Hormones and bioactive substances in milk: a rudiment or a message?

The composition of milk is the result of an evolutionary process with both positive and negative natural selection mechanisms where variants with components supporting offspring development from birth to reproduction (but not later) are preferred. Almost all known hormones and many other bioactive substances are present in milk of various species. Some of them are synthesized by the mammary gland, the others are passively or selectively transported (and eventually modified) from blood into milk. Hormones in milk are of importance for the regulation of the functions of the mammary gland itself. There are indications that they affect (sometimes permanently) both functions of suckling GIT and other functions after their resorption during the defined developmental periods. The definite role of these messages for further offspring development remains to be established.