Abstract

The authors estimated the association between asthma and body mass index in a 1963–2002 study of 135,000 Norwegians aged 14–60 years who were followed on average for 21 years. Cox proportional hazards regression models were fitted to estimate the relative risk of asthma adjusting for smoking, education, and physical activity. Compared with persons with a body mass index (weight (kg)/height (m)2) of less than 25, overweight (body mass index: 25–29) men and women had relative risks of asthma of 1.27 (95% confidence interval (CI): 1.13, 1.43) and 1.30 (95% CI: 1.17, 1.45), respectively, while obese (body mass index: ≥30) men and women had relative risks of 1.78 (95% CI: 1.35, 2.34) and 1.99 (95% CI: 1.67, 2.37), respectively. Stratified analyses revealed a similar association between body mass index and asthma for never smokers, ever smokers, persons with less than or equal to 12 years of education, and persons with more than 12 years of education. Analyses including all the covariates gave results similar to those not adjusting for these factors. The risk of asthma increased steadily with body mass index, from a body mass index of 20 in men and of 22 in women. In men, the risk of asthma increased by 10% with each unit of increased body mass index between 25 and 30. The similar value for women was 7%. Overweight or obese persons reported asthma more often than did thinner persons after adjustment for smoking, education, and physical activity.

Received for publication January 14, 2004; accepted for publication June 3, 2004.

Studies have suggested that obesity is a risk factor for adult asthma (15). Most of these studies have been cross-sectional. However, having asthma may lead to a sedentary lifestyle with increased risk of developing obesity. Associations between obesity and asthma should thus be addressed in longitudinal studies ensuring that obesity develops before asthma. Few studies have explored the relation between body weight and adult asthma prospectively (2, 3, 6). The follow-up periods have been short, information on important covariates and/or potential confounders has been inadequate, and measures of height and weight have been mainly self-reported. Some studies have found a relation between asthma and obesity in females but not in males (2, 3). The association between body mass index and asthma should be explored further in studies with a longitudinal design and with sufficient statistical power to ascertain potential confounding.

During the 1960s and 1970s, a large proportion of the Norwegian population took part in health surveys (7, 8). These surveys included measures of height and weight. A total of 135,000 of these participants have also taken part in more recent health surveys, which also have collected information on asthma. Linking information from the different surveys gave us the opportunity to explore the association between body mass index and adult asthma prospectively in a large cohort of Norwegians.

MATERIALS AND METHODS

Between 1963 and 1999, 1.8 million Norwegians aged 14–59 years participated in different population-based, government-run health surveys that included height and weight measurements. Participation rates varied between 75 percent and 90 percent. Since 1994, health surveys have also collected information on asthma and, at the end of 2002, a total of 320,333 persons had participated in these latter surveys. The combined cohort has been described in more detail elsewhere (9, 10). Briefly, a major part of the health surveys in the period 1963–1975 was included in a nationwide screening program aimed at detecting tuberculosis (8). This screening was compulsory for all Norwegians aged 15 years and above. The main objective of most of the other studies was to assess risk factors for coronary heart disease, but items of other exposures and health outcomes were also included (7, 11).

The current study population includes participants from the health surveys with information on asthma who also have had their height and weight measured in a previous population-based, government-run health survey. Inclusion criteria were as follows: 1) answered questions on asthma in 1994–2002 before the age of 80 years, 2) height and weight measured in a health survey prior to the survey where they answered questions on asthma, 3) height and weight measured at ages 14–59 years, and 4) height and weight measured before the development of asthma.

Altogether, 135,405 persons fulfilled the inclusion criteria. These persons were first measured, and then they participated in a health survey and answered questions on asthma. A subcohort of 30,799 persons also answered a question about their usual leisure-time activity. The observation time in the study was defined as the time from the date of measurement of height and weight until the presence of asthma, age 60 years, or the date of answering questions on asthma.

Variables

Asthma

Information on asthma and the onset of asthma in this study was based on the following questions in a self-administered questionnaire: 1) Have you ever had asthma? 2) If yes, at what age did you get asthma for the first time?

Body mass index

Height and weight were measured in a standardized way by a trained staff. Height was measured without shoes, and weight was measured with the subject wearing light clothing. About 3.5 percent of the measurements were excluded because of irregularities (persons wearing shoes, being pregnant, being disabled, and so on).

