Obesity and asthma: An association modified by age

Abstract

Background

Some studies indicate some causal relationship between obesity and asthma, while others show inconsistent results. Our objective was to evaluate the prevalence of asthma according to obesity in children.

Methods

A cross-sectional study, following the ISAAC study methodology, was conducted on two randomly selected groups consisting of 6–7 year-old children (n = 7485) and 13–14 year-old adolescents (n = 8496).

The asthma symptoms and potential risk factors were determined from the questionnaire. Overweight and obesity were defined based on the body mass index.

Multiple logistic regression was used to obtain adjusted prevalence odds ratios (OR) and 95% confidence intervals.

Results

Obesity was associated with an increase in wheezing ever (OR: 1.35) and exercise-induced asthma (OR: 1.62) in the 6–7 year-old group. No significant relationship was observed in the adolescent population.

Conclusion

Obesity was associated with a higher prevalence of asthma in young children, but not in adolescents.

Introduction

Both the prevalence of asthma and that of obesity appear to have increased in the childhood population in the last few years, particularly in developed countries.^1,2

The studies published on the childhood population are inconsistent, as some show an increase in asthma prevalence in those who are obese,^3,4 while others did not observe any relationship between obesity and asthma prevalence.^5,6

Moreover, the importance of environmental and socioeconomic factors in asthma prevalence is known, as such that the influence of a risk factor may be different between one population and another.^7,8

The purpose of our study was to evaluate the prevalence of asthma according to obesity in a large sample of the child and adolescent population in our region.

Material and methods

A study was conducted using the methodology of the International Study of Asthma and Allergies in Childhood (ISAAC) (http://isaac.auckland.ac.nz). A written questionnaire previously translated and validated in Spanish was used for the purpose of this study.^9,10

The population studied were two groups of children, one with 6–7 year-olds (young children), and another with 13–14 year-olds (adolescents), from six of the main Health Areas in Galicia (Spain), which include a population of 1.9 million (69%) of the total population of the Autonomous Community. The schools required to include 1000 validated questionnaires from each Health Area were randomly selected, and included all children of the targeted age range in each school. Schools that refused to participate were replaced by others.

The field work was done between October 2006 and February 2007. The A Coruña Health Area was excluded, as the study was conducted there in 2003 and its results are included in other publications.^9,10

Permission was requested from the parents or guardians, who also answered the questionnaire for the 6–7 year-old group, while in the older age group the questionnaire response were made by the children themselves.

The questionnaire data were entered manually into a data base in accordance with the ISAAC protocols, using double entry with subsequent validation.

The environmental questionnaire included questions about asthma symptoms, self-reported height and weight, presence of a dog or cat in the home, parental smoking habits, parental asthma history, and maternal education level (Tables 1 and 2).

Table 1.

Prevalence of asthma symptoms.

	6–7 years		13–14 years
	N	%	N	%
Wheezing ever
No	4809	61.33	6915	76.7
Yes	3038	38.7	2099	23.3
Current asthma
No	6815	86.8	7822	86.8
Yes	1032	13.2	1192	13.2
Exercise-induced asthma
No	7368	93.9	7204	79.9
Yes	479	6.1	1810	20.1
Severe asthma
No	7485	95.4	8496	94.3
Yes	362	4.6	518	5.7

All values as number of cases (N) and percentage (%).

Table 2.

Main characteristics of the children and prevalence of risk factors.

	6–7 years		13–14 years
	N	%	N	%
Gender
Male	3983	50.8	4599	51.0
Female	3864	49.2	4415	49.0
Obesity
Normal weight	5261	67	7421	82.3
Overweight	1834	23.4	1396	15.5
Obesity	752	9.6	197	2.2
Cat keeping in the past 12 months
No	7152	92.9	7501	84.0
Yes	545	7.1	1431	16.0
Cat keeping in the first year of life
No	7371	94.6	5641	88.8
Yes	423	5.4	710	11.2
Dog keeping in the past 12 months
No	6749	88.1	6367	71.3
Yes	910	11.9	2558	28.7
Dog keeping in the first year of life
No	6999	90.1	5073	79.8
Yes	772	9.9	1287	20.2
Maternal education
No education/elementary	2099	27.2	1848	21.3
High school	2955	38.2	3766	43.4
University	2672	34.6	3069	35.3
Parental asthma
Neither parent asthma	4304	87.9	4269	90.1
Some parent asthma	590	12.1	468	9.9
Parental smoking
Neither parent smoke	3795	49.8	4271	48.5
Father only	1427	18.7	1254	14.3
Mother only	994	13.0	1578	17.9
Both parents	1404	18.4	1696	19.3

All values as number of cases (N) and percentage (%).

