Diet, rumen biohydrogenation and nutritional quality of cow and goat milk fatThe potential to modify the milk fatty acid (FA) composition by changing the cow or goat diets is reviewed. Ruminal biohydrogenation (RBH), combined with mammary lipogenic and D-9 desaturation pathways, considerably modifies the profile of dietary FA and thus milk composition. The pasture has major effects by decreasing saturated FA and increasing FA considered as favorable for human health (c9-18:1, 18:3n-3 and c9t11-CLA), compared to winter diets, especially those based on maize silage and concentrates. Plant lipid supplements have effects similar to pasture, especially linseed, but they increase to a larger extent, simultaneously several trans isomers of 18:1 and, conjugated or non-conjugated 18:2, especially when added to maize silage or concentrate-rich diets. The goat responds better for milk 18:3n-3 and c9t11-CLA, and sometimes less for c9-18:1, and is less prone to the RBH trans-11 to trans-10 shift, which has been shown to be time dependent in the cow. The respective physiological roles of most milk trans FA have not been studied to date, and more studies in rodents and humans fed dairy products modified by changing ruminant diet are required before recommending a larger use of lipid sources and how to combine them with the different feeding systems used by dairy farmers.Keywords: Diet composition, biohydrogenation intermediates, mammary metabolism, fatty acid desaturation, milk fatty acids. 828 DOI 10.1002/ejlt.200700080 Eur. J. Lipid Sci. Technol. 109 (2007 IntroductionMilk fat secretion and milk fatty acid (FA) composition are of great interest with regard to human nutrition. Apart from their contribution to dairy products' sensorial quality and to the amount of dietary energy, different lipid and FA compounds (short-and medium-chain saturated, branched, mono-and polyunsaturated, cis and trans, conjugated FA, etc.) present in ruminant milk fat are indeed potentially positive or negative factors for the health of consumers [1][2][3].Dairy products provide indeed 25-60% of the overall saturated fat consumption in Europe, which makes them, since decades, a target of dieticians' criticism due to the negative effects of excessive consumption of saturated FA on human health [4]. The image of saturated FA should, however, be weighed by the fact that C 12 -C 16 saturated FA are thought to be atherogenic only when consumed in excessive amounts, that 18:0 has no atherogenic effect and that saturated fat could even be protective when compared to a low-fat, high-carbohydrate diet [5,6]. The allegedly atherogenic effect of certain trans monounsaturated FA (MUFA) [7] has not been confirmed for vaccenic acid (t11-18:1), the main isomer present in milk [8,9]. The intake of some trans isomers of 18:2 seems to be particularly harmful, although further research is needed to discriminate between industrial and ruminant isomer profiles [10]. In other respects, it has been shown in humans that the consumption of milk fat [11] could sometimes d...

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Numerous experiments have studied the use of oilseed supplements in cow diets to alter milk fatty acid (FA) composition, but no quantitative synthesis of these studies is currently available. This article reports a meta-analysis of the response of cow milk FA composition to oilseed lipid supplements from linseed, rapeseed, soybeans, and sunflower seed. First, from a database of 145 oilseed supplementation experiments, we collected the mean FA percentages observed with unsupplemented diets and diets supplemented with the 4 oilseeds given as seeds (after various types of processing), as oils (including Ca salts and amides), or in protected forms. Second, we studied the response of the major milk FA percentages to increasing amounts of supplemental lipids from the 4 oilseeds. Responses were nonsignificant, linear, or quadratic, depending on the FA studied and the supplement. Effects of interfering factors, such as supplement form, forage component of the diet, or lactation stage, were difficult to assess from the available data. Third, we studied the response of the major milk FA percentages to increasing dietary intakes of linoleic or linolenic acids, taken separately. Overall, these results confirm the high plasticity of milk FA composition, with the widest variations being observed in the percentages of medium-chain versus C18 FA, and among the C18 in 18:0, cis-18:1, and trans-18:1. The percentages of the polyunsaturated FA cis-9 cis-12-18:2 and 18:3 were less variable, except when protected lipids (mostly formaldehyde treated) were supplied. However, trans-18:1 and polyunsaturated FA (including conjugated linoleic acid) exhibited the greatest variations when expressed relative to their respective basal values (for unsupplemented diets). Oils, compared with seeds, induced greater percentages of trans-18:1 and tended to decrease C6 to C12 FA more. Intakes of 18:2- and 18:3-rich lipid sources did not differ greatly in their effects on short- and medium-chain FA and trans-18:1 percentages, although the profiles of individual 18:1 and 18:2 isomers in milk differed. This meta-analysis provides quantitative estimates, obtained from the extensive literature produced over more than 40 yr, of the impact of oilseed supplements on milk FA composition.

Forages, through the amount and composition of their fatty acids (FA), and because they represent a major part of ruminant diets, can help improve the nutritional quality of milk and meat. However, no comprehensive dataset is available to estimate fat and FA content and composition of forages. This study used the available data on fat and FA content and composition of forages to (i) compute mean composition values for the main forages, and (ii) estimate the influence of forage conservation, cultivation and harvest conditions on fat and FA content and composition. We report mean values for the main forage species in the form of fresh forage, silage or hay. The main factor influencing fat and FA composition was vegetation stage of forage at harvest (estimated by the month of harvest or regrowth interval). Compared with fresh forage at harvest, wilting or drying forages (especially in bad drying conditions) altered their FA, whereas unwilted silage, the use of ensiling additives and N fertilization had only minor effects. The differences between grass (except corn) and legume species were lower than those induced by vegetation stage and wilting or drying. We gave equations to estimate the effects of these factors and thus refine the estimation of the FA content and composition of the forages. Total FA content and proportion of linolenic acid were positively related to crude protein, and negatively related to fiber content of the forages.