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Background: The rising prevalence of asthma and allergic disease remains unexplained. Several risk factors have been implicated including diet, in particular poly-unsaturated fats and antioxidant intake.
Methods: A nested case–control study comparing the dietary intake of sensitized children with recurrent wheeze (age 3–5 years) and nonsensitized children who had never wheezed was carried out within an unselected population-based cohort. Cases and controls were matched for age, sex, parental atopy, indoor allergen exposure and pet ownership. Dietary intake was assessed using a validated semi-quantitative food frequency questionnaire and nutrient analysis program.
Results: Thirty-seven case–control pairs (23 male, mean age 4.4 years) participated. Daily total polyunsaturated fat intake was significantly higher in sensitized wheezers (g/day, geometric mean, 95% confidence intervals: 7.1, 6.4–7.9) compared with nonsensitized nonwheezy children (5.6, 5.0–6.3, P = 0.003). Daily omega-3 and omega-6 fat intakes were not significantly different between the two groups. No significant differences were found in intake of any antioxidant or antioxidant cofactors between the groups.
Conclusions: Young sensitized wheezy children had a significantly higher total polyunsaturated fat intake compared with nonsensitized nonwheezy children. However, we were unable to distinguish a significant difference in specific poly-unsaturated fat intakes. Otherwise the children in both groups had a very similar nutritional intake.
The rapid increase in the prevalence of asthma and atopic disease is likely because of changes in lifestyle and/or in the environment. Amongst other factors, nutrition may have played a role. Two hypotheses have been put forward regarding different aspects of diet(1): (1) reduction in dietary antioxidants (vitamin A, C and E) and/or antioxidant cofactors (selenium, copper and zinc) consequent to reduction in intake of fresh fruit and vegetables and an increase in convenience foods; (2) shift in fatty acid intake from omega-3 to omega-6 polyunsaturated fats because of an increase in margarine consumption and decline in animal fat consumption (2, 3).
We aimed to investigate whether there are any differences in dietary macro- and micro-nutrient intake between sensitized children with wheeze and nonsensitized asymptomatic children. We used a case–control study nested within the context of a prospective birth cohort, which allowed us to control for a number of risk factors, which have previously been associated with asthma and sensitization (parental sensitisation status and history of allergic disease, indoor allergen exposure, pet exposure, gender).
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In this case–control study of preschool sensitized children with recurrent wheeze and nonsensitized nonwheezy controls we have examined dietary intake using a semi-quantitative food frequency questionnaire. The only difference we found was that polyunsaturated fat intake was significantly higher in the symptomatic children compared with healthy controls. However, we could not find a significant difference in intake of any of the specific poly-unsaturated fatty acids (PUFA). Intakes of the major omega-3 fatty acids [α-linolenic acid, eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA)] were similar in cases and controls. However, there was a nonsignificant trend for intakes of the major omega-6 fatty acids (linoleic acid and arachidonic acid) to be higher in cases than controls.
We found no difference between dietary antioxidants (vitamin A, C and E) and/or antioxidant cofactors intake between cases and controls. Previous studies on this topic reported conflicting results. Vitamin C consumption has been associated with a reduction in wheezing and an increase in lung function in some (6, 7) but not all studies (8). Other studies have found an inverse relationship between vitamin E consumption and wheeze, but no relationship between vitamin C or A (9, 10).
The strength of our study is the very careful definition of the outcomes and matching of the cases and controls according to a number of factors which are associated with the development of atopic sensitization and wheeze. Furthermore, we used a semi-quantitative food frequency questionnaire along with a nutrient analysis program that allowed calculation of actual nutrient intake for each individual. The questionnaire and program have been validated in adults (11, 12). Portion sizes were altered accordingly for the use in young children. Although this method of calculating nutrient intake gives somewhat more information than many previous studies that have only inquired about types of foods eaten and not about frequency or quantity, semi-quantitative food frequency questionnaires have their limitations. They rely heavily on recall and are only as good as the nutritional database that supports them. In addition they remain a ‘snap-shot’ of current diet, albeit a more overall picture than some other methods of assessing dietary intake (e.g. 48 h recall) used in some other studies (13).
