>>280788052>Canonically her Dragon Core lets her eat as much as she wants and the excess food