For the purpose of this study, Wheezing ever was defined as a positive answer to the question “Has your child ever had wheezing or whistling in the chest at any time in the past?”

Current asthma was defined as a positive answer to the question “Has your child had wheezing or whistling in the chest during the last 12 months?”

Severe asthma was defining as a combination of the three questions assessing the severity of asthma: “How many attacks of wheeze has your child had during the last 12 months? (none, 1–3, 4–12, >12)”, “In the last 12 months, how often on average has your child's sleep been disturbed due to wheezing? (never, <1 night/week, ≥1 nights/week)”, and “In the last 12 months, has wheezing been severe enough to limit your child's speech to only one or two words at a time between breaths?”. Children were considered to have current severe asthma when there were ≥4 asthma attacks or when sleep was disturbed ≥1 nights/week or when there had been an episode of speech limitation.

Exercise-induced asthma was defined as a positive answer to the question, “In the last 12 months, has your child's chest sounded wheezy during or after exercise?”.^9,11,12

Obesity and overweight were defined in accordance with the Body Mass Index (BMI) cut-off points set by Cole et al., for each group by age and sex.¹³ Based on this, three groups were established; normal weight, overweight, and obese.

Data analysis

Multiple logistic regression was used to obtain adjusted prevalence odds ratios (OR) and 95% confidence intervals (95% CI) between asthma symptoms of schoolchildren and obesity. Children with normal weight were taken as a reference for calculating the ORs of the asthma symptoms for the overweight and obese categories.

These calculations were preferred to other methods (for example, linear regression) which do not provide information on risk increase, and which require statistical assumptions (such as linearity) that are unverifiable in empirical conditions.

In the multivariate analysis, the results are presented adjusted for gender, dog or cat in the home, parental smoking, parental asthma, and maternal education.

The children with incomplete data were excluded from this study.

The statistical analysis was performed using the software SPSS 17.0

The study was approved by the Clinical Research Ethics Committee of Galicia.

Results

The response rate in the 6–7 year-old group was 72.4%, including 7847 valid cases. The response rate was higher (84.4%, 9014 cases) in the adolescent group.

The prevalence of asthma in the lower age group was 39.0%, that of current asthma 13.5%, severe asthma 4.9%, and exercise-induced asthma 6.4%. In the 13–14 year-olds these prevalences were 23.0%, 13.2%, 5.8% and 20.0%, respectively (Table 1).

The prevalence of overweight was 9.6% in the younger children, and 2.2% in the adolescents, while 23.4% of the 6–7 year-old group and 15.5% of the 13–14 year-olds were obese (Table 2).

Obesity was associated with an increase in the probability of wheezing ever (OR: 1.35, 95% CI: 1.10–1.66) and exercise-induced asthma (OR: 1.62, 95% CI: 1.10–2.36) in the 6–7 year age group. No significant influence of obesity was observed in the rest of the variables analysed in this group (Table 3).

Table 3.

Odds ratio for prevalence of asthma symptoms according to obesity and overweight in children and adolescents.

	Wheezing ever	Current asthma	Exercise-induced asthma	Severe asthma
	OR (95% CI)	OR (95% CI)	OR (95% CI)	OR (95% CI)
6–7 years old
Normal weight	1	1	1	1
Overweight	1.07 (0.92–1.25)	1.10 (0.88–1.36)	1.10 (0.80–1.52)	0.96 (0.66–1.41)
Obesity	1.35 (1.10–1.66)	1.31 (0.98–1.74)	1.62 (1.10–2.36)	1.20 (0.74–1.94)
13–14 years old
Normal weight	1	1	1	1
Overweight	1.03 (0.84–1.28)	0.88 (0.66–1.16)	1.09 (0.88–1.36)	1.01 (0.68–1.51)
Obesity	0.62 (0.34–1.13)	0.52 (0.22–1.21)	1.04 (0.60–1.79)	0.86 (0.31–2.41)

OR: Odds Ratio. CI: Confidence interval.

