Effects of diet and physiological factors on milk fat synthesis, milk fat composition and lipolysis in the goat. A short review



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Effects of diet and physiological factors on milk fat synthesis, milk fat composition and lipolysis in the goat. A short review.

Y. Chilliarda,b,*, P.G. Torala,b, K.J. Shingfieldc, J. Rouela,b, C. Lerouxa,b, L. Bernarda,b,*


a INRA, UMR1213 Herbivores, Site de Theix, F-63122 Saint-Genès-Champanelle, France

bClermont Université, VetAgro Sup, BP 10448, F-63000 Clermont-Ferrand, France

c Institute of Biological, Environmental and Rural Sciences, Aberystwyth University, Aberystwyth, Ceredigion, SY23 3EB, United Kingdom.
* Corresponding authors.

INRA, UMR1213 Herbivores, Site de Theix, F-63122 Saint-Genès-Champanelle, France. Tel: +33 473 62 41 14 and 40 51 ; fax: +33 473 62 45 19.

E-mail addresses: yves.chilliard@clermont.inra.fr and laurence.bernard@clermont.inra.fr Abstract.

The current short review summarizes recent data on the specificities of goats compared with cows, of milk fatty acid (FA) secretion and milk fat lipolysis responses to physiological and nutritional factors. The influence of lactation stage on milk fat yield and FA composition is similar between goats and cows. In contrast, changes in milk fat yield and composition to diet, lipid supplements in particular, differs between the two ruminant species. In almost all cases, dietary lipid supplements increase milk fat content in goats, but not in cows. The goat is much less sensitive to diet-induced alterations in ruminal biohydrogenation pathways causing trans-10 18:1 to replace trans-11 18:1 as the major intermediate relative to the cow. Mammary lipid secretion in the goat is also less sensitive to the anti-lipogenic effect of trans-10,cis-12 conjugated linoleic acid (CLA) compared with the cow. Consistent with these observations, mammary lipogenic gene expression is less affected by diets rich in starch and polyunsaturated FA (PUFA) in goats than cows. However, diets containing PUFA induce much greater changes in delta-9 desaturase gene expression in goats compared with cows, that may be related to differences in the availability of biohydrogenation intermediates at the mammary glands (e.g. trans-9,trans-11-CLA). The development of either goat flavour or rancidity is related to the inherent peculiarities of milk FA composition and lipolytic system in this species. In contrast with cows, milk LPL activity and lipolysis are low during early and late lactation in goats, and are decreased when animals are underfed or receive a diet supplemented with plant oils. In goats the alpha-s1-casein (CSN1S1) gene polymorphism is associated with a decrease in milk fat content and 8:0-12:0 concentrations in the low CSN1S1 genotype. Conversely, milk fat product/substrate concentration ratios for delta-9 desaturation and spontaneous lipolysis are increased in the low genotype.


Keywords: goat milk, fatty acids, lipolysis, diet, physiological factors

The current short review summarizes the specificities of goats compared with cows, of milk fatty acid (FA) secretion and milk fat lipolysis responses to physiological and nutritional factors, and considers the mechanisms underlying the differences between ruminant species. Emphasis is placed on the findings from recent reports in the literature including several review articles (Chilliard et al., 2003, 2007; Bernard et al., 2008; Mele et al., 2008a; Shingfield et al., 2010). This subject is of academic interest in terms of using interspecies comparisons to develop a more comprehensive understanding of the regulation of physiological processes and also as a means to develop diets and management practices for the production of dairy products with a predefined nutritional composition and sensory attributes. Stage of lactation has a similar influence on milk fat yield and FA composition in goats (Chilliard et al., 1986) and cows (Palmquist et al., 1993).

