This editorial discusses the findings of the paper in this issue by E. A. Mitchell et al. , pp. 73–84.
Lifestyles of the fat and lazy
Article first published online: 24 DEC 2012
© 2012 Blackwell Publishing Ltd
Clinical & Experimental Allergy
Volume 43, Issue 1, pages 2–4, January 2013
How to Cite
Clinical & Experimental Allergy, 2013 (43) 2–4.and ,
This logo highlights the Editorial article on the cover and the first page of the article.
- Issue published online: 24 DEC 2012
- Article first published online: 24 DEC 2012
Non-communicable disease makes up an increasing proportion of the health-care burden of both developed and developing countries, and a large proportion of this burden is attributed to ‘lifestyle’ factors. Obesity and lack of physical activity, including an increasing amount of time spent in front of television and other screens, are associated with diabetes, cardiovascular disease and various forms of cancer. Understanding how lifestyle factors interact with one another and exert their detrimental effects on health is therefore an essential prerequisite for the development of interventions to reduce the burden of disease. In this issue of Clinical & Experimental Allergy, Mitchell and colleagues  have used data from a very large international study to consider the effect of these lifestyle factors on three allergic diseases, namely asthma, rhinoconjunctivitis and eczema, in children and adolescents.
The International Study of Asthma and Allergies in Childhood (ISAAC) is a large and very well-standardized study. The data reported by Mitchell and colleagues  are from ISAAC phase III, involving over 580 000 participants in 53 countries, and as such provide high quality evidence about risk factors for asthma, rhinoconjunctivitis and eczema. Mitchell and colleagues confirm the findings of previous smaller studies in children , where the highest body mass index quintile was a risk factor for asthma symptoms, with odds ratios between 1 and 2, suggesting that although the association is significant, the effect size is modest. The strength of this study, arising from the large sample size, is its ability to separate the overweight and obese groups and thus demonstrate a dose response for both children and adolescents. Obesity was also associated with increased risk of eczema, but not rhinoconjunctivitis. Physical activity and TV viewing had less consistent relationships with risks for allergic disease than for overweight and obesity, which might reflect the fact that lifestyle factors such as physical activity and television viewing are difficult to measure objectively at a population level. As noted by Mitchell and colleagues , some significant associations, such as the increased risk of asthma associated with vigorous physical activity, might be attributed to methodological factors arising from the way the questions about physical activity were framed. Nevertheless, the possibility that changes in lifestyle contribute to an increase in prevalence or severity of allergic disease not only suggests opportunities for interventions to reduce the risk but could also provide some clues to a better understanding of the mechanisms involved in these diseases.
There are some caveats to the possible interpretation of these data. One of the important outcomes of the ISAAC research programme has been the understanding that asthma, rhinoconjunctivitis and eczema are not necessarily allergic diseases, and that their increasing prevalence may have little to do with allergy, especially in the developing world. Furthermore, in the study by Mitchell and colleagues , no data were available for allergic status of the participants, so no conclusions can be made about the role of allergic mechanisms in this study. In addition, although there is a well-recognised pathway between sedentary behaviour and obesity, it remains clear that obesity, physical activity and TV watching cannot be regarded as part of a single ‘lifestyle’ factor, and are likely to be independent factors that each contribute to increasing disease risk via different mechanisms. Mitchell and colleagues provide two pieces of evidence to suggest that each of these lifestyle factors have their effects via different mechanisms. Firstly, the effects of obesity, physical activity and TV watching were independent of each other, and the interaction terms were not significant modifiers of the risk of any of these diseases. Secondly, these factors had inconsistent effects on different diseases. Notably, both vigorous physical activity and prolonged television viewing increased risk for all three diseases in adolescents.
The potential of obesity to up-regulate the Th2 allergic inflammatory pathways conventionally associated with asthma has been considered a possible mechanism for the association between obesity and asthma. However, although obesity is associated with increased systemic inflammation , there is little evidence that obesity is associated with increased airway inflammation [4, 5], except for one study suggesting that neutrophilic, but not eosinophilic, inflammation may be increased in obese asthmatics . Adipokines, such as leptin and adiponectin, also modify inflammatory processes and are weakly associated with asthma incidence  and symptoms . If obesity increases asthma symptoms by up-regulating inflammatory pathways, then one would expect that obesity would also be associated with increased severity of other inflammatory diseases, such as rhinoconjunctivitis and eczema. Interestingly, Mitchell et al. found that obesity was associated with symptoms of asthma and eczema, but not rhinoconjunctivitis. They speculated that the association between obesity and increased skin irritation was due to friction caused by the increased presence of skin folds due to obesity, rather than to any inflammatory process. Consistent with this, bariatric surgery reduces symptoms of asthma and increases serum adiponectin, without altering inflammatory proteins in bronchoalveolar lavage . In contrast, weight loss due to dietary interventions and exercise reduces both asthma symptoms and inflammatory proteins in sputum, suggesting that obesity may be a marker for poor diet. Indeed, excessive intake of both dietary fat and reduced intake of antioxidant vitamins can modify airway function [9, 10]. Alternatively, the mechanical effects of obesity on operating lung volume  may increase airway closure  and expiratory flow limitation  during bronchoconstriction, and increase symptoms independently of any effect on eosinophilic airway inflammation and airway hyperresponsiveness . Although the mechanisms by which obesity alters asthma symptoms remain unknown, the study by Mitchell and colleagues  joins a growing body of evidence suggesting that we may have to look outside the airway inflammatory ‘square’.
