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In their study, Haby and colleagues1 found a high intake of dietary polyunsaturated fats to be a risk factor for asthma in preschool children. They suggest that reducing the consumption of these fats represents an intervention that has great potential for lowering asthma rates. However, this may be much too broad a conclusion as it is not the total content of polyunsaturated fats in the diet but their composition that is likely to be causing adverse health effects. The problem most probably stems from the imbalance of n-6 to n-3 fatty acids.
It is estimated that, throughout much of human existence, we ate diets that contained ratios of n-6 to n-3 fatty acids of approximately 1:1 to 2:1.2 However, today in the US the ratio is estimated to be greater than 10:1,3 and some have estimated that it is actually more like 20:1 to 30:1.2 Could it be that the increased intake of polyunsaturated fats seen in preschool asthmatics by Haby et al is actually just a marker for an even greater imbalance in this ratio?
In Japan the n-6:n-3 ratio in the typical diet is about 4:1, several times better than in the US.4 One epidemiological study found that the rate of childhood asthma in Tokyo is 0.7% compared with a worldwide average of roughly 5%,5 and there are confounding factors that could contribute to higher childhood asthma rates in Japan. For example, because a higher percentage of the Japanese population live in urban settings, the average air quality to which they are exposed is poorer. In addition, smoking is much more prevalent in Japan which means that children are more exposed to passive smoking.
Several studies support the assertion that greater consumption of oily fish, which contain high amounts of n-3, may protect against childhood asthma and can improve lung function.6, 7 In addition to a decrease in fish consumption, there in another less widely acknowledged factor in the large imbalance in the intake of fatty acids today. Modern livestock management techniques, which rely heavily on the use of grain feedlots, have caused great reductions in the omega-3 composition of our commercial meat supply. When cattle are allowed to graze freely in the warmer months, as historically they have always done, their fatty acid composition is favourably enhanced. While improving the fatty acid profile of the meat supply may not fully compensate for the many shortcomings of the modern western diet, it would certainly be a step in the right direction. Since it is estimated that a fourfold increase in fish consumption would be required to bring n-3 fatty acid consumption up to recommended levels, improvements in any other food sources would be of great value.
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