In marked contrast, milk fat yield and composition responses to diet, lipid supplementation in particular, differs between these species. In almost all cases, lipid supplements increase milk fat content in goats (Chilliard et al., 2003; Bouattour et al., 2008; Martínez Marín et al., 2011; Nudda et al, 2013), whereas the effects in cows are much more variable (Bauman and Griinari, 2001). For both species, changes in milk fat composition is dependent on complex interactions between the composition of basal diet (forages, starchy concentrates) and amount and FA profile of lipid supplements. It has been shown in the cow that these interactions have a profound influence on ruminal biohydrogenation of dietary unsaturated FA and the formation of specific intermediates in the rumen. High starch diets containing relatively high proportions of polyunsaturated FA (PUFA) are known to promote shifts in biohydrogenation pathways resulting in trans-10 18:1 replacing trans-11 18:1 as the major intermediate (e.g. Zened et al., 2013). Such changes are also accompanied by alterations in the synthesis of minor polyenoic biohydrogenation intermediates, including isomers of conjugated linoleic acid (CLA). Indirect comparisons of milk fat composition indicate that the goat is much less sensitive than the cow to alterations in ruminal biohydrogenation pathways (Chilliard et al., 2007; Mele et al., 2008b; Schmidely and Andrade, 2011; Serment et al., 2011; Bernard et al., 2012), even when diets rich in starch and PUFA from plant oils are fed, which led to lower increases in trans-10 18:1 in goats than in cows (Shingfield et al., 2010; Figure 1). It is notable that diet-induced changes in milk fat melting point are small, and of a similar magnitude in goats and cows, despite of differences in milk fat secretion and FA profile responses between the two species (Toral et al., 2013a). For both cows and goats, milk cis-9,trans-11-CLA concentrations are increased several-fold when conserved forages are replaced with fresh grass or diets are supplemented with plant oils s rich in 18-carbon PUFA, whereas changes to whole untreated oilseeds are marginal (Chilliard et al., 2003, 2007; Bernard et al., 2009; Martínez Marín et al., 2011; Lerch et al., 2012; Renna et al., 2012).

In goats, milk fat content and yield are not altered by dietary fish oil supplementation (alone or in combination with plant oils) at doses which induce milk fat depression (MFD) in cows (Figure 2). However, significant reductions in milk fat synthesis do occur in goats fed high-starch diets containing high amounts of fish oil, but the relative decreases are lower compared with cows. Even during high starch and fish oil induced MFD in goats, milk trans-10 18:1 concentration did not increase (contrary to cows receiving the same diet in a direct comparison), whereas cis-9,trans-11-CLA was substantially increased (more than in cows) (Toral, P.G., Bernard, L., Chilliard, Y., unpublished results).

Species differences in the molecular and biochemical regulation of mammary lipid metabolism have been identified (Bernard et al., 2008; Shingfield et al., 2010). Diet has sometimes effects on the transcription of the major lipogenic genes (mRNA abundances of genes involved in FA uptake (LPL), de novo synthesis (ACACA and FASN) and delta-9 desaturation (SCD1)) in mammary tissue. However, the influence of diet on mammary transcript abundances do not always correspond with observed changes in milk FA secretion. In goats as for cows, data suggest i) that the availability of substrates is more limiting than the lipoprotein lipase (LPL) activity in the uptake of long-chain FA (except rather extreme diets fed to cows that induce MFD, in which mammary LPL expression is decreased; Harvatine and Bauman, 2006; Angulo et al., 2012), and ii) that other proteins involved in the uptake and intracellular transport of FA (e.g., fatty acid translocase, CD36; fatty acid-binding protein, FABP) may be implicated (Peterson et al., 2003; Toral et al., 2013b). In cows and goats ACACA and FASN mRNA abundances are related to short- and medium-chain FA synthesis, even though the abundance of these transcripts are not always decreased by the addition of PUFA in the diet at least in the goat. In this species, ACACA and FASN mRNA are regulated by dietary factors at a transcriptional level, and SCD1 is regulated at a transcriptional and/or post-transcriptional level, depending on the type of lipid supplement fed (Shingfield et al., 2010; Bernard et al., 2013a). However, the abundance of SCD1 mRNA varies little with diet composition in cows, except when "rumen-protected" fish oil or docosahexaenoic acid (DHA)-rich algae were fed (Angulo et al., 2012; Bernard et al., 2013a).

Post-ruminal infusions in cows have demonstrated that trans-10,cis-12-CLA or a mixture of fatty acids containing trans-10 18:1 have anti-lipogenic effects (Baumgard et al., 2002; Shingfield et al., 2009a). Even though much of the evidence suggests that trans-10 18:1 may inhibit milk fat synthesis experimental data are still equivocal (Lock et al, 2007; Shingfield et al., 2009a).

