We have previously shown that regular fish consumption and particularly consumption of oily fish is associated with reduced risk of children having asthma.  Fish oil is high in a fatty acid of the omega-3 class known as eicosapentaenoic acid.  This fatty acid is important in the inflammatory process and the products of its metabolism are less inflammatory than its omega-6 fatty acid counterpart - arachidonic acid, which is found in meat, eggs and dairy products.  Linoleic acid is a precursor to arachidonic acid and is found in many vegetable oils and margarines.  The use of vegetable oils and margarines has increased up to five-fold in the past 30 years and this period coincides with a doubling of the prevalence of childhood asthma.

Since inflammation in the airway walls is a fundamental abnormality in asthma, the association between oily fish consumption and reduced risk of asthma led to the hypothesis that the change in the ratio of omega-3 to omega-6 fatty acids in the diet may be one of the factors responsible for the increase in childhood asthma.  It is also possible that a diet high in omega-6 fatty acids and low in omega-3 fatty acids can increase the severity of asthma or, alternatively, that a diet high in omega-3 fatty acids can reduce the severity of asthma.

In order to investigate the hypothesis that diets high in omega-3 fatty acids can reduce the severity of childhood asthma, thirty-nine asthmatic children aged 8-12 years participated in a double-blind, randomised, controlled trial for six months during which they received fish oil capsules plus canola oil and margarine (omega-3 group) or safflower oil capsules plus sunflower oil and margarine (omega-6 group).  Plasma fatty acids, markers of inflammation (stimulated TNF production and circulating eosinophil numbers) and lung function were measured at baseline and after three and six months of dietary modification.  Day and night symptoms, peak flow rates and medication use were recorded for one week prior to laboratory visits.  The major findings of this study were :
1. Plasma omega-3 fatty acids (eicosapentaenoic acid, docosahexaenoic acid) were not different at baseline between groups but were significantly greater in the omega-3 group than in the omega-6 group at three and six months (p<0.00001).
2. Of the omega-6 fatty acids, arachidonic acid was unchanged in both groups, but linoleic acid decreased in the omega-3 group and increased in the omega-6 group.  These levels were significantly different between groups at three months (p <0.01) but not at six months.
3. In the omega-3 group TNF production fell significantly compared with baseline at six months (p=0.026) but the magnitude of change between groups did not reach significance (p=0.075).
4. Circulating eosinophil numbers were significantly reduced in the omega-3 group compared with the omega-6 group.  This reduction was not significant after log-transformation and the eosinophil numbers at 6 months in both groups were still above normal levels.
5. There were no significant changes in the clinical outcome measures of symptoms, lung function, peak flow rates and use of medication.

We conclude that a fish oil supplement along with the addition to the diet of oils and margarines high in omega-3 fatty acids increased the plasma levels of eicosapentaenoic acid and produced a downward trend in the production of inflammatory cells (eosinophils) and chemicals (TNF) over 6 months, but had no effect on the clinical severity of asthma in the children studied.  It is possible that clinical improvement may not be achieved until the levels of inflammatory cells and chemicals reach the normal range, suggesting that a longer period of supplementation may be required.  Alternatively, fish oil may prevent the development of asthma and needs to be introduced earlier in life, before the disease becomes established.  Further studies are required to investigate this latter explanation.