Nutrition of children in modern Western societies has changed in recent decades. The modern diet includes more provitamin A carotenoids, whereas nonprovitamin A carotenoids are less abundant (1). The dietary intake of vitamin A and particular carotenoids determines their serum concentration (2), and therefore carotenoids have been used as a biomarker reflecting differences in diet. In an immigrant population to a more affluent Western country we observed a significantly lower serum level of provitamin A carotenoids than in the domestic population (3).
Concomitant with dietary changes, the prevalence of atopy has risen and is particularly high in countries with a westernized lifestyle (4). The reasons are not clear yet. Immigrants from less affluent regions to these countries suffer less often from atopic disease than their domestic peers (5), but they catch up with adaptation to their new environment (6,7). It is thus tempting to consider nutrition patterns in this respect.
Dietary vitamin A and provitamin A carotenoids can be metabolized in vitro and in vivo into bioactive retinoic acids, which activate nuclear hormone receptors, including the retinoic acid receptor (RAR) (8). The endogenous RAR agonist is all-trans retinoic acid (ATRA). A recent study indicates that ATRA exerts proinflammatory effects. A synthetic RAR agonist upregulates thymic stromal lymphopoietin expression in the skin (9). Thymic stromal lymphopoietin is an important mediator in the pathogenesis of atopic disease (10). In mice fed with a vitamin A–deficient diet, the TH2-type response is downregulated (11) and ATRA supplementation before allergic sensitization promotes TH2-dependent allergic inflammation (12).
Recently, we demonstrated among children from Turkish migrants to Germany a progressively increasing serum concentration of various provitamin A carotenoids with better adaptation to the German culture. The ratio of provitamin A to nonprovitamin A serum levels tended to be higher among atopics (3). Whether serum levels of ATRA are also higher among German children and whether differences in atopy rates are better explained by serum levels of ATRA than by levels of precursor carotenoids is unknown yet.
To our knowledge, the concentration of serum concentrations of ATRA has not been investigated in this respect. We therefore aimed the present study at investigating the hypothesis that a higher prevalence of atopy among German children in comparison with Turkish children growing up in Germany is attributable to a higher serum concentration of ATRA.
The catchment area for the present cross-sectional study was a subdistrict of Berlin, Germany, with about 30% Turkish immigrants. Children were recruited in 1998 before school entry for a population-based survey of atopic disease. For the present study, we included sera from children who were born in Germany and whose parents were either both of German or both of Turkish nationality. Informed written consent was obtained from the parents, and the study protocol was approved by the institutional ethics committee (6).
We included all of the available sera from children with atopy from 2 groups: German and Turkish children. From each of the groups, we included at random a matching number of sera from the remaining children without allergic sensitization.
Analysis of ATRA and Carotenoids
Serum samples were stored at −20°C until analysis. Analysis of ATRA and carotenoids was performed by high-performance liquid chromatography as described before (13,14). Provitamin A carotenoids included α-carotene, β-carotene, and β-cryptoxanthin. Nonprovitamin A carotenoids included lycopene, lutein, and zeaxanthin.
Serum samples were stored at −20°C until analysis. All of the sera were screened with a fluorescence immunoassay (CAP FEIA, Phadia, Freiburg, Germany) for the presence of specific immunoglobulin E (IgE) to common indoor and outdoor inhalant allergens (Phadiatop; Phadia). In case of a positive test result (IgE titer ≥0.35 ku/L), the child was defined as atopic.
Statistical analysis was performed with PASW Statistics 18 (2009) (SPSS Inc, Chicago, IL). Fisher exact test was used when the expected frequency of any cell was <5. The nonparametric Mann-Whitney U test was used for the association of continuous variables. Nonparametric correlation coefficients were expressed as Spearman rho. Statistical significance was defined by a 2-sided α level of 0.05.
The sera of 36 German and 50 Turkish children were included in the present study. In the German group, 22 children (61%) were born in 1992 and 14 children (39%) were born in 1991. In the Turkish group, 13 children (26%) were born in 1993, 27 (54%) in 1992, and 10 (20%) in 1991. Twenty-five children (29.1%) were atopic: 19 (53%) in the German group and 6 (12%) in the Turkish group. There was no statistically significant group difference in regard to sex, dampness, or tobacco smoke exposure at home (data not shown). There was inhomogeneity in regard to presence of sibs (German 64%, Turkish 88%; P = 0.017) and pets (German 61%, Turkish 19%, P < 0.001).
