Body Mass Index and Asthma Among Low-Birth-Weight Children
Body Mass Index and Asthma Among Low-Birth-Weight Children
The primary measures were concentrated around expected norms. Birth weights for the majority of subjects were between 3,000 and 4,000 g with appropriate gestational ages (37–40 weeks) and a healthy current body mass index (17–20 kg/m). Birth weights less than 3,000 g were more common in girls (22.9%) than boys (16.9%). SGA was also more likely among girls (11.5% vs. 9.5%). When overweight and obese students were combined, the distribution between genders was similar: 35.7% for boys and 35.5% for girls.
Parental education and environmental tobacco smoke exposure were similar between genders. For instance, in the largest educational category, 36.1% of boys and 36.8% of girls were raised in households where the parents had 6 or fewer years of education. Environmental tobacco smoke exposure was high at 60.4% for boys and 60.8% for girls.
As expected, the physically larger boys had greater lung capacities (Table 1). The mean FEV1 was 3,167 mL for boys and 2,538 mL for girls. Also, the mean FVC was 3,473 mL for boys and 2,749 mL for girls. When normalized, however, the mean FEV1/FVC ratio was similar, 91% for boys and 92% for girls. Again, not unexpectedly, the peak and mid expiratory flows were higher in boys (4,071 and 6,413 mL/second) than in girls (3,562 and 5,167 mL/second).
Low birth weight and SGA were associated with an increased prevalence of physician-diagnosed asthma (Table 2). In contrast, pregnancies longer than 40 weeks were also at increased risk. Among boys, birth weights below 3,000 g, SGA, and gestations beyond 40 weeks were associated with an increased prevalence of physician-diagnosed asthma, with adjusted odds ratios of 1.26, 1.21, and 1.25, respectively. For girls, the odds ratios were very similar at 1.21, 1.18, and 1.21, respectively. Higher body mass indexes were suggestive of increased likelihood of asthma throughout, but only those for girls between 21 and 23 kg/m were significant with an odds ratio of 1.26.
When the questionnaire was used to identify those with asthma, the relations were less consistent (Table 2). Boys with birth weights below 3,000 g, SGA, and prematurity (<37 weeks) were more likely to have asthma with accompanying odds ratios of 1.18, 1.13, and 1.22, respectively. Similar comparisons in girls were not observed. Questionnaire-based asthma risk was, however, more closely associated with body mass in both sexes. Boys weighing more than 60 kg or having a body mass index greater than 23 kg/m were at greater risk of having asthma, with odds ratios of 1.10 and 1.18, respectively. Similarly, girls greater than 60 kg in weight and those with body mass indexes between 21 and 23 kg/m or greater than 23 kg/m were all associated with an increased prevalence of questionnaire-diagnosed asthma and accompanying odds ratios of 1.20, 1.21, and 1.29, respectively.
Adjusted odds ratios for the risk of physician-diagnosed and questionnaire-determined asthma by birth weight and adolescent body mass index are shown in Figure 1. Both boys and girls who were low birth weight and currently overweight or obese were consistently at a higher risk of asthma. However, a sex difference appeared. The odds ratios of asthma for boys were consistently reversely associated with birth weights regardless of body mass index levels with a narrow variation. Higher birth weight showed a trend of protection of being nonasthmatic. On the other hand, there was a much larger variation in asthmatic risks from the interaction between birth weights and body mass index levels in girls. Higher birth weight girls were consistently at a lower risk of asthma; the odds ratio was particularly lower in those with a low body mass index. However, a U-shaped association appeared in girls with body mass indexes of 21–23 kg/m; the odds ratios were greater in those with both low birth weights and higher birth weights: 1.63 and 1.54 by the physician diagnoses and 1.30 and 1.47 by the questionnaire reports, respectively.
(Enlarge Image)
Figure 1.
