Determination of insulin-like growth factors in bovine milk and colostrum by radioimmunoassay

Bovine milk is an important ingredient in human nutrition as well as in the diet of the young calf. In view of the role of insulin-like growth factors (IGFs) in growth and development attempts were made to determine the concentrations of both IGF-I and IGF-II in colostrum and milk. Colostrum was collected on the day of calving from six dairy cows on a large cc-operative farm. Milk samples were taken directly from nine cows at different stages of lactation on two private farms. Pasteurized milk and various milk powders available commercially were also examined. The mean concentrations of IGF-I and IGF-II in colostrum were found to be 345 +/- 200 ng/mL and 406 +/- 46 ng/mL, respectively. When milk samples were centrifuged to remove the fat before radioimmunoassay, IGF-I levels were found to be lower than in non-centrifuged samples, whereas no differences were detected concerning IGF-II concentrations. This indicates that these growth factors are distributed differently in the milk emulsion. Thus, concentrations of IGF-I decreased from 38.7 +/- 3.5 ng/mL and 24.0 +/- 2.8. ng/mL at four days postpartum to 7.2 +/- 6. ng/mL and 4.1 +/- 3. 7 ng/mL in whole and centrifuged mature milk samples, respectively. Mean IGF-II concentrations were 14.1 +/- 6.0 ng/mL and 13.7 +/- 6.1 ng/mL in whole and centrifuged milk samples, respectively. Pasteurized milk contained lower concentrations of both factors (IGF-I - 1.1 +/- 0.9 ng/mL; IGF-II - 6.5 +/- 3.2 ng/mL). Even lower amounts, near or below the limit of the sensitivity of the assay systems (1 ng/mL) were found for humanised milk powders intended for human infants. Thus, while consumption of colostrum leads to considerable intake of IGF-I and IGF-II, commercially available milk and milk products contain negligible amounts.