bavaria reported a statistical relation ingirls but not in boys.12 However, no test of interaction wasgiven, without which no real conclusion can be drawn.38 Insecond-grade children in Israel a stronger relation wasfound in boys than in girls.19 Again, no test of the differencewas reported. However, Oddy et al. – in their study ofchildren in western Australia21 – found no significantinteraction between BMI and gender on asthma at 6 years.Hence the results are either inconsistent or – overall –suggest no difference in the asthma–obesity relation atyoung ages.

All other cross-sectional studies included children of 10years or older. Most, but not all, reported results by sex,others reported non-significant tests for interaction.Slightly larger associations were found in 4- to 11-year-old girls than in same-aged boys in the NSHG representa-tive sample,11 although the differences were not statisti-cally significant. However, in the inner-city sample, whichincluded a high proportion of children of African–Carib-bean and Indian subcontinent origin, there was evidencefor associations in girls but not in boys. NHANES IIIshowed no effect modification by ethnicity or sex inchildren aged 4–17 years.13 Gold et al. studied sex andrace differences in prevalence of respiratory illness andenvironmental exposures in 7- to 14-year-olds,17 but donot appear to have looked at differences in the cross-sectional association of BMI and asthma. Schachter et al.found a significant trend in wheeze and cough with increas-ing BMI in girls but not in boys aged 7–12,18 but did not testthe interaction, and trends in ever-diagnosed asthma andrecent asthma were not statistically significant in eitherboys or girls. In 2000, in New Zealand, associations ofwheeze and asthma outcomes with BMI appeared stron-ger in 11- to 12-year-old girls than in same-aged boys.25

Outcomes analysed included hospital admission, two med-ication variables, bronchial hyperresponsiveness (BHR)and exercise, and differences in relations with BMIbetween boys and girls were not significantly differentwhen P-values were adjusted for nine multiple compar-

isons. Mai et al. found a statistically significant relation ofwheeze with BMI in Swedish boys aged 11 to 13, but not ingirls; however, the interaction was not significant.29 Norelevant information was available from the other cross-sectional studies. Overall, even in older children, theevidence for a stronger association in girls than in boysis weak.

The relative risk for incidence of asthma over 4 years forobese compared to normal-weight children was similar forboys and girls in the NSHG.6 These findings are borne outby the net change results shown in Table 1. Gold et al.divided children into quintile group categories of BMI atbaseline and change in BMI.34 Comparison of the highestquintile group with that of lowest incidence appears toshow that the incidence rate ratios were similar for boysand girls. In the Children’s Health Study, the relation ofincidence to overweight or obesity seemed confined toboys33 but the formal test of interaction of gender andobesity was of borderline significance (P = 0.09). Resultsfrom the Tucson study have often been quoted as evidencefor a gender difference in children32 but are difficult tointerpret. The outcome variable was ‘wheezing’, dividedinto mutually exclusive categories of none, infrequent andfrequent. Outcome data were available at ages 6, 8, 11 and13 years. BMI at 6 and 11 was used to categorise children asoverweight/obese or not. The cross-sectional relation ofwheeze category to overweight/obese age 11 years wasstronger in girls than in boys but no interaction wasreported. Relations at other ages were not statisticallysignificant for boys or girls. Based on the weakly significantgreater prevalence of either wheezing category at age 11 inthose who became overweight or obese in girls, but not inboys, incident wheeze was analysed only in girls. It seemsunlikely, in this relatively small study of 600 children at age11, that any gender–obesity interaction was statisticallysignificant. Overall, there seems to be no convincing evi-dence for a gender difference in the asthma–obesity rela-tion in children or adolescents.

ALTERNATIVE MEASURES OFOBESITY

As noted above, BMI is a very rough and ready measure offatness in children but few studies have included otherassessments. In NHANES II, Schwartz et al. found wheezeto be associated with increasing BMI and triceps skinfoldthickness but current doctor-diagnosed asthma withneither.9 In the 1994 NSHG representative white sample,asthma and wheeze were strongly associated with increas-ing BMI but not with the sum of triceps and subscapularskinfolds, whereas in the multi-ethnic inner-city samplethere were strong associations with each in girls but withneither in boys.11 All other studies used a definition basedon BMI alone. Thus the limited information as to whetherthe observed relations with BMI are found with fatness isconflicting.