The role of physical activity in the development and/or severity of asthma is much less clear and the results of Mitchell and colleagues  in this regard may only pose further questions. In their teenage population, any amount of ‘vigorous’ physical activity was associated with increased symptoms of asthma, rhinoconjunctivitis and eczema. This association was not apparent in the infant group. An adverse effect of vigorous physical activity on asthma symptoms is surprising since previous epidemiological evidence suggests that increased physical activity is associated with reduced risk of airway hyperresponsivenees . Indeed, a 10 weeks indoor rowing programme can reduce airway responsiveness for up to 6 weeks after completion of the programme in asthmatic  and non-asthmatic subjects . However, it may be that exercise is beneficial up to a certain intensity, after which it becomes deleterious to the lung. The prevalence of asthma and airway hyperresponsiveness is increased amongst elite athletes, particularly cross-country skiers and swimmers . Intense exercise may negatively affect lung function , such as the reduced elastic recoil at functional residual capacity observed in elite female rowers  and increased serum inflammatory cytokines in adolescent males following intense exercise . The question then is, should infants and teenagers be encouraged to exercise but discouraged from ‘vigorous’ exercise during years where lung development is still taking place? On the other hand, the authors point out that the association between physical activity and asthma may simply be confounded by the way the question about physical activity was framed.
Sedentary behaviour, or too much time spent sitting, is not simply the inverse of physical activity. Even in people who meet physical activity guidelines, increased sedentary time, particularly television-viewing time, is associated with increased metabolic risk . In the ISAAC study, increased television viewing (five or more hours) was associated with increased symptoms of all three diseases in the teenage population, whereas in the infant group television viewing was only associated with current wheeze and symptoms of severe asthma. Importantly, there was no interaction between either vigorous physical activity or television viewing and obesity. The magnitude of the risk was small, and the clinical relevance of the finding remains unclear. However, an association between asthma and TV viewing has been observed previously , and the mechanisms remain unclear. Sighs, or deep breaths, are a regular feature of the respiratory cycle, occurring approximately every 3–6 min . In normal healthy people, deep breaths both relieve existing bronchoconstriction and reduce the response to subsequent challenge, effects which are either absent or reduced in asthma . The frequency of sighs is reduced during television viewing  and one could speculate that an absence of these deep breaths, and their modulatory effect on bronchoconstriction, could lead to the development of asthma symptoms. On the other hand, it may not be the television viewing, per se, that is the problem but where that television viewing occurs. Dust mites, moulds and household pets are all common aeroallergens, and it is likely that increased television viewing simply translates to increased exposure to any or all of these allergens.
There are many good reasons for wanting to reduce the risks associated with a ‘fat and lazy’ lifestyle. The absence of significant interaction terms between obesity, physical activity and television viewing in the risk regressions of Mitchell and colleagues  does not preclude the possibility that these factors exist on the same causal pathway. Further analysis of these data is required to determine the extent to which obesity is attributable to a lazy lifestyle, which will, in turn, inform the design of interventions to reduce the burden of these diseases. Whether such interventions can stem the rising tide of asthma, rhinoconjunctivitis and eczema remains to be seen. Nevertheless, the observation by Mitchell and colleagues  that measures of excess – of weight, of television viewing or of vigorous exercise – were associated with increased risk of asthma, eczema and rhinoconjuctivitis, reminds us that, in the words of the eighteenth century poet Alexander Pope, ‘health consists with temperance alone’.
Conflict of interests: The authors declare no conflicts of interests.
- 1The association between BMI, vigorous physical activity and television viewing and the risk of symptoms of asthma, rhinoconjunctivitis and eczema in children and adolescents: ISAAC Phase Three. Clin Exper Allergy 2012; 43:73–84., , et al.