脂質の榮養價に関する研究-VIII

Abstract

According to the result from the experiments on nutritive values of fish oils conducted by Yoshida (1) and Ozaki (2), highly unsaturated fatty acids in the fish oils had toxic effect upon the growth of rats. So far this has been generally believed right. And existance of highly unsaturated fatty acids in fish oils have been accounted for the reason why fish oils are inferior to other oils from nutritive point of view. It is a well known fact that highly unsaturated acids are rapidly oxidized when they come in contact with the air. For this reason, a special attention must be paid when we carry out a feeding experiment by giving highly unsaturated acids to test animals. Otherwise, there will be a danger of supplying more or less oxidized acid to the animals and calculating nutritive value of such degenerating acids, even if the experiment has started with genuine highly unsaturated acids. As reported in the previous paper (3), we experimented the nutritive value of ethyl ester of highly unsaturated acids with iodine value of 365 and its partially reduced products. During the experiment many hairs around the mouth and legs were falling off in the result of feeding on the ethyl ester of these acids. (see Fig. 1) The ethyl ester used for the test was preserved in a brown bottle. Every time the bottle was opened, CO2 gas was poured in so that the acids might be kept from oxidation. We found, however, in the later course of the experiment that it was impossible to prevent the acids from deterioration by such a treatment as that. Such being the case, the acids we used for the test were not genuine as we had taken for granted, while measuring nutritive value of slightly oxidized acids. Another experiment carried out on the same occasion revealed that the partially reduced products of highly unsaturated acid, even though iodine value being fairly high, have nutritive value almost same as oleic acids. It is understood that oxidation of natural unsaturated acids around C18 starts from the carbon which is suspended under the most unstable condition in the atoms of unsaturated acids. On the other hand, oleic acid is regarded from its stracture to be one of the stable unsaturated acids, a fact which may suggest the following. In the course of the experiment the partially reduced products of highly unsaturated acids have turned to the ones which, despite of their fairly high iodine value, are as nearly stable as oleic acid when unstable factors have been removed out of them in consequence of reduction. On the basis of these findings we assumed that natural unsaturated acids around C18, except one which has conjugate double bond, would not always become less nutritious by the increase of double bond, as it has been generally believed. In other words, the unsaturated acids with the same number of carbon, even if different in the unsaturation degree, are almost same in their nutritive values. While the long-believed difference of nutritive value among the unsaturated acids should certainly be attributable mainly to the degree of oxidation affected on each unsaturated acid. In order to prove this assumption, we chose, as test materials, some of the highly unsaturated acids which had been considered nutritively much lower than others. Because highly unsaturated acids such as contained in the fresh sardine kept unoxidized are supposed to be not only harmless, but also highly nutritious. However, it must be the oxidized ones that show a retarding action as often observed in the past experiments. With this view in mind, we extracted highty unsaturated acids from sardine oil as genuinely as possible to carry out our experiment.