OBESITY AND ASTHMA 227

ASTHMA OR ASTHMA-LIKESYMPTOMS AND OTHEROUTCOMES

Many studies had questions on wheeze as well as recentasthma attacks or doctor-diagnosed asthma. Results werenot always consistent. As already noted, in NHANES IIthere was an association between wheeze and BMI but notasthma and BMI.9 In the NSHG in 1994, odds ratios forpersistent wheeze were greatest in the representativesample and largest for asthma attacks in inner-city girls.11

In children in Bavaria, the relation with BMI category wasstronger for lifetime doctor-diagnosed asthma than forwheeze in the last 12 months.12 In NHANES III, therewas a clear trend with increasing BMI for asthma outcomesbut not for wheeze in the last 12 months, which did notincrease monotonically with BMI.13 Gold et al. found similarodds ratios with BMI for asthma in the past year and forwheeze apart from colds or for most days or nights.17

Schachter et al. found stronger evidence for trends inwheeze with increasing BMI than for asthma outcomes.18

Bibi et al. found associations with obesity for asthma andwheeze without a cold.19 Sulit et al. found a strongerrelation of asthma than of wheeze to obesity.23 Wickenset al. found strong evidence for trends in wheeze andasthma with BMI category in girls.25

Results are inconsistent for wheeze and asthma indifferent directions. Wheeze is not specific to asthma.Increased wheeze in obese children could be related tothe mechanics of breathing,5 whereas asthma might bediagnosed more frequently in obese children.19 Lack ofconsistency either way can be argued to indicate that theassociation is not one of true asthma with obesity. Only anobjective measure, such as BHR, can provide furtherevidence. Schachter et al. found no trend of increasingBHR to histamine with greater BMI, in boys or in girls.18

Wickens et al. found no trend of BHR to exercise, even ingirls who showed strong relations of wheeze and asthma toBMI.25 In teenagers in Taiwan there was no relation of BHRto methacholine to BMI in boys or girls after adjustment forother variables.39 Mai et al. found no difference in BHR tohypertonic saline between children above and below the75th centile of BMI, in those with or without wheeze.However, this case-control study, with selection on currentwheeze, gave no overall result for the relation of BHR toBMI.29 A case-control study in South Africa selected onBHR to exercise found a strong relation between BHR andBMI.40 However, only 0.6% of the African children wereobese according to UK criteria, and 4.5% overweight. Thisstudy, therefore, does not provide evidence of a relation ofBHR to BMI in the context of the majority of studies. Therelation between BHR to exercise and BHR to histamine isnot strong.41 This might also have some bearing on theresults of Calvert and Burney40 and on the lack of relationfound by Wickens et al. in contrast to other outcomes.25

Kaplan and Montana found a greater exercise-induced fall in

forced expiratory flow between 25% and 75% of forcedvital capacity in non-asthmatic obese children compared tonon-asthmatic non-obese, with obesity defined using tri-ceps skinfold thickness.42

INTERVENTION STUDIES

One paper reported a resolution of co-morbid conditionsincluding asthma in four surgically treated morbidly obeseadolescents, aged 17 to 19 years,43 but it was unclearwhether all had asthma at the outset and surgical inter-vention was not carried out in younger patients. No non-surgical intervention study in obese asthmatic childrenreporting change in asthma or symptoms has been identi-fied. There have been a number of surgical and non-surgicalintervention studies in adults but a study that measuredBHR before and after the intervention found no relationbetween change in weight and change in BHR.44 All otherstudies reported subjective outcomes and changes inweight were generally associated with dietary changes,so these studies are not direct evidence for a causal relationbetween asthma and obesity.6

CONCLUSIONS

Schachter et al. concluded that increased symptoms inobese children were unlikely to be due to asthma18; othershave also expressed scepticism.19 There is no convincingevidence to support a real relation. There are many goodreasons to reduce obesity in children and interventionstudies that include respiratory outcomes, and BHR couldprovide better evidence for or against an associationbetween asthma and obesity.

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