Response of milk fatty acid pro fi le to various protein levels in goat diets with similar fatty acid content

The aim of the study was to evaluate the infl uence of feeding different protein levels (11.4, 13.3 and 16.9%) in diets for goats on the fatty acid (FA) profi le in milk. The experimental design was 3 × 3 Latin square. Each 22-day experimental period consisted of 14 days adaptation to the diets and 8 days for milk sample collection. Separation of methylated FAs was achieved using GLC. Milk production was nearly the same when the lowand medium-protein diets were fed, while the highprotein diet resulted in a tendency to increase milk production. The concentrations of total saturated fatty acids (SFA) were similar in milk from goats fed the diets containing lowand medium-protein contents, while the diet containing the highest content of protein resulted in a slight increase in the concentration and daily production of SFA milk. The concentration and daily production of monounsaturated fatty acids, CLA isomers, other polyunsaturated fatty acids and the sum of FAs in milk increased as the dietary protein level rose. The highest dietary protein content resulted in an increase in the Δ9-desaturase index.


INTRODUCTION
Milk and dairy products make a signifi cant contribution to human nutrition, including essential fatty acids (e.g., linoleic and linolenic acid), high-quality protein, and numerous vitamins and minerals.Although milk and dairy products provide essential nutrients, there is growing demand to increase the content of health-promoting components in milk and dairy products.Altering the milk fatty acid (FA) profi le through dietary manipulations has gained signifi cant attention because of its implications for human health.Dietary manipulations should result in increasing polyunsaturated fatty acids (PUFA), CLA isomers and the value of the PUFAn-3/PUFAn-6 ratio (Chilliard et al., 2003).Differences between goats and cows in milk fat content and FA profi le in response to dietary manipulations were recently reviewed (Chilliard and Ferlay, 2004;Chichlowski et al., 2005).To our knowledge, there are few studies on the effect of dietary manipulation on the CLA isomer profi le in goats' milk.Thus, the aim of this study was to evaluate the effect of feeding different dietary protein levels on the FA profi le, PUFAn-3 and CLA isomers in goats' milk.

MATERIAL AND METHODS
The study was performed on 3 dairy primiparous Alpine goats (~12 months old) in similar lactation phases.The experimental design was 3×3 Latin squares.Goats were housed in separate metabolic cages, fed every 6 h with three diets containing 11.4, 13.3 and 16.9% of crude protein in DM; water was freely available.The 22day period consisted of 14 days adaptation to the diets (Table 1) and 8 days for milk collection.Goats were milked twice a day at 06.00 and 18.00, weighed and pulled for 24 h.Milk samples were stored at -20 o C until analysis.Thawed milk samples were warmed to 38 o C and sonicated for 1 min.Milk hydrolysis, extraction, FA methylation and separation of methylated FAs was carried out using the GLC-FID method as previously described (Czauderna et al., 2007).The data were statistically analysed using the nonparametric Mann-Whitney U test for comparing independent experimental groups (fatty acids concentration and the protein level in the diet).The Statistica v. 6 package was used (Statistica by StatSoft, 2002. Web: www.statsoft.pl ).

