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Article Excerpt
Byline: Xinqing Deng, MD, MPH, Daniel Venarske, MD, Terryl Hartman, PhD, RD, and Tina V. Hartert, MD, MPH
Abstract: Although additional studies are needed to clarify the impact of diet on asthma and allergy, some patterns have emerged. For example, there is evidence that a diet high in omega-6 fatty acids (found in margarine, for example) is associated with increased risk of childhood atopy and asthma. In contrast, consumption of omega-3 fatty acids (found in fresh oily fish) appears to have a protective effect. There also is evidence that consumption of antioxidant nutrients, including vitamin E, vitamin C, zinc, and selenium, is associated with reduced risk of asthma. Maternal intake of vitamin E during pregnancy may reduce the risk of childhood asthma. Maternal intake of oily fish (not fish sticks) also may help protect children from asthma, especially when the mother has a history of asthma. (J Respir Dis. 2007;28(10):448-459)
Key Words: Diet, Nutrition, Asthma, Atopy, Pregnancy, Antioxidants
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Asthma is an important public health concern, affecting more than 20 million persons in the United States.1 There has been a dramatic increase in the worldwide prevalence of asthma over the past several decades. In the United States, the prevalence of asthma increased from 3.6% in 1980 to 5.8% in 2003.2
The rapid rise in asthma prevalence is probably the result of changes in our environment, since genetic changes would take more than several generations to occur.2 Dietary changes in particular have been extensively studied, with sometimes conflicting results, depending on the specific factor investigated and the population studied. Recently, studies have shown that different environmental exposures that influence the risk of asthma interact with different genetic polymorphisms.3 Therefore, part of the impact of environmental factors on asthma depends on the genetic makeup of the individual person. Fatty acids, antioxidants, minerals, nutrients, and probiotics have been the dietary factors most frequently investigated.
It is the aim of this 2-part review to discuss the possible role of these dietary factors in the asthma epidemic. In part 1, we review the role of fatty acids and antioxidants, and we discuss the impact of maternal dietary intake during pregnancy on respiratory outcomes in children. In part 2, to be published in a coming issue of The Journal of Respiratory Diseases, we will review the role of vitamin D and probiotics.
FATTY ACIDS
The immunomodulatory effects of fatty acids on the immune system have been known for more than 30 years. In 1997, Black and Sharpe4 proposed that the increase in the prevalence of asthma and allergy was a consequence of changes in the consumption of dietary fatty acids. They suggested that an increased dietary intake of omega-6 polyunsaturated fatty acids (found in margarine and vegetable oils) and a decreased intake of omega-3 fatty acids (found in oily fish) can alter inflammatory cell membrane lipid composition. Since then, research on the effects of fatty acids on asthma has focused on increased dietary omega-6 fatty acids and decreased omega-3 fatty acids.
Molecular mechanisms
Omega-6 fatty acids in cell membranes are converted into arachidonic acid molecules that are metabolized into mediators of inflammation, such as leukotrienes and prostaglandins.4 Prostaglandin E2 (PGE2) suppresses the differentiation of T helper (TH) 1 cells and promotes the TH2 phenotype associated with asthma by increasing class switching to IgE and promoting production of interleukin (IL)-4.5 Fatty acids may also affect B- and T-cell differentiation by binding and activating nuclear receptors.6
Study findings
-Animal studies: Animal studies have demonstrated a positive relationship between the level of arachidonic acid in inflammatory cells and the ability of those cells to produce PGE2.7 Increasing the amount of arachidonic acid in the diets of rats increased the amount of arachidonic acid in inflammatory cells.
-Observational studies: Epidemiological data link high omega-6 fatty acid consumption or low omega-3 fatty acid consumption with childhood atopy and asthma.8,9 In a longitudinal case-control study of more than 1000 children, Dunder and associates8 found that atopic children consumed more margarine (rich in omega-6 fatty acids) than nonatopic children.
In a case-control study of more than 400 Australian children aged 8 to 11 years, the regular consumption of fresh oily fish was associated with a reduced risk of current asthma (odds ratio [OR], 0.26; 95% confidence interval [CI], 0.09 to 0.72;
P < .01).9 These observational studies support the hypothesis that the rise in asthma prevalence may be related to dietary fatty acid consumption patterns.
-Intervention trials: Several randomized trials have prospectively investigated the relationship between fatty acid consumption and asthma, and their results are summarized in Table 1.10-16 Overall, the results are difficult to interpret because the studies were generally small (12 to 39 participants), included persons with a wide range of ages, and the durations of the interventions ranged from 10 weeks to 1 year.
In one randomized controlled study of 10 months' duration, Nagakura and associates15 found that children with asthma whose diets were supplemented with fish oil in the strictly controlled environment of a hospital had a reduction in asthma symptom scores and decreased responsiveness to acetylcholine compared with children in the control group. Overall, the collective results of these studies suggest that larger well-controlled trials are needed.
Mickleborough and colleagues17 reported that supplemental fish oil capsules containing 3.2 g of eicosapentaenoic acid (EPA) and 2 g of docohexaenoic acid (DHA) (n = 8), compared with placebo (n = 8), had a protective effect against exercise-induced bronchoconstriction for 3 weeks in adults with asthma....
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