Moreover, in cows, mRNA abundances of genes involved in de novo FA synthesis, FA uptake, transport and esterification in the mammary glands during post-ruminal trans-10,cis-12-CLA infusion or on diets causing MFD, were found to be decreased, changes that occurred prior to any decrease in SCD1 mRNA (Shingfield et al., 2010). Conversely, in goats, administration of trans-10,cis-12-CLA at the duodenum (Andrade and Schmidely, 2006) or when fed as calcium salts (Shingfield et al., 2009b) or methyl esters (Baldin et al., 2013) lowered milk FA product/substrate ratios for SCD even in the absence of changes in milk fat secretion. This suggests that the expression of mammary lipogenic genes is less sensitive to the anti-lipogenic effect of trans-10,cis-12-CLA in goats than cows (Figure 3), which was supported by recent studies in vitro using bovine and caprine mammary slices (Bernard et al., 2013b, Figure 4). However, the reverse is true for changes in SCD mRNA, with a higher sensitivity being reported in goats (Bernard et al., 2013a). Indirect comparisons of milk fat composition between studies in goats and cows fed maize silage or hay diets supplemented with plant oils showed that milk FA product/substrate ratios for SCD were decreased in goats (Bernard et al., 2009), but increased in cows (Roy et al., 2006), consistent with the relative increase in milk cis-9 18:1 concentration being lower in goats than for cows.

Furthermore, post-ruminal infusion studies in cows have shown that in addition to trans-10,cis-12-CLA, trans-10,trans-12-CLA and trans-9,trans-11-CLA lower milk FA product/substrate ratios for SCD (Bernard et al., 2013a). This suggests that these two biohydrogenation intermediates could be specific inhibitors of SCD activity, since neither appear to be involved in diet-induced MFD (Shingfield et al., 2010). It is possible that the relatively high increase in milk trans-9,trans-11-CLA concentration in goats receiving diets supplemented with sunflower oil could contribute to the typical decrease of milk FA product/substrate ratios for SCD in this species. In contrast, increases in milk trans-9,cis-11-CLA concentrations that may lower milk fat synthesis in cows, did not occur in goats receiving high starch-high PUFA diets (Bernard et al., 2009; Shingfield et al., 2010).

Altogether, these data suggest that variations between ruminants in mammary FA secretion and lipogenic responses to changes in diet composition reflect inherent inter-species differences, not only with respect to ruminal lipid metabolism (although no in vivo data are available in goats), but also in mammary specific regulation of cellular processes involved in the synthesis of milk fat (Bernard et al., 2008; Shingfield et al., 2010).

The development of either goat flavour (linked to free, branched and medium-chain FA release) or rancidity (due to excessive release of free butyric acid) is related to the peculiarities of the goat milk FA composition and lipolytic system (Chilliard et al., 2003; Raynal-Ljutovac et al., 2008). The milk LPL activity is lower in the goat compared with the cow. This enzyme has a higher affinity for fat globules with an activity more closely correlated with post-milking spontaneous lipolysis of milk fat in goats compared with cows. In contrast to cows (Chazal and Chilliard, 1986; Lerch et al., 2012), milk LPL activity and lipolysis in the goat are low during early and late lactation, and decrease when animals are underfed or receive a diet supplemented with plant oils (Chilliard et al. 2003; Eknæs et al., 2009; Dønnem et al, 2011). Among putative mechanisms, it was hypothesized that during dietary lipid supplementation the partition of the LPL synthesized by the mammary secretory cells increased towards the basal membrane (near the blood vessels) at the expense of the flow of LPL towards the apical membrane and milk (Figure 5). This could explain, at least in part, the observed decreases in the goat flavour of dairy products when animals receive lipid-rich diets (Chilliard et al., 2003).

In goats the alpha-s1-casein (CSN1S1) gene polymorphism affects milk composition, being linked to a decrease in milk fat content and 8:0-12:0 concentrations. These changes are also associated with higher milk FA product/substrate ratios for SCD (without changing milk fat melting point), and an increase in milk fat spontaneous lipolysis in the low CSN1S1 genotype (Chilliard et al., 2006; Valenti et al., 2010). Moreover, with diets supplemented or not with extruded linseeds, several genotype × diet interactions were observed, with lower responses in milk fat content and FA concentrations, and a much higher response in milk fat spontaneous lipolysis in the low than high CSN1S1 genotype (Chilliard et al., 2013; Table 1). Diet × genotype interactions may also explain the smaller mammary gene expression, milk production and composition responses to food-deprivation, in goats carrying the low compared with the high genotype (Ollier et al., 2006).

Research conducted during the last 10 years has confirmed and defined our previous statement (Chilliard et al., 2003) that changes in milk FA secretion and milk fat lipolysis to physiological and nutritional factors differ markedly between the goat and cow. An increasing amount of data are now available on the underlying mechanisms that occur in mammary secretory tissue. Future research involving direct comparisons of cows and goats receiving similar diets and analysis of tissue biopsies, ruminal digesta and milk lipids using the same analytical methods will be required to provide a more complete understanding of the between-species differences in lipid metabolism.


