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Nutrient Requirements

The Amino Acid Need for Milk Synthesis Is Defined by the Maximal Uptake of Plasma Amino Acids by Porcine Mammary Glands^1,2,3

Department of Animal Sciences, Michigan State University, East Lansing, MI 48824 and ^* Department of Animal and Avian Sciences, University of Maryland, College Park, MD 20742

⁴To whom correspondence should be addressed. E-mail: xguan{at}bcm.tmc.edu.

	ABSTRACT

TOP ABSTRACT MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

 
To define dietary indispensable amino acid (IAA) needs for milk synthesis by the mammary glands (MG), 16 lactating sows were fed 1 of 4 isocaloric diets varying in protein concentrations (from 78 to 235 g/kg) with an ideal amino acid (AA) pattern. On d 9, 13, 17, and 21 of lactation, blood samples were obtained simultaneously from a carotid artery and the main mammary vein every 30 min over 6 h. A quadratic regression model of the log mammary arteriovenous difference (AVD) of plasma IAA () against daily intake of dietary IAA (X) was established. First, the reverse log intercept, defined as the mammary AVD at zero dietary AA supply, was used to quantify the contribution of endogenous IAA. The quantification was validated by body N balance coupled with AA composition analysis. Then, the estimated vertex (_max, X_i) was used in 2 aspects: 1) The maximal mammary uptake of plasma IAA, quantified by multiplying the maximal mammary AVD and plasma flow rate, was considered the physiological IAA need for milk synthesis. 2) Corresponding to the _max, dietary IAA intake (X_i) would represent the total dietary IAA requirement, i.e., the sum of maintenance need and milk synthesis need after adjustment for body weight loss. Thus, dietary IAA needs for milk synthesis were derived. Moreover, the estimate of lysine need for milk synthesis in this study was identical to an estimate obtained from multiple regression analysis of feeding trial data. We conclude that dietary IAA needs for milk synthesis can be quantified by the maximal uptakes of plasma IAA by porcine MG.

KEY WORDS: • amino acid requirement • mammary arteriovenous difference • amino acid uptake • pigs • lactation

Lactating sows must be provided with adequate amounts of indispensable amino acids (IAA)⁶ in optimal proportions for maternal health, reproductive performance, and dietary protein utilization. Dietary IAA requirements (i.e., adequate intakes) of sows were estimated using both an empirical method and a factorial approach. The empirical method involves exploring physiological and/or reproductive response curves in animals fed graded levels of the test amino acid (AA). This method only estimates total dietary requirement for 1 AA under the specified conditions. Moreover, it is difficult to extrapolate total dietary requirement to other conditions such as different rates of lactation performance and body weight change. In contrast, the factorial approach derives total dietary requirements of the test AA by summing up individual physiological needs of various functional components (e.g., body maintenance, milk synthesis, and body protein accretion). The factorial approach was used to predict dietary IAA requirements of sows under various conditions (1); however, none of the individual physiological needs was defined experimentally.

Of the dietary IAA requirements, lysine has been the most intensively investigated because it is usually the first limiting AA for lactating sows. Empirical estimates of dietary lysine requirements of sows vary widely from 40 to 55 g/d (2–5). The wide variation in estimates may be attributed to changes in maternal body weight, differences in lactation performance, and/or diversity in response criteria used (6,7). When dietary intake of protein and/or lysine is inadequate during lactation, sows mobilize body proteins to support milk synthesis (8,9). In this situation, the contribution of endogenous lysine mobilized from body protein reserves will compensate for part of the total dietary lysine requirement. It is this lactation homeorrhesis that renders an inherent uncertainty of any empirical estimates of minimal IAA requirements of lactating sows. The total dietary lysine requirement was also estimated by the factorial approach (1). However, neither endogenous lysine contribution nor dietary lysine need for milk synthesis by the mammary glands (MG) per se was defined experimentally.