Multivariate odds ratios of physician (A, boys; B, girls)- and questionnaire (C, boys; D, girls)-diagnosed asthma associated with birth weight and body mass index in Taiwanese adolescents screened for asthma during 1995–1996. Odds ratios were adjusted for the child's age, paternal education, and environmental tobacco smoke. The line types (thick, thin, dashed, and dotted) represent the different body mass index groups: <17, 17–20, 21–23, and >23, respectively. Body mass index: weight (kg)/height (m).
Adjusted odds ratios for the risk of physician- and questionnaire-diagnosed asthma by birth percentile and adolescent weight are shown in Table 3. Those born as SGA who later became obese had a consistently increased prevalence of physician- and questionnaire-diagnosed asthma, with respective adjusted odds ratios of 1.42 and 1.44 for boys and 1.53 and 1.37 for girls.
SGA increased the asthma likelihood for physician- and questionnaire-diagnosed asthma among normal-weight adolescent males (odds ratios = 1.25 and 1.16, respectively). This was also true for low-weight adolescent males but only for the physician-based diagnoses (odds ratio = 1.43). Females who were born SGA and were now overweight had a significantly higher questionnaire-diagnosed asthma with an adjusted odds ratio of 1.30. Males and females born at AGA who were overweight and obese in adolescence all had a significantly higher prevalence of questionnaire-based asthma with similar adjusted odds ratios for boys of 1.12 and 1.20 and for girls of 1.24 and 1.22. A similar relation was observed among AGA, body mass index, and increased prevalence of physician-diagnosed asthma except paradoxically for obese males, which did not reach significance.
Low birth weight was associated with statistically significant but minor reductions in FEV1 and FVC across all groups (Table 4). As expected, the values were uniformly higher as the current adolescent body mass increased. When normalized to the FEV1/FVC ratio, the low-birth-weight association resolved, and the obesity effect, though still significant, was similarly reduced in magnitude. When the more dynamic measures of lung function (FEF25%–75% and peak expiratory flow) were analyzed, the low-birth-weight effect was absent or smaller among adolescents, particularly boys currently underweight or normal weight. However, as the dynamic lung function increased with body mass, the difference between low- and high-birth-weight groups increased progressively, reaching a maximum in obese adolescents. This disadvantage associated with low birth weight and increasing body mass was exaggerated in girls.
Results
The primary measures were concentrated around expected norms. Birth weights for the majority of subjects were between 3,000 and 4,000 g with appropriate gestational ages (37–40 weeks) and a healthy current body mass index (17–20 kg/m). Birth weights less than 3,000 g were more common in girls (22.9%) than boys (16.9%). SGA was also more likely among girls (11.5% vs. 9.5%). When overweight and obese students were combined, the distribution between genders was similar: 35.7% for boys and 35.5% for girls.
Parental education and environmental tobacco smoke exposure were similar between genders. For instance, in the largest educational category, 36.1% of boys and 36.8% of girls were raised in households where the parents had 6 or fewer years of education. Environmental tobacco smoke exposure was high at 60.4% for boys and 60.8% for girls.
As expected, the physically larger boys had greater lung capacities (Table 1). The mean FEV1 was 3,167 mL for boys and 2,538 mL for girls. Also, the mean FVC was 3,473 mL for boys and 2,749 mL for girls. When normalized, however, the mean FEV1/FVC ratio was similar, 91% for boys and 92% for girls. Again, not unexpectedly, the peak and mid expiratory flows were higher in boys (4,071 and 6,413 mL/second) than in girls (3,562 and 5,167 mL/second).
Low birth weight and SGA were associated with an increased prevalence of physician-diagnosed asthma (Table 2). In contrast, pregnancies longer than 40 weeks were also at increased risk. Among boys, birth weights below 3,000 g, SGA, and gestations beyond 40 weeks were associated with an increased prevalence of physician-diagnosed asthma, with adjusted odds ratios of 1.26, 1.21, and 1.25, respectively. For girls, the odds ratios were very similar at 1.21, 1.18, and 1.21, respectively. Higher body mass indexes were suggestive of increased likelihood of asthma throughout, but only those for girls between 21 and 23 kg/m were significant with an odds ratio of 1.26.
