Effect of Medium-Chain Fatty Acid Positional Distribution in Dietary Triacylglycerol on Lymphatic Lipid Transport and Chylomicron Composition in Rats
  【摘要】本研究通过胸导管插管评估日粮合成脂肪的中链脂肪酸（MCF）位置分布是否会影响大鼠膳食脂肪的淋巴转运和乳糜微粒的化学组成。本次实验准备了四种类型的合成甘油酯: (i) sn-1(3) MCF-sn 2亚油酸，(ii)酯化sn-1(3) MCF-sn 2亚油酸，(iii)sn-2 MCF-sn-1(3)亚油酸，(iv) 酯化sn-2 MCF-sn-1(3)亚油酸。通过淋巴管插管，由等量的合成脂肪和可可脂组成的纯化日粮强饲大鼠。研究结果显示：膳食脂肪中MCF的位置分布对淋巴回流，甘油三酯、磷脂的生成，乳糜微粒的脂质组成，或微粒大小均没有显著影响。合成脂肪中的MCF的位置分布在乳糜脂肪中仍然存在。以上结果表明：在膳食脂肪中的MCF被转运到了淋巴管并且其位置分布乳糜脂肪中同样存在。

在进行外科手术并且发生胰腺功能不全情况下，中链脂肪酸（MCF）的代表：辛酸和癸酸被认为在此情况下能够及时满足机体对能量需求的物质。与长链脂肪酸相比，因为MCF具有相对较高的极性和易于β-氧化，MCF通过胃肠道时容易被肠系膜静脉吸收。MCF以甘油三酯的形式在哺乳动物乳汁和椰子油中的含量相对较高。含有MCF的合成脂肪现在被广泛用于各种领域以提高MCF的生理功能。MCF的吸收似乎比长链脂肪酸更好，但是很少有人会关注MCF在长链脂肪酸与人造脂肪和膳食脂肪在吸收方面的相互作用。长链饱和脂肪酸在甘油三酯分子中的位置极大地影响了相关的肠道吸收率，并因此改变了其他长链脂肪酸的吸收速率。我们和其他的研究已经揭示了MCF在脂肪分子中位置的重要性。
偶然的一次机会，通过将脂肪乳剂注入胃或十二指肠，发现了大鼠的淋巴系统转运膳食脂肪。这种方法使数据的解释更简单，因为它不需要考虑脂肪乳剂与其他膳食成分的相互作用。相反，这也是该方法的一个缺点，因为膳食组分特别是蛋白质、碳水化合物和磷脂会从口腔到肠道对膳食脂肪的乳化的影响没有考虑进去。加之，用于此研究的大鼠并不总是从手术相关的压力中恢复过来的。为了解决这些问题，采用大鼠胸导管插管的方法，但应用这种方法，在膳食脂肪吸收时去分析膳食脂肪的淋巴运输是不普遍的。
在本研究中，饲养的大鼠日粮中添加了特殊结构脂肪含有sn-2或sn-1（3）的MCF及其酯化脂肪。通过淋巴管插管，大鼠吸收体内的这些膳食脂肪时，膳食脂肪的淋巴运输或淋巴中乳糜微粒的化学组成被检测出来。
 

以下是实验中的相关图表
 
 
表1：合成脂肪中总脂肪酸和sn-2脂肪酸
 
表2：日粮脂肪中总脂肪酸和sn-2脂肪酸     
 
图1：大鼠的日粮中添加sn-1(3)中链脂肪酸MCF+可可脂（sn-1(3)MCF-S）；酯化sn-1(3)MCF+可可脂（sn-1(3)MCF-I）；sn-2 MCF+可可脂（sn-2 MCF-S）；或酯化sn-2 MCF+可可脂（sn-2 MCF-I），从而计算淋巴流动速率。箭头表示食物消耗的终止。数值±SEM，n=6.
 