Height was categorized according to percentiles in a US reference population (12, 13). For ages 14–20 years, the categories were as follows: less than 25th(low), 25th–74th(medium), 75th–94th (high), and greater than or equal to 95th(very high). Above the age of 20 years, the categories were defined by the boundaries defined by the above percentiles at age 20 years.

Body mass index (weight (kg)/height (m)2) was categorized into less than 25.0 (normal weight), 25.0–29.9 (overweight), and 30.0 or more (obese). For ages 14–18 years, growth curves linked to the same adult body mass index categories were used (14).

Covariates

The study subjects were divided into never smokers and ever smokers and into those having 12 years of education or less and those having 13 or more years of education. This information was collected when the persons answered questions on asthma. The study subjects were divided by age at measurement (14–16, 17–19, 20–24, ..., 45–49, and 50–59 years) and year of birth (1910–1919, 1920–1929, ..., 1950–1959, ≥1960).

A question with four alternatives was used to describe level of physical activity: sedentary (reading, watching television, or other sedentary activity); moderate (walking, bicycling, or moving around in other ways at least 4 hours a week); intermediate (participating in recreational athletics, heavy garden work, and so on, at least 4 hours a week); and intensive (participating in hard training or athletic competitions, regularly and several times a week) (15).

Data linkage

As a result of the unique 11-digit identification number assigned to all individuals living in Norway, it was possible to link the height and weight measurements with the person’s answers on questions about asthma.

Statistical analysis

Cox proportional hazards regression models with time since measurement as the time variable were fitted to estimate the hazard rate ratio, hereafter denoted relative risk, of asthma. These analyses were performed with the SPSS statistical program (16). The results were presented as the relative risk of asthma with 95 percent confidence intervals.

The hazard function of asthma by body mass index in the Cox model was estimated by using penalized spline functions in S-PLUS statistical software (17) with 6 df. Body mass index values from ages less than 18 years were transferred to predicted body mass index values at age 18 years using growth curves (14).

RESULTS

A total of 135,405 persons aged 14–59 years (mean age: 28 years) at height and weight measurements were included in the present study (table 1). The participants were followed for an average of 21 years (range: 0–39 years) constituting 2,802,227 person-years, and they answered questions on asthma at a mean age of 52 years. A total of 4,218 persons developed asthma at age 15–60 years (mean age at onset: 41 years; mean time from measurements of height and weight to the onset of asthma: 15 years).

The proportion of overweight or obese (body mass index: ≥25) individuals at the start of the observation time was 26 percent and 21 percent in men and women, respectively. Compared with persons with a body mass index below 25, overweight men and women had relative risks of asthma of 1.27 (95 percent confidence interval (CI): 1.13, 1.43) and 1.30 (95 percent CI: 1.17, 1.45), respectively, while obese men and women had relative risks of 1.78 (95 percent CI: 1.35, 2.34) and 1.99 (95 percent CI: 1.67, 2.37), respectively (table 2). By including height in the analyses, we found that there was a somewhat higher risk of asthma in the shortest persons.

Stratified analyses revealed a similar association between body mass index and asthma for never smokers and ever smokers (table 3). For persons with 12 years of education or less and persons with more than 12 years of education, a similar association was observed. There were fewer persons with information on physical activity. In analyses stratified by level of physical activity, overweight and obese persons tended to have a higher risk of asthma in all strata except among men with an intensive physical activity level.

Since the date of diagnosis was uncertain and a diagnosis of asthma might influence body mass index, analyses were also performed with start of follow-up 5 years after measurement of body mass index. The results were similar (data not shown). Furthermore, analyses were performed to reveal possible changes due to diagnostic changes or increased awareness of asthma over time. Inclusion of only those who were at least 18 years at measurement or inclusion of only persons born before 1950 also gave very similar results (data not shown).

To deal with possible misclassification of chronic obstructive pulmonary disease, we also performed analyses including only persons aged 40 years or less, and we did stratified analyses on smokers and nonsmokers, which produced similar results.

