Sustenance, n.
Etymology: [OF. sustenance, sostenance, soustenance: cf. L. sustenentia endurance. See Sustain.]
1. The act of sustaining; support; maintenance; subsistence; as, the sustenance of the body; the sustenance of life.
2. That which supports life; food; victuals; provisions; means of living; as, the city has ample sustenance. "A man of little sustenance." Chaucer. For lying is thy sustenance, thy food. Milton.
Shades of meaning
Fernald has no entry of its own for sustenance; he lists it under food.
Where he writes about it:
Thus, we speak of food and drink, of wholesome, unwholesome, or indigestible food; but in a more scientific sense whatever, when taken into the digestive organs, serves to build up structure or supply waste may be termed food; the word is extended to plants to signify whatever taken in any way into the organism serves similar purposes; thus, we speak of liquid food, plant food, etc.; in this wider sense food is closely synonymous with nutriment, nourishment, and sustenance. — from food
Nourishment and sustenance apply to whatever can be introduced into the system as a means of sustaining life; we say of a convalescent, he is taking nourishment. — from food