Scientific Definition of Colostrum

Scientific Definition of Colostrum

Colostrum is the first lacteal secretion produced by the mammary glands of mammals at the beginning of lactation and is intended for ingestion by the newborn during the first hours of life. In most mammals, such as humans, many of the biologically active substances essential to development and survival, such as growth promoting substances and immunoglobulins, cross the placental barrier and are transferred to the fetus in utero. In sharp contrast, in bovine essentially none of these biologically active substances cross the placental barrier and, thus, must be acquired by the calf through suckling during the early hours of its life. As such, bovine colostrum represents a unique, concentrated resource for a wide variety of biologically active substances.

Colostrogenesis, the formation of colostrum, and lactogenesis, the process which mammary tissue undergoes when changing from a non-lactating to a lactating state, are events associated with pregnancy that are controlled by a specific complex of hormones and influenced by physical factors associated with the mammary gland. In the cow, development of mammary tissue and its ability to synthesize and produce secretions is promoted primarily by growth hormone and its mediators, the insulin-like growth factors IGF-1 and IGF-2 and the transforming growth factors TGF-beta 1 and TGF-beta 2. The process is regulated by a series of other hormones, one of the most important being progesterone, which attaches to special receptors on the cells lining the mammary gland and prevents them from secreting any fluids into the gland during most of pregnancy.

The formation of colostrum in the pregnant cow is initiated about 3-4 weeks before parturition (birth) when a limited amount of fluid is released into the developing mammary tissue containing small amounts of the growth factors and other transforming substances. These substances influence the appearance of specific receptors on the surface of the cells lining the mammary gland that will facilitate the transfer of materials from the mother's blood into the gland. The substances include, among others, the IgG (antibodies) necessary to convey passive immunity to the calf and various hormones and growth promoters required to induce development of the newborn calf.

About two weeks before birth, the receptors for IgG become fully active. IgG from the mother's blood attaches to the receptors and is transferred via special vessels through the cells into the fluid in the gland. The IgM and IgA found in colostrum are actually produced by cells (B cells) from the mother's immune system that have moved into the mammary tissue. These molecules are transferred into the fluid in the gland by a similar receptor mechanism. The additional receptors on the cells lining the mammary gland capable of transporting other substances become fully activated about 3-5 days before birth.

About 2 days before birth, the hormonal balance begins to shift when the mother's blood concentration of prolactin and glucocorticoid hormones increases sharply, overriding some of the inhibitory effects of progesterone. This initiates the production of copious secretions and switches on the ability of cells in the mammary tissue to synthesize various substances, including lactose.

At birth, when the placenta is eliminated, progesterone levels fall dramatically in the mother and its inhibitory control of the secretions is removed. Simultaneously, a protein-based substance develops in the cells lining the mammary gland that essentially blocks any further transfer of substances from the mother's blood into the gland.

The composition of the fluid in the mammary gland at birth is that of true colostrum and reflects the functional changes that have occurred in the gland up to that time; it a) has a high protein concentration, most of which is IgG; b) contains the highest concentration of growth promoters, other hormones and additional metabolically active substances; c) is low in lactose content; and d) is rich in milk fat.

In addition to containing a high concentration of maternally-derived immunoglobulins, first milking bovine colostrum contains a number of other biologically active substances. Although all of these substances have not as yet been characterized, many of the seemingly most important ones have been identified, including, among others, the following:

  • Insulin-like growth factors that act on various metabolic processes that affect the uptake of glucose to derive a source of energy and utilization of amino acids to build proteins by cells in the body.
  • Subunits of the hormone thymosin that act independently and in concert on the thymus gland to stimulate development and activation of the immune system.
  • Cytokines that stimulate the development and activation of various cells associated with the immune system.
  • Lactoferrin and transferrin, which are proteins that bind iron and act primarily in the gut to impede the growth of certain potentially pathogenic aerobic bacteria, like E. coli.
  • Xanthine oxidase and lactoperoxidase, which are enzymes that can inhibit the growth of certain potentially pathogenic bacteria in the gut.


After birth, one of the most influential factors on the composition of subsequent secretions is physical removal of the fluid from the mammary gland. The removal of even small quantities of fluid triggers the production of copious amounts of secretion from the cells in the mammary gland.

Since the transfer of biologically-active substances from the mother's blood is blocked, replacement fluid will contain primarily substances synthesized by cells in the mammary gland and, thus, will be of a different composition than the fluid originally contained in the mammary gland at birth. This fluid is known as transitional milk. The more original fluid, which is true colostrum, that is removed, the more adulterated the residual fluid becomes with transitional milk.

This becomes more complex since the mother's system will begin to reabsorb certain of the biologically-active substances, including some of the hormones and the immunoglobulins, from the mammary gland back into her bloodstream within 6-8 hours after birth of her calf. Therefore, the composition of the mammary secretions changes rapidly during the hours and days after birth so that there is a continuous transition from true colostrum to mature milk.

The average yield of colostrum taken from a mature Holstein dairy cow during the first six hours after birth will be approximately ten liters. This removes about 80% of the true colostrum while the remaining 20% usually cannot be milked out during this period due to the normal congestion of the udder of the mother. After removal of the first harvest, the udder quickly fills with "transitional milk", which is lower in fat and protein and higher in lactose.

The milking taken during the 6-12 hour period after birth again removes about 80% of the lacteal secretions present in the udder, including the 20% residual from the first milking. Again, 20% of this "colostral" material remains and is further diluted as the udder refills with "transitional milk". The only unadulterated true colostrum is, therefore, derived upon harvesting the first milking from the mother within six hours after birth and before the calf has suckled.

To assure that calves receive sufficient quantities of high quality true colostrum, most dairy producers in the United States do not allow the calf to suckle its mother immediately after birth, but prefer to physically remove the true colostrum, and feed the required amount by nursing bottle. The remainder of this material is what is then used to make colostrum products for human consumption.



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