Fats in breastmilk provide most of the caloric energy of milk and can vary
considerably during the course of a single lactation, and over time, and
among different women.  The fats in infant formula are usually provided by
vegetable oils and are, of course, constant if the formula is prepared
according to instruction.  Mature human milk contains about 4% lipid which
supplies about 40% of the total calories in milk.  Individual milk lipid
percentage varies widely, however, from about 1% to 10%.  This gives rise
to large differences in caloric content of breastmilk.

 The origins of fats in milk are interesting, too.  About 30% of milk fats
originate from the maternal diet (recent ingestion), about 60% from adipose
(fat) tissue storage and 10% from de novo synthesis in the mammary gland.
Mobilization of adipose tissue stores takes place even if the mother has
adequate caloric intake, although further adipose mobilization may take
place if maternal energy requirements are not met.  Differences in maternal
diet lead to differences in some fatty acids in breastmilk, but the
composition of breastmilk is remarkably constant despite dietary
differences.  Certain fatty acids are produced only by de novo synthesis in
the mammary gland, for example, are found in similar amounts in breastmilk
of vegetarian and non-vegetarian women.

Judy Schreiber, NYSDOH