Dietary requirements of IAA other than lysine have not been fully established in the lactating sow. The NRC (1) derived dietary requirements for IAA other than lysine by multiplying dietary lysine needs and ideal ratios of other IAA to lysine for maintenance, milk synthesis, and body protein accretion (or body tissue), respectively. The AA pattern in milk was assumed to be an ideal profile of dietary AA for milk synthesis (1). However, the AA pattern in milk is not identical to the mammary AA uptake pattern (10). Specifically, the mammary uptake of branched-chain AA and arginine exceeds the output in milk by 20 to 50% (10). In addition to their incorporation into milk, branched-chain AA may provide -amino nitrogen, carbon skeleton, and energy for synthesis of dispensable AAs and other compounds (e.g., lactose and fatty acids) in the MG (11). Arginine is also the precursor for de novo synthesis of nitric oxide, which regulates mammary blood flow (12). Furthermore, a recent study indicated that the ideal ratios of dietary IAA are dynamic and highly related to exogenous intake of dietary IAA and endogenous contribution of body protein (13). Therefore, estimates of dietary IAA needs for milk synthesis based only on the AA pattern in milk are appropriate in the absence of empirical data, but they are the minimum values.

Our recent study indicates that the mammary net uptake of some IAA (e.g., lysine) is a rate-limiting step for the synthesis of milk protein in vivo (14). Maximal mammary uptake of plasma IAA will theoretically reflect the intramammary IAA potentially required for milk synthesis. Thus we hypothesized that the maximal mammary uptake of plasma IAA can be defined as the mammary physiological need for IAA and further presumed as the dietary true digestible IAA (TDIAA) need for milk synthesis. Dietary TDIAA needs for milk synthesis defined by the maximal mammary uptake not only include their output in milk, but also their metabolic need by the MG. Moreover, a set of dietary true digestible IAA needs for milk synthesis could be concurrently estimated by this method. In the present study we quantified the maximal mammary uptakes of plasma IAA and estimated a set of dietary true digestible IAA needs for milk synthesis. In addition, our estimate of dietary requirement for true digestible lysine for milk synthesis was supported by a multiple regression analysis of the total dietary lysine requirements against litter weight gain and maternal body weight loss. The estimates of this study can be used to construct an ideal pattern of dietary IAA for milk synthesis and predict total dietary IAA requirements of lactating sows.

	MATERIALS AND METHODS

TOP ABSTRACT MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

 
The All-University Committee on Animal Use and Care of Michigan State University approved all procedures used in the study.

    Dietary treatments. On d 1 of lactation, 16 sows (Landrace x Yorkshire, parity 2 or 3) were allocated to dietary treatments according to a completely randomized block design. Each block consisted of 4 sows. Each sow in 1 block was allowed to eat ad libitum from 1 of 4 isocaloric diets (14.3 MJ ME/kg) containing different protein concentrations (78, 130, 182, and 235 g/kg) with an ideal pattern of dietary IAA (1). The composition of the diets is given in Table 1.

    Milk yield estimation. Milk yield was estimated on d 11 and 21 by the D₂O dilution method of Pettigrew et al. (16). In brief, piglets were deprived for 1 h, weighed, and injected i.m. at a dose (1 mg/kg BW) of D₂O (Cambridge Isotope Laboratories). At 1 h after the dose, piglets were bled from a jugular vein. At 24 h after the dose, the piglets were deprived of food for 1 h, weighed, and bled as before. During this 24-h period, piglets were not given access to water. Blood samples were centrifuged at 1500 x g for 15 min at 4°C, and then serum samples were separated and stored at –20°C. Serum D₂O concentration was determined by an infrared spectrophotometer (Foxboro) using a fixed filter length of 4 µm. Milk yield was calculated on the basis of the D₂O dilution principle (16).

    Sampling protocol. Blood samples were collected on d 9 ± 1, 13 ± 1, 17 ± 1, and 21 ± 1 (referred to as d 9, 13, 17, and 21, respectively). Sows were fed 1 h prior to blood sampling and still given ad libitum access to food and water. Carotid arterial and main mammary venous blood samples (10 mL each) were simultaneously collected every 30 min over a 6-h period. Thus a total of 13 blood samples were collected from each sow per sampling day. Blood samples were centrifuged at 1500 x g for 15 min at 4°C. Plasma samples were removed and stored at –20°C. On sampling days, milk was collected following an i.v. administration of oxytocin (10 IU) and pooled from all functional teats on each sow. An aliquot of pooled milk was stored at 4°C until assayed for milk composition; the remainder was defatted by centrifugation at 1500 x g for 15 min at 4°C and stored at –20°C until assayed for AAs.

