Milk Secretion

The purpose of milk secretion, for all terrestrial mammals, is to meet the needs of the neonate, the natural habitat, and the nursing habits of the particular species.

From: Non-Bovine Milk and Milk Products , 2016

LACTATION | Dietary Requirements

N.M.F. Trugo , C.M. Donangelo , in Encyclopedia of Human Nutrition (Second Edition), 2005

Introduction

Milk secretion imposes a considerable nutritional demand on lactating women. The challenge to the maternal organism to sustain milk production and nutrient composition while maintaining an adequate nutritional status is high and must be met by increased dietary intake of energy and nutrients. Otherwise, maternal nutrient depletion may occur due to excessive mobilization of maternal stores. Owing to the major gaps in the knowledge on maternal nutrient requirements and the impact of lactation on maternal nutrient status, and the quantitative and qualitative importance of milk production for the incremental nutrient requirements, the recommended nutrient intakes for lactating women are based mainly on the volume of milk secreted and its nutritional content. The high nutritional demands for milk production result in recommended intakes of most nutrients for lactation that are higher (10%–90%) than in nonreproductive stages.

The dietary recommendations for lactating women considered in this article are those of the FAO/WHO Reports on fats (1994) and micronutrients (2001), and the Dietary Reference Intakes (DRIs) of the Institute of Medicine (US) on micronutrients (1997, 1998, 2000, 2001) and macronutrients (2002). The rationale for recommended nutrient intakes, and the nutrient requirements and dietary recommendations for energy, fat, protein, calcium, zinc, folate, and vitamin A are specifically addressed.

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LACTATION | Physiology

M. Neville , in Encyclopedia of Food Sciences and Nutrition (Second Edition), 2003

Regulation of Milk Secretion

Prolactin is necessary for milk secretion, and suckling promotes its secretion. However, the volume of milk secretion is not directly regulated by the concentration of prolactin in the blood. Rather, local mechanisms within the mammary gland related to the amount of milk removed by the infant are responsible for the day-to-day regulation of milk volume. A protein factor called feedback inhibitor of lactation is secreted with other milk components into the alveolar lumen. If milk is not removed from the gland, this substance, whose identity is not yet entirely clear, interacts with the mammary alveolar cell and inhibits milk secretion possibly by altering the sensitivity of the cells to prolactin.

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Volume 1

Ø. Holand , ... M. Nieminen , in Encyclopedia of Dairy Sciences (Third Edition), 2011

Future

Maintenance of milk secretion requires repeated exposure of the mammary cells to hormones released during milking, in addition to a continuous removal of milk. In semidomestic species, it may be hard to maintain normal secretion rate in the absence of young, unless the milking frequency is high during the first days after separation of mother–offspring. Hence, permitting limited suckling by calves may be a viable strategy for keeping up the production. This will secure a flexible management system, for example, in periods of shortage of labor force or insect harassment, the milking can be reduced without jeopardizing future yield. Also from an ethical perspective, calves partly at foot would be preferable.

Reindeer milking is an intensive form of reindeer husbandry, and is hardly feasible within the extensive free roaming meat production system practiced today, as selection for milk yield and training of the milking does would be severely constrained. But in isolate areas, a partly free-ranging milking system may be viable if a herdsman and his dog(s) manage the roaming animals during the daytime and pen them close to the camp at night. The animals should be milked both morning and evening. To secure the females' homing, the calves could be kept separated close to the milking camp and allowed to join their mothers after the milking session. The homing could be further enhanced by supplementary feeding, which also will contribute to a positive energy balance and hence a sufficient milk output. The summer milking farm has to be highly mobile to ensure high-quality natural forages and reduce the parasite load and risk of contagious diseases.

Reindeer milk is and will be a valuable niche product, but a commercially sound production is dependent on continuing high returns. The marked potential lies in an exclusive niche within the tourist industry as well as in the gourmet and cosmetic market. A new milking industry would need to adapt new techniques and management regimes within the biological and ecological constraints of the animals and the environment and within an acceptable ethical framework.

