After the depletion of hepatic glycogen in newborns, which compounds can be used as precursors to sustain the blood glucose level?

Nutrients from breast milk.

Explanation:

Healthy newborns make blood glucose from sugar and several nutrients from the colostrum, a type of liquid that mother's breasts produce before breast milk itself. Later, the babies make glucose from mature breast milk.

Most healthy babies, born after 37 weeks of gestation do not risk hepatic glycogen drops. They can easily compensate for normal drops in blood sugar, in other words, whenever the baby is breastfed when needed, he/she will be able to keep his/her glucose levels stable.