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Serotonin and the Body

    An anatomical display of the upper half of the human body. The picture was chosen to represent the amount of serotonin receptors in lactating mothers' milk.

    Written by: Anna Petherick, Ph.D. | Issue # 26 | 2014

    • Beta-casomorphins are present in colostrum in much higher levels than they are in later breast milk.
    • Because beta-casomorphins affect serotonin receptors, scientists are beginning to wonder whether they could have functional roles specific to young infants.
    • At present these roles are unclear, although they could include effects on the lungs, heart and digestive system.

    Serotonin is well known as a chemical that elicits effects on the brain. But it has a vast number of other roles; for example, serotonin affects heart function, as well as milk release from the mammary gland, bladder control, and how long it takes a man to ejaculate during sex. In fact, most of the body’s serotonin is not in the central nervous system—and almost all of the 15 serotonin receptors that have been identified in the brain are also found outside of it.

    A potentially related curiosity to serotonin’s long list of corporal functions is that mothers feed their babes molecules that affect1 serotonin receptors. No one is entirely clear why this is. Moreover, colostrum, the sweet, golden milk produced in the first few days after birth, has much higher levels of at least two of these molecules compared with normal breast milk.

    In a paper2 discussing this strange bounty, Beata Jarmolowska and colleagues speculate as to what might be going on. The two molecules in question are structurally similar, going by the names of beta-casomorphin-5 (BCM-5) and beta-casomorphin-7 (BCM-7). There is more known about the biological roles of the latter than the former.

    Jarmolowska and her team measured the levels of these molecules in the milk of 30 Polish women from the town of Bialystock in the northeastern part of the country. For a typical Bialystock woman, the concentration of BCM-5 in her colostrum was five to eight times higher than in her normal breast milk; the concentration of BCM-7 was eight to nine times higher.

    Not only do beta-casomorphins arrive in the gut in greater concentrations during an infant’s first few days, but the odds that a given molecule will make it through the gut wall and into the bloodstream are higher in young infants because they have especially leaky gut walls. “The high level of both beta-casomorphins in colostrum suggests that these peptides may affect not only the gastrointestinal tract but also the whole organism after passing the gut barrier,” Jarmolowska et al conclude.

    So what biological effects might these be? The authors pick up on a few of them, the first of which is the regulation of gastrointestinal function and development. Suggestive evidence for this comes from other work that has, for instance, linked casomorphins to changes in intestinal electrolyte transport that prevent diarrhea.

    The second corporal effect is a potential role in allergic reactions of the skin. This is because BCM-7 has been found to cause skin rashes in a test where it was administered by injection. But the effects of allergens in the development of the immune response over time are complex, and for evolutionary reasons it seems much more probable that, on balance, the BCM-7 in breast milk is good for the health of its tiny consumers. Similarly, BCM-7 has been linked to autism and to sudden infant death syndrome, or SIDS. However, a review by the European Food Safety Authority3 reported that recent data do not provide any support for either theory.

    Indeed, the beta-casomorphins in milk are potentially doing a whole lot more good than Jarmolowska et al speculate. For example, serotonin is important for energy balance and digestion. Serotonin’s roles outside of the brain are diverse. Depending on the type of serotonin receptor in the walls of blood vessels, the presence of serotonin (and presumably of beta-casomorphins) causes the vessels to either contract or dilate. These effects appear to extend to the lungs, where serotonin is known to help in the control of respiration. Therefore, it is conceivable that beta-casomorphins have a role in calming an infant’s breathing.

    Moreover, beta-casomorphins may affect serotonin receptors that modify the function of the heart. Research in this field has arisen, in part, from observations of the side effects of a kind of tumor that pumps out serotonin. Patients with “serotonin-producing carcinoid tumors” often have fast and irregular heart beats for several mechanistic reasons, including modifying how electricity is conducted across the organ and in the timing of when the heart valves close.4

    There is simply so much more to know and such diversity among serotonin receptors that the literature yields conflicting answers to the question of whether beta-casomorphins activate or inhibit them. “It is likely that these molecules do something. [That] they act on all kind of systems and the result is a mixed effect,” says Renger Witkamp of Wageningen University, the Netherlands, who was one of the authors on the European Food Safety Authority report. It is also unclear why colostrum contains particularly high levels of BCM-7 and BCM-5 compared with normal breast milk, as revealed by Jarmolowska et al’s study in Bialystock. Perhaps you, the reader, will find out.


    1.  Sokolov OY, Pryanikova NA, Kost NV, Zolotarev YA, Ryukert EN, Zozulya AA (2005). Reactions between beta-casomorphins-7 and 5-HT2-serotonin receptors. Bull Exp Biol Med 140: 582-584.

    2.  Jarmołowska B, Sidor K, Iwan M, Bielikowicz K, Kaczmarski M, Kostyra E, Kostyra H (2007). Changes of beta-casomorphin content in human milk during lactation. Peptides 28: 1982-1986.

    3.  EFSA DATEX Working Group (2009). Review of the potential health impact of β-casomorphins and related peptides. EFSA Scientific Report 231: 1-107.

    4.  Berger M, Gray JA, Roth BL (2009). The expanded biology of serotonin. Annu Rev Med 60: 355-366.