Diet influences the content of bioactive peptides in goat milk

Leptin, insulin, ghrelin and insulin-like growth factor-1 (IGF-1) are circulating peptide hormones concerned with energy homeostasis and the regulation of GH axis. They are present in human milk, and are thought to promote neonatal development. The aim of the present study was to detect these substances in goat milk and determine whether their levels can be modified by changing the macronutrient content of the lactating animals’ diet. Sixteen Saanen goats in mid lactation were divided into two equal groups, one was given a diet containing 17% starch (LS) and the other a diet of 33% starch (HS). Eighty days later, leptin, insulin, ghrelin and IGF-1 were determined by human RIA kits in plasma before and after feeding, and in sonicated milk centrifuged to remove fat from morning and evening milkings. The HS diet was associated with higher plasma and milk insulin and IGF-1, and plasma ghrelin. Leptin, insulin and ghrelin in milk were two-three times higher than in plasma; milk IGF-1 was only 13% of plasma level. Plasma insulin correlated positively with plasma IGF-1; morning milk IGF-1 and insulin correlated positively with morning plasma levels. These findings demonstrate that human immuno-activities of bioactive peptides are present in goat milk, and also that levels of insulin and IGF-1 in milk can be altered by changing the macronutrient content of the diet. Further research is required to determine whether these substances can be transferred from the milk to suckling animals and humans, and whether they have biological activity in such animals.