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Reproductive and Endocrine Toxicology

E.J. Begg , ... C.M.J. Kirkpatrick , in Comprehensive Toxicology, 2010

11.22.5 Effect of Xenobiotics on Milk Production

Drugs may affect milk secretion by influencing the hormonal regulation of lactation. Dopamine agonists such as cabergoline and bromocriptine reduce prolactin and have been used therapeutically to stop lactation. Sex hormones such as estrogens and high doses of ethanol may decrease lactation. Dopamine antagonists such as domperidone and metoclopramide have been used to increase milk production, particularly in mothers who have delivered prematurely. Galactorrhea may be an undesirable side effect in men and in nonbreastfeeding women treated with drugs such as antipsychotics that antagonize dopamine receptors as part of their mode of action and agents like the selective serotonin reuptake inhibitors that can cause hyperprolactinemia via indirect dopamine antagonism.

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Risk factors as biomarkers of susceptibility in breast cancer

Casey E. Reed , Suzanne E. Fenton , in Biomarkers in Toxicology, 2014

Lactation

Lactogenesis, or milk secretion, includes all the necessary changes that the mammary epithelium goes through to develop into a fully functional lactational gland after birth ( Neville et al., 2001). This process begins during mid-pregnancy and continues until weaning (Neville et al., 2001). During the pregnancy–lactation cycle the breast matures from nonfunctional tissue into a mature milk-producing gland (Faupel-Badger et al., 2013). When breastfeeding is completed, there is apoptosis of the secretory epithelium to return the tissue to a resting state (Watson, 2006; Faupel-Badger et al., 2013).

Research has shown that breastfeeding, especially for a longer time span, is protective against breast cancer development when compared to parous women who never breastfed (Bernier et al., 2000; Kobayashi et al., 2012). The mechanisms of how breastfeeding decreases breast cancer risk are unknown; however, theories generally focus on the fact that breastfeeding and sustained lactation decrease the number of menstrual cycles a woman has and therefore decrease the lifetime exposure to estrogen and that pregnancy and lactation change the TDLU morphology (Kobayashi et al., 2012; Faupel-Badger et al., 2013). Nulliparous women have fewer TDLUs per unit area of breast tissue than do parous women and parous women have more TDLU involution (Faupel-Badger et al., 2013). Increased amounts of TDLU involution is associated with decreased risk, especially for basal-like breast cancers, because the TDLUs are the site where most breast cancers originate (Faupel-Badger et al., 2013). Each year that a woman breastfeeds infers a decrease in relative breast cancer risk of 4.3% with an additional reduction in risk of 7% for each birth the woman completes (Collaborative Group on Hormonal Factors in Breast Cancer, 2002). Prolonged breastfeeding and subsequent gradual weaning may also provide protection by decreasing the inflammatory reaction caused by rapid involution of secretory epithelium (Kobayashi et al., 2012). However, it is hard to determine the effects of breastfeeding alone on breast cancer risk because the strongest reduction of risk is seen in multiparous women (Faupel-Badger et al., 2013).

Studies have also found protective effects of breastfeeding in high-risk groups such as those with genetic mutations and a family history of breast cancer. Women who carry the BRCA1 mutation are at a higher risk for early onset breast cancer; however, those with the mutation who breastfed for at least 1 year had a 32% risk reduction in the development in early- and late-onset cancers (Kotsopoulos et al., 2012). The same protection was not found in breastfeeding women with the BRCA2 mutation (Kotsopoulos et al., 2012). Breastfeeding, regardless of length of duration, was found to reduce the incidence of premenopausal breast cancer by 59% in women with a first-degree relative with breast cancer, which is a comparable reduction in risk to using hormonal treatments for those who are at high risk for developing breast cancer (Stuebe et al., 2009).

Breastfeeding, then, can be said to be protective against breast cancer development. Therefore, not-breastfeeding is linked to breast cancer risk. The Carolina Breast Cancer Study found that the strongest risk factors among premenopausal black women for basal-like breast cancers were "not breastfeeding" and increased waist-to-hip ratios (attributable fraction of 68%) (Millikan et al., 2008). Increased risk for estrogen receptor-negative/progesterone receptor-negative breast cancers in the Black Women's Health Study and increased risk of triple-negative breast tumors in a predominantly white population were also linked with lack of breastfeeding in parous women (Gaudet et al., 2011; Palmer et al., 2011).