    Amino acid analysis. Food samples were finely ground using a sample mill. Total nitrogen content in food and milk was determined using a nitrogen analyzer (FP-2000, LECO) using EDTA (Sigma) as a calibration standard. Milk lipid and protein contents were assayed by a mid-infrared spectrophotometry (Multispec M, Berwind Instrument). Concentrations of AAs in the defatted milk and food were analyzed using reversed-phase HPLC by precolumn derivatization with phenylisothiocyanate (14). Concentrations of AAs in the pooled plasma samples were determined using a Beckman 6300 amino acid analyzer (14).

    Calculations Mammary plasma flow rate. Mammary plasma flow rate was estimated by Fick’s principle as described previously (14). Given that the amount of lysine output in milk contributed from plasma peptides is negligible (17), mammary uptake of plasma lysine can be calculated as

(1)

In addition to incorporation into milk, lysine has 2 major metabolic pathways: oxidation and accretion in the MG. Because dietary ME intake was adequate, lysine as the first limiting AA might be minimally oxidized in the MG. In addition, accretion of lysine in the MG was calculated from the study of Kim et al. (18) to be around 1 g/d, which is negligible compared to the measured lysine output in milk (41 g/d) in the present study. Therefore, Eq. (1) was simplified as follows:

(2)

(3)

The vertex of the quadratic regression. The relationship of log mammary AVD of plasma IAA () and daily intake of each dietary IAA (X) was fitted by a quadratic regression curve: = ß₀ + ß₁X + ß₂X², with being the predicted variable, X being the predictor variable, and ß₀, ß₁, and ß₂ being the parameter estimates. The vertex (_max, X_i) of this regression was predicted at d/dX = 0, i.e., X_i = –ß₁/2ß₂, thus _max = ß₀ – ß₁²/4ß₂.

Dietary TDIAA needs for milk synthesis derived by the factorial approach. Based on the factorial approach, the total dietary requirement of true digestible IAA (i.e., daily intake of true digestible IAA at _max) = maintenance need + milk synthesis need – the contribution of endogenous IAA. Thus dietary true digestible IAA need for milk synthesis = daily intake of true digestible IAA at _max + the contribution of endogenous IAA – maintenance need. Daily intake of true digestible IAA at _max was converted from daily intake (X_i) of IAA at _max (1).

    Modeling and statistical analyses The regression models were computed with the REG and NLIN procedures (SAS/STAT Version 8, SAS Institute). Mammary AVD of plasma IAA were log-transformed based on their residual error distribution. Based on the sum of squared residuals (see the supplemental table), the quadratic regression model was better fitted than a nonlinear exponential model [ = b₀(1 – EXP(–b₁X)]. Confidence intervals were estimated with a total of 750 bootstrapping samples using the bootstrapping reflection method (19). Milk yield and plasma flow rate were analyzed by the MIXED procedure (SAS/STAT), in which dietary treatment and lactation day as the fixed effects and sow as the random effect were included in the model; lactation day as repeated measures was included in the REPEATED statement; and least-square means and SEM were presented.

	RESULTS

TOP ABSTRACT MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

 
    Mammary plasma flow. Plasma flow rate across the MG was estimated to be 6440 ± 765 L/d based on Fick’s principle using lysine as an internal indicator and was not different from that using phenylalanine plus tyrosine as an internal indicator. In the present study, the AVD of plasma lysine over the 21-d lactation period was 43.4 ± 3.6 µmol/L whereas the concentration of lysine in skin milk was 26.0 ± 0.9 mmol/L. The milk yield was 11.48 ± 0.98 L/d, which did not differ (P > 0.10) between d 11 and 21 of lactation (11.7 ± 0.6 and 11.4 ± 0.6 L/d, respectively). The mammary plasma flow rate was not affected (P > 0.10) by dietary treatments. The mean ratio of mammary plasma flow to milk yield was 560:1 (v:v) over the 21-d lactation period.

