Linoleic biohydrogenation, milk

It is not certain that the presence of CLA in tissue lipids is due entirely to the production of cis-9, trans-11 as an intermediate during the biohydrogenation of linoleic acid in the rumen. However, the amount of CLA in milk (7 J) and butter (14) is positively correlated to the level of dietary linoleic acid. Some long chain fatty acid intermediates reach the small intestine and are normally absorbed and deposited into adipose tissue (75). There is seasonal variation in CLA content of milk, with the highest values occurring usually in summer (76). 

Cw-polyenoic acids are present at low concentrations in milk fat, because of the biohydrogenation reactions that take place in the rumen. These acids are comprised almost exclusively of linoleic acid (9c, 12c-18 2), about 1.2 to 1.7% and a-linolenic acid (9c, 12c, 15c-18 3), about 0.9 to 1.2% (Table 1.2). These two fatty acids are essential fatty acids they cannot be synthesised within the body and must be supplied by the diet. In recent times, the usage of the term essential has been extended to include derivatives of these fatty acids, which are not synthesised in significant quantities (e.g., eicosapentaenoic acid, 20 5 and docosahexaenoic acid, 22 6). The proportion of a-linolenic acid appears to be affected by the cow s diet the concentration is higher in milk from pasture-fed cows than in milk from barn-fed cows (Hebeisen et al., 1993 Wolff et al., 1995). In the case of linoleic... 

The term conjugated linoleic add (CLA) refers to a mixture of positional and geometric isomers of linoleic add with a conjugated double bond system milk fat can contain over 20 different isomers of CLA. CLA isomers are produced as transient intermediates in the rumen biohydrogenation of unsaturated fatty acids consumed in the diet. However, cis-9, trans-11 CLA, known as rumenic acid (RA), is the predominant isomer (up to 90% of total) because it is produced mainly by endogenous synthesis from vaccenic acid (VA). VA is typically the major biohydrogenation intermediate produced in the rumen and it is converted to RA by A9-desaturase in the mammary gland and other tissues. 

Information on the effect of diet on the production of minor isomers of CLA in the rumen and alterations in their content in milk fat is limited. Diet-induced changes in trans-10, cis-12 CLA have been best described, and its biological effects in the dairy cow will be discussed in Section 3.6.1. Griinari and Bauman (1999) presented a putative pathway for the biohydrogenation of linoleic acid where the initial isomerization involved the cis-9 double bond, thereby resulting in the production of trans-10, cis-12 CLA and trans-10 18 1 as intermediates. As discussed earlier, rumen bacteria have been identified that produce trans-10, cis-12 CLA when incubated with linoleic acid (Verhulst et al., 1987 Kim et al., 2002), and the addition of trans-10, civ-12 CLA to the rumen results in the increased formation of trans-10 18 1 (Loor and Herbein, 2001). 

Figure 3.5. Generalized scheme of ruminal biohydrogenation of linoleic acid under normal conditions (solid line) and during diet-induced milk fat depression (dotted line). Adapted from Griinari and Bauman (1999).

Fatty acids with two or more conjugated double bonds are found in some plants and animals. In tissues of ruminant animals (and, hence, in meat and dairy products), fatty acids with conjugated diene system were detected as intermediates or by-products in the biohydrogenation of linoleic acid by microorganisms in the rumen. The main isomer, 9-cis, ll-fran -octadecadienoic acid, may account for up to 1% of the total fatty acids of milk fat. 9-cis, ll-fran5-15-cw-octadecatrienoic acid, derived from a-linolenic acid, is present in ruminant tissues only in trace levels. This fatty acid has been shown to have several medical properties, especially anti-cancer and anti-atherosclerosis effects. 

Reports suggest that feeding lipid sources rich in linoleic and linolenic acids either as seeds or free oil increases the CLA content of milk when oil is accessible to the rumen microorganisms for biohydrogenation (Dhiman et al., 2000). The scientists found that supplementing the dietary dry matter with 2% or 4% soybean resulted in a 237% or 314% increase in CLA content of milk compared with the control. 