Finally, an analysis including physical activity, education, and smoking in addition to body mass index gave very similar results as the analysis not adjusting for these factors regarding the association between body mass index and asthma (table 4). This final analysis included 17,619 men and 12,408 women with complete information. Of these, 385 men and 436 women developed asthma at ages 15–60 years (mean age at onset: 47 years).

The association between body mass index and asthma was also explored in detail using penalized spline functions (figures 1 and 2). The incidence of asthma increased steadily with body mass index after a body mass index of 20 in men and of 22 in women. Furthermore, in men, the risk of asthma increased by 10 percent (95 percent CI: 2, 18) with each unit of increased body mass index between 25 and 30. The similar figure for women was 7 percent (95 percent CI: 0, 14).

The oldest persons with asthma may have a higher risk of dying compared with persons without. To indicate the impact of this, we performed an analysis including only persons who were less than 60 years of age while answering questions on asthma. Compared with the results in table 2, the results on the association between body mass index and asthma were somewhat strengthened; the overweight persons had a relative risk of 1.39 (95 percent CI: 1.18, 1.63) and 1.43 (95 percent CI: 1.26, 1.62) in men and women, respectively. The corresponding values for obese persons were 1.88 (95 percent CI: 1.28, 2.76) and 2.21 (95 percent CI: 1.75, 2.80) in men and women, respectively. Furthermore, cardiovascular disease could also cause symptoms suggestive of bronchial asthma (18). Stratified analysis including only persons without any cardiovascular disease and diabetes (n = 107,000) revealed, however, similar results (data not shown).

DISCUSSION

In this longitudinal study of 135,000 Norwegian men and women, we found that overweight and obese persons had an increased risk of adult asthma. This finding was consistent across different strata of smoking, education, and physical activity. In contrast to other studies, our study showed similar results for men and women. The risk of asthma increased steadily from a body mass index of 20 in men and of 22 in women.

Obesity has many negative health consequences (19). Our findings support the hypothesis that obesity is a risk factor for adult asthma. Few prospective studies have explored the association between adult asthma and obesity, adjusting for potential confounders (1, 3, 6). The first adult study with a longitudinal perspective was the US Nurses’ Health Study (20). That study, including women aged 24–46 years with self-reported body mass index with 4 years of follow-up, found a positive association between body mass index and asthma. We found an increased risk in both sexes. In contrast, Beckett et al. (1) found that gain in body mass index increased the risk of asthma in females but not in males. However, as underlined by Chinn (20) in a recent review, the separate analyses for men and women were not shown. Chen et al. (3) reported that obesity increased the risk of asthma in women but not in men. That study was also based on self-reported body mass index in a 2-year follow-up.

Our study is not a regular cohort study. We have, however, measurements of body mass index among persons taking part in health screenings with a diagnosis of asthma after measurement of height and weight. Consequently, we have a longitudinal study, where the exposure variable is measured independently of the self-report of asthma. We thus have included individuals in the present study from the date of body mass index measurement and followed them until they reported asthma, including the age at onset, or until our defined end of follow-up.

One important dimension of our study is that the study is based on standardized measures of body mass index. The validity of self-reported height and weight has been shown to vary systematically with body mass index (21, 22). Furthermore, our questions on asthma have been used in other Norwegian population-based studies of asthma in children and adults (23). Questionnaire-based information on asthma and physician diagnosis of asthma in Norwegian population-based studies of asthma tend to give similar results (24).

One important issue in our study is whether the self-reported age at onset of asthma might be uncertain. We thus performed additional analyses with the start of follow-up 5 years after the body mass index measurements, which did not influence the results. Furthermore, almost 500 individuals answered the health survey twice (most of these after the measurements of height and weight), including age at onset. In 24 percent of the asthma cases, there was no difference in reported age at onset and, in 45 percent of the cases, age at onset varied by less than 5 years. A concern might also be if there was a misclassification of chronic obstructive pulmonary disease. Analyses restricted to persons less than 40 years of age and among smokers and nonsmokers, however, offer support that the findings in the present study are not biased by misclassification of chronic obstructive pulmonary disease.

Another aspect is whether adults with new onset of asthma actually are incident asthma rather than cases of the disease that has relapsed after a symptom-free period. Adults with childhood asthma, with symptom-free periods during adolescence, may have an increased risk to develop obesity-associated asthma later in life compared with adults without childhood asthma because of a sedentary lifestyle. We cannot exclude that this might have influenced our results.