    Contributions of endogenous IAA (IAA_c) predicted by the mammary uptake of plasma IAA. A quadratic regression model for mammary AVD (MAVD) of plasma IAA was developed to predict the IAA_c. The relationship was best fitted by a quadratic regression model between log mammary AVD of plasma IAA and daily intake of IAA over the 21-d period of lactation (Table 2). Uptake of indispensable AAs by the MG at zero intake of protein can be attributed to 2 sources: 1) release from (muscle) protein breakdown and 2) endogenous de novo synthesis (e.g., arginine). Thus estimation of the mammary uptake by the model in the present study includes both sources of endogenous indispensable AAs. Because arginine can be synthesized in the body (1), the contribution of endogenous arginine released from body proteins could not be defined. In addition, either estimates of intercepts for histidine and tryptophan or estimates of the quadratic term for isoleucine and leucine were not different from zero (P > 0.05). Endogenous contributions of lysine, methionine, phenylalanine, threonine, and valine were determined in the study. The relationship between log mammary AVD of plasma lysine and daily intake of lysine is shown as an example in Figure 1. The reverse log intercept (ß₀) is defined as the mammary AVD of plasma lysine when lysine intake is extrapolated to zero. The mammary uptake of plasma IAA at zero intake of dietary IAA was attributed to the IAA_c (mobilized from extramammary tissue proteins). A small proportion of endogenous IAA would be first used for obligatory metabolism, and a predominant proportion of endogenous IAA would be then taken up by the MG for milk synthesis. Thus, the IAA_c is defined as the sum of these 2 parts. The amount of IAA_c used for obligatory metabolism was considered maintenance need (1). The amount of IAA_c used for milk synthesis was predicted using the regression equations (Table 3).

View larger version (13K): [in this window] [in a new window]   FIGURE 1 Relationship between the log mammary AVD of plasma lysine and daily intake of dietary lysine in lactating sows over a 21-d lactation period. The solid curve is best fitted by a polynomial regression model: = 2.4104 + 0.07902X - 0.001059X2 (P = 0.036; R2 = 0.49). Extrapolation of the curve (dotted line) is predicted on the basis of this model. Each diamond represents an individual sow. The reverse log intercept is defined as the plasma lysine AVD0 when dietary lysine intake is extrapolated to zero.

View larger version (15K): [in this window] [in a new window]   FIGURE 2 The relation between log mammary AVD of plasma Val () and daily intake of dietary total Val (X) in lactating sows over a 21-d lactation period. The curve was best fitted by a quadratic regression model: = 1.7441 + 0.1120X - 0.001308X2 (P = 0.0066, R2 = 0.63). Each dot represents an individual sow. The vertex (max, Xi) derived from this regression had 2 implications: 1) The maximal mammary uptake of plasma Val could be quantified by multiplying the maximal mammary AVD of plasma Val (i.e., the reverse log max) and mammary plasma flow rate. 2) The maximal mammary uptake of plasma Val was defined as mammary physiological need for Val and further considered the dietary true digestible Val need for milk synthesis by the MG. Daily intake of dietary Val (Xi) would represent the total requirement of dietary Val by lactating sows, i.e., the sum of maintenance need and dietary need for milk synthesis after adjusted by the maternal body weight loss in this study. Thus dietary true digestible Val need for milk synthesis could be derived by the reverse factorial approach. A bootstrapping reflection method was employed to compute the 95% CI for daily intake of dietary Val (19).

	DISCUSSION

TOP ABSTRACT MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

Mammary IAA uptake and their dietary intake.

