Nutritional and Technological Factors Influencing Melatonin Contents in Cow’s Milk: A Systematic Review

Melatonin is a bioactive indoleamine produced primarily by the pineal gland in vertebrates as the body’s natural response to the perception of darkness in the environment; it is also known as the “darkness hormone” or “sleep hormone” due to its role in regulating the circadian rhythm and promoting good-quality sleep. In addition to being secreted by the pineal gland, melatonin is also produced extrapineally in numerous tissues of vertebrates, as well as by other living organisms, including plants and various microorganisms. Melatonin is also present in varying amounts in food products, including milk. In this systematic review, the authors set out to critically evaluate the influence of feeding strategies and housing conditions for dairy cows on melatonin levels in the animals’ blood and milk. The paper also includes an analysis of the possible effects of milk processing technologies on melatonin concentrations in the final products. A systematic review of the scientific literature was conducted using the PubMed/MEDLINE, Web of Science, and ScienceDirect databases. We have identified the circadian rhythm as the main factor for melatonin content; milk collected in the early morning hours have significantly higher melatonin concentrations than milk collected in the afternoon, while the bioavailability of tryptophan is a secondary factor that nevertheless contributes to the total amount of melatonin in the milk. The use of tryptophan for melatonin synthesis is limited by the physiological state of the bovine organism and its metabolic priorities. Under stressful conditions, tryptophan is preferentially directed toward the metabolic pathways involved in the response to this physiological state, a situation that may limit its availability for melatonin synthesis. Milk processing methods, particularly high-temperature heat treatment, can affect the stability of melatonin to a certain extent and, consequently, the levels of this molecule present in food.