The global incidence of asthma among adolescents is estimated at 12.6%, with considerable variations depending on the region of between 5.1% and 22%. In Mexico, the incidence varies from 1.2% to 14.9%, with tropical regions being the ones presenting the largest percentage values.1 Many risk factors have been implicated in the onset of asthma, the most frequent being the family history of allergic illness.2–5 However, it is the variability of the incidence of asthma that appears to indicate the presence of non-genetic factors associated with its appearance. Among these factors are: the breakdown of the family structure, food intake at birth, childhood history of respiratory infection, the presence of humidity in the home, low weight at birth.6–10 Two additional factors which have attracted particular attention in recent years are disorders characterized by overweight and exposure to cigarette smoke.

Both excess weight and obesity are considered very serious problems on a global scale, and recently they have gained in prevalence in the developing world.11 The frequency of both problems has greatly increased. In the USA, the incidence of excess weight doubled among children, and tripled among adolescents, after a period of almost 20 years.12 In our country, more than one-third of the adolescents suffer from excess weight or obesity.13 Recently many studies have been published with a view to explaining the link between asthma and obesity, but so far there has been some discrepancy between the results obtained.14–17 The role played by active or passive exposure to cigarette smoke in the incidence of asthma, like that of obesity, also appears to be unconsciousness.2,3,18–22

The vast majority of the research studies aimed at determining the incidence of asthma have centered their attention on late adolescence. This is a stage of life of physical, physiological, social and psychological change, as adolescents increasingly acquire the characteristics of an adult; it is functional activities that cause asthma and its associated features to behave differently. The aims of this study are twofold; in the first place to determine the incidence of asthma and its symptoms in a sample population of late adolescents; secondly, to determine whether obesity and exposure to cigarette smoke and a family history of allergies are among the factors associated with it.

1600 adolescents were included in the study, ranging in age from 15 to 18, and the level of participation was 100%; 55.8% of the subjects were women. Table 1 shows the comparison of certain demographic aspects, the allergic antecedents and the smoking habit by gender. Approximately 44% of the subjects had a family member with some sort of allergic condition. A history of allergy in the mother was considerably more frequent in the female subjects than in the male subjects. 15.1% of the adolescents were active smokers.

Table 2 shows the incidence o allergic conditions, which in all cases was lower than 10%. The symptoms of asthma were significantly more frequent in the female than in the male subjects. In general, severe asthma was present in fewer than 2% of the subjects. When the frequency of allergic conditions was compared by gender, asthma was found to be predominant in males and atopic dermatitis significantly predominant in females. More than a quarter of the subjects were overweight, with the males presenting a greater frequency of overweight and obesity.

When the presence of symptoms of asthma during the previous year was measured against the nutritional status of the adolescents (Table 3), a significant association was found between the presence of overweight (P=.004) and weight excess with nocturnal coughing (P=.05), but in the case of obesity there was no such correlation.

The association of the incidence of asthma and other atopic illnesses with hereditary and exposure to cigarette smoke is shown in Table 4. In the case of asthma, both the history of allergic illness and asthma in the mother and the history of atopic disorders in the family were associated with it. Regarding exposure to tobacco smoke, the frequency of cases of adolescents with asthma confirmed by a doctor was not found to be associated with active smoking or with the fact of living with smokers.

Table 5 shows the different models of logistical regression for the incidence of asthma as a dependent variable. In the model which included excess weight, male sex and age, no significant association was found. The subsequent models, 2 and 3, made it possible to show that the male sex was significantly associated with asthma, but they also showed the maternal history of allergy to be an associated factor. A fourth model showed that the presence of asthma in the mother was associated with asthma. The final model was constructed from the covariables that had been identified previously, and shows that the maternal history of allergy, a personal history of allergic rhinitis and the male sex remained significantly associated with asthma.

This investigation shows that the incidence of asthma in late adolescence in the area in which the study took place is 7.8% (95% CI: 6.5%–9.1%). When this figure is compared with other parts of Mexico, we find variations, such as in the central region where it oscillates between 3.6% and 4.4%,4,24 while in the north east it reaches 6%.14 Recently the results were published corresponding to phase III of ISAAC, a study which included a population of 798685 adolescents, in which the global incidence of asthma was shown to be 12.6%, with geographical variations of between 5.1% and 22%, with the lowest frequencies being recorded for Africa, India and Eastern Europe, in sharp contrast with the higher figures for North America and Oceania. The highest incidences were recorded in English-speaking regions (19.9%). In the same study, the results for Mexico were between 1.2% and 14.9%, with the highest values being recorded for tropical regions.1 Other studies using different methods from ISAAC continue to show substantial differences in the incidence of asthma.2,20 In the case of the late adolescents assessed for our study, the presence of wheezing in the year preceding the study (7.3%) was close to the Mexican average (6.9%), but it was much lower than in the south east, where it varied between 13% and 14.9%.1 The risk factors involved in this case would appear to be the same as those for asthma. The risk factors invoked to explain the differences in incidences of asthma are numerous. Our study offers additional information from an often neglected group, concerning the relationship between asthma and some of these factors. With regard to the importance of gender in the evolution of asthma, it is known that during childhood asthma is more common in males than in females and in adulthood it is clearly predominant in women, but it is during early adolescence that the beginning of this reversal has been located25; in our population, asthma was common amongst men, with male gender remaining the single most significant factor associated with it. This same behavior has been documented in the United Arab Emirates,26 where the group of males aged between 13 and 19 was associated with the incidence of asthma, adjusted OR of 1.45 (95% CI 1.10–1.9, P=.008), in which the group of men aged between 13 and 19 presented a significantly greater incidence of asthma, adjusted OR of 1.45 (95% CI 1.10–1.90, P=.008). However, this did not occur with men over the age of 19, adjusted OR of 0.75 (95% CI 0.60–0.95, P=.02). On the opposite end of the scale, our results differ from those obtained in northeast Mexico in a study carried out with a similar age group, in which no significant correlation was found between asthma and gender OR of 1.02 (95% CI 0.80–1.30, P=NS).14 By means of a cross-sectional study carried out with adolescents of Argentinean origin aged between 15 and 18, no correlation was found either between gender and the incidence of asthma, OR of 1.16 (95% CI 0.94–1.45, P=.18).27 What we can deduce from this, at least as far as our region is concerned, is that the greater frequency of asthma among women than among men tends to be associated with the later stages of life. The factors that account for these differences have yet to be elucidated, but the role played by hormones would appear to be significant.

