Effects of dietary soybean lecithin oil on the immunoglobulin level and fat globule size of milk in lactating sows

ABSTRACT

This study was conducted to investigate the effects of dietary soybean lecithin (SL) on immune function and milk quality in lactating sows. The dietary treatments included a control group and three groups that replace soybean oil with 1%, 2% and 3% SL respectively in the diets. The weight of piglets at weaning and the average daily gain (ADG) were increased by supplementation of 3% SL (P < 0.05). The content of phisphatidylcholine in milk was increased by supplementation of 2% and 3% SL (P < 0.05), and the average diameter of milk fat globule was decreased by supplementation of 1%, 2% and 3% SL (P < 0.05). The immunoglobulin levels of milk and plasma were enhanced when 2% or 3% SL was added (P < 0.05). Collectively, dietary SL decreased the milk fat globule size and improved the content of phospholipid in milk, as well as enhanced the immunoglobulin levels in milk and plasma of sows and piglets.

KEYWORDS:

• Soybean lecithin
• phospholipid
• milk fat globule
• immunoglobulin
• sow

Introduction

Chemically, Soybean lecithin (SL) is a phospholipid commercially extracted from soybeans and primarily consists of three types of phospholipids: phosphatidylcholine (PC), phosphatidylethanolamine (PE) and phosphtidylinositol (PI), with similar amounts of each phospholipid (Giovanna & Milena, 2013). The health benefits of lecithin have long been appreciated, and diet rich in PC has been reported to improve lipid absorption and enhance immune function. Immunoglobulins cannot cross the placenta in pregnant sows, and neonatal pig must acquire maternal immunoglobulins from ingested colostrum and milk for passive immune protection until the immune system of the piglet becomes fully developed (Rooke & Bland, 2002). Protection by maternal immunity is mediated by a number of factors involving mostly immunoglobulin can be transferred from blood to colostrum (Bontempo, Sciannimanico, Pastorelli, Rosi, & Corino, 2004; Salmon, Berri, Gerdts, Meurens, & Summerfield, 2009). As a substitute for lipid in diets, SL was shown to be an important nutriment and has been reported to improve lipid absorption in the body with its ready availability and outstanding functionalities (Wu & Wang, 2003). The addition of lecithin to piglet diet have improved average daily gain and feed digestibility (Danek et al., 2005). SL also have an effect on immune function of animals due to it contains more linoleic acid, linolenic acid, etc., as well as PC, inositol and vitamin E (Wei, Li, Bi, & Zhang, 2016). A higher phospholipid content in milk may be desirable due to the recognized positive health effects related to plasma lipid profile and overall metabolism (Burgess et al., 2005; Dillehay, Webb, Schmelz, & Merrill, 1994), as well as beneficial effects on immune system, heart and brain functions (Vesper et al., 1999).

Furthermore, the size of the milk fat globules is related to the digestion and absorption of milk (Gallier, Gordon, & Singh, 2012; Michalski, Briard, Michel, Tasson, & Poulain, 2005). Generally speaking, small fat globule tend to be more easily digested and absorbed by the body than larger fat globule (Brown, Van, & Odle, 1998). Milk fat globule size is related to the amount of fat globule membrane. SL can provide sufficient raw materials for milk fat globule membrane. Improving the availability of dietary fats and decreasing the size of MFG have important practical significance. However, it is not well known whether the sow dietary supplementation of SL has an effect on the size of the milk fat globules. Improving the availability of dietary fats and decreasing the size of MFG have important practical significance. So there might be better effect on the size of the milk fat globules by the supplementation of SL in sows.

Pigs (Sus scrofa) and humans (Homo sapiens) are omnivorous animals and share similar organs, nutritional requirements and functional features, which leads to the application of pigs as an excellent model for the research of human nutrition and health (Nielsen et al., 2014; Roura et al., 2016; Tian, Wu, Chen, Yu, & He, 2017). Therefore, the objective of this study was to examine the effects of supplementation of 1%, 2% and 3% SL in late gestation and lactation basal diets on sow and litter performance, milk composition and phospholipid content, milk fat globule size, and immunoglobulin levels in sows.

