Although milk fat depression (MFD) has been observed and described since the beginning of the last century, all the molecular and biochemical mechanisms involved are still not completely understood. Some fatty acids (FA) originating during rumen biohydrogenation have been proposed as causative elements of MFD. However, contradictory results were obtained when studying the effect of single FA on MFD. An alternative could be the simultaneous evaluation of the effect of many FA using a multivariate approach. The aim of this study was to evaluate the relationship between individual milk FA of ruminal origin and MFD using canonical discriminant analysis, a multivariate technique able to distinguish 2 or more groups on the basis of a pool of variables. In a commercial dairy herd, a diet containing 26% starch on a DM basis induced an unintentional MFD syndrome in 14 cows out of 40. Milk yielded by these 14 animals showed a fat content lower than 50% of the ordinary value, whereas milk production and protein content were normal. The remaining 26 cows secreted typical milk fat content and therefore were considered the control group, even though they ate the same diet. The stepwise discriminant analysis selected 14 milk FA of ruminal origin most able to distinguish the 2 groups. This restricted pool of FA was used, as variables, in a run of the canonical discriminant analysis that was able to significantly discriminate between the 2 groups. Out of the 14 FA, 5 conjugated linoleic acid isomers (C18:2 trans-10,trans-12, C18:2 trans-8,trans-10, C18:2 trans-11,cis-13, C18:2 cis-9,cis-11, C18:2 cis-10,cis-12) and C15:0 iso were more related to the control group, whereas C18:2 trans-10,cis-12, C16:1 trans-6–7, C16:1 trans-9, C18:1 trans-6–8, C18:1 trans-9, C18:1 trans-10, C18:1 cis-11, and C18:3n-3 were positively associated with the MFD group, allowing a complete discrimination. On the basis of these results, we can conclude that (1) the shift of ruminal biohydrogenation from C18:1 trans-11 to C18:1 trans-10 seemed to be strongly associated with MFD; (2) at the same time, other C18:1 trans isomers showed a similar association; (3) on the contrary, conjugated linoleic acid isomers other than C18:2 trans-10,cis-12 seemed to be associated with a normal fat secretion. Results confirmed that MFD is the consequence of a combined effect of the outflow of many ruminal FA, which collectively affect mammary fat synthesis. Because the animals of the 2 groups were fed the same diet, these results suggested that factors other than diet are involved in the MFD syndrome. Feeding behavior (i.e., ability to select dietary ingredients in a total mixed ration), rumen environment and the composition of ruminal bacteria are additional factors able to modify the products of rumen biohydrogenation. Results of the present work confirmed that the multivariate approach can be a useful tool to evaluate a metabolic pathway that involves several parameters, providing interesting suggestions about the role of some FA involved in MFD. However, results about the MFD syndrome obtained in the present research require a deep molecular investigation to be confirmed.

尽管自上个世纪初以来就已经观察到并描述了乳脂降低（MFD），但是仍未完全了解所涉及的所有分子和生化机制。已提出在瘤胃生物氢化过程中产生的某些脂肪酸（FA）作为MFD的致病因素。但是，当研究单一FA对MFD的影响时，得出了矛盾的结果。一种替代方法是使用多变量方法同时评估许多FA的效果。这项研究的目的是使用规范判别分析来评估瘤胃来源的单个乳FA与MFD之间的关系，该判别分析是一种多变量技术，能够基于一组变量来区分2个或更多组。在一个商业奶牛场，DM中含有26％淀粉的日粮在40头母牛中有14头会引起意外的MFD综合征。这14只动物生产的牛奶的脂肪含量低于正常值的50％，而牛奶产量和蛋白质含量却正常。其余26头母牛分泌典型的牛奶脂肪含量，因此即使它们吃相同的饮食也被认为是对照组。逐步判别分析选择了最能区分两组的14种瘤胃乳FA。在有限的经典判别分析中，FA的这种受限池被用作变量，从而能够在两组之间进行明显的判别。在14种FA中，有5种共轭亚油酸异构体（C18：2 牛奶产量和蛋白质含量正常。其余26头母牛分泌典型的牛奶脂肪含量，因此即使它们吃相同的饮食也被认为是对照组。逐步判别分析选择了14个瘤胃来源的牛奶FA，最能区分这两组。在有限的经典判别分析中，FA的这种受限池被用作变量，从而能够在两组之间进行明显的判别。在14种FA中，有5种共轭亚油酸异构体（C18：2 牛奶产量和蛋白质含量正常。其余26头母牛分泌典型的牛奶脂肪含量，因此即使它们吃相同的饮食也被认为是对照组。逐步判别分析选择了14个瘤胃来源的牛奶FA，最能区分这两组。在有限的经典判别分析中，FA的这种受限池被用作变量，从而能够在两组之间进行明显的判别。在14种FA中，有5种共轭亚油酸异构体（C18：2 在有限的经典判别分析中，FA的这种受限池被用作变量，从而能够在两组之间进行明显的判别。在14种FA中，有5种共轭亚油酸异构体（C18：2 在有限的经典判别分析中，FA的这种受限池被用作变量，从而能够在两组之间进行明显的判别。在14种FA中，有5种共轭亚油酸异构体（C18：2反式-10，反式-12，C18：2反式-8，反式-10，C18：2反式-11，顺式-13，C18：2顺式-9，顺式-11，C18：2顺式-10，顺式- 12）和C15：0 iso与对照组更为相关，而C18：2反-10，顺-12，C16：1反-6-7，C16：1反-9，C18：1反-6– 8，C18：1反式-9，C18：1反式-10，C18：1顺式-11和C18：3n-3与MFD组呈正相关，从而可以完全区别对待。根据这些结果，我们可以得出以下结论：（1）瘤胃生物氢化从C18：1 trans -11转变为C18：1 trans -10似乎与MFD密切相关；（2）同时，其他C18：1反式异构体显示相似的缔合；（3）与此相反，共轭亚油酸比C18异构体的其他：2反式-10，顺式-12似乎与正常的脂肪分泌有关。结果证实MFD是许多瘤胃FA流出共同作用的结果，这些作用共同影响乳腺脂肪的合成。由于两组动物的饮食相同，这些结果表明MFD综合征与饮食无关。进食行为（即以总混合比例选择饮食成分的能力），瘤胃环境和瘤胃细菌的组成是能够改变瘤胃生物氢化产物的其他因素。本工作的结果证实，多元方法可以作为评估涉及多个参数的代谢途径的有用工具，为某些脂肪酸在MFD中的作用提供有趣的建议。然而，