Among the factors constantly associated with asthma is the family history of allergic and atopic disorders.2–5 In this study we show that maternal history is strongly associated with asthma, and this was also found to be the case with allergic rhinitis. From the genetic point of view there are several gene polymorphisms which explain why heredity on the mother's side has a greater impact than on the father's in the development of asthma: the presence of polymorphisms in the maternal antioxidant genes (GSTP1 and GSTM1, GSTT1) and the high affinity IgE receptor beta chains (FceRI-beta), as well as in the atopic alleles located in the 11q13 chromosome.28 The effect of family atopic disorders could not be evaluated in our study, so for the time being we are unable to say what role this factor plays in the development of asthma.

The harmful effects of exposure to active or passive smoking on a person's health have been documented on numerous occasions; but the role that it plays in the incidence of asthma is a matter of debate. In our study, one-third of the adolescent subjects lived with parents who smoked, but what was even more important was the high proportion of adolescents who were smokers themselves. Nevertheless, we show in this study that in no case was the incidence of smoking related to exposure to tobacco smoke; previous research has produced similar findings.2,18–20

Conversely, a multicentric study carried out with several cohorts of children, whose development was tracked from pregnancy to the age of 4 years old, found a significant correlation between postnatal exposure to tobacco smoke and an asthma diagnosis, OR of 1.69 (95% CI, 1.01–2.82)21; a similar link was found between the presence of more than one smoker at home and the development of wheezing during the 12 months prior to the study, OR of 1.27 (95% CI, 1.01–1.59).3 Hawkins and Berkman22 also found that the presence of a member of the family who smoked increased the likelihood of the children developing asthma.

It would seem that exposure to tobacco smoke is related with the intensity of the symptoms and the number of aggravations of the asthma, as was demonstrated by Mannino et al.,29 through the Third National Health and Nutrition Examination Survey, who showed that children exposed to large amounts of tobacco smoke were more likely to have moderate to severe asthma (OR 2.7, 95% CI, 1.1–1.68), and that they also suffered from a more significant diminishing of the lung function. On account of their consequences, disorders characterized by excess weight (overweight and obesity) are the most serious health problems in different parts of the world.11 In Mexico, the National Survey of Health and Nutrition (ENSANUT 2006) showed that more than 30% of adolescents (12–19 years of age) were either overweight or obese.13 In our sample population 25% presented excess weight.

These high rates of overweight or obesity, as observed in our population, led us to consider the possibility of a link between this factor and the incidence of asthma, but our results did not indicate that there was any such link. The information provided in this respect has thrown up conflicting results. In our country alone, Vázquez-Nava et al.14 in a sample of late adolescents found a link between asthma and obesity, OR of 1.47 (95% CI, 1.08–1.99; P=.01), but not between asthma and overweight, OR of 0.99 (95% CI, 0.69–1.42; P=non-significant). These same researchers, this time with children of 4–5 years old, found that asthma was associated neither with obesity (OR of 1.47; 95% CI: 1.16–0.64; to 2.08), nor with overweight (OR of 1.58; 95% CI: 0.78–3.19).15 In our opinion, the causes of this inconsistency lie in the way in which the studies are designed, the age group studied, the way in which the weight and height of the participants are measured, the cut-off points used to classify overweight and obesity, among other factors. One area in which a stronger link has been demonstrated between asthma and obesity is the severity of the asthmatic condition in the obese, a more severe intensity of the condition having been observed in obese sufferers.30

This study suffers from certain limitations. One of them has to do with the lack of confirmation of an asthma diagnosis by more objective means, such as clinical history or the performance of respiratory function tests. Another is recall bias, a condition which is very common in epidemiological studies based on questionnaires. This might explain, to a large extent, why such a low rate of wheezing has been observed among these subjects. Although the sample of adolescents surveyed only took into account those who come to the city of Guadalajara to study, we believe that the behavior of the variables studied are a clear reflection of the reality of this age group as a whole, as the rate of participation was high; a range of different administrative and geographical districts of the city were included, and it covered schools in both the public and private sector.

As no previous results were available to us in relation to the epidemiological behavior of asthma in late adolescents, we find ourselves unable, for the time being, to evaluate the general tendency of one of the most common respiratory disorders. Our results will therefore serve as a departure point for these tendencies to be determined. With regard to the risk factors studied in this paper, the maternal history of asthma, the presence of allergic rhinitis, and being male were all significantly associated with the incidence of asthma.

The authors have no conflict of interest to declare.