RESULTS AND DISCUSSION
The results (Table 1) demonstrated that there were substantial differences between groups in crude protein intake, while negligible differences in FA and metabolizable energy intake.Milk production was practically the same when the low-and medium-protein diets were fed, while the high protein diet resulted in a tendency (P<0.083) to increase milk production in comparison with goats fed the low-protein diet.As shown in Table 2, the concentration (g/l) of the sum of saturated fatty acids (SFA), atherogenic SFA (A-SFA; i.e.: C12:0, C14:0 and C16:0) and thrombogenic SFA (T-SFA; i.e.: C14:0, C16:0 and C18:0) represented a substantial proportion of all assayed fatty acids (ΣFA) in milk and their concentrations were practically the same in milk from goats independently of the level of protein in the diet.Feeding the diet with high content protein resulted in the highest daily production (g) of SFA, A-SFA and T-SFA in milk.Increasing the protein content in the diet resulted in an elevated concentration and daily production of C18:0 in milk.The concentration and daily production of MUFA, MUFA containing trans double bonds (MUFAt), PUFA and ΣFA in milk increased as the level of protein rose in the diet.The highest content of protein in the diet resulted in an increase of values of PUFAn-6/PUFAn-3 ratio, while tending to decrease the concentration of PUFAn-3 in milk.
Increasing amount of protein in the diet numerically decreased the Δ9desaturase C14:1/C14:0 index compared with this index in milk of goats fed the diets containing medium and low levels of protein.Surprisingly, the values of the Δ9-desaturase index, determined from cis9C14:1 (c9C14:1), c9C16:1, c9C18:1, C14:0, C16:0 and C18:0, numerically increased as the content of protein in the diet increased.The Δ9-desaturase C14:1/C14:0 index based on nearly completely de novo synthesized C14:1 is a better criterion for evaluation of the magnitude of the Δ9-desaturation compared with the Δ9-desaturase index determined from c9C14:1, c9C16:1, c9C18:1, C14:0, C16:0 and C18:0.The concentrations and daily production of c9C18:1, c9t11CLA and the sum of detected CLA isomers (ΣCLA) in milk were higher by feeding the diet containing the high level of protein.
The changes in the c9t11CLA concentration correlate well with the changes in the concentration of CLA isomer precursors (i.e.t11C18:1).
The concentrations and daily production of c9c12C18:2, c11c14C20:2 and c11c14c17C20:3 were increased (P<0.05) or numerically elevated in milk as the content of protein in the diet increased, while the decrease in the concentration and daily production of other non-CLA PUFA decreased in milk with increasing the dietary protein content.Correlations (r g/l and r g ) between crude protein intake (Table 1) and the concentration and daily production of FAs in milk are higher and positively associated with products of Δ9-, Δ6-, Δ5-, Δ4-desaturation and elongation of dietary fatty acids (Rioux et al., 2005): c6c9c12C18:3, c11c14 C20:2, c11c14c17C20:3, c5c8c11c14c17C20:5, c7c10c13c16c19C22:5 and c4c7c10c13c16c19C22:6 (i.e.r g/l and r g >0.91) than with other assayed FAs in milk (Table 2).The inverse correlations were associated only with the concentration of A-SFA and the value of the Δ9-desaturase C14:1/C14:0 index.The current results clearly show that the concentration and daily production of PUFA, CLA isomers and the value of the PUFA/SFA ratio depended on the level of protein in the diet.As can be seen from the obtained results, the increase in the PUFA concentration and the value of the PUFA/SFA ratio were mainly due to the increase in the concentrations of c9t11CLA, c9c12C18:2, c11c14C20:2 and c11c14c17C20:3 in milk.We suggest that the increase of the dietary protein content increased the capacity of Δ9-desaturation and elongation.We also hypothesize that increasing the protein content in the diets decreased the yield of complete biohydrogenation of PUFA in the rumen, therefore, the concentration of t11C18:1 in milk increased.All these effects are responsible for the increase in the concentration and daily production of c9t11CLA, c9c12C18:2, c11c14C20:2 and c11c14c17C20:3.Consequently, the PUFA concentration and the PUFA/SFA ratio increased in milk.Therefore, we argue that increasing the protein concentration in the diet resulted in improving the nutritional quality of milk due to increases in the concentration of PUFA and the sum of CLA isomers, particularly c9t11CLA (Leiber et al., 2005).Unfortunately, enhancement of the protein content in the diet caused a decrease in the concentration of healthy PUFA n-3 and resulted in an increase in the value of the PUFA n-6/PUFA n-3 ratio in milk.Unexpectedly, the concentration and/or daily production of c6c9c12 C18:3, c5c8c11c14C20:4 c5c8c11c14c17C20:5 and c7c10c13c16c19C22:5 were numerically lowest in the milk of goats fed the highprotein diet.

CONCLUSIONS
The increase the content of protein in the diet improves the health promoting properties of milk such as an increased content of PUFA and, in particular, the sum of CLA isomers and PUFA/SFA ratio.We hypothesized that decreasing the yield of biohydrogenation in a rumen of goats fed diets containing the higher content of protein is mainly responsible for increase of the level of t11C18:1.In consequence, the c9t11CLA content in milk is positively correlated with the content of protein in the diet.In order to further increase the value of milk, in the future we intend to investigate the correlation between the content of protein in diets and contents of PUFAn-3, CLA isomers and their metabolites in milk and blood plasma, as these FAs are important due to their potential health benefi ts, such as anticarcinogenic properties.

Table 1 .
Diet intake, ingredients, chemical composition of diets and milk production

Table 2 .
Fatty acid profi le in milk of goats fed diets containing various protein concentrations