When the questionnaire was used to identify those with asthma, the relations were less consistent (Table 2). Boys with birth weights below 3,000 g, SGA, and prematurity (<37 weeks) were more likely to have asthma with accompanying odds ratios of 1.18, 1.13, and 1.22, respectively. Similar comparisons in girls were not observed. Questionnaire-based asthma risk was, however, more closely associated with body mass in both sexes. Boys weighing more than 60 kg or having a body mass index greater than 23 kg/m were at greater risk of having asthma, with odds ratios of 1.10 and 1.18, respectively. Similarly, girls greater than 60 kg in weight and those with body mass indexes between 21 and 23 kg/m or greater than 23 kg/m were all associated with an increased prevalence of questionnaire-diagnosed asthma and accompanying odds ratios of 1.20, 1.21, and 1.29, respectively.
Adjusted odds ratios for the risk of physician-diagnosed and questionnaire-determined asthma by birth weight and adolescent body mass index are shown in Figure 1. Both boys and girls who were low birth weight and currently overweight or obese were consistently at a higher risk of asthma. However, a sex difference appeared. The odds ratios of asthma for boys were consistently reversely associated with birth weights regardless of body mass index levels with a narrow variation. Higher birth weight showed a trend of protection of being nonasthmatic. On the other hand, there was a much larger variation in asthmatic risks from the interaction between birth weights and body mass index levels in girls. Higher birth weight girls were consistently at a lower risk of asthma; the odds ratio was particularly lower in those with a low body mass index. However, a U-shaped association appeared in girls with body mass indexes of 21–23 kg/m; the odds ratios were greater in those with both low birth weights and higher birth weights: 1.63 and 1.54 by the physician diagnoses and 1.30 and 1.47 by the questionnaire reports, respectively.
(Enlarge Image)
Figure 1.
Multivariate odds ratios of physician (A, boys; B, girls)- and questionnaire (C, boys; D, girls)-diagnosed asthma associated with birth weight and body mass index in Taiwanese adolescents screened for asthma during 1995–1996. Odds ratios were adjusted for the child's age, paternal education, and environmental tobacco smoke. The line types (thick, thin, dashed, and dotted) represent the different body mass index groups: <17, 17–20, 21–23, and >23, respectively. Body mass index: weight (kg)/height (m).
Adjusted odds ratios for the risk of physician- and questionnaire-diagnosed asthma by birth percentile and adolescent weight are shown in Table 3. Those born as SGA who later became obese had a consistently increased prevalence of physician- and questionnaire-diagnosed asthma, with respective adjusted odds ratios of 1.42 and 1.44 for boys and 1.53 and 1.37 for girls.
SGA increased the asthma likelihood for physician- and questionnaire-diagnosed asthma among normal-weight adolescent males (odds ratios = 1.25 and 1.16, respectively). This was also true for low-weight adolescent males but only for the physician-based diagnoses (odds ratio = 1.43). Females who were born SGA and were now overweight had a significantly higher questionnaire-diagnosed asthma with an adjusted odds ratio of 1.30. Males and females born at AGA who were overweight and obese in adolescence all had a significantly higher prevalence of questionnaire-based asthma with similar adjusted odds ratios for boys of 1.12 and 1.20 and for girls of 1.24 and 1.22. A similar relation was observed among AGA, body mass index, and increased prevalence of physician-diagnosed asthma except paradoxically for obese males, which did not reach significance.
Low birth weight was associated with statistically significant but minor reductions in FEV1 and FVC across all groups (Table 4). As expected, the values were uniformly higher as the current adolescent body mass increased. When normalized to the FEV1/FVC ratio, the low-birth-weight association resolved, and the obesity effect, though still significant, was similarly reduced in magnitude. When the more dynamic measures of lung function (FEF25%–75% and peak expiratory flow) were analyzed, the low-birth-weight effect was absent or smaller among adolescents, particularly boys currently underweight or normal weight. However, as the dynamic lung function increased with body mass, the difference between low- and high-birth-weight groups increased progressively, reaching a maximum in obese adolescents. This disadvantage associated with low birth weight and increasing body mass was exaggerated in girls.
