Primate and bovine colostrum and milk contain a variety of growth factors and hormones that stimulate the growth of cells.112-114 Studies carried out across a full lactation indicate that growth factor content and growth-promoting activity of milk are greatest immediately after parturition and decline as lactation progresses.86115 The neonate benefits from maternal-derived growth factors as they play an essential role in postpartum development.115 The profile of growth factors in bovine and primate milk differ. For example, primate milk contains more epidermal growth factor than does bovine milk.86116-118 Concentrations of IGF-1 vary from 8-28 ng/ml in human colostrum, and 5-10 ng/ml thereafter in milk.119120
In bovine milk, IGF-1 concentrations vary considerably dependent on stage of lactation, milk somatic cell count and protein content, and age and nutritional status of the cow. The greatest concentrations are present in prepartum mammary secretions and colostrum, ranging from 55 to 2949 ng/ml.89121-122 Over an entire lactation, individual milk IGF-1 concentrations ranged from 1 to 30 ng/ml in a survey of 409 cows that were never treated with bST.123 Milk IGF-1 concentrations varied from 1 to 83 ng/ml in 5777 samples from a Bavarian dairy cow population that was never treated with bST.124 Milk concentrations generally decline with stage of lactation123 and are elevated again in late-lactation cows.124125 After a review of a number of milk surveys, it was concluded that concentrations of IGF-1 in the milk of untreated cows are quite variable, ranging from 0.7 to 8.1 ng/ml, depending on parity and stage of lactation of the cow.39 However, as shown in the literature, the concentration of IGF-1 in individual milk samples can be much higher, which may be due to individual cow variability or differences in analytical methodology used by various investigators.
Composition of the major constituents of milk generally do not affect IGF-1 concentration. However, a positive correlation has been reported between milk IGF-1 and both milk somatic cell count and protein content.124 Age and nutritional status of the cow also affect blood and/or milk IGF-1 levels. Multiparous (two or more lactations) cows generally have slightly higher milk IGF-1 concentrations than primiparous cows (first lactation).74 120 124 Severe feed restriction in cows decreases blood IGF-1 concentrations, but alterations in protein and energy intake also affect basal IGF-1 levels and limit IGF-1 responses to bST14121126 and presumably milk IGF-1 concentrations.
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