Materials and methods

The experimental proposals and procedures for the care and treatment of animal were approved by the Institutional Animal Care and Use Committee of Northeast Agricultural University (NEAU-[2011]-9).

Animals and experimental design

A total of 48 crossbred pregnant sows [Large White × Landrace], on day 107 of gestation, were randomly allocated to one of four dietary treatments accounting for parity (in the range of 3–5) and expected delivery date (n =12 sows/treatment). The dietary treatments included a basal diet group (control) and groups that replace soybean oil with 1%, 2% and 3% SL respectively in the diet. The experimental diets were provided from day 107 of gestation until day 21 of lactation (weaning), for a total of 28 days. All diets were formulated to meet or exceed the requirements for all nutrient standards (NRC, 2012). Ingredient composition and chemical analyses of the experimental diets are shown in Table 1. According to Official methods of analysis of AOAC international (2007), diets samples were analysed in terms of crude protein (CP), ethanol extract (EE). The soybean lecithin and soybean oil were provided by Harbin Gushi A&H Group, which digestive energy contents were 30.29 and 36.31 MJ/kg respectively according to the AOAC (2007) and Pu et al. (2017).

Table 1. Ingredients and chemical composition of the experiment diets (as-fed basis)^a.

Ingredients, %	Soybean lecithin, %
	0	1.0	2.0	3.0
Corn	64.85	64.85	64.85	64.85
Soybean meal (43% CP)	18.5	18.5	18.5	18.5
Wheat bran	6.5	6.5	6.5	6.5
Fish meal (67% CP)	3.0	3.0	3.0	3.0
Soybean oil	3.0	2.0	1.0	0
Soybean lecithin	0	1.0	2.0	3.0
Dicalcium phosphate	1.2	1.2	1.2	1.2
Limestone	0.7	0.7	0.7	0.7
Choline chloride, 50%	0.25	0.25	0.25	0.25
Salt	0.5	0.5	0.5	0.5
Premix^b	1.5	1.5	1.5	1.5
Chemical composition,^c %
DE, MJ/kg	13.93	13.87	13.81	13.74
CP	16.25	16.25	16.25	16.25
EE	6.81	6.81	6.81	6.81
Lys	0.83	0.83	0.83	0.83
Met	0.28	0.28	0.28	0.28
Ca	0.75	0.75	0.75	0.75
T-P	0.65	0.65	0.65	0.65
A-P	0.36	0.36	0.36	0.36

^aAbbreviation: DE, digestible energy; CP, crude protein; EE, ether extract; Lys, lysine, Met, Methionine; Ca, calcium; T-P, total phosphorus; A-P, available phosphorus.

^bSupplied per kilogram of diet: vitamin A, 13,500 IU; vitamin D₃, 3000 IU; vitamin E, 44 IU; vitamin K₃, 3 mg; vitamin B₁, 1.8 mg; vitamin B₂, 6 mg; vitamin B₆, 0.3 mg; vitamin B₁₂, 0.024 mg; biotin, 0.03 mg; niacin, 24 mg; pantothenic acid, 15 mg; folic acid, 0.9 mg; 20 mg of Cu as CuSO₄·5H₂O; 120 mg of Zn as ZnSO₄; 0.3 mg of Se as Na₂SeO₃·H₂O; 40.07 mg of Mn as MnSO₄·H₂O; 100.50 mg of Fe as FeSO₄·H₂O; and 0.3 mg of I as Ca(IO3)₂.

^cDE, Lys, Met, Ca, T-P, A-P were calculated values. CP, EE were analysed values.

Housing, feeding and management

The experiment was initiated on day 107 of gestation, when sows were moved to the same farrowing house, and sows were offered experimental supplements until day 21 of lactation. The sows and piglets were individually housed in farrowing pens (2.2 × 2.4 m) with crates and slatted floors. The average farrowing room ambient daily temperature was approximately 18–20°C. The parturitions were watched, but the observers interfered as little as possible in the farrowing process. A piglet corner with a heating lamp was available for the piglets. The sows and piglets had free access to drinking water throughout the experimental period.

