Physiology of Lactation

Ronald S. Kensinger

Department of Dairy and Animal Science, Pennsylvania State University, University Park, Pennsylvania, U.S.A.


Mammals undergo numerous metabolic and physiological adaptations as they make the transition from the pregnant to the lactating state. In the process, they are confronted with the challenge of maintaining maternal tissues, producing enough milk for the young (or for the farmer in the case of dairy species), and also preparing for the next pregnancy. This adaptation is evident as voluntary food intake can increase from a level that is just above maternal maintenance requirements in late pregnancy to as much as three times maintenance requirements after a few weeks into lactation. If mammals fail to make timely physiological adjustments, then the outcome can be reproductive failure, metabolic disease, or even death. Fortunately, the incidence of these undesirable outcomes is relatively low, indicating that mammals have evolved with very sophisticated physiological mechanisms for cell-to-cell (and even offspring to parent) communication. These include communication through direct cell contacts, the endocrine system, and through neural systems.


Numerous metabolic changes must occur as mammals make the transition into lactation. Examples of this adaptation include mobilization of fatty acids, Ca, and amino acids from body stores to meet the needs of mammary cells, and an increase in hepatic gluconeo-genesis. Typically, these changes depend on alterations in hormonal secretory rate, changes in receptor numbers on target cells, or modified receptor signaling mechanisms to change the responsiveness of tissues to normal signals. Examples of this in postpartum animals include the increased ability of adipose tissue to release fatty acids in response to stimulation by epi-nephrine, the decrease in rate of lipogenesis in adipose tissue in response to insulin, and an increase in the capacity of the liver to produce glucose in response to growth hormone and epinephrine. The reader is referred to more thorough reviews by Bauman and Currie,[1] and Bauman and Vernon[2] in which the concept of homeorhesis, or orchestrating metabolism in the face of changing metabolic states, is discussed.

Physiologic regulation of appetite is a critical feature of homeorhesis.


It is desirable that mammals eat adequate amounts of food in order to meet nutrient needs during all physiological states, and yet there are times, such as early lactation, when this does not occur. A number of authors have reviewed the literature to describe the metabolites, hormones, and neuropeptides involved in appetite regulation,[3-5] and continued progress in our knowledge of the regulation of appetite will likely advance both animal productivity as well as animal well being. If producers could stimulate food intake around the time of parturition, there should be a reduction in metabolic diseases, like ketosis or milk fever. Furthermore, managing animals to avoid excessive accumulation of body fat may preclude a host of problems that are common to overly conditioned animals as they undergo parturition. There appears to be consistent agreement that important appetite control centers of the hypothalamus include the arcuate nucleus and the lateral basal hypothalamus, although other regions may also contribute to food intake. There are numerous circulating compounds that have been implicated in appetite control including cholecystokinin, non-esterified fatty acids, galanin, neuropeptide Y, orexins, bombesin, estrogen, and glucagons.[3] Promising recent research with leptin (summarized in Ref.[4]) suggests that it may be an important regulator of appetite in large animals, but how these findings can be practically applied is not yet clear.


A physiological adjustment common to many mammals is that there is an increase in cardiac output and a redistribution of blood flow during late pregnancy. Much of the change in blood flow distribution is due to the needs of the developing gravid uterus and mammary glands. In many animals, there is a tendency toward temporary accumulation of fluid near the mammary glands as parturition approaches. Cows are frequently observed with minor cases of abdominal edema. The loss of immunoglobulins from the blood that are transported into colostrum results in a reduction of colloidal oncotic pressure in plasma. Thus, fluids are more likely to move into the interstitial spaces in the extremities. Linzell[6] and his associates developed many techniques for measuring blood flow to the mammary glands. They demonstrated that there is a very high correlation between blood flow and milk yield, and that mammary blood flow was affected by a number of endogenous compounds. Prosser et al.[7] extended these findings to studies on the microvascula-ture in mammary glands and have reviewed a number of agents that are good candidates for local regulation of mammary blood flow including IGF I, PTHrp, prostacyclin, and nitric oxide.


