Electrolytes and free fatty acids in milk during mammary involution in the cow.

Measurements have been made of the concentrations of triglycerides, free fatty acids, sodium, chloride and potassium in milk samples obtained from cows during acute mammary involution and after parturition. Free fatty acid levels in milk increased more than 10-fold during involution. Their appearance did not immediately follow the cessation of milking but followed the increase in permeability of the mammary epithelium, evidenced by changes in the electrolyte content of the milk, that occurs after a delay of a few days. However, the concentration of free fatty acids did not remain high throughout the dry period but had declined to low levels before the change in permeability was reversed at the next parturition. It is concluded that the high level of free fatty acids in milk during mammary involution does not result from equilibration with an extra-mammary pool, but most likely arises from breakdown of triglycerides remaining in the gland which may be accelerated in some manner by the increase of permeability of the mammary epithelium.