Adjusted by gender, cat and dog keeping, maternal education, parental asthma and parental smoking.

No significant influence of obesity on asthma prevalence was observed in the adolescent group.

Discussion

The results of this study suggest that obesity is associated with a higher asthma prevalence in 6–7 year old children, but not in the 13–14 year-old adolescents.

The results of previous studies are inconsistent, given that some studies did not find a relationship between obesity and asthma,^5,6,14 while others observed an increase in the prevalence of asthma in obese children.^3,4,15

Some studies even suggest the possibility of reverse causality, in that the asthma would be the cause of the obesity.^16,17

Several mechanisms have been established that could justify the increase in prevalence in obesity.

Obesity is associated with various comorbidities that could increase asthma symptoms such as gastroesophageal reflux disease (GERD) or obstructive sleep apnoea syndrome (OSAS).^18,19

On the other hand, some regions of the human genome, such as chromosomes 5, 6, 11 and 12 are associated with both asthma and obesity.²⁰

Some authors suggest that obesity restricts mechanical conditions of the respiratory system, reducing chest wall compliance, increasing respiratory effort, or altering lung elastic retraction.^21,22

Obesity also increases the prevalence of atopy, due to changes in cytokines, modification of lymphocyte activity, and favouring Th2 differentiation.^20,23

Another mechanism could be the increase in systemic inflammation, since adipose tissue can produce cytokines such as, TNF-alpha, IL-6 or eotaxin.^19,24,25 An increase has also been observed in inflammation markers, such as CRP or fibrinogen in obese asthmatics, compared to non-obese asthmatics.²⁶

Some hormonal mechanisms seem to take part in these processes, since high levels of leptin and a low adiponectin have been associated with a higher asthma prevalence.^27,28,29 Leptin is involved in breathing control, as well as in systemic inflammatory processes, whilst adiponectin would produce the opposite effect.^19,24,27,28

It has also been observed that obese asthmatics have a lower response to glucocorticoids, measured by a lower expression of protein-kinases.^30–32

The difference in the effect of obesity that we observed between the 6–7 year-old group (where the asthma prevalence is increased), and the adolescents, in whom no significant effect could be associated, at least in part, with the low prevalence of obesity in this population. This obesity prevalence, which was 2.2%, places us in the lowest values in the European population. Meanwhile, 9.6% of the 6–7 year-olds are obese.³³

Another aspect to consider is the methodology of the ISAAC study, as the parents answered the questionnaire in the 6–7 year age group, while the adolescents gave their own responses. It is known that this may lead to some differences in the perception of the symptoms.^11,34

The inconsistencies in the literature could be partly related to methodological differences between the different studies. On the other hand, it includes different population ages. Thus, in the study by Brenner, a group of 12–18 year-olds¹⁴ were analysed, two studies included a 4–11 year-old population,⁶ van Gysel between 3.4 and 14.8 years,³⁵ and Chu 16–16 year-olds.⁴

The definition of obesity is also different, since some authors used a pre-defined cut-off point as in our case^15,35 and others a percentile of the distribution of their population.^3,4,6,14

Limitations and strengths

The main strengths of our study are based on the use of a validated methodology, with a large sample size, and the random selection of the subjects included.

As limitations, it should be noted that on being a cross-sectional study it could only identify the relationships but not determine the causality. On the other hand, the data obtained came from a questionnaire, with no objective measurements of the disease or the risk factors.

Conclusion

In conclusion, the results of our study reinforce the hypothesis that obesity is associated with the increase in prevalence of asthma symptoms in the child population, although this relationship disappears in adolescents.

Ethical disclosures

Patients’ data protection

Confidentiality of data. The authors declare that they have followed the protocols of their work centre on the publication of patient data and that all the patients included in the study have received sufficient information and have given their informed consent in writing to participate in that study.

Right to privacy and informed consent

Right to privacy and informed consent. The authors have obtained the informed consent of the patients and/or subjects mentioned in the article. The author for correspondence is in possession of this document.

Protection of human subjects and animals in research

Protection of human and animal subjects. The authors declare that no experiments were performed on humans or animals for this investigation.

Conflict of interest

The authors have not any conflict of interest.

Acknowledgment

This work was funded by Maria Jose Jove Foundation.