We measured the concentrations of
protein,
insulin, and
epidermal growth factor (
EGF) in human milk from mothers delivering at term. Samples were obtained from d 1 (colostrum) to d 42 after birth. Human colostrum contains very high concentrations of
protein [83.7 +/- 7.4 g/l (SEM)],
insulin (3.75 +/- 0.88 nM), and
EGF (53.9 +/- 6.9 nM). Similar concentrations have been measured in prebirth milk.
Insulin,
EGF, and
protein in milk decline rapidly during the first few days of lactation but remain constant thereafter. Although the concentrations of
insulin and
EGF in mature milk are only 10% of those in colostrum, they are considerably higher than in serum. We also showed that human milk has a growth-promoting activity in cultured cells, causing a stimulation of
protein synthesis in L6 myoblasts and 3T3-L1 fibroblasts and an increase in
DNA synthesis in L6 cells and T47D
breast cancer cells. This mitogenic activity declines as lactation progresses, with a similar time-course to the fall in
insulin and
EGF; however, the cell lines used here are not responsive to either of these two
growth factors in the range of concentrations found in milk. This indicates that human milk also contains high concentrations of additional, unidentified
growth factors. The occurrence of high concentrations of
growth factors in milk suggests that they may be important for the proliferation and differentiation of infant tissues.