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Determinants of Milk Volume and Composition

MARGARET C. NEVILLE , in Handbook of Milk Composition, 1995

IV. The Implications of Changes in Milk Composition During Lactogenesis

The rate of milk secretion in the early postpartum period ( Figure 1) started low, less than 100 ml/day up to 36 hr postpartum, then began to rise almost linearly to level off at about 600 ml/day at 96 hr postpartum (Neville et al., 1988). The concomitant change in the citrate concentration is shown in Figure 5 replotted from Neville et al. (1991). When mean values are examined the change in milk volume and the change in the citrate concentration are almost precisely parallel as noted many years ago by Peaker and Linzell (1975). This parallelism led Peaker to refer to citrate as the "harbinger of lactogenesis." The increase in citrate concentration clearly parallels the metabolic activity of the mammary gland as it increases its production of milk lipid (Linzell et al., 1976; Neville and Peaker, 1979).

Figure 5. Changes in the concentration of citrate in human milk in the early postpartum period compared to the increase in milk volume.

 Data replotted from Neville et al. (1991). Copyright © 1991

In Figure 6 the temporal changes in the concentration of several other milk components are compared to the change in citrate concentration. After an initial rapid fall the phosphate concentration generally parallelled milk volume as did the glucose concentration. The changing concentrations of free phosphate may reflect the increased utilization of UDP-galactose for lactose synthesis with the subsequent generation of uridine monophosphate and phosphate in the Golgi compartment of the mammary alveolar cell (Neville, 1983). We have elsewhere provided evidence that the change in glucose concentration reflects an increase in glucose transport into the mammary alveolar cell across the basolateral cell membrane (Neville et al., 1990). Casein (Patton et al., 1986) as well as calcium and magnesium (Neville et al., 1991) concentrations appear to increase coordinately with this metabolic sequence (data not shown).

Figure 6. Changes in milk composition during the early postpartum period (closed symbols). The milk volumes during the corresponding period are plotted for comparison (open circles). Data replotted from Neville et al. (1991). The vertical lines in the graphs on the right hand side of the figure indicate the time at which the lactose concentration reaches a maximum. Note that this occurs about 24 hr before the milk volume reaches a constant value. Protein, chloride, and sodium have also largely stabilized by this time.

On the other hand, not all concentration changes parallel the increase in milk synthesis rates. For example, as shown in Figure 6, the concentrations of lactose, sodium, chloride, and protein begin to change immediately after birth and achieve nearly stable values about 24 hr before peak milk volume is attained (note dotted line marking the point at which the lactose concentration stabilizes). These immediate changes likely reflect closure of the paracellular pathway with a corresponding decrease in the direct flux of interstitial constituents into the milk.

The composition changes during lactogenesis shown in Figure 6 can be explained by a simple two-step process in which junctional closure is followed by the onset of secretory activity. Unfortunately, other changes in milk composition fall less easily into this neat pattern. The abrupt postpartum fall in the protein content of the milk, for example, cannot be the result of closure of the junctions between the cells. Thus, the major proteins in human colostrum are secretory IgA and lactoferrin. Secretory IgA reaches the milk via a specific transcytotic pathway that ferries dimeric IgA molecules from the interstitial space across the mammary cells themselves (Solari and Kraehenbuhl, 1987). Lactoferrin is actually synthesized in the mammary alveolar cells (Teng et al., 1989; Schanbacher et al., 1992). Quantitative data on the concentrations of these two proteins in human breast milk during the first week of lactation have been provided by Lewis-Jones and co-workers (1985). The concentrations of both fall in the early postpartum period (Figure 7, top) and are responsible for the decline in total protein concentration during this period. However, as shown in Figure 7 (bottom), the secretion rate of both proteins actually rises on the second or third day postpartum. sIgA secretion falls rapidly again on the third day but lactoferrin continues to be secreted at a more or less constant rate after the second day postpartum. Thus, the time courses of the changes in the secretion rates of these two proteins do not coincide with the other events taking place during the first week of lactation in women.

Figure 7. (Top) lactoferrin and secretory IgA concentrations in human milk during the first week postpartum. Data from Lewis-Jones et al. (1985). (Bottom) estimate of lactoferrin and secretory IgA secretion rates using data from Figure 6 multiplied by the mean volumes given in Figure 1.

Examination of the concentration of cells in the mammary secretion (Figure 8) shows yet another pattern. The concentration of cells in milk is highest on Day 1 at about 3 × 106 cells/ml and falls by 50% on Day 3. However, there is substantial cellular secretion, particularly of polymorphonuclear leukocytes and macrophages, up to Day 10 after the cellular junctions have closed. This observation implies that the passage of cells into milk may involve more than passive transfer through open intercellular spaces.

