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疾病是如何影響營養需求的

作者：Ioannis Mavromichalis, Ph.D. 譯者：劉彩紅

序言：生病的動物吃很少的食物,但是很明顯,他們也有不同的營養需求。

在生病時,從合成代謝到分解代謝的代謝改變被認為是免疫系統應答成功的關鍵因素,從生長到支持免疫系統功能控制病原感染,營養必須重新組合分布。然而,目前,存在很少專門為生病動物制定加強營養的飼料日糧配方資料。

免疫系統應答

廣泛地認為,暴露于病原體觸發了免疫系統,在應答中釋放很多前炎癥細胞因子,這些因子不僅調節免疫應答,而且調動整體代謝。研究已經清洗地表明免疫系統被這些細胞因子激活后(甚至在亞臨床水平),抑制了食欲,減少了肌肉蛋白的合成。同時也減少了肌肉蛋白質的降解。因此,生病動物的代謝狀態連通合成代謝(生長)到分解代謝(肌肉損失)。

當然,當免疫系統激活時,新陳代謝直接指向免疫系統成分的合成。這種情況發生時,這些過程的營養需求與自然生長不同。今天,還是真正缺乏生病動物的營養需求的有意義資料。所有的“標準”營養需求是從傳統的劑量調整試驗中得到的,這些試驗是使用健康飼養豬在接近最佳狀態的條件進行的。
氨基酸

免疫系統有它自己獨立的一套氨基酸需求方案,這不同于生長需求。比如說,急性期細胞因子應答時肝臟產生的蛋白質與骨骼肌肌肉蛋白相比有不同的氨基酸組成，急性期蛋白質有高濃度的芳香族氨基酸(苯丙氨酸、酪氨酸和色氨酸)存在，這些是疾病發作時，在肝臟中大量產生。這些實際上表明肌肉過度退化是由于組成肌肉蛋白和急性蛋白的氨基酸功能不同。
此外，蘇氨酸是免疫G蛋白的主要組成，日糧成分中為豬抗體生產所需的蘇氨酸是高于自然生長的需求。除此之外，免疫應激導致氨基酸的持續需求如同氨基酸總需求一起增加?？偠灾?，抵抗疾病的豬，氨基酸需求可能會增加，然而，賴氨酸需求相對來說低點。

提到礦物質，鋅減少免疫競爭，增加感染的機會，這是眾所周知的。然而，高濃度鋅也可能會影響感染豬的免疫應答，這是由于它可能會干擾正常免疫功能。事實上，很長一段時間，氧化鋅的藥理劑量常常與性能缺陷相關。
此外，因為感染期大多數病原微生物生長和增殖需要鐵，血液中鐵的含量大幅下降。因此，減少日糧中鐵的供應可能會是一種有效的干預策略，這樣能幫助生病動物抵抗疾病，甚至防止感染。實驗管理中，抗病豬的過剩鐵被證明是導致恢復時間延長的原因。最后，免疫應激動物血液中銅含量增加，因為IL-1增強了銅藍蛋白的合成，這是一種急性期蛋白，它增強了淋巴細胞的反應。
維生素

很少有資料是關于疾病期維生素需求的。維生素E和硒的缺乏降低感染動物淋巴細胞的增殖，這一信息已經被證明，而這些營養過剩增加了抗體生長和淋巴細胞的增殖，這些都是非常有益的成分。其他天然抗氧化劑目前是未知的，但是也被認為是有相似作用功能的。另外，維生素A缺乏與減少上皮完整度有關，這樣潛在地促進了病原體物質通過胃腸道或者呼吸道系統的感染。

以上信息只是針對這個令人興奮的話題作一個簡短的介紹，也是對許多研究的一個指導，研究需要充分理解這個主題。對于在疾病時期營養需求更深的認識將促進動物更快地恢復，減少疾病對生長性能的消極影響。

英文原文

How disease affects nutrient requirements

Date: 2013-02-04

Ioannis Mavromichalis, Ph.D.,

Sick animals eat less feed, but it appears they also have different nutrient requirements.

During disease a metabolic alteration from anabolism to catabolism is considered essential for the success of the immune system response, as nutrients must be redistributed away from growth to the support of immune system functions to control the pathogenic infection. However, today there is very little quantitative data to enable nutritionists to formulate diets specifically for sick animals.
Immune system responses
It is widely understood that exposure to pathogens triggers the animal’s immune system, which in response releases a host of pro-inflammatory cytokines that modulate not only the immune response, but also, overall metabolism. Research has demonstrated clearly that activation of the immune system by these cytokines (even at subclinical levels) depresses appetite and reduces muscle protein synthesis, while increasing muscle protein degradation. Thus, the metabolic state of the diseased animal switches from anabolism (growth) to catabolism (muscle loss).
Naturally, when the immune system is activated, metabolism is directed towards the synthesis of immune system compounds. As it happens, nutrient requirements for these processes are quite different than requirements for normal growth. Today, there is a real lack of meaningful information regarding the nutrient requirements of diseased animals. All “standard” nutrient requirements have been traditionally derived from dose-titration experiments with relatively healthy pigs reared under near optimal conditions.
Amino acids
The immune system has its own unique set of amino acid requirements, which differ from that required for growth. For instance, acute-phase proteins produced by the liver in response to cytokines have a different amino acid composition than skeletal muscle proteins: the aromatic amino acids (phenylalanine, tyrosine, and tryptophan) are found in larger concentrations in acute phase proteins, which are produced in massive quantities in the liver at the onset of disease. It has actually been suggested that excessive muscle degradation is due to the differences in amino acid profile between muscle protein and acute phase proteins.
In addition, threonine is a major component of immunoglobulin-G and obviously higher dietary concentrations of threonine are required for antibody production than for body growth in pigs. Additionally, immunological stress causes an increase in amino acid maintenance needs as a proportion of total requirement. Overall, it appears that requirements for many amino acids may be increased in challenged pigs, whereas lysine requirements could be much lower.
Minerals
When it comes to minerals, it is already known that a deficiency of zinc greatly reduces immunocompetency and increases the chances of infection. However, zinc hyper-alimentation may also complicate the immune response of infected pigs because it may interfere with normal immune system functions. Indeed, pharmacological dosages of zinc oxide for a prolonged period of time have been often associated with depressed performance.

In addition, as iron is required for the growth and proliferation of most pathogenic microorganisms, blood iron levels are dramatically reduced during infection. Thus, reducing dietary iron supplementation may be a useful intervention strategy to help diseased animals combat or even prevent an infection. Experimental administration of excess iron in challenged pigs has been shown to result in increased time to recovery. Finally, immunologically stressed animals exhibit increased copper levels in plasma, because IL-1 enhances synthesis of ceruloplasmin, an acute phase protein that enhances lymphocyte response.
Vitamins
There is very little information on vitamin requirements during disease. A deficiency of vitamin E and selenium has been known to greatly reduce proliferation of lymphocytes in infected animals, whereas hyper-alimentation of these nutrients increases antibody production and lymphocyte proliferation; something that is highly beneficial. The role of other natural antioxidants is currently unknown, but it is believed to be along similar lines of action. In addition, vitamin A deficiency is associated with reduced epithelial integrity that could potentially facilitate the invasion of pathogenic agents via the gastrointestinal tract or the respiratory system.
Conclusions
The above can only serve as a brief introduction to this exciting topic and also as a guide towards more research that is needed to fully understand this issue. A deeper understanding of nutrient requirements during disease will enable animals to recover faster and reduce the negative effects of disease on growth performance.

英文來源：wattagnet.com

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