The prevalence of asthma is low in our study because we include only incident asthma cases, which do not represent the lifetime prevalence of asthma in the general population. The strengths of our study are the large number of persons included and recruited from the general population and the long observation period.

Information on education and smoking from the last survey was used, that is, after an eventual diagnosis of asthma. However, since the mean age at diagnosis was 41 years and 90 percent of the cases were at least 30 years at onset, education was probably completed, and those smoking had probably started smoking prior to onset. Additional analyses including only persons aged less than 60 years also suggested that the association between body mass index and asthma in our study might be stronger than in the main analysis in the present study.

Our analyses applying a spline function revealed that using the standardized categories of body mass index in relation to the definition of overweight (body mass index: ≥25) and obesity (body mass index: ≥30) is not necessarily the optimal way of exploring the association between asthma and body mass index. Furthermore, we show how the risk of asthma increases in the population with each unit of increased body mass index. The associations seen in this study add insight to the relation between obesity and asthma.

The causal links between asthma and obesity need to be further examined with a longitudinal design, including secondary asthma phenotypes and the life course of obesity. However, the present study found that overweight or obese men and women report asthma more often than do thinner persons, even after adjustment from smoking, education, and physical activity.

ACKNOWLEDGMENTS

The authors are grateful to those who during almost 40 years have collected the data used in the present study. These are persons connected to the former National Health Screening Service, the Nord-Trøndelag Health Survey (HUNT), the Hordaland Health Survey (HUSK), and the Tromsø Study.

Correspondence to Dr. Wenche Nystad, Division of Epidemiology, Norwegian Institute of Public Health, P.O. Box 4404 Nydalen, NO-0403 Oslo, Norway (e-mail: wenche.nystad@fhi.no).

FIGURE 1. Logarithm of the relative risk of asthma by body mass index (BMI) for men, using a Cox model estimated with a penalized spline function in S-PLUS statistical software, with 6 df, adjusted for birth year and age at measurement, Norway, 1963–2002.

FIGURE 1. Logarithm of the relative risk of asthma by body mass index (BMI) for men, using a Cox model estimated with a penalized spline function in S-PLUS statistical software, with 6 df, adjusted for birth year and age at measurement, Norway, 1963–2002.

FIGURE 2. Logarithm of the relative risk of asthma by body mass index (BMI) for women, using a Cox model estimated with a penalized spline function in S-PLUS statistical software, with 6 df, adjusted for birth year and age at measurement, Norway, 1963–2002.

FIGURE 2. Logarithm of the relative risk of asthma by body mass index (BMI) for women, using a Cox model estimated with a penalized spline function in S-PLUS statistical software, with 6 df, adjusted for birth year and age at measurement, Norway, 1963–2002.

TABLE 1.

Number of observed person-years and cases of asthma in a study of the association of body mass index and asthma, Norway, 1963–2002