Figure 8. The concentration of leukocytes in milk. In addition to the leukocytes shown here epithelial cells are thought to be present at a concentration of about 104 cells per milliliter throughout lactation. The dotted line represents the total leukocyte secretion rate obtained by multiplying the total number of cells by the mean volume for the corresponding day.

 Data replotted from Ho et al. (1979). Used by permission of Plenum Press. Copyright © 1979

It is likely that the gradual fall in progesterone during the postpartum period in women combined with maintained prolactin levels is responsible for the observed changes in mammary cell morphology and activity (Kuhn, 1977). However, the molecular mechanisms involved in both transduction of the secretory signal and initiation of the diverse components of lactogenesis remain almost totally unknown.

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Improving the quality and safety of sheep milk

R. Bencini , ... G. Pulina , in Improving the Safety and Quality of Milk: Improving Quality in Milk Products, 2010

13.5.2 Milking interval and frequency

As the rate of milk secretion is controlled locally by the feedback inhibitor of lactation (FIL), a fraction of the whey proteins present in milk ( Wilde and Peaker 1990, Wilde et al. 1996), the interval and frequency of milking assume paramount importance in affecting the yield of milk. The findings of Denamur and Martinet (1961) and Grigorov and Shalichev (1962) provided some indirect evidence that an autocrine control of milk secretion is present in sheep. This was later confirmed by Bencini et al. (2003). Therefore, the interval between milking, the milking frequency and the adoption of stripping methods to remove additional milk and ensure completeness of milking increase both the daily output of milk and the total lactation yield of dairy ewes by removing the inhibitory effect of milk accumulated in the alveolar tissue of the mammary glands.

In dairy sheep, reducing the frequency of milking results in a loss in milk production (Morag 1968, Karam et al. 1971, Bencini et al. 2003), but there are few and contrasting reports on the effect of milking frequency and interval on the composition of the milk. Casu and Labussiere (1972) and Huidobro (1989) reported that omitting one or both of the milkings on a particular day of the week did not affect the composition of the milk. When the milking frequency was reduced from twice to once daily the composition of the milk was not affected in the studies of Casu and Boyazoglu (1974), De Maria-Ghionna et al. (1982) and Cannas et al. (1991), but fat and protein concentrations were increased in the studies of Battaglini and De Maria (1977) and Battaglini et al. (1977, 1979), and reduced in a study by Morag (1968). When the milking frequency was increased from two to three times daily, milk composition was not affected according to Morag (1968) and Cannas et al. (1991), but the concentrations of fat and protein were increased according to Mikus and Masar (1978).

Such contrasting reports may be due to the fact that these studies were conducted with different breeds of sheep, which would have efficient or inefficient autocrine control of milk secretion according to the degree of selection for dairy production. Bencini et al. (2003) reported that milk composition was affected by the milking interval and frequency in breeds that are not selected for dairy production because they have an efficient control of milk secretion. On the contrary, breeds selected for dairying have an impaired autocrine control mechanism and therefore do not respond as much to changes in milking frequency (Nudda et al. 2002).

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Lactation: Physiology

J.L. McManaman , M.C. Neville , in Encyclopedia of Human Nutrition (Third Edition), 2013

Delays in Secretory Activation

A delay in the onset of milk secretion is a problem for the initiation of breast-feeding in a significant number of parturient women. A number of pathological conditions may delay secretory activation in women, including cesarean section, diabetes, obesity, and stress during parturition. The role of cesarean section is controversial, but if there is one, it is likely to have only a modest effect. However, poorly controlled diabetes, stress from delivery, or obesity are associated with significant decreases in early milk production. Because each of these conditions is related to higher blood glucose, hyperglycemia may be an underlying factor in the delay in lactation. However, once it is established, women with diabetes do not have a problem in maintaining lactation. Thus, compensatory factors may override initiation defects to ensure infant nutrition in these disorders.

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Diseases of the Mammary Glands

Robert N. White , in Handbook of Small Animal Practice (Fifth Edition), 2008

Definition and Causes

I.

Galactostasis is either cessation of milk secretion or an abnormal collection of milk in the mammary glands.

II.

The condition may be associated with sterile or septic mastitis.

Pathophysiology and Clinical Signs

I.

Galactostasis is a condition of uncertain etiology and is seen just before or shortly after parturition, after weaning, or during pseudopregnancy.

II.

The condition may also develop in a dam after abrupt weaning of the litter.

III.

