Kathy:

100 g of mature milk  has approximately :
    3.8 g fat
    1.0 g protein
    7.0 g lactose
Taking into consideration the energy value of each, that means that
yes, fat contributes a little over 50% of energy.  (See R. Lawrence's
chapter on "Biochemistry of human milk").

Grace S. Marquis, Ph.D.
University of Alabama at Birmingham