Mammary net uptake (transport) of plasma AAs is a tightly regulated and highly coordinated process that provides AAs to the organ for milk protein synthesis and specific metabolic functions. The process is not only determined by arterial AA availability and mammary blood flow, but also controlled by AA transporter capacity in the mammary epithelium, AA transporter competition, intracellular metabolic driving (e.g., metabolic activity and milk synthesis potential), and hormone action (21,22). However, the quantitative relationship between mammary AA uptake and dietary AA intake has not been described in vivo. To quantify mammary uptake of plasma IAA, we measured the product of mammary AVD for IAA and plasma flow estimated by Fick’s method (10,14). In the present study, lysine was used as the internal indicator to estimate mammary blood flow, based on the assumption that lysine metabolic loss and accretion in the MG are negligible relative to its output in milk. Mammary plasma flow rate was estimated to be 6440 L/d, equivalent to a volume ratio of plasma flow to milk yield of 560:1, which is in agreement with an estimate of 540:1 in lactating sows (10). It is important to note that mammary plasma flow was not affected by dietary IAA intake. Therefore, mammary IAA uptake was largely driven by increases in arterial IAA concentrations, which were positively correlated with their dietary intake. For most IAA the relationship between mammary AVD (i.e., uptake) and dietary intake was quadratic, reaching a vertex at 182 g protein/kg diet; however, above this dietary intake, the mammary IAA AVD declined. In contrast, the mammary AVD of leucine and isoleucine increased linearly with increasing dietary intake. We postulate that the decline in the mammary AVD of most IAA at their highest intake was not due to substrate saturation of the AA transport system, but to competitive inhibition of AA transport by large neutral amino acids (e.g., leucine and isoleucine). In fact, arterial concentrations of those AAs (e.g., lysine and methionine) were much lower than their K_m at the highest intake. Increasing evidence suggests that there is a substantial interaction in mammary AA transport between cationic and neutral amino acids. It has been previously demonstrated in our laboratory that high physiological concentrations of valine significantly inhibit in vivo net uptake of lysine by the porcine MG (14). In addition, high physiological concentrations of leucine strongly inhibit in vitro net uptake of both valine by the porcine mammary tissue (23) and lysine by rodent mammary tissue (24).

Estimation of endogenous IAA contribution.

Sows mobilize body protein reserves to support milk synthesis when the intake of protein or IAA is not adequate. This mobilization is mediated mainly through the regulation of interorgan protein metabolism [e.g., decreasing protein synthesis and increasing protein degradation in muscle tissues (25,26)]. Through this mobilization, IAA from extramammary tissue proteins are partitioned to the MG for milk synthesis. Moreover, nutrients are used first for maintenance and second for milk synthesis in sows (27). Therefore we hypothesized that IAA_c mobilized from extramammary tissue proteins can be predicted as the sum of the amount of the IAA captured for obligatory metabolism (or maintenance need) and the amount of the IAA taken up by the MG when dietary intake of the IAA is zero.

To validate the MAVD estimation, we quantified the contributions of endogenous IAA by multiplying body protein loss and the concentrations of IAA in body protein. In the present study, body protein loss was estimated at 177 g/d or 150 g/kg BW loss, assuming that negative N balance of the whole body represented body protein loss. This estimate is in agreement with previous studies (9). In the present study, lysine concentration in body protein was assumed to be 70 g/kg body protein, which was based on estimates of 70 (20) and 70.5 (28) g/kg body protein. Moreover, AA composition of the whole body protein was assumed to represent that of mobilized body protein (i.e., mainly muscle protein) during lactation. However, AAs are not released from muscle in an exact proportion to the occurrence in AA sequence of muscle protein. Within muscle branched-chain AAs are metabolized substantially through transamination and decarboxylation and methionine to some extent through transulfuration; however, no other AA (including lysine, phenylalanine, and threonine) are metabolized (29,30). The first 2 enzymes (i.e., branched-chain aminotransferase and branched-chain -keto acid dehydrogenase) in the catabolic pathway are highly expressed in skeletal muscle (29,31) and upregulated significantly in the MG during lactation (32). As a result, these highly metabolized AAs (especially valine) from muscle proteolysis will become less available to the MG. Therefore, the estimates of lysine, phenylalanine, and threonine predicted by the MAVD could be close to those defined by the BCCA, whereas the estimates of methionine and valine predicted by the MAVD are slightly lower than those defined by the BCCA. It should be pointed out that the MAVD predicted the available amount of IAA from body protein loss contributed to the MG at zero dietary AA supply, whereas the BCCA estimated the maximal amount of IAA released from body protein loss at a negative body N balance. Furthermore, measurements of the nonmetabolized AA AVD and blood flow across the hindquarter would be a direct estimation of the AAs released from muscle proteolysis, which can be used to validate the MAVD estimates. Finally, the contribution of endogenous lysine (a mean of 10.5 g/kg BW loss) is in agreement with results from the study of Jones and Stahly (9), but is much higher than an estimate of 6.2 g/kg BW loss assumed by the NRC (1). However, contributions of endogenous AAs are not only determined by gross loss of body weight, but also by the lean proportion of mobilized body reserves and the aforementioned AA metabolism in muscle. Thus, it is not appropriate to estimate the contributions if they are only based on body weight loss and AA composition in body protein as modeled by the NRC (1).