Throughout lactation, the sows were fed twice daily at 08:00 and 15:00, and the feed refusals for each sow were recorded and subtracted from the total feed intake. The sows were initially fed 3.0 kg before partum. On the day of farrowing, the sows were not fed; after farrowing, the sows were initially fed 1.5 kg on day 1. This amount was increased daily by 0.5 kg until day 7 postpartum, depending on the maternal feed consumption and the recovery after partum. From day 7 postpartum, the sows had free access to their diets until weaning. The piglets did not receive a solid diet and were fed only with colostrum and milk throughout the experimental period.

Sow and litter performance

Sows were weighed (using scales with an accuracy of ±100 g) when moving from the gestation barn to the farrowing room (day 107 of gestation) and at day 21 of lactation (weaning). Record the daily feed intake of sows and calculate the average daily feed intake (ADFI). Backfat thickness was measured at the P2 position (left and right side of the 10th rib and 6 cm lateral to the spine) during times of weighing using a B-mode ultrasound (Renco Lean Meater type 7, Minneapolis, MN, USA), and the average was calculated and recorded. After weaning, estrus detection was conducted at 08:30, 15:30, and 22:30 daily using mature boar stimuli. The beginning of the estrus period was characterized as the midpoint between the time of the first observed positive response to back pressure (immobilization reflex) and the previous period of estrus detection. The end of estrus was the midpoint between the time when a negative response to back pressure was first identified and the previous period of estrus detection. The weaning to estrus interval (WEI) was the average of the starting and the ending of the estrus period.

Reproductive performance

The following parameters were recorded: total number of piglets born, born alive, stillborn and mummified, and litter weights and piglet weights at birth and on day 21 of lactation. The litter piglets were weighed from parturition until weaning to calculate average daily gain (ADG) per litter. The individual piglet body weight (BW) was recorded at birth and weaning (day 21), and ADG was calculated.

Blood sample collection and analysis

Blood samples (10 mL) from eight sows per treatment were obtained using heparin tubes from the ear vein on day 21 of lactation and were immediately placed on ice until they were centrifuged at 1000 × g for 15 min. The plasma was immediately stored at −20°C until further analysis. During the suckling period, blood samples (5 mL) from 12 piglets per litter were collected, after having an empty stomach for 24 h, from the vena jugularis by puncture into heparin tubes at weaning (day 21) to facilitate immunoglobulin quantification. The blood samples were immediately placed on ice until they were centrifuged at 1000 × g for 15 min. The plasma was immediately stored at −20°C until further analysis.

The plasma samples from sows and piglets were analysed for immunoglobulin G (IgG), immunoglobulin A (IgA) and immunoglobulin M (IgM). The immunoglobulins were analysed with immunoturbidimetry using immunoglobulin-specific kits (Sanwei Biological Engineering Co., Ltd, Shandong, China) as described previously (Liao et al., 2017). The optical density values of standards and samples at 700 nm (IgG) or 340 nm (IgA, IgM) were measured with UV-2401PC (UV-vis recording spectrophotometer, SHIMADZU Corporation, Japan). The immunoglobulin of samples (g/L) was calculated with the standard curve.

Milk collection and analyses

Colostrum samples (30 mL) were collected one hour after the birth of the first piglet. At day 14 of lactation, 30–40 mL of milk were collected from the functional glands of each sow after an injection of 2 mL oxytocin (Inter-Chemical Ltd., China). The milk samples were immediately frozen at −20°C for later analysis.

The colostrum and milk samples were analysed for lactose, protein, fat and total solids with a fully automatic milk analyser (Milko ScanTM FT + Analyser, Foss). The milk phospholipid content was determined through ³¹P Nuclear Magnetic Resonance (³¹P-NMR) spectroscopy with Bruker Avance Spectrometer AV500 (Bruker, Germany). The product content was calculated using the integral relative strength of each peak. The fat globule size distribution of the milk were measured by laser light scattering using a Mastersizer (CILAS L1064, France). Experimental procedures to dissociate casein micelles and refractive index values were detailed previously (Michalski, Briard, & Michel, 2001). The whey samples were analysed by diluting them 1:2 for colostrum IgG, 1:1 for colostrum IgA and 1:1 for colostrum IgM. Other experimental procedures were the same as measurements of plasma immunoglobulin.