Engorgement of the glands with residual milk induces localized inflammation and discomfort of the mammae, leading to a further failure of milk let-down.

IV.

The cranial glands may be more commonly affected in lactating queens (Colby and Stein, 1983).

V.

The condition is not generally associated with signs of systemic illness, although in cases of septic mastitis, severe systemic disease can occur.

Diagnosis

I.

Galactostasis is a likely sequela to mastitis.

II.

The condition is more commonly seen in bitches on a high plane of nutrition.

III.

Galactostasis is most likely to develop during the early period of lactation, when the nursing young are inexperienced at suckling.

IV.

The teat anatomy of affected glands (commonly, inverted nipples) may play a role in the development and progression of the condition.

V.

Cytological examination of the milk taken from affected glands may be unremarkable, although an elevated white blood cell (WBC) count (>3000/μL) can be found (Wheeler et al., 1984).

VI.

Other cytological findings, such as nondegenerate neutrophils and macrophages, are considered nonspecific, although the presence of phagocytosed fat droplets can indicate the stasis of milk within an affected gland.

Differential Diagnosis

Galactostasis must be differentiated from agalactia because the treatment and outcome for these conditions are different.

Treatment

I.

Treatment depends on the underlying cause of the galactostasis.

II.

When the condition is seen in early lactation and is possibly associated with inexperienced neonate sucking behavior, do the following:

A.

Confirm the neonates' ability to suckle.

B.

Confirm the dam's acceptance of the offspring and her ability to nurse.

C.

Attempt to evert any inverted nipples.

D.

Fast the dam for 24 hours, followed by a more limited feeding regimen.

E.

Consider manual milking of affected glands.

III.

When the condition is related to mastitis, consider the following measures:

A.

Treat the mastitis as described (see Mastitis).

B.

Apart from the notable exceptions discussed under Mastitis, the neonates may be encouraged to suckle.

IV.

When the condition is associated with abrupt weaning or pseudopregnancy, institute the following:

A.

In these instances the suckling of offspring is unlikely to relieve congestion; therefore the goal is to reduce further production of milk.

B.

Do not encourage manual milking of glands, because this causes additional milk production and let-down.

C.

Lower the dam's plane of nutrition and institute a limited feeding regimen for several days.

D.

Remove any further stimulus for milk production (e.g., the local presence of the recently weaned offspring).

E.

Cool compresses are applied to the affected gland(s) to reduce local inflammation and provide some comfort to the dam.

F.

Diuretics and glucocorticoids are, in most cases, of no benefit.

Monitoring of Animal

I.

Most cases respond to treatment within a few days.

II.

The sooner the condition is recognized and treatment initiated, the quicker the response to therapy.

III.

Cases that are not recognized and treated promptly have the greatest likelihood of developing complications, such as mastitis.

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Diseases of the Teats and Udder

Lisle W. George , ... Frank L. Welcome , in Rebhun's Diseases of Dairy Cattle (Second Edition), 2008

California Mastitis Test.

The CMT qualitatively estimates the amount of DNA in milk secretions. The concentrations of DNA and white blood cells (WBCs) in milk are directly correlated. The CMT reagent lyses the cells and gels the DNA. The amount of gel formation can be used to estimate the numbers of WBCs in the milk sample. The test is subjectively read as negative, trace, 11, 12, and 13; these scores equate well with somatic cell levels as listed in Table 8-3. The CMT is most helpful in detecting subclinical mastitis and, although accurate, serves little purpose in acute clinical mastitis.

A CMT will tend to have a high score in recently fresh cows and in cows at the end of lactation just before dry off. A CMT is also elevated in secretions from cows whose milk production has decreased precipitously because of illness. For example, cows in peak lactation that become acutely ill as a result of traumatic reticuloperitonitis may have milk production plummet acutely. Although these cows do not have mastitis, based on normal udder palpation and strip plate evaluation, the CMT will be positive. The high CMT scores represent a failure of fluid milk production to dilute the somatic cells.

Because results of the CMT are interpreted subjectively, discrepancies may arise between evaluators, and estimates of SCC that correlate with CMT score vary greatly. Therefore the values listed in Table 8-4 are composites of several reported values of CMT scores versus SCC. Ample evidence demonstrates that loss of production correlates directly with CMT scores. This factor may be useful when convincing owners to use mastitis detection aids. Production losses from quarters with CMT trace values may be 5% or more, and losses from quarters having CMT 13 values may be 25% to 50%.

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