Variable Men  Women 
No. No. of asthma cases No. of person-years Rate  No. No. of asthma cases No. of person-years Rate 
Year of birth          
1914–1929 12,155 269 181,397 148  10,359 212 162,929 130 
1930–1949 20,593 659 468,369 141  19,405 982 471,315 208 
≥1950 33,975 704 711,460 99  38,918 1,392 806,758 173 
Age (years) at measurement          
14–19 23,970 565 617,331 92  26,719 1,100 684,519 161 
20–29 14,367 356 303,081 117  17,350 768 369,131 208 
30–39 11,267 334 221,519 151  11,740 453 233,291 194 
40–49 15,594 350 208,598 168  11,474 244 143,983 169 
50–59 1,525 27 10,696 252  1,399 21 10,078 208 
Education (years)*          
≤12 46,535 1,175 953,514 123  50,857 1,991 1,075,796 185 
≥13 18,635 418 381,221 110  15,830 533 329,431 162 
Smoking*          
Not answered 887 13 16,866 77  1,899 42 36,908 114 
Never smokers 21,103 447 431,869 104  26,940 782 553,964 141 
Ever smokers 44,733 1,172 912,490 128  39,843 1,762 850,130 207 
Height†          
Low 15,092 459 315,119 146  11,539 527 244,764 215 
Medium 34,501 792 706,116 112  37,591 1,389 791,042 176 
High 14,302 314 284,640 110  15,858 533 331,810 161 
Very high 2,828 67 55,351 121  3,694 137 73,386 187 
Body mass index (kg/m2)‡          
<25 49,442 1,137 1,067,524 107  54,183 1,983 1,177,599 168 
25–29 15,685 439 269,730 163  11,653 457 217,046 211 
≥30 1,596 56 23,972 234  2,846 146 46,357 315 
Physical activity          
Sedentary 3,465 75 45,511 165  2,764 124 34,502 359 
Moderate 9,662 220 131,019 168  8,726 281 105,642 266 
Intermediate 4,145 86 56,430 152  1,132 44 12,805 344 
Intensive 774 13 11,311 115  131 1,419 141 
Age (years) at onset          
Not answered 65,091 1,337,996   66,096 1,401,447  
<20 24 24 31   48 48 85  
20–39 597 597 7,333   1,194 1,194 15,072  
40–59 1,011 1,011 15,865   1,344 1,344 24,398  
Total 66,723 1,632 1,361,225 120  68,682 2,586 1,441,002 179 
Variable Men  Women 
No. No. of asthma cases No. of person-years Rate  No. No. of asthma cases No. of person-years Rate 
Year of birth          
1914–1929 12,155 269 181,397 148  10,359 212 162,929 130 
1930–1949 20,593 659 468,369 141  19,405 982 471,315 208 
≥1950 33,975 704 711,460 99  38,918 1,392 806,758 173 
Age (years) at measurement          
14–19 23,970 565 617,331 92  26,719 1,100 684,519 161 
20–29 14,367 356 303,081 117  17,350 768 369,131 208 
30–39 11,267 334 221,519 151  11,740 453 233,291 194 
40–49 15,594 350 208,598 168  11,474 244 143,983 169 
50–59 1,525 27 10,696 252  1,399 21 10,078 208 
Education (years)*          
≤12 46,535 1,175 953,514 123  50,857 1,991 1,075,796 185 
≥13 18,635 418 381,221 110  15,830 533 329,431 162 
Smoking*          
Not answered 887 13 16,866 77  1,899 42 36,908 114 
Never smokers 21,103 447 431,869 104  26,940 782 553,964 141 
Ever smokers 44,733 1,172 912,490 128  39,843 1,762 850,130 207 
Height†          
Low 15,092 459 315,119 146  11,539 527 244,764 215 
Medium 34,501 792 706,116 112  37,591 1,389 791,042 176 
High 14,302 314 284,640 110  15,858 533 331,810 161 
Very high 2,828 67 55,351 121  3,694 137 73,386 187 
Body mass index (kg/m2)‡          
<25 49,442 1,137 1,067,524 107  54,183 1,983 1,177,599 168 
25–29 15,685 439 269,730 163  11,653 457 217,046 211 
≥30 1,596 56 23,972 234  2,846 146 46,357 315 
Physical activity          
Sedentary 3,465 75 45,511 165  2,764 124 34,502 359 
Moderate 9,662 220 131,019 168  8,726 281 105,642 266 
Intermediate 4,145 86 56,430 152  1,132 44 12,805 344 
Intensive 774 13 11,311 115  131 1,419 141 
Age (years) at onset          
Not answered 65,091 1,337,996   66,096 1,401,447  
<20 24 24 31   48 48 85  
20–39 597 597 7,333   1,194 1,194 15,072  
40–59 1,011 1,011 15,865   1,344 1,344 24,398  
Total 66,723 1,632 1,361,225 120  68,682 2,586 1,441,002 179 

* Information collected at or after end of follow-up.

† Defined by percentiles in a US reference population (http://www.cdc.gov/growthcharts/) (13): low (<25th), medium (25th–74th), high (75th–94th), and very high (≥95th).

‡ Defined by percentiles in an international reference population (T. J. Cole et al. BMJ 2000;320:1240–3) (14).

TABLE 2.