Statistical analysis

The data were analysed by a one-way analysis of variance (ANOVA) and multiple comparisons with Tukey’s test using SPSS 20.0 (2012, IBM-SPSS Inc., Chicago, Illinois, USA). The results were presented with mean values and the standard error of the mean (SEM). Differences were considered significant if P < 0.05.

Results

Body condition of sows

The effect of maternal dietary treatment on body condition of sows during the suckling period is presented in Table 2. ADFI, BW change, backfat change and WEI in sows from day 107 of gestation to day 21 of lactation (weaning) were similar across treatments (P > 0.05).

Table 2. Effect of supplementation of soybean lecithin during gestation and lactation on body condition of sows.

Items	Soybean lecithin, %				SEM	P-value
	0	1	2	3
No. of sows	12	12	12	12
Sow BW at farrowing, kg	272.7	274.0	280.3	275.3	3.300	0.891
Sow BW at weaning, kg	242.0	245.0	252.7	249.3	3.200	0.716
Sow BW change, kg	30.7	29.0	27.7	26.0	0.900	0.312
Backfact at farrowing, mm	20.56	20.63	20.81	20.60	0.151	0.859
Backfact at weaning, mm	17.13	17.56	17.81	17.19	0.164	0.416
Sow backfact change, mm	3.44	3.06	3.00	3.38	0.078	0.105
ADFI, kg	4.85	5.07	5.01	5.14	0.057	0.352
WEI, d	5.16	5.00	4.83	5.00	0.104	0.760

^a,bMeans within a row with different superscripts differ (P < 0.05).

Abbreviation: ADFI, average daily feed intake; BW, body weight; WEI, weaning to estrus interval.

Litter performance

The effect of maternal dietary treatment on piglet performance during the suckling period is presented in Table 3. Similarly, sow dietary treatments have not influenced on the number of piglets born alive, number of piglets at weaning per litter, survival rate of piglets, and weight of piglets at birth from day 107 of gestation to day 21 of lactation (weaning) (P > 0.05). However, the addition of 3% SL to diets significantly improved the weight of piglets at weaning, as well as the ADG compared with the control group (P < 0.05).

Table 3. Effect of supplementation of soybean lecithin during gestation and lactation on piglet performance.

Items	Soybean lecithin, %				SEM	P-value
	0	1	2	3
No. of sows	12	12	12	12
Number of piglets born alive/ litter	11.38	11.50	11.25	11.00	0.213	0.871
Number of piglets at weaning/ litter	9.38	9.38	9.88	9.50	0.168	0.705
Survival rate of piglets, %	82.91	82.55	88.85	87.01	0.021	0.670
Piglet weight at birth, kg	1.36	1.45	1.58	1.48	0.041	0.342
Piglet weight at weaning, kg	5.31^b	5.72^ab	5.70^ab	6.15^a	0.097	0.013
Day 0–21 ADG, g/d	186.70^b	203.30^ab	196.70^ab	223.30^a	4.38	0.011

^a,bMeans within a row with different superscripts differ (P < 0.05).

Abbreviation: ADG, average daily gain.

Content of milk phospholipid and fat globule size

The effects of SL supplementation during gestation and lactation on milk phospholipid content and fat globule size are presented in Table 4. Supplementation of 2% and 3% SL produced significant increase of PC compared with the control in milk (P < 0.05). No significant differences among dietary treatments were observed in the content of PE and PI. Sows fed 1%, 2% and 3% SL diets significantly decreased the average diameter of milk fat globule compared with the control group (P < 0.05).

Table 4. Effect of supplementation of soybean lecithin during gestation and lactation on the content of milk phospholipid and fat globule size in sows.