The main weakness of our study is the relatively small number of participants. This may explain why we found the intake of PUFA to be significantly greater in cases than controls, but could not find a significant difference in individual omega-3 and omega-6 fatty acid intake.
Over the last few decades the type of fat consumed in western diets has changed with a reduction in saturated fats (animal fats) and an increase in PUFA in the form of vegetable-oil products rich in omega-6 fatty acids. The major omega-6 fatty acid consumed in the human diet is linoleic acid and this is converted to arachidonic acid. Human inflammatory and immune cells contain high proportions of arachidonic acid (14) and its principal functional role is as a substrate for the two-series prostaglandins (e.g. PGE2) and the four-series leukotrienes. It has been suggested that this pattern of change in fat intake over recent decades is responsible for the corresponding increase in atopic disease (15). This is because PGE2 exerts effects that could promote the development of a Th2-type phenotype. PGE2 has been reported to increase the production from T-cells of IL-4 (16), IL-5 and IL-10 (Th2 cytokines) (17) and to decrease the production of IFN-γ and IL-2 (Th1 cytokines) (18).
As the consumption of vegetable oils (rich in omega-6 fatty acids) has increased, the consumption of omega-3 fatty acids has decreased both in absolute and relative (to omega-6 fatty acids) terms (19). For example, linoleic acid contributes 50–80% of fatty acids found in corn, sunflower and soybean oils, whereas good sources of α-linolenic acid such as rapeseed and soybean oils contain only 5–15% of these fatty acids. EPA and DHA are found in relatively high proportions in the tissues of ‘oily fish’, but in the absence of significant consumption of oily fish, α-linolenic acid is the major dietary omega-3 fatty acid. Since there is a competition between the omega-6 and omega-3 fatty acids for the enzymes that metabolise them, the relative reduction in omega-3 fatty acid intake may exacerbate the effect of the increase in omega-6 fatty acid intake. Thus, both relative and absolute increases in omega-3 fatty acid intake have the effect of reducing the amount of arachidonic acid produced.
A number of studies to date have found an association between margarine consumption and atopic disease. Margarine contains up to 20 times more linoleic acid than butter (20). Von Mutius et al. reported that the rising prevalence of hay fever and atopy in children in East Germany in the 1990s had coincided with, amongst other things, a change in diet, in particular a dramatic rise in the use of margarine over butter (21). Subsequently, other investigators in Europe have used dietary surveys to show an association between atopic disease in childhood and increased consumption of margarine over butter (22–24).
Several studies using food frequency questionnaires have shown an association between oily fish consumption and a reduction in asthma symptoms (24, 25) and also an association between the consumption of fish in the first year of life and a reduction in risk of developing asthma in early childhood (26). However, no studies in childhood so far have analysed actual nutrient intake. A study from Australia has shown that a high dietary intake of PUFA (information derived from enquiring about type, but not quantity of fat used) was associated with an increased risk of recent asthma in preschool children (27).
Although to date there is still little evidence suggesting a role of dietary fatty acids in the development of asthma and allergies, randomized controlled trials have been initiated, with interesting early results (28). Supplementation of omega-3 fatty acid (with and without dust mite avoidance) significantly reduced wheeze in the first 18 months of life (27). At age 3 years there was no effect of dietary intervention on wheeze, but there was a significant reduction in cough in atopic children(29). Fish oil supplementation in pregnancy may reduce neonatal cytokine responses to dust mite, cat, ovalbumin and PHA (30). Although this study was not powered for clinical outcomes, the infants in the fish oil supplementation group were less likely to be sensitized to egg and had milder eczema.
The results of our study add further to the evidence that dietary intake of PUFA plays a role in the development of atopic wheeze in young children. Further studies in larger groups of children, using semi-quantitative food frequency questionnaire and nutrient analysis programs, may help to answer this question.