Estimation of IAA need for milk synthesis.

Output of IAA in milk might not be a sensitive endpoint because of the inherent lactation homeorrhesis. A certain level of milk synthesis can be maintained at the cost of body protein loss when intake of dietary protein (IAA) is not adequate (8,9). In contrast, mammary uptake of plasma IAA is specifically responsive to intake of dietary IAA (14), which can be considered a new response criterion for estimating dietary IAA needs for milk synthesis (7). In the present study, we defined the maximal mammary uptake of plasma IAA as its mammary physiological need for milk synthesis and further considered the dietary true digestible IAA need for milk synthesis. Estimates of dietary true digestible IAA (except for phenylalanine) needs for milk synthesis defined by the maximal mammary uptake are comparable to those derived independently using the reverse factorial approach (see Tables 5, and 7). Quantitatively, the maximal mammary uptakes of plasma IAA not only encompass their output in milk, but also includes their metabolic need by the MG. The metabolic need may include oxidation, tissue protein accretion, de novo synthesis of dispensable AAs, and/or conversions to other substrates (33).

The total requirement for dietary lysine of lactating sows was estimated empirically based on reproductive performance and nutritional status of body nitrogen (7). The total requirements of dietary lysine are strongly related to lactation performance (6), which can be met by both exogenous intake of dietary lysine and endogenous contribution of lysine mobilized from body protein reserves. It is this endogenous contribution that complicates the empirical estimation of total requirements of dietary lysine for lactating sows (6). To compare empirical estimates of total requirements of dietary lysine, we developed a response surface model to fit data reported in the literature (2–5,8,34–39). The response surface of the total requirements of dietary true digestible lysine was regressed against litter weight gain and maternal body weight loss during lactation. The total requirements (Y) of dietary true digestible lysine by lactating sows can be predicted by the response surface model: Y (g/d) = 0.83 + 20.20 litter weight gain (kg/d) – 7.28 body weight loss (kg/d) (P < 0.0001, R² = 0.64). The partial coefficient of litter weight gain (20.20 g/kg, P < 0.0001) can be considered the dietary true digestible lysine need for milk synthesis at zero body weight loss and agrees closely with an estimate of 20.15 g/kg predicted by the maximal mammary uptake in this study. The total requirement of dietary true digestible lysine increases with increasing level of lactation performance (i.e., with increasing litter weight gain); but decreases with increasing endogenous contribution of lysine from body protein reserves (i.e., with increasing maternal body weight loss) (Fig. 3).

View larger version (61K): [in this window] [in a new window]   FIGURE 3 A 3-D response surface of dietary true digestible lysine requirements (Y) of lactating sows fitted by the multiple regression model: Y = 0.83 + 20.20 litter weight gain – 7.28 body weight loss (P < 0.0001, R2 = 0.64). Each dot represents an individual estimate of dietary true digestible lysine requirement in the literature (2–5,8,35–39). Dietary lysine requirements are expressed on true digestible basis as cited from the literature or calculated from its total basis (1).