Scientists have known for many years that suckling intensity in mice and rats had a significant positive effect on milk yield. There is now increasing evidence that this phenomenon is also true in large animals. King[8] reviewed this literature and reported that there is a strong linear relationship between milk yield and litter size in sows. Heavier (more vigorous) pigs stimulate greater milk production by sows, and increasing nursing frequency also increased milk yield.[8] There is also broad agreement that increasing milking frequency from twice per day will increase milk production in dairy cows,[9] and this remains an active area for research. These observations suggest that there are also physiological mechanisms that communicate to the dam that more milk is needed. It is widely accepted that the process of milking or nursing stimulates oxytocin and prolactin secretion, but mammary stimulation also induces secretion of many other hormones including galanin and vasoactive intestinal peptide.[10] It is possible that one mechanism for the increased milk secretion is the milking induced secretion of additional hormones. It may be that part of the mechanism for increased milk yield is the removal of an inhibitor of milk synthesis (such as the feedback inhibitor of lactation), and it is also possible that simply reducing intramammary pressure leads to increased rates of milk synthesis. One wonders if local mammary stimulation leads to production of vasodilatory agents in the mammary vasculature, which in turn leads to an increase in mammary blood flow.[7] That proposed response would surely contribute to additional milk synthesis.


One of the challenges to the lactating female is to address the apparent conflicting needs between its neonates (or dairy farmer) and its desire to return to the fertile state to commence another pregnancy. There are interesting species differences in how rapidly the female can return to a fertile estrous cycle after parturition. These range from swine and rodents, where lacta-tional anestrous is nearly absolute, to dairy cows that are expected to conceive again while they remain in ''early lactation.'' This is an important question as it relates to reproductive efficiency, a variable that is routinely measured on farms. There is general agreement that negative energy balance during lactation affects the hypothalamic pituitary axis, and gonadotropin secretion in particular.[11] However, the presence of the offspring alone, with no suckling allowed, may also affect the likelihood that the female returns to estrus.[11]


The female mammal in early lactation exhibits numerous physiological changes that are required to maintain her health, produce milk for her neonate, and respond to various environmental cues. Therefore, the central nervous system of the lactating dam must integrate numerous signals from her own tissues, as well as cues from her environment, to thrive. Her response to those stimuli can have a very profound impact upon the amount of milk she produces.


1. Bauman, D.E.; Currie, W.B. Partitioning of nutrients during pregnancy and lactation: a review of mechanisms involving homeostasis and homeorhesis. J. Dairy Sci. 1980, 63, 1514 1529.

2. Bauman, D.E.; Vernon, R.G. Effects of exogenous bovine somatotropin on lactation. Ann. Rev. Nutr. 1993, 13, 437 461.

3. Ingvartsen, K.L.; Andersen, J.B. Integration of metabolism and intake regulation: a review focusing on periparturient animals. J. Dairy Sci. 2000, 83, 1573 1597.

4. Ingvartsen, K.L.; Boisclair, Y.R. Leptin and the regulation of food intake, energy homeostasis and immunity with special emphasis on peripar-turient ruminants. Domest. Anim. Endocrinol. 2001, 21, 215 250.

5. Akers, R.M. Management and nutritional impacts on mammary development and lactational performance. In Lactation and the Mammary Glands; Iowa State Press: Ames, Iowa, 2002; 219 239.

6. Linzell, J.L. Mammary blood flow and methods of identifying and measuring precursors of milk. In Lactation a Comprehensive Treatise; Larson, B.L., Smith, V.R., Eds.; Academic Press: New York, 1974; Vol. 1, 143 225.

7. Prosser, C.G.; Davis, S.R.; Farr, V.C.; Lacasse, P. Regulation of mammary blood flow in the mammary vasculature. J. Dairy Sci. 1996, 79, 1184 1197.

8. King, R.H. Factors that influence milk production in well-fed sows. J. Anim. Sci. 2000, 78 (Suppl 3), 19 25.

9. Hale, S.A.; Capuco, A.V.; Erdman, R.A. Milk yield and mammary growth effects due to increased milking frequency during early lactation. J. Dairy Sci. 2003, 86, 2061 2071.

10. Algers, B.; Madej, A.; Rojanasthien, S.; Uvnas-Moberg, K. Quantitative relationships between suckling induced teat stimulation and the release of prolactin, gastrin, somatostatin, insulin, glucagons and vasoactive intestinal poly-peptide in sows. Vet. Res. Commun. 1991, 15, 395 407.

11. Stevenson, J.S.; Lamb, G.C.; Hoffman, D.P.; Minton, J.E. Interrelationships of lactation and postpartum anovulation in suckled and milked cows. Livestock Prod. Sci. 1997, 50, 57 74.

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