Conjugated linoleic acid (CLA) is naturally present in milk, dairy products, and the meat of ruminants (1). Ruminants are the major dietary source of this fatty acid because of the unique abihty of rumen bacteria to convert linoleic acid into cis-9,trans- CLA (c9,tll-CLA) (1). This reaction is part of a process that takes place in the rumen it is called biohydrogenation and it converts linoleic acid [or, less efficiently, other 18-carbon polyunsaturated fatty acids (PUFA) with double bonds located at 9 and 12 positions] to stearic acid (1). During this process, vaccenic acid (tl 1-18 1) is formed. This acid can be converted to CLA in all organisms that possess A9-desaturase (2). 

Elaidic acid (18 1 9t) is easily made from oleic acid (Section 10.9) but is very rare among natural acids. Vaccenic acid (18 1 Ilf), present in small quantities in depot and milk fats of ruminant animals is probably produced from dietary linoleic acid by biohydrogenation occurring in the rumen. The cis isomer is a major bacterial fatty acid (Section 3.8). 

In the mid-1900s there was increased interest in the process of biohydrogenation, which occurs in the rumen, and is responsible for the formation of both the trans monoenoic acids and the conjugated linoleic acids found in the rumen and milk. 

In food, CLA is found in milk fat, the tissue fat of ruminant animals, and products derived from them, although there are exceptions. About 90% of the CLA is represented by the cis-9, trani-11-isomer or rumenic acid (RA) (4). This isomer is produced in the rumen by the action of a linoleic acid isomerase on dietary linoleic acid as a first step in the biohydrogenation process. Further hydrogenation produces trans- - %. or vaccenic acid (VA), the predominant trans monounsaturat-ed fatty acid of milk and animal tissue fat. The major polyunsaturated fatty acids of pasture, a-and y-linolenic acid, cannot be converted to RA, but they do produce VA. A proportion of RA and VA escape further hydrogenation in the rumen and after absorption pass via the circulatory system to adipose tissue and the mammary gland where A -desaturases convert VA to RA. About 70% of RA in milk fat is produced by this pathway (5). 

Not only does dietary linoleic acid result in increases in the CLA content of milk fat, but it is also thought to inhibit the biohydrogenation of trans-waccenic... 

In contrast to cis-9, trans-11 and trans-1, cis-9, the other isomers of CLA found in milk and body fat of ruminants appear to originate exclusively from rumen output. These are detected in rumen fluid (61) and duodenal fluid (39), and estimates of duodenal flow indicate that rumen output of these minor cis/trans, cis-cis, and trans-trans CLA isomers is greater than the trace amounts secreted in milk fat (39). The common theme to endogenously synthesized CLA isomers is A -desaturase and the cis-9 double bond that is added to trans-1 and trans- 1 monoenes. In contrast, there has been no demonstration that other mammalian desaturases act in a manner analogous to A -desaturase to synthesize CLA endogenously from mono-unsaturated fatty acids. Thus, these other CLA isomers found in trace levels in ruminant fat are of rumen origin and must represent intermediates in the ruminal biohydrogenation of linoleic and linolenic acids. 

Shingfield, K.J., Ahvenjarvi, S., Toivonen, V., Arola, A., Huhtanen, P., and Griinari, J.M. (2002) Fish Oil Inhibits the Biohydrogenation of Fatty Acids in the Rumen Causing an Increase in Milk frans-Octadecenoic and Conjugated Linoleic Acid Content, J. Dairy Sci. 85 (Suppl. 1), 143 (Abstr.). 

Unpublished results of own experiments also did not confirmed the potential of essential oils to modulate rumen biohydrogenation. None of used essential oils originating from Eugenia caryophyllata, Abies sibirica or Vanilla planifolia improve concentration of conjugated linoleic acid isomers in the rumen and milk of dairy cows. 

A higher intake of concentrate, especially when associated with maize silage, induces a shift in rumen biohydrogenation towards the synthesis of trans- 0, cis- 2 C18 2 and trans-lO C18 1 (Chilliard et al, 2007). Among animal sources, fish oil is more effective than plant oils in enhancing CLA, VA and omega-3 FA in milk fat, especially when fish oil is fed in combination with oil supplements rich in linoleic acid (Figure 1). 

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