乙酯型DHA 含量较高,消化吸收困难,还可能存在安全隐患。甘油酯型是 DHA 的天然存在形式,因此食用安全,易被人体消化吸收,是被人们普遍接受的一种产品形式。具体原理如下:
长链脂肪酸甘油三酯进入小肠上段后,经胰脂肪酶及胆酸盐的作用,水解为甘油、甘油一酯和脂肪酸等[14]。经小肠上皮细胞吸收再重新合成为甘油三酯,与载脂蛋白和胆固醇结合成乳糜微粒后,经淋巴进入血液循环。但乙酯缺少甘油骨架,且脂肪酸-乙醇之间的酯键对胰脂肪酶的抵抗力远远大于脂肪酸-甘油酯键[15]。乙酯进入小肠后被分解为乙醇和脂肪酸,脂肪酸被小肠上皮细胞吸收后只有转化为甘油三酯才可以进入血液,甘油三酯型DHA存在属于自己的甘油一酯底物,乙酯型鱼油正是因为缺少这种甘油一酯底物,甘油三酯的合成被推迟而造成了脂肪酸的分解,因此,甘油三酯型鱼油才呈现出更好的生理活性[16]。其次,乙酯型 DHA 的在体内的代谢存在安全隐患。乙酯型 DHA要通过肠上皮细胞转化成甘油三酯型 DHA/EPA 才能被人体吸收,此过程的代谢产物是乙醇,虽然产生乙醇的量比较低,但是对酒精过敏或对酒精耐受性差的人群,尤其是儿童都是不宜的;其次,经过氧化作用研究发现,乙酯型 DHA/EPA 的氧化速率高出甘油三酯型 33%[17]。
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