Serum Levels of ATRA and Carotenoids
The mean serum level of ATRA was 1.62 ng/mL (95% confidence interval [CI] 1.44–1.81 ng/mL). There was a significant difference of the mean serum level between the German group (1.93 ng/mL, 95% CI 1.57–2.29 ng/mL) and the Turkish group (1.40 ng/mL, 95% CI 1.23–1.57 ng/mL, P = 0.006; Fig. 1). The correlation of ATRA serum levels with those of provitamin A carotenoids was rather moderate (rs = 0.642, P < 0.001; see supplemental digital content at http://links.lww.com/MPG/A78). Correspondingly, levels of ATRA correlated with levels of α-carotene (rs = 0.534), β-carotene (rs = 0.662), and β-cryptoxanthin (rs = 0.557). Mean serum levels of retinol were similar among German (0.279 ng/mL, 95% CI 0.257–0.301 ng/mL) and Turkish children (0.262 ng/mL, 95% CI 0.236–0.287 ng/mL; P = 0.186). There was only weak correlation of ATRA with retinol (rs = 0.391, P < 0.001), nonprovitamin A carotenoids (rs = 0.316; P = 0.004), or any of the particular nonprovitamin A carotenoids (data not shown). Levels of ATRA did not differ between children with or without pets or sibs, respectively (data not shown).
Serum Levels of ATRA in Relation to Atopy
There was no statistically significant difference in ATRA serum levels between atopic and nonatopic individuals (Fig. 2). Moreover, there was no significant correlation of total serum IgE (rs = 0.063, P = 0.936) or the IgE titer of the screening test for IgE against inhalant allergens (rs = 0.201, P = 0.336).
We found that ATRA levels correlate with serum levels of provitamin A carotenoids and were higher in domestic German children than in their peers of Turkish descent. These results support the view that diet differs between these populations and that diet differences translate into differences of the immunologically active provitamin A metabolite ATRA (3). Retinol and nonprovitamin A carotenoids, however, are similarly distributed in both subpopulations (6) and do not correlate well with ATRA.
Remarkably, domestic German children have a higher atopy risk (6) and higher ATRA serum levels; however, we observed only a weak association of ATRA serum level and atopy (Fig. 2). On a systemic level, this finding argues against a strong promotional role of ATRA for atopy and is in accordance with the weak association between provitamin A serum carotenoids and atopy in this population (3). The number of subjects investigated in this explorative study was limited. Studies in larger patient samples, however, may allow the detection of a significant association of atopy and ATRA serum level. By contrast, a reduced concentration of ATRA in target organ tissue of established atopic disease (but not in serum) has been demonstrated recently (15).
The correlation of ATRA serum levels with those of provitamin A carotenoids was much stronger than with nonprovitamin A carotenoids. Nonprovitamin A carotenoids are not ATRA precursors and are probably of no biological relevance for atopy. Retinol can be metabolized to ATRA, but its weak association with ATRA in our study suggests no superior role in this respect.
All of the serum samples were stored at −20°C until analysis. Although possible degradation of carotenoids should be ruled out at this temperature, a systematic bias not affecting the direction of group differences between domestic and migrant subpopulations or atopic and nonatopic subpopulations would be expected.
In summary, we found that ATRA serum levels differ between different cultural groups and are related to serum levels of provitamin A carotenoids, but they are rather weakly related to atopy. The present study suggests dietary differences between groups of different atopy risk. Further study is warranted to identify alternative dietary risk modifiers in the development of atopy and its clinical manifestations.
The authors appreciate the contributions of Dana Remmler, Anja Ingwers, Dr Mohamad Raai (field work), and Gabi Schulz (serum IgE analysis), Department of Pediatric Pneumology and Immunology, Charité–Universitätsmedizin Berlin. The Child Health and Youth Health Service, District Wedding, City Senat of Berlin, provided the opportunity to perform clinical examinations in its institutions and was helpful during the realization of the WAS-98 project.
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