Relative risk of asthma with 95% confidence intervals obtained from a Cox regression analysis including age at measurement, year of birth, and either body mass index or height, Norway, 1963–2002

Variable Men  Women 
No. of asthma cases Relative risk 95% confidence interval  No. of asthma cases Relative risk 95% confidence interval 
Body mass index (kg/m2       
<25 1,137 1.00 Referent  1,983 1.00 Referent 
25–29 439 1.27 1.13, 1.43   457 1.30 1.17, 1.45 
≥30 56 1.78 1.35, 2.34   146 1.99 1.67, 2.37 
Height*        
Low 459 1.28 1.14, 1.44   527 1.23 1.12, 1.37 
Medium 792 1.00 Referent  1,389 1.00 Referent 
High 314 0.99 0.87, 1.13   533 0.91 0.82, 1.00 
Very high 67 1.11 0.87, 1.43   137 1.04 0.87, 1.24 
Variable Men  Women 
No. of asthma cases Relative risk 95% confidence interval  No. of asthma cases Relative risk 95% confidence interval 
Body mass index (kg/m2       
<25 1,137 1.00 Referent  1,983 1.00 Referent 
25–29 439 1.27 1.13, 1.43   457 1.30 1.17, 1.45 
≥30 56 1.78 1.35, 2.34   146 1.99 1.67, 2.37 
Height*        
Low 459 1.28 1.14, 1.44   527 1.23 1.12, 1.37 
Medium 792 1.00 Referent  1,389 1.00 Referent 
High 314 0.99 0.87, 1.13   533 0.91 0.82, 1.00 
Very high 67 1.11 0.87, 1.43   137 1.04 0.87, 1.24 

* Defined by percentiles in a US reference population (http://www.cdc.gov/growthcharts/) (13): low (<25th), medium (25th–74th), high (75th–94th), and very high (≥95th).

TABLE 3.

Relative risk of asthma with 95% confidence intervals obtained from a Cox regression analysis including age at measurement, year of birth, and body mass index in subgroups, Norway, 1963–2002

Variable by body mass index (kg/m2Men  Women 
Relative risk 95% confidence interval  Relative risk 95% confidence interval 
Never smokers      
<25 1.00 Referent  1.00 Referent 
25–29 1.55 1.22, 1.97  1.31 1.08, 1.59 
≥30 1.61 0.85, 3.05  2.21 1.66, 2.95 
Ever smokers      
<25 1.00 Referent  1.00 Referent 
25–29 1.22 1.06, 1.40  1.34 1.18, 1.52 
≥30 1.84 1.36, 2.49  1.92 1.53, 2.41 
Persons with 12 years of education or less      
<25 1.00 Referent  1.00 Referent 
25–29 1.23 1.07, 1.41  1.30 1.16, 1.47 
≥30 1.47 1.06, 2.04  1.94 1.61, 2.35 
Persons with more than 12 years of education      
<25 1.00 Referent  1.00 Referent 
25–29 1.32 1.02, 1.71  1.15 0.86, 1.53 
≥30 2.50 1.36, 4.62  2.18 1.22, 3.92 
Persons with sedentary physical activity      
<25 1.00 Referent  1.00 Referent 
25–29 1.12 0.69, 1.82  1.19 0.76, 1.87 
≥30 1.43 0.60, 3.41  2.13 1.27, 3.58 
Persons with moderate physical activity      
<25 1.00 Referent  1.00 Referent 
25–29 1.66 1.26, 2.20  1.22 0.91, 1.62 
≥30 2.70 1.61, 4.53  1.62 1.03, 2.54 
Persons with intermediate or intensive physical activity      
<25 1.00 Referent  1.00 Referent 
25–29 1.30 0.86, 1.97  2.37 1.20, 4.68 
≥30 1.00 0.24, 4.11  3.45 1.03, 11.5 
Variable by body mass index (kg/m2Men  Women 
Relative risk 95% confidence interval  Relative risk 95% confidence interval 
Never smokers      
<25 1.00 Referent  1.00 Referent 
25–29 1.55 1.22, 1.97  1.31 1.08, 1.59 
≥30 1.61 0.85, 3.05  2.21 1.66, 2.95 
Ever smokers      
<25 1.00 Referent  1.00 Referent 
25–29 1.22 1.06, 1.40  1.34 1.18, 1.52 
≥30 1.84 1.36, 2.49  1.92 1.53, 2.41 
Persons with 12 years of education or less      
<25 1.00 Referent  1.00 Referent 
25–29 1.23 1.07, 1.41  1.30 1.16, 1.47 
≥30 1.47 1.06, 2.04  1.94 1.61, 2.35 
Persons with more than 12 years of education      
<25 1.00 Referent  1.00 Referent 
25–29 1.32 1.02, 1.71  1.15 0.86, 1.53 
≥30 2.50 1.36, 4.62  2.18 1.22, 3.92 
Persons with sedentary physical activity      
<25 1.00 Referent  1.00 Referent 
25–29 1.12 0.69, 1.82  1.19 0.76, 1.87 
≥30 1.43 0.60, 3.41  2.13 1.27, 3.58 
Persons with moderate physical activity      
<25 1.00 Referent  1.00 Referent 
25–29 1.66 1.26, 2.20  1.22 0.91, 1.62 
≥30 2.70 1.61, 4.53  1.62 1.03, 2.54 
Persons with intermediate or intensive physical activity      
<25 1.00 Referent  1.00 Referent 
25–29 1.30 0.86, 1.97  2.37 1.20, 4.68 
≥30 1.00 0.24, 4.11  3.45 1.03, 11.5 
TABLE 4.