Items	Soybean lecithin, %				SEM	P-value
	0	1	2	3
No. of sows	12	12	12	12
PC, g/L	0.47^b	0.57^ab	0.70^a	0.71^a	0.036	0.017
PE, g/L	0.75	0.85	0.90	0.97	0.044	0.386
PI, g/L	0.21	0.13	0.15	0.17	0.014	0.208
Total phospholipid, g/L	1.43^a	1.55^a	1.75^b	1.85^b	0.041	0.037
AD of milk fat globule, μm	3.34^a	2.75^b	2.91^b	2.72^b	0.092	0.025

^a,bMeans within a row with different superscripts differ (P < 0.05).

Abbreviation: PC, phisphatidylcholine; PE, phosphatidylethanolamine; PI, phosphatidylinositol; AD, average diameter.

Composition and immunoglobulin levels of colostrum and milk

As presented in Table 5, dietary SL had no significant effect on the concentrations of lactose, fat, protein and total milk solids in colostrum and milk (P > 0.05). Sows in the supplementation of 1%, 2% and 3% SL groups had higher levels of IgA in colostrum than sows in the control group (P < 0.05). The concentration of IgM in colostrum was significant higher in sows fed 2% and 3% SL diets compared with the other groups (P < 0.05). Similarly, the addition of 3% SL significantly increased the concentration of IgA in milk (P < 0.05). No significant effects of SL supplementation on IgG and IgM were observed in milk (P > 0.05).

Table 5. Effect of supplementation of soybean lecithin during gestation and lactation on the composition and immunoglobulin levels of colostrum and milk in sows.

Items	Soybean lecithin, %				SEM	P-value
	0	1	2	3
No. of sows	12	12	12	12
Colostrum
Lactose, g/kg	32.40	32.40	32.73	31.60	0.481	0.897
Fat, g/kg	44.07	44.80	44.73	47.67	0.746	0.363
Protein, g/kg	192.00	193.47	192.67	194.30	0.853	0.843
Total solids, g/kg	270.53	262.53	268.6	273.53	1.905	0.222
IgG, g/L	52.00	55.40	54.33	55.87	0.590	0.061
IgA, g/L	7.93^b	8.33^a	8.23^a	8.40^a	0.062	0.015
IgM, g/L	2.81^b	2.80^b	3.15^a	3.07^a	0.010	<0.001
Milk
Lactose, g/kg	57.00	59.53	59.13	59.40	0.413	0.071
Fat, g/kg	68.60	71.13	67.47	69.67	0.598	0.152
Protein, g/kg	52.73	52.13	51.73	54.33	0.471	0.227
Total solids, g/kg	192.33	191.87	194.27	191.40	1.426	0.927
IgG, g/L	0.44	0.45	0.47	0.46	0.005	0.205
IgA, g/L	3.38^b	3.39^b	3.52^ab	3.57^a	0.032	0.041
IgM, g/L	1.25	1.28	1.27	1.28	0.006	0.151

^a,bMeans within a row with different superscripts differ (P < 0.05).

Abbreviation: IgG, immunoglobulin G; IgA, immunoglobulin A; IgM, immunoglobulin M.

Plasma immunoglobulin levels of sow and piglet

As presented in Table 6, the addition of SL did not produce significant increase in IgG and IgM for sow and piglet. However, IgA was significantly increased with 3% SL supplemented diets compared with the other groups for sow and piglet (P < 0.05). Plasma IgA concentration was higher with 3% SL supplemented diets compared with the other three groups (P < 0.05).

Table 6. Effect of supplementation of soybean lecithin during gestation and lactation on sow and piglet plasma immunoglobulin levels on day 21 of lactation in sows.

Items	Soybean lecithin, %				SEM	P-value
	0	1	2	3
No. of sows	12	12	12	12
Sows
IgG, g/L	6.71	6.70	6.71	6.73	0.005	0.321
IgA, g/L	0.26^b	0.28^b	0.30^a	0.31^a	0.006	0.001
IgM, g/L	0.30	0.30	0.31	0.31	0.002	0.752
Piglet
IgG, g/L	0.67	0.69	0.70	0.69	0.005	0.321
IgA, g/L	0.28^b	0.30^b	0.31^b	0.32^a	0.005	0.011
IgM, g/L	0.39	0.39	0.40	0.39	0.004	0.990

^a,bMeans within a row with different superscripts differ (P < 0.05).

