Evolution of the circadian profile of human milk amino acids during breastfeeding

Human milk is a living fluid that changes with time, composition and volume. Circadian rhythms regulate a variety of biological processes in living organisms; and perhaps the most evident function is the sleep-wake cycle. The aim of the present study was to evaluate the circadian rhythm of breast milk amino acids and their evolution throughout the breastfeeding period. Human breast milk samples from 77 donors were collected every 3 hours over a 24-h period. The rhythmicity of the amino acids was determined by cosinor analysis. Colostrum samples showed no circadian rhythm in most amino acids except tryptophan. However, daily variations were observed in tryptophan and methionine at transitional phase, according to the newborn's pattern of intake every 3 hours regardless of whether it is day or night. During the last stage (mature milk), when breast milk has fully stabilized, most amino acids showed a circadian rhythm. In conclusion, breast milk should be given to the baby at the same time of day it is expressed. Thus, the baby would be adjusting its circadian pattern in harmony with his environment (day/night), which is crucial for the proper functioning and synchronization of all systems in the human body.