Relative risk of asthma with 95 percent confidence intervals obtained from a Cox regression analysis adjusting for age at measurement and year of birth in addition to the variables listed below, Norway, 1963–2002

Variable Men (n = 17,619)  Women (n = 12,408) 
No. of asthma cases Relative risk 95% confidence interval  No. of asthma cases Relative risk 95% confidence interval 
Body mass index (kg/m2       
<25 204 1.00 Referent  297 1.00 Referent 
25–29 160 1.45 1.17, 1.79  96 1.28 1.02, 1.62 
≥30 21 1.76 1.11, 2.77  43 1.99 1.43, 2.77 
Physical activity        
Sedentary 73 0.94 0.72, 1.23  118 1.26 1.02, 1.57 
Moderate 215 1.00 Referent  275 1.00 Referent 
Intermediate or intensive 97 0.92 0.72, 1.18  43 1.25 0.90, 1.73 
Education (years)*        
≤12 271 1.00 Referent  345 1.00 Referent 
>12 114 0.90 0.72, 1.12  91 0.95 0.75, 1.21 
Smoking*        
Never smokers 83 1.00 Referent  124 1.00 Referent 
Ever smokers 302 1.44 1.12, 1.84  312 1.56 1.27, 1.93 
Variable Men (n = 17,619)  Women (n = 12,408) 
No. of asthma cases Relative risk 95% confidence interval  No. of asthma cases Relative risk 95% confidence interval 
Body mass index (kg/m2       
<25 204 1.00 Referent  297 1.00 Referent 
25–29 160 1.45 1.17, 1.79  96 1.28 1.02, 1.62 
≥30 21 1.76 1.11, 2.77  43 1.99 1.43, 2.77 
Physical activity        
Sedentary 73 0.94 0.72, 1.23  118 1.26 1.02, 1.57 
Moderate 215 1.00 Referent  275 1.00 Referent 
Intermediate or intensive 97 0.92 0.72, 1.18  43 1.25 0.90, 1.73 
Education (years)*        
≤12 271 1.00 Referent  345 1.00 Referent 
>12 114 0.90 0.72, 1.12  91 0.95 0.75, 1.21 
Smoking*        
Never smokers 83 1.00 Referent  124 1.00 Referent 
Ever smokers 302 1.44 1.12, 1.84  312 1.56 1.27, 1.93 

* Information collected at or after end of follow-up.