Abbreviation: IgG, immunoglobulin G; IgA, immunoglobulin A; IgM, immunoglobulin M.

Discussion

The effect of dietary lecithin on animal performance has received considerable attention over the last few years (Danek et al., 2005; Li, Liu, Zhao, & Kim, 2017; Soares & Lopez-Bote, 2002). In agreement with previous reports, adding SL to sows diet did not affect feed intake, weight loss, and BW gain or backfat thickness (Bontempo et al., 2004; Lauridsen & Danielsen, 2004). In several studies, lecithin has been shown to improve ADG of piglets (Danek et al., 2005). Moreover, Li et al. (2014) reviewed that 1% or 2% or 4% SL added in diets could significantly improve weight gain and final BW compared to the control group. Another experiment showed that the ADG and ADFI were increased when lecithin was added to the diet for piglets (Jones, Nelssen, & Hines, 1990). In our trial, consistent with previous studies mentioned above, the weights of piglets at weaning and ADG were significantly improved by adding soybean lecithin in maternal diet. Dietary fat utilization for nursery pigs is limited by insufficient digestion and fat absorption (Cera, Mahan, & Reinhart, 1988). As an emulsifier, the lecithin enhances the utilization of fat, as well as a highly digestible energy source (Ho et al., 2008). This might be explained that the addition of SL led to the higher ADG and ADFI.

PC, PE and PI are the main components of SL (Giovanna & Milena, 2013). The majority of animals can synthesize lecithin, but the level of synthesis is sufficiently low that it cannot meet metabolic requirements (Kanazawa, 1985). Sun, Wu, Zhang, Yue, and Qi (2010) showed that the contents of total phospholipid and PC in eggs were significantly increased relative to the control group when 0.5%, 1.0%, 2.0% and 4.0% SL was added in laying hens. Sink and Lochmann (2014) reported that PC content was greater in catfish fed 2% or 4% lecithin than fed 0% lecithin. Additionally, SL is one of the sources of choline and fatty acids. Tsiagbe, Cook, Harper, and Sunde (1988) by analysis of egg samples at peak production showed significant elevations in total phospholipid, PC, and the ratio of PC to PE, when choline was added in diets. Similarly to our results, the content of PC and total phospholipid was significantly increased when 2% and 3% SL was added to diets, and the contents of PE increased numerically as SL supplementation increased, although it did not reach a significant level. The effect of SL on total phospholipid and PC synthesis in milk may be through its metabolites, such as choline. Choline was recognized as a component of lecithin, and as such, animals will release essential nutrients and choline in the body when assimilating lecithin (Blusztajn, Zeisel, & Wurtman, 1979). Parts of choline will participate in fat metabolism, and others will stimulate the CDP-choline way for the synthesis, transport, and deposition of PC in animal products, thus increasing the PC, PE and the total phospholipid contents in milk (Zeisel, 1981). These might be the logical explanations for the increase of total phospholipid, PC and PE in milk.