Dietary true digestible threonine need for milk synthesis was estimated at 14.1 g/kg litter wt gain at zero body weight loss in the present study, which is comparable to an estimate of 12.08 g/kg litter wt gain at minimal body weight loss (47). The ratio (0.70:1) of dietary threonine need for milk synthesis is in agreement with the study of Cooper et al. (47). Threonine can also be oxidized to CO₂ and converted to dispensable AAs in the MG (48). Therefore, the estimate of its dietary need for milk synthesis may only be a minimum if it is based on its output in milk. The ratio (0.60:1) of dietary phenylalanine need for milk synthesis is very close to a ratio of 0.63:1 for its total requirement when based on milk yield and plasma AA profile (1,49). Dietary phenylalanine need for milk synthesis predicted by the maximal mammary uptake was substantially lower than that derived from the reverse factorial approach. Whole body hydroxylation of phenylalanine to tyrosine is upregulated by increasing systemic phenylalanine availability in lactating goats (17). The hydroxylation would decrease the efficiency of dietary phenylalanine utilization for milk synthesis. Thus, dietary phenylalanine need for milk synthesis might be overestimated by the reverse factorial approach. Dietary true digestible methionine need for milk synthesis was estimated to be 14.9 g/d, which is in agreement with a value of 17 g/d for total requirement of dietary methionine at minimal body weight loss (50). However, the dietary methionine need for milk synthesis is related to the oxidation of methionine and conversion to cysteine in the MG.

In summary, dietary IAA needs for milk synthesis can be determined by their maximal mammary uptake, and these estimates were further validated by the reverse factorial approach. Dietary true digestible lysine need for milk synthesis was estimated to be 20.15 and 20.26 g/kg litter wt gain, respectively, by these 2 methods, which is supported by the multiple regression analysis of empirical estimates of total requirements of dietary lysine. Dietary ratios of true digestible arginine, histidine, methionine, phenylalanine, threonine, tryptophan, and valine to lysine required for milk synthesis were higher than values recommended by the current NRC (1). Dietary IAA needs for milk synthesis defined by their maximal mammary uptake not only include their output into milk, but also cover their mammary metabolic needs. Therefore an estimate of dietary IAA need for milk synthesis might be a minimum if it is solely based on the AA profile in milk, especially for arginine and the branched-chain AA.

	ACKNOWLEDGMENTS

 
The authors thank the Michigan State University Swine Farm and the College of Veterinary Medicine staff for their assistance and care in animal handling. The authors also thank Douglas G. Burrin and Peter J. Reeds of the USDA/ARS Children’s Nutrition Research Center at Baylor College of Medicine for their contribution in reviewing the manuscript.

	FOOTNOTES

 
¹ Part of this work was published in abstract form and presented orally at the Joint Meeting of ASAS and ADSA, July 27–31, 1998, Denver, CO [Guan, X. F., Ku, P. K., Pettigrew, J. E., Ames, N. K., Tempelman, R. J. & Trottier, N. L. (1998) Limiting amino acid requirements of lactating sows estimated by plasma arterio-venous difference of free amino acids across the mammary gland. J. Anim. Sci. 76 (Suppl. 1): 161A (abs.)].

² Funded by the Minnesota Soybean Council and the Michigan Agricultural Experiment Station.

³ The fitting analysis between the 2 models is included in a supplemental table with the online posting of this paper at www.nutrition.org.

⁵ Present address: U.S. Department of Agriculture, Agricultural Research Service, Children’s Nutrition Research Center, Department of Pediatrics, Baylor College of Medicine, Houston, TX 77030.

⁶ Abbreviations used: AA, amino acid; AVD, arteriovenous difference; BCCA, body composition change approach; IAA, indispensable AA; IAA_c, contribution of endogenous IAA; MAVD, regression model for mammary AA AVD; MG, mammary glands; TDIAA, true digestible IAA.

Manuscript received 4 February 2004. Initial review completed 28 March 2004. Revision accepted 14 June 2004.

	LITERATURE CITED

TOP ABSTRACT MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

 

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