「红叶论坛一你的专属六」评估围产牛的脂肪动员和炎症反应

时间:2020-01-11 14:42:24

「红叶论坛一你的专属六」评估围产牛的脂肪动员和炎症反应

红叶论坛一你的专属六,作者: andres contreras

密歇根州立大学 兽医兽药学院

译者:郭闪闪,北京奥特奇生物制品有限公司

在脂肪细胞中,脂肪酸以甘油三酯分子的形式储存,且在能量负平衡时期(如围产期),作为主要的能量储备之一。脂肪动员是奶牛在脂肪细胞中释放或储存脂肪酸的代谢过程。

脂肪动员包括脂肪生成和脂肪分解。脂肪生成是由脂肪酸和甘油合成甘油三酯。在脂肪分解过程中,脂肪细胞的酶分解甘油三酯分子,产生nefa(非酯化脂肪酸)。

在围产期,低干物质采食量和由分娩和开始泌乳驱动的高能量的需求促进了脂肪分解,减少了脂肪细胞的脂肪生成。因此,nefas被释放到血液中。正常情况下,随着泌乳期的延长,脂肪分解减少,脂肪生成增加补充甘油三酯的储备。然而,过多的脂肪动员会使奶牛更容易患上酮症、真胃移位和脂肪肝等疾病。

脂解诱发炎症

围产期奶牛脂肪分解增加和脂肪生成减少的后果不仅仅是血液循环中nefa的出现。脂肪分解还会诱导脂肪中的炎症反应,其特点是免疫细胞,尤其是巨噬细胞的浸润(见图1)。在有适度脂肪分解的健康围产牛中,巨噬细胞在脂肪组织总细胞占比低于1%。然而,在那些动员过多脂肪的奶牛中,例如患有酮病的奶牛,巨噬细胞占脂肪细胞总数的3%以上。

适度的巨噬细胞浸润脂肪是有益的,因为这些细胞清除脂肪分解产生的有毒产物的脂肪组织,包括nefa,双甘油酯和单甘油酯。然而,巨噬细胞的过度浸润会增加炎症,并进一步促进脂肪分解。这是因为脂肪巨噬细胞会分泌抑制胰岛素功能的阻滞剂。此外,过多的脂肪分解也会损害血液免疫细胞的功能。例如,高nefa浓度降低了血液白细胞杀死和清除入侵病原体的能力,减少了抗体的产生,减缓了它们向感染部位的移动。因此,产生过多脂肪分解的围产牛更容易发生乳房炎和子宫炎。

调节围产期的脂肪动员

一个在围产期减少脂肪动员促进脂肪生成的基本管理和营养策略就是最大化干物质采食量。此外,产前几周通常观察到采食量的突然下降,如何减小采食量的降低幅度很重要。平衡产前日粮以满足但不超过能量需求也很重要。这通常是通过饲喂高水平的纤维来实现的。

在平衡干奶牛日粮时,有必要考虑到妊娠最后几周的能量的过多摄入会增加产后脂肪分解和脂肪肝风险。干奶期体况增加的奶牛的脂肪细胞更大,在产犊和泌乳早期对脂肪分解刺激会更敏感。

作为平衡日粮补充策略,可以考虑在围产牛日粮中加入限制脂肪动员的营养添加剂。饲喂烟酸可以通过阻断分解甘油三酯分子酶(激素敏感脂肪酶)的激活来减少脂肪组织的分解。在整个围产期饲喂烟酸可以有效减少脂肪分解。

评估围产期的脂肪动员

体况评分(bcs)是一种很好的皮下脂肪的测量方法,产后bcs的减少可以反映出脂肪分解的情况。成年牛在产犊时的bcs值应在3.0-3.5,而头胎牛的bcs值应在3.25-3.75,无论过瘦还是过肥都容易发生疾病。成母牛在产后的第一个月里损失的体况不能超过0.5分。

通过测定血浆nefa和β-羟丁酸(bhb)的浓度,还可以间接判断脂肪分解。理想情况下,在泌乳期开始第一周,牛的nefa值应低于500 meq/l, bhb浓度应低于12.5 mg/dl。脂解率也可以用群体或个体牛只的脂蛋比判断。乳脂随着血浆nefa的增加而增加。泌乳开始第一周脂蛋比高于2的奶牛产后发生胎衣不下、真胃移位、临床子宫内膜炎以及在泌乳末期被淘汰的风险更高。

有必要指出的是单独的脂肪动员指标并不能提供足够的信息来决定围产期的管理策略。但是将多个指标一起分析并结合健康、生产、营养和环境数据时,则更容易识别长期高度脂肪动员相关的代谢问题。