References

1.
Beckett WS, Jacobs DR Jr, Yu X, et al. Asthma is associated with weight gain in females but not males, independent of physical activity.
Am J Respir Crit Care Med
 
2001
;
164
:
2045
–50.
2.
Celedon JC, Palmer LJ, Litonjua AA, et al. Body mass index and asthma in adults in families of subjects with asthma in Anqing, China.
Am J Respir Crit Care Med
 
2001
;
164
:
1835
–40.
3.
Chen Y, Dales R, Tang M, et al. Obesity may increase the incidence of asthma in women but not in men: longitudinal observations from the Canadian National Population Health Surveys.
Am J Epidemiol
 
2002
;
155
:
191
–7.
4.
Jarvis D, Chinn S, Potts J, et al. Association of body mass index with respiratory symptoms and atopy: results from the European Community Respiratory Health Survey.
Clin Exp Allergy
 
2002
;
32
:
831
–7.
5.
Xu B, Pekkanen J, Laitinen J, et al. Body build from birth to adulthood and risk of asthma.
Eur J Public Health
 
2002
;
12
:
166
–70.
6.
Camargo CA Jr, Weiss ST, Zhang S, et al. Prospective study of body mass index, weight change, and risk of adult-onset asthma in women.
Arch Intern Med
 
1999
;
159
:
2582
–8.
7.
Bjartveit K. The National Health Screening Service: from fight against tuberculosis to many-sided epidemiological activities. (In Norwegian).
Nor Epidemiol
 
1997
;
7
:
157
–74.
8.
Waaler HT. Height, weight and mortality. The Norwegian experience.
Acta Med Scand Suppl
 
1984
;
679
:
1
–56.
9.
Engeland A, Bjørge T, Søgaard AJ, et al. Body mass index in adolescence in relation to total mortality: 32-year follow-up of 227,000 Norwegian boys and girls.
Am J Epidemiol
 
2003
;
15
:
517
–23.
10.
Engeland A, Bjørge T, Selmer RM, et al. Height and body mass index in relation to total mortality.
Epidemiology
 
2003
;
14
:
293
–9.
11.
Bjartveit K, Foss OP, Gjervig T, et al. The cardiovascular disease study in Norwegian counties. Background and organization.
Acta Med Scand Suppl
 
1979
;
634
:
1
–70.
12.
Kuczmarski RJ, Ogden CL, Grummer-Strawn LM, et al. CDC growth charts: United States.
Adv Data
 
2000
;
(314)
:
1
–27.
13.
National Center for Health Statistics. 2000 CDC growth charts: United States. Hyattsvile, MD: National Center for Health Statistics, 2002. (http://www.cdc.gov/growthcharts/). (Accessed August 12, 2002).
14.
Cole TJ, Bellizzi MC, Flegal KM, et al. Establishing a standard definition for child overweight and obesity worldwide: international survey.
BMJ
 
2000
;
320
:
1240
–3.
15.
Meyer HE, Søgaard AJ, Tverdal A, et al. Body mass index and mortality: the influence of physical activity and smoking.
Med Sci Sports Exerc
 
2002
;
34
:
1065
–70.
16.
SPSS for Windows, release 11.0.1. Chicago, IL: SPSS, Inc, 2001.
17.
S-PLUS 6.1 for Windows. Seattle, WA: Insightful Corporation, 2002.
18.
Kiss D, Veegh W, Schragel D, et al. Bronchial asthma causing symptoms suggestive of angina pectoris.
Eur Respir J
 
2003
;
21
:
473
–7.
19.
Must A, Strauss RS. Risks and consequences of childhood and adolescent obesity.
Int J Obes Relat Metab Disord
 
1999
;
23(suppl 2)
:
S2
–11.
20.
Chinn S. Obesity and asthma: evidence for and against a causal relation.
J Asthma
 
2003
;
40
:
1
–16.
21.
Nawaz H, Chan W, Abdulrahman M, et al. Self-reported weight and height: implications for obesity research.
Am J Prev Med
 
2001
;
20
:
294
–8.
22.
Plankey MW, Stevens J, Flegal KM, et al. Prediction equations do not eliminate systematic error in self-reported body mass index.
Obes Res
 
1997
;
5
:
308
–14.
23.
Harris JR, Magnus P, Samuelsen SO, et al. No evidence for effects of family environment on asthma. A retrospective study of Norwegian twins.
Am J Respir Crit Care Med
 
1997
;
156
:
43
–9.
24.
Nystad W, Magnus P, Søyseth V. Occurrence of asthma among school children in Norway during the period 1985–94. (In Norwegian).
Tidsskr Nor Laegeforen
 
1997
;
117
:
644
–7.