Acknowledgements

P. G. Toral was granted a fellowship from the Fundación Alfonso Martín Escudero (Madrid, Spain).


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Table 1

Effect of goat casein alpha-s1genotype and diet on milk fat content, free fatty acids, fatty acid composition and delta-9 desaturation ratio (adapted from Chilliard et al., 2013).

Genotype

High

High

Low

Low

Effect

Diet

Control

ELa

Control

EL

Genotype

Diet

G×D

N. goats

23

23

24

24


































Fat content (g/kg)

34.9b

44.5d

29.4a

36.9c

0.001

0.001

0.09

Free fatty acidsb

0.85ab

0.43a

2.61c

1.06b

0.001

0.001

0.001

























Fatty acids (g/100 g total FA)






















10:0

12.66c

9.05a

11.22b

8.96a

0.001

0.001

0.003

16:0

29.5c

17.7a

33.0d

19.5b

0.001

0.001

0.02

18:0

5.96b

13.30d

5.20a

11.02c

0.001

0.001

0.002

cis-9 18:1

12.9a

15.9b

13.2a

15.0b

NS

0.001

0.10

























Delta-9 desaturation ratio






















cis-9 14:1 / 14:0

0.014c

0.009a

0.016b

0.010a

0.001

0.001

0.01

Within a row, different letters indicate significant differences due to genotype (G), diet (D), and G×D interaction (P<0.05).

a EL, extruded linseeds.

b mmol of free fatty acids/100 g fat in milk after storage at 4°C for 34 hours post-milking.
Figure 1

Relationship between increases in milk fat trans-10 18:1 content and changes in milk fat yield in lactating goats (□, n = 63) and cows (○, n= 82). Each point represents the difference between treatment group and control calculated from studies reported in the literature. Fitted line indicates the relationship in cows. No obvious association was observed in goats (adapted from Shingfield et al 2010).


Figure 2

Percentage change in milk fat content in response to dietary supplementation with fish oil (FO) alone or in combination with sunflower oil (SO) in lactating cows (data from Pennington and David, 1975; Offer et al., 1999; Shingfield et al., 2006; Cruz-Hernandez et al., 2007) and goats (Chilliard, Y., Rouel, J., Toral, P.G., unpublished data).


Figure 3

Association between the percentage decrease in milk fat yield with milk fat trans-10,cis-12 conjugated linoleic acid (CLA) content in response to rumen protected CLA supplements in lactating goats (□) and cows (○). Each point represents one experimental group from studies reported in the literature. Fitted lines indicate the relationship in cows (solid line) and goats (dotted line) (adapted from Shingfield et al., 2010).


Figure 4

Effect of the addition of long-chain fatty acids, either cis-9 18:1 (0.17 mM), 18:2n-6 (0.16 mM), 18:3n-3 (0.16 mM), cis-9,trans-11 conjugated linoleic acid (CLA) (0.20 mM), and trans-10,cis-12-CLA (at C1: 0.11 mM, C2: 0.16 mM, and C3: 0.37 mM) compared with an authentic control (without addition), on the total lipogenic activity measured by the incorporation of 14C-acetate into the lipid fraction of mammary slices incubated during 20 h from goats (a) and cows (b). Values are means ± SE for n = 6 goats and n = 5 cows. Uncommon letters above bars within a panel indicate differences between treatments (P < 0.05) (adapted from Bernard et al., 2013b).


Figure 5

Origin of milk lipoprotein lipase: putative mechanisms.

Milk-LPL likely arises from the partitioning of mammary synthesized LPL (m-LPL) between secretion into milk with either caseins or fat globules (1) and migration to the basal membrane (2) followed by interaction with VLDL substrate and/or leakage into blood stream. Alternatively, LPL may originate from adipose or other body tissues (a-LPL) and secretion either by endocytosis of LPL at the basal membrane, followed by transport in secretory vesicle and exocytosis at the apical membrane (3) or by paracellular leakage into milk serum (4) (adapted from Chilliard et al., 2003).

FIGURE 1



y = -61.97 + 59.116exp-0.1554x

r2 = 0.68, n = 82



Increase in milk trans-10 18:1 content [g/100 g fatty acids]

Percentage change in milk fat yield

FIGURE 2

FIGURE 3



y = -62.93 + 53.913exp-4.276x

r2 = 0.73, n = 17



y = -31.88 + 35.312exp-4.862x

r2 = 0.94, n = 5



Percentage change in milk fat yield

FIGURE 4


FIGURE 5





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