Fat is present in milk in the form of spherical droplets known as the milk fat globules (Contarini & Povolo, 2013). They are comprised of a triglyceride core and are surrounded by a natural biological membrane (Altenhofer et al., 2015). The mean size of natural milk fat globules is approximately 4 μm, and their broad diameter distribution spans from approximately 0.2–15 μm (Mesilati-Stahy, Mida, & Argov-Argaman, 2011). The size of milk fat globules has crucial effect on the stability and technological properties of milk, and it is related to the digestion and absorption of milk (Gallier et al., 2012; Michalski et al., 2005). Generally speaking, small milk fat globules gastric emptying is slow and has a greater contact area with digestive enzymes (Michalski, 2009). SL is a natural surfactant, and the presence of a single surfactant results in a dramatic lowering of the size of the fat globules, the near absence of surfactants results in a larger value for size of the fat globules (Robin, Blanchot, Vuillemard, & Paquin, 1992). In this work, adding 1%, 2% and 3% SL significantly decreased the average diameter of the milk fat globules than that of the control group. This may be due to the higher addition of SL to diets and content of PC in milk. The other reason might be that the addition of emulsifiers in the diet promoted digestibility of fat and broke fat down into smaller fat droplets in the body, which ultimately reduce fat globule particle size in milk. No influence of colostrum and milk sampling was obtained with regard to composition of colostrum and milk. In agreement with the findings of the present study, Lauridsen and Danielsen (2004) showed no significant differences were observed between the contents of fat in sow milk supplemented with different fat resources. The addition of multiple types of fats in sow diet had no effect on milk fat content (Laws et al., 2009). One possible reason was that it seems unlikely that the variations in dietary supplemental emulsifier or fat within a short period of feeding could change the milk composition. Lactating mammary glands are part of the secretory immune system, and IgA antibodies in breast milk reflect antigenic stimulation of gut-associated lymphoid tissue and nasopharynx-associated lymphoid tissue such as the tonsils. Breast-milk antibodies are thus highly targeted against infectious agents and other exogenous antigens in the mother’s environment, which are those likely to be encountered by the infant (Brandtzaeg, 2010).

Maternal immunity is of vital importance in maintaining pig health and production (Dividich, Lyons, Jacques, & Hower, 2006). The concentrations of immunoglobulins are used as vital response criteria to evaluate the humoral immune response in humans and animals (Yuan et al., 2015). Immunoglobulins cannot cross the placenta in pregnant sows. Therefore, neonatal pig must acquire maternal immunoglobulins from ingested colostrum and milk for passive immune protection until the immune system of the piglet becomes fully developed (Rooke & Bland, 2002). Protection by maternal immunity is mediated by a number of factors, involving mostly immunoglobulin can be transferred from blood to colostrum (Salmon et al., 2009), which maybe one possible explanation for significantly increased concentrations of IgA and IgM in colostrum by adding 2% and 3% SL to the diets. As lactation proceeded, IgG concentrations decreased and IgA became the major immunoglobulin in sow milk, and most of which was produced in the mammary gland (Ariza-Nieto et al., 2011). Milk immunoglobulin cannot be absorbed into the blood, but it protects against local pathogens, commensal bacteria, and food antigens in the topical digestive tract (Brandtzaeg, 2010). IgM in colostrum plays a role in initiating the immune response system of piglets to fight viruses (Chau, Collier, Welsh, Carroll, & Laurenz, 2009). In our experiment, supplementation of 2% and 3% SL in the diets significantly increased concentrations of IgA and IgM in colostrum and IgA in milk, which might better protect the humoral and mucosal immunities of piglets.

We found that the content of IgA of sows and piglets increased, especially with 2% and 3% SL supplemented diets. A researcher reported that concentrations of immunoglobulins in piglet plasma depend on the amount of colostrum ingested (Rooke & Bland, 2002). This may be a reason to explain the content of IgA of sows and piglets increased when fed SL to sow. SL are rich in linoleic, linolenic acids and other unsaturated fatty acids and choline, inositol and VE and other essential nutrients, which has a significant role to promote and enhance immune function. Previous studies have shown that SL can promote the proliferation of T-cell, increase the number and conversion of lymphocytes, and promote phagocytosis of macrophages to tumour cells by adding SL to rat diets, and thus improve the immune function of the body (Shao, 2003). This may be another explanation for increasing concentrations of IgA in sows and piglets plasma when fed SL to sow diet. However, little research has been performed to determine the effects of SL supplementation to sow diets on immunoglobulin of pigs, and the further research is warranted.

Conclusions

In summary, the present study indicated that addition of SL in lactating sow diets increase the content of PC and total phospholipid in milk, decrease the milk fat globule size, and improve the weight of piglets at weaning, ADG, immunoglobulin levels of plasma, colostrum and milk. These findings will also helpful to enhance our understanding of the role of SL in nutritional health of animal and human.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This work was supported by the National Natural Science Foundation of China [grant number 31672429] and the earmarked fund for China Agriculture Research System [grant number CARS-35].