总之,分娩时脂肪的过度动员会促进脂肪组织的炎症反应,继而引发恶性循环,过多脂肪分解加剧脂肪炎症反应,进而加剧脂肪分解,减少脂肪生成反应。

附原文

assessing fat mobilization and inflammation in transition cows

author: andres contreras

college of veterinary medicine

michigan state university

within fat cells, fatty acids are stored as triglyceride molecules and constitute one of the main energy reserves that are mobilized during times of negative energy balance, such as the transition period. fat mobilization is the metabolic process that cows use to either release or store fatty acids within fat cells.

fat mobilization includes lipogenesis and lipolysis. lipogenesis is the assembly of triglycerides from fatty acids and glycerol. during lipolysis, fat cells’ enzymes break down the triglyceride molecule and release fatty acids as nefa (non-esterified fatty acid).

during the transition period, low dry matter intake (dmi) and high energy needs driven by parturition and the onset of lactation promote lipolysis and reduce lipogenesis in fat cells. as a result, nefas are released into the blood. normally, lipolysis decreases and lipogenesis replenishes fat depots’ triglyceride reserves as lactation progresses. however, cows that mobilize too much fat will become more susceptible to diseases such as ketosis, displaced abomasum and fatty liver.

lipolysis induces inflammation

the consequences of enhanced lipolysis and reduced lipogenesis in fat of transition cows go beyond the release of nefa into circulation. lipolysis also induces an inflammatory response in fat that is characterized by infiltration of immune cells, especially macrophages (see figure 1). in healthy transition cows that have moderate lipolysis, macrophages account for less than 1 percent of the total number of cells in fat tissues. however, in cows that mobilize too much fat, such as those with ketosis, macrophages account for greater than 3 percent of the total number of cells in fat.

a moderate infiltration of macrophages into fat is beneficial as these cells clean up the fat tissues from the toxic products that lipolysis produces, including nefa, diglycerides and monoglycerides. excessive infiltration of macrophages, however, will increase inflammation and further promote lipolysis. this is because fat macrophages secrete potent blockers of insulin function. in addition, excessive lipolysis also impairs the function of blood immune cells. for example, high nefa concentrations reduce the capacity of blood leukocytes to kill and clear invading pathogens, impair the production of antibodies and slow down their migration to sites of infection. therefore, transition cows with high lipolysis may be more susceptible to mastitis and metritis.

modulating lipid mobilization in the transition period

a basic management and nutritional strategy that reduces lipolysis and promotes lipogenesis in the transition period is maximizing dmi. in addition, it is necessary to limit the sudden drop in feed intake commonly observed during the final weeks of the dry period. it is also imperative to balance prepartum diets to meet but not exceed energy requirements. this is usually accomplished by feeding high levels of fiber.

when balancing rations for dry cows, it is necessary to consider that overfeeding energy in the last weeks of gestation enhances lipolysis postpartum and increases the risk of fatty liver. cows that gain excessive body condition during the dry period have larger fat cells that are more sensitive to lipolysis stimuli at calving and early lactation.

to complement ration balancing strategies, the inclusion of nutritional supplements that limit lipid mobilization in the diet of transition cows can be considered. feeding niacin (as nicotinic acid) reduces adipose tissue lipolysis by blocking the activation of the enzyme that breaks down triglyceride molecules, hormone-sensitive lipase. in order to be effective, niacin needs to be fed throughout the entire transition period in order to reduce lipolysis effectively.

evaluating lipid mobilization during the transition period

body condition score (bcs) is a good measure of subcutaneous fat, and the reduction in bcs after parturition is a reflection of lipolysis rates. mature cows should approach calving with a bcs of 3.0 to 3.5 and heifers with 3.25 to 3.75, as excessively thin or overconditioned cows are more susceptible to disease. mature cows should not lose more that 0.5 bcs points during the first month after calving.

lipolysis is also measured indirectly by the concentration of plasma nefa and beta hydroxybutyrate (bhb). ideally, during the first week of lactation, cows should have nefa values of less than 500 millequivalent per liter (meq/l) and bhb concentrations below 12.5 milligrams per deciliter (mg/dl). the rate of lipolysis can also be evaluated at the group or individual animal level using the milkfat-to-milk-protein percentage ratio. milkfat increases as plasma nefa rise. cows with milkfat-to-milk-protein ratio values higher than 2 during the first week after calving are at a higher risk for developing retained fetal membranes, a displaced abomasum, clinical endometritis and being culled before the end of lactation.

it is necessary to note that a single marker of lipid mobilization does not provide enough information to support management decisions during the transition period. however, when multiple markers are analyzed together and combined with health, production, nutritional and environmental data, it is easier to identify metabolic problems related to extended periods of intense fat mobilization.

in conclusion, excessive fat mobilization around parturition promotes inflammation in fat tissues that triggers a vicious cycle where excessive lipolysis can exacerbate fat inflammation, which in turn further intensifies lipolysis and reduces lipogenesis.

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