图2：大鼠的日粮中添加sn-1(3)中链脂肪酸MCF+可可脂（sn-1(3)MCF-S）；酯化sn-1(3)MCF+可可脂（sn-1(3)MCF-I）；sn-2 MCF+可可脂（sn-2 MCF-S）；或酯化sn-2 MCF+可可脂（sn-2 MCF-I）从而计算脂肪，胆固醇，磷脂在淋巴管中的生成速率。箭头表示食物消耗的终止。数值±SEM，n=6.在两个小组中，每个时期对应字母的数值显著不同（P<0.05）。缩写参见图1。
 
表3：大鼠淋巴管乳糜的化学组成和微粒大小
     日粮中添加Sn-1（3）MCF或Sn-2MCF类型的合成脂肪
 
表4：日粮中添加Sn-1（3）MCF或Sn-2MCF类型的合成脂肪饲喂小鼠之后检测淋巴管乳糜脂肪的脂肪酸组成
     综上所述，该研究证实了日粮脂肪中的MCF能够被运输到淋巴管内，并且其位置分布在乳糜脂肪中也同样存在。
  The present study was carried out to examine ifthe positional distribution of medium-chain fatty acid (MCF) indietary synthetic fat influences lymphatic transport of dietary fat and the chemical composition of chylomicrons in rats with permanent cannulation of thoracic duct. Four types of synthetic triacylglycerol were prepared: (i) sn-1(3)MCF-sn2 linoleic acid, (ii) interesterified sn-1(3)MCF-sn2 linoleic acid, (iii)sn-2MCF-sn-1(3)linoleic acid, and (iv)interesterified sn-2 MCF-sn-1(3) linoleic acid. A purified diet composed of equal amounts of the synthetic fat and cocoa butter was given to rats with permanent lymph duct cannulation. The positional distribution of MCF in the dietary fat had no significant effect on the lymph flow, triacylglycerol output, phospholipid output, lipid composition of chylomicrons, or the particle size. The positional distribution of MCF in the synthetic triacylglycerol was maintained in the chylomicron triacylglycerol. These results showed that MCF in the dietary triacylglycerol is transported into lymphatics and the positional distribution is well preserved in chylomicron triacylglycerol.
Octanoic and decanoic acids are representative medium-chain fatty acids (MCF) that meet a prompt demand for energy under conditions where surgical operations are carried out and pancreatic insufficiency occurs (1,2). MCF, compared with longer-chain fatty acids, are readily absorbed through the stomach as well as intestine via the portal route, as they have a relatively high polarity and susceptibility to β-oxida- tion (1,2). MCF are included to a relatively greater extent in mammalian milk and coconut oils as a form of triacylglyc- erol. Synthetic fats containing MCF are now used in various fields to improve the physiological properties of MCF (3–11). Absorption of MCF seems to be better than longer-chain fatty acids, but little attention has been directed to the interactive effects of MCF on longer-chain fatty acids with regard to ab- sorption of synthetic fats and dietary fats (6,7,12). The position of longer-chain saturated fatty acids in a triacylglycerol molecule greatly influences related intestinal absorption and consequently modifies absorption rates of other longer-chain fatty acids (13–19). The importance of the position of MCF in a triacylglycerol molecule has also been advocated by us (5) and others (6,7,9–12,20,21).
Occasionally, lymphatic transport of dietary fats in rats has been measured by infusing fat-emulsion into the stomach or duodenum (22). This method has an advantage of making the interpretation of data easy since it does not need to take the in- teraction of lipid emulsion with other dietary components into account. Conversely, this is a drawback of this method, since the interaction of dietary components, particularly proteins, carbohydrates, and phospholipids, which influence emulsifi- cation of dietary fats from the mouth to intestine, are not taken into consideration (23). In addition, rats used for this purpose do not always recover from surgery-related stress (24). To ad- dress these problems, permanent cannulation of the thoracic duct has been done in rats (24,25), but application of this method, to characterize the lymphatic transport of dietary lipids during active dietary fat absorption, is not widespread.
In the present studies, rats were kept on diets supplemented with structure-specific fats containing MCF either in sn-2 or sn-1(3) and their interesterified fats. The lymphatic transport of dietary fats or chemical composition of lymph chylomi- crons during active absorption of these dietary fats was deter- mined in rats with a permanent lymph duct cannulation. In summary, our studies confirmed that MCF in the dietary triacylglycerols are transported into lymphatics and the positional distribution is well preserved in chylomicron triacylglycerol










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