Causes of fatty liver in farmed fish: a review and new perspectives
Causes of fatty liver in farmed fish: a review and new perspectives
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作者：
Du Zheny
脂肪肝是一种生理或病理现象，目前在养殖鱼类中很常见。鱼类严重的脂肪肝会导致生长和饲料效率降低、免疫反应受损和营养品质下降。然而，到目前为止，人们仍然不知道养殖鱼脂肪肝的确切原因，也没有找到很好的解决方案来消除脂肪肝。本文系统综述了鱼类脂肪肝的研究成果，并对当前水产养殖系统中鱼类脂肪肝的起源提出了一些新的观点。本文从营养和配方饲料、环境污染、生理过程、物种差异和基因突变等5个方面综述了引起养殖鱼类脂肪肝的主要原因。其中，营养成分不平衡和能量摄入过多，如过度摄食、饲料脂质和碳水化合物含量过高、缺乏维持正常脂质代谢所需的维生素等，被认为是造成鱼类脂肪肝的主要原因。此外，在一些水产养殖区，最近有报告称，大量的水污染物，特别是重金属和持久性有机污染物（POPs）是造成养殖鱼类脂肪肝流行的原因之一。脂肪肝的发生还与鱼类的性别和发育有关。例如，幼鱼和/或雌鱼肝脏脂肪含量的变化通常比老年鱼和/或雄鱼肝脏脂肪含量的变化更敏感。此外，有些鱼类会在肝脏中沉积脂肪，因此在预防养殖鱼类脂肪肝时应考虑鱼类的差异。通过对上述研究结果的回顾，本文提出了一些新的观点：1)根据营养失衡或氧化应激引起的脂肪肝症状，大多数脂肪肝可分为“营养性脂肪肝”和“氧化性脂肪肝”；2)养殖鱼类脂肪肝的普遍发生与人类对水产养殖产量的高要求与鱼类自然生理特性的矛盾有着根本的联系。因此，只要目前的鱼类品种没有经过转基因改造以适应现代高密度的水产养殖业，养殖鱼类中常见的脂肪肝现象将在未来很长一段时间内持续存在；3)脂肪肝不能完全视为病理症状。实际上，肝脏甘油三酯合成的初始阶段增加是避免高游离脂肪酸在循环中引起的脂肪毒性的保护机制。综上所示，以营养均衡的饲料喂养鱼类，保持饲料和原料的新鲜度，维持可持续健康的水环境是预防鱼类脂肪肝的最终正确措施。
Fatty liver is a physiological or pathological phenomenon and is commonly seen in farmed fish at present. Severe fatty liver in fish results in reduced growth and feed efficiency,impaired immune response and decreased nutritional quality. However,so far,people still don't know exactly the causes of fatty liver in farmed fish,and also have not found good solutions to eliminate fatty liver. The present article systemically reviews the results of fish fatty liver studies and has put forward some new perspectives to discuss the origins of fish fatty liver in the current aquaculture system. In the present review,five main causes of fatty liver in farmed fish are summarized: nutrition and formulated diet,environmental pollution,physiological process,species difference and genetic mutation. Among them,the unbalanced nutrients composition and excess energy intake,such as overfeeding,excess dietary lipid and carbohydrate contents,deficiency of vitamins which are needed to maintain normal lipid metabolism,are regarded as the principal causes of fish fatty liver.Beside,in some aquaculture regions,high amount of water contaminants,especially heavy metals and persistent organic pollutants( POPs),have been recently reported to contribute to the prevalence of fatty liver in farmed fish. The occurrence of fatty liver is also related to the gender and development of fish. For example,the change of fat content in liver of young and /or female fishes normally is more sensitive to dietary energy content than that in liver of older and /or male fishes. It is also noted that some fish species specifically deposit fat in liver,thus fish species difference should be considered in preventing fatty liver in farmed fish. Through reviewing the above results,the present article brings some new perspectives: 1) Most of fatty liver could be classified as"nutritional fatty liver"and"oxidative fatty liver"depending on the fatty liver symptom caused from nutrient unbalance or oxidative stress; 2) The common occurrence of fatty liver in farmed fish fundamentally correlates with the contradiction between the requirement of high aquaculture production of human and natural physiological features of fish. Therefore,as long as the present fish species have not been genetically modified to adapt to modern high-density aquaculture industry,the common occurrence of fatty liver in farmed fish would last in future for a long time; 3) Fatty liver could not be completely regarded as a pathological symptom. Actually,initial stage of increasing hepatic triglyceride synthesis is a protective mechanism to avoid the lipotoxicity induced by high free fatty acids in circulation. In conclusion,feeding fish with optimal nutrients-balanced diets,keeping freshness of feed and raw feed materials,and maintaining a sustainable healthy water environment are the ultimately right measures to prevent fish fatty liver.