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Note: The following letter was written in response to a study Thorax published on polyunsaturated fats and asthma last August. The letter below was published in the March issue of Thorax.
Researchers found a high intake of dietary polyunsaturated fats as a risk factor for asthma in preschool children. They then suggest that reducing the consumption of these fats represent an intervention that has great potential for lowering asthma rates.
However, this conclusion in in error, as it is not the total polyunsaturated fat content of the diet, but the composition of the polyunsaturated fats, that is likely causing adverse health effects. The problem most likely stems from the imbalance of omega-6 to omega-3 fatty acids.
It is estimated that throughout much of human existence, we ate diets that contained omega-6:omega-3 ratios of approximately 1:1 to 2:1 (1). However, today in the US, the ratio is estimated to be greater than 10:1 (2), although some have estimated that it is actually more like 20:1 to 50:1 (1).
It is likely that the increased poly intake seen in preschool asthmatics by Haby is actually just a marker for an even greater imbalance in this ratio?
In Japan, the omega-6:omega-3 ratio in the typical diet there is about 4:1, several-fold better than the US (3). One epidemiological study found that the childhood asthma rate in Tokyo is 0.7% as compared to a worldwide average of roughly 5% (4).
And there are confounding factors that could contribute to higher childhood asthma rates in Japan. For example, because a greater percentage of Japanese live in urban settings, the average air quality they are exposed to is lower. In addition, smoking is much more prevalent in Japan, which means more second-hand smoke exposure for children.
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 (5,6).
In addition to a decrease in fish consumption, there is 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 they have historically always done, their fatty acid composition is favorably 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. Considering the fact that it is estimated that a 4-fold increase in fish consumption would be required to bring n-3 fatty acid consumption to up to recommended levels, any other food sources would be of great value.
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