Effects of Dietary Fatty Acids on Milk Quality and Reproduction
Prof. DR. Phil Garnsworthy
Professor of Dairy Science, University of Nottingham, School of Biosciences,
Sutton Bonington Campus, Loughborough LE12 5RD, UK
Email: Phil.Garnsworthy@nottingham.ac.ukThis e-mail address is being protected from spambots, you need JavaScript enabled to view it
Origins of milk fatty acids
Shorter-chain fatty acids (C4:0 – C14:0 and half of the C16:0) in milk originate by de novo synthesis in the mammary gland from circulating acetate and b-hydroxybutyrate (Mansbridge and Blake, 1997). Acetate and butyrate are produced in the rumen during microbial fermentation of cellulose, hemicellulose and sugars, and butyrate is converted to b-hydroxybutyrate in the rumen wall.
Longer-chain fatty acids (half of the C16:0 and C18 – C24) in milk are absorbed from the bloodstream. They originate mainly from dietary fatty acids that have undergone biohydrogenation in the rumen to protect rumen microorganisms from the deleterious effects of unsaturated fatty acids (Garnsworthy, 1997). Some longer-chain fatty acids (C16:0, C18:0 and C18:1) are synthesised in adipose tissue and appear in milk following body fat mobilisation (Salter et al. 2002). A proportion of longer-chain saturated fatty acids, particularly C18:0, are converted to mono-unsaturated fatty acids by the Δ9-desaturase enzyme within the mammary gland (Jensen, 2002). This enzyme is also responsible for converting vaccenic acid (t11-C18:1) to conjugated linoleic acid (CLA; c9,t11-C18:2) (Bauman et al., 2001).
Trans fatty acids (mainly t11-C18:1 and t10-C18:1) are formed by rumen microorganisms as intermediates during biohydrogenation of linoleic (c9,c12-C18:2) and linolenic (c9,C12,C15-C18:3) acids to stearic acid (C18:0), although they can also originate from hydrogenated vegetable oil in the diet (Griinari and Bauman, 2003).
Dietary factors affecting milk fatty acids
In general, factors that influence volatile fatty acid concentrations in the rumen affect fatty acid synthesis in the mammary gland through the availability of precursors. Therefore, milk fat concentration varies directly with the forage to concentrate ratio or the fibre content of the diet (Sutton, 1985). However, it is also suggested that synthesis of short-chain fatty acids can be inhibited by diets with high proportions of starchy concentrates or long-chain fatty acids (Thomas and Martin, 1988; Beaulieu and Palmquist, 1995). High starch, low fibre diets result in milk with a low fat content, which has characteristic increases in trans fatty acids, especially t10-C18:1 and t10,c12 CLA (Griinari and Bauman, 2003).
The proportions of long-chain fatty acids in milk are determined primarily by their dietary concentrations, although they can also reflect rate of body fat mobilisation (Salter et al. 2002). Polyunsaturated fatty acids from vegetable or fish oils have a low rate of transfer from diet to milk due to rumen biohydrogenation (Garnsworthy, 1997).
Lock and Garnsworthy (2002) found that biohydrogenation of C18 fatty acids varied with the number of double bonds when comparing dietary intake with duodenal flow. The extent of biohydrogenation in the rumen was 95-97% for C18:3, 85-91% for C18:2 and 48-76% for c9-C18:1. Duodenal flow of C18:0, the normal end product of biohydrogenation, was 240% of dietary intake.
The natural diet of dairy cows normally has a fat content of less than 50 g/kg. Rumen fermentation is disrupted if supplementary fat increases total dietary fat content to more than 100 g/kg (Garnsworthy, 1997). Bypass fats have been developed that largely overcome these problems. Examples of bypass fats include whole oilseeds, calcium salts of palm acid oil (CaPO), fatty acids encapsulated in formaldehyde-treated casein, and fatty acids with high melting point and small particle size. In a recent study, five levels of CaPO (0 to 0.8 kg/d) were added to a high-starch TMR. As CaPO level increased, we observed a quadratic decrease in de novo synthesis of FA (C6-14) and a quadratic increase in C18:1c9, reflecting differences in supply of C18:1c9. Proportions of C16:0, CLA and trans fatty acids increased with CaPO level; proportions of n3-PUFA decreased with CaPO level (Garnsworthy, 2006).
The n3 fatty acids C20:5 and C22:6 are of interest because of potential benefits for human health. However, Wonsil et al. (1994) found that although plasma concentrations of these fatty acids were increased by supplementation with menhaden oil, they were not detectable in milk. Mansbridge and Blake (1997) found that only 5% of dietary C20:5 and 3% of dietary C22:6 appeared in milk fat. Recently, we have studied the effects of fish oil on milk fat depression and found that fish oil increases the proportions of all trans fatty acids, but the degree of depression is more closely related to concentration of c11-C18:1 in milk than t10-C18:1 (Gama et al. 2008).
Conjugated linoleic acid is also of interest in human health because of possible reductions in cancer, diabetes, plasma lipids and fat deposition (Whigham et al. 2000; Pariza et al. 2001). Some CLA is produced in the rumen as a biohydrogenation intermediate, but CLA concentration in milk is mainly related to production of t11-C18:1 in the rumen, and to activity of Δ9-desaturase enzymes in the mammary gland. Lock and Garnsworthy (2002) showed that C18:2 increased CLA in milk through production of rumen CLA, but C18:3 increased CLA in milk through production of t11-C18:1. They also showed that 75-80% of CLA in milk was produced by the Δ9-desaturase enzyme in the mammary gland. CLA is doubled by grass-based diets, which are high in C18:3 compared with silage-based diets, because C18:3 is partially hydrogenated to t11-C18:1, which is subsequently desaturated to CLA (Lock and Garnsworthy, 2003).
Animal factors affecting milk fatty acids
Ruminant adipose tissue is rich in C16:0, C18:0 and C18:1, large proportions of which are synthesized de novo (Salter et al. 2002). The primary product of fatty acid synthesis is C16:0, which is synthesised from acetyl-coA by acetyl-coA carboxylase and fatty acid synthetase. In adipose tissue, but not mammary tissue, C16:0 may be further elongated by elongase to produce C18:0; mammary tissue lacks the elongase enzyme which catalyses this reaction. In both tissues, a double bond may then be inserted to form c9-C18:1. This last step is catalysed by D9-desaturase, which appears to be rate limiting in the production of c9-C18:1 in adipose tissue (Ntambi, 1995).
In addition to dietary factors, therefore, animal factors that alter deposition and mobilisation of body fat reserves will also influence supply of fatty acids to the mammary gland. Hormonal control of energy partition is by growth hormone, insulin, glucagon, IGF and leptin. All of these animal factors that influence mobilisation of body fat will influence the supply of C16:0, C18:0 and C18:1 to the mammary gland, and hence their appearance in milk.
Another animal factor that influences the fatty acid profile of milk is D9-desaturase activity in the mammary gland. Lock and Garnsworthy (2002) showed that there was considerable variation among individual cows in the activity of this enzyme, independent of dietary effects. In a large genetic study of D9-desaturase activity in dairy cows (Royal and Garnsworthy, 2005), we found significant effects of season, parity, days postpartum and farm, reflecting differences in diet and body fat mobilisation. When these factors were included in the analysis, we found that heritability of D9-desaturase activity is approximately 30 %. Stage of lactation does not affect D9-desaturase activity, although it does affect de novo synthesis of fatty acids (Garnsworthy et al. 2006).
Insulin has been shown to increase expression of the SCD gene in sheep adipose tissue by up to 5 times (Salter et al. 1999; Daniel et al. 2001; 2002). Euglycaemic-clamp studies suggested that D9-desaturase activity is also increased by insulin in the lactating bovine mammary gland (Corl et al. 2001). Our recent studies suggest, however, that dietary-induced changes in insulin do not affect D9-desaturase activity in dairy cows (Garnsworthy, 2006).
Effects of fatty acids on reproduction in dairy cows
Supplementary fat can increase dietary energy concentration and energy status of the cow, thereby improving potential fertility. However, fatty acids also appear to affect reproductive processes in ways that are not related to energy. For example, increased availability of fatty acid precursors allows increased steroid and eicosanoid secretion which can alter ovarian and uterine function. At the cell level, fatty acids may have direct effects on transcription of genes that encode proteins essential to reproductive events, and may also affect membrane structure.
Prostaglandin synthesis
The precursor for dienoic prostaglandins, such as PGF2α, is arachidonic acid (C20:4), which is synthesised from linoleic acid (C18:2); the precursor for trienoic prostaglandins, such as PGF3α, is eicosapentaenoic acid (C20:5), which is synthesised from linolenic acid (C18:3) (Abayasekara and Wathes, 1999). Competition between omega-3 and omega-6 precursors for desaturation and elongation, as well as for the site of prostaglandin synthesis, means that increasing the supply of C18:3 will decrease production of dienoic prostaglandins (Barnouin and Chassagne, 1991).
Embryo mortality during early stages of pregnancy is an important cause of infertility in dairy cows. This period of embryonic loss coincides with inhibitory effects of embryos on prostaglandins and evidence suggests that a proportion of embryos are unable to inhibit PGF2α leading to regression of CL and reduced progesterone secretion (Thatcher et al. 1995). Cheng et al. (2001) examined the effects of a control, a high 18:2 diet (protected soya) and a high C18:3 diet (protected linseed) on uterine PG synthesis in the lactating dairy cow. Basal concentrations of PGs were significantly lower (>50% reduction) in the 18:2-fed group compared with the other groups. The C18:3 diet did not alter basal PGF2α or PGE2 but increased 6-keto-PGF1α. They concluded that dietary PUFA intake can inhibit PG production in bovine endometrial explants, with a more pronounced effect following C18:2 rather than C18:3 supplementation. The authors suggest that a high C18:2 diet reduces endometrial capacity to produce PGs and may therefore have implications for control of luteolysis and ovulation (Cheng et al. 2001). It is also possible that dietary fatty acids reduce the sensitivity of the corpus luteum to PGF2α (Burke et al. 1997).
Progesterone concentration
There is evidence for a reduced rate of clearance of progesterone when animals are given diets supplemented with fats. Supplementation of diets for lactating dairy cows with Ca salts of long-chain fatty acids doubled the number of corpora lutea, reduced time to first rise in progesterone, doubled the number of normal luteal phases, and restored the pattern of accumulated plasma progesterone concentrations (Garcia-Bojalil et al. 1998). Pregnancy rate was increased from 52.3 to 86.4% with fat supplementation, and cows fed fat tended to have a larger corpus luteum and more accumulated plasma progesterone. In another study, granulosa cells collected from follicles of fat-supplemented cows showed increased progesterone and androstenedione secretion (Wehrman et al. 1991).
However, it is possible that only saturated or monounsaturated fatty acids enhance circulating progesterone concentrations. Incubation of dispersed bovine luteal cells with PUFAs decreased secretion of progesterone (Hinckley, et al. 1996), and in another study PUFA-supplemented diets reduced plasma progesterone, particularly in the early luteal phase (Robinson et al. 2002).
Ovarian follicular dynamics
Supplementary fat in the diet can increase the total number of follicles, and stimulate growth and size of the preovulatory follicle (Lucy et al. 1991; Lucy et al. 1993). In a study by Robinson et al. (2002), supplementation with PUFAs increased the number of medium-sized follicles. The diameter of the first dominant follicle, IGF-I concentrations at oestrus and cholesterol concentrations were all higher in cows fed a diet supplemented with C18:2, but there was an increase in oestradiol during the follicular phase in cows fed a diet supplemented with C18:3 (Robinson et al. 2002). In a study of bypass-fat supplementation in high-yielding cows, however, we found no effect of fat supplementation on follicle numbers or ovulation rate (Garnsworthy, et al. 2008b).
Fatty acids in follicular fluid and oocytes
Homa and Brown (1992) studied the fatty-acid composition of follicular fluid from small and large developing follicles. Linoleic acid was the major fatty acid in follicular fluid, constituting about a third of total fatty acids. Oleic acid constituted 19% of the total fatty acids in small follicles and 17% in large follicles. Saturated fatty acids accounted for less than 30% of total fatty acids in follicular fluid. The proportion of linoleic acid was significantly lower in follicular fluid from large follicles (31% of total fatty acids) than from small follicles (35% of total fatty acids) and there was a significant inverse correlation between follicle diameter and proportion of linoleic acid in follicular fluid.
Adamiak et al. (2006) compared the FA composition of plasma to that of granulosa cells (GCs) and cumulus–oocyte complexes (COCs) in heifers supplemented with 0% or 6% calcium soaps of palm oil. High levels of dietary FA, reduced (P < 0.05) blastocyst yields in low, but not in moderate, BCS heifers. Diet-induced alterations to the FA content of plasma were less apparent in GCs and COCs. Although dietary lipids increased the FA content of COCs, the selective uptake of saturated FAs at the expense of mainly polyunsaturated FAs within the follicular compartment ensured that the FA composition of COCs was largely unaffected by diet. However, the concentration of saturated FAs within COCs was inherently high, and so further increases in FA content may have impaired post-fertilisation development.
We have studied the effects of dietary fatty acids on oocyte quality in lactating dairy cows. Cows were fed on diets containing a high (0.8 kg/d) or low (0.2 kg/d) level of calcium soaps of palm oil and oocytes were collected by ovum pickup. During subsequent in vitro culture, a significantly higher proportion of fertilized oocytes from the high-fat treatment developed to the blastocyst stage (Fouladi-Nashta et al. 2007). In another study (Fouladi-Nashta et al. 2008), we compared calcium soaps of palm oil with protected fats in the form of soya (high in linoleic acid) or linseed (high in linolenic acid). Significant differences were observed in fatty acid profiles of serum and milk, which were related directly to dietary fatty acid profiles. Oocyte cleavage rate and oocyte quality were significantly lower in cows given fats with high linoleic or linolenic acid content. It is not known whether these effects are due to differences in oocyte membrane structure, oxidative status or fatty acids as energy sources.
Fatty acids and pregnancy rate
Feed supply is the main constraint to milk production and reproduction in the tropics, especially in the dry season. We conducted research in Mexico to study the effect of supplementation with a calcium soap of long chain fatty acids (ByFat ®) on energy balance, milk production and reproduction of suckling crossbred cows, grazing star grass in the tropics (Aguilar-Pérez et al. 2009a). ByFat was supplied at 3% of live weight. Milk yield was not different between diets (12.1 kg/d for ByFat, 11.7 kg/d for C). Milk protein concentration was lower for ByFat (2.9 vs. 3.1%), but no difference was found in protein yield, milk fat or lactose. ByFat had no effect on reproductive performance, due to lower (P<0 .05="" 10.70="" a="" acids="" balance="" because="" br="" calcium="" chain="" concluded="" cows="" d="" depressed="" dm="" effect="" effective="" energy="" fat="" fatty="" for="" grass="" grazing="" hormones.="" improving="" in="" intake.="" intake="" is="" kg="" long="" low-merit="" metabolic="" no="" not="" of="" on="" or="" performance="" reproductive="" resulted="" soap="" supplementation="" that="" the="" treatment="" tropics="" vs.="" we="" which="" with="">
In a subsequent study (Aguilar-Pérez et al. 2009b), we found that supplementation with cereal at 0.9% of live weight increased total energy intake, leading to increased milk yield (by 30%), proportion of cows that showed oestrus (74% vs. 39%), that ovulated (58% vs. 30%), and that were pregnant rate at 90 days (47% vs. 22%). We concluded that cereal supplementation can eliminate negative energy balance in these cows and is more effective than fat supplementation, probably because cereal increases plasma insulin concentrations (Garnsworthy, et al. 2008a).
We have performed several experiments to investigate the importance of insulin for reproductive performance in dairy cows. A high-fat diet can depress insulin and delay first ovulation postpartum (Gong, et al. 2002), whereas a high-fat diet fed after a period of insulin stimulation with cereal can significantly increase pregnancy rate (Garnsworthy, et al. 2009).
Conclusions
Dietary fatty acids influence milk fatty acid content by supplying precursors for incorporation into milk triglycerides, by altering rumen volatile fatty acid production, and by inhibition of de novo synthesis of shorter chain fatty acids. Rumen biohydrogenation saturates a large proportion of unsaturated fatty acids, but there is some desaturation in the mammary gland and adipose tissue. Rumen protection of fats partially overcomes adverse effects on rumen microorganisms, but does not fully protect fats from biohydrogenation. The main animal factors affecting milk fatty acid profile are rate of body fat mobilisation and desaturase activity in the mammary gland, both of which are under genetic control.
Fatty acids influence reproductive performance through several different mechanisms. Not only do they have direct and indirect effects on ovarian and uterine functions, but they also have direct effects on the developmental competence of oocytes. Fat supplementation has major effects on energy balance and metabolic hormones in both high-yielding and low-yielding dairy cows, which have consequences for reproductive performance.
Acknowledgements
Fatty acid and Fertility research at Nottingham are funded by Defra, SEERAD, LINK, ABNA, BOCM PAULS and Provimi Ltd.
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