It is currently accepted that acetate produced from fiber fermentation in the rumen, supplies nearly 40% of the building blocks for de-novo milk fatty acid synthesis in the mammary gland. Of the remaining milk fat, 10 % comes from circulating fatty acids mobilized from fat stores, and 50% coming arises from the diet.
Dietary phospholipids have health-promoting effects in humans such as inhibition of colorectal cancer, reduce the prevalence of gastric ulcers, and decrease the absorption of cholesterol. The latter likely results from the physical intestinal interaction between phospholipids and cholesterol, decreasing the absorption of both.
The fatty acid (FA) composition of phospholipids influences this bonding since their long-chain saturated FA interact more strongly with cholesterol than the unsaturated form. These phospholipids are in the milk fat globule membrane and dairy products rich in this membrane (i.e. buttermilk) are good sources of these phospholipids.
Dairy cows’ milk fat has 0.2 to 1% phospholipids with sphingomyelin constituting approximately 20% of them. Since sphingomyelin has a high proportion of long-chain saturated FA, this allows for a better interaction with cholesterol. These strong sphingomyelin-cholesterol bonding is essential for cell function. They are also present in the milk fat globule membrane, suggesting that this complex may play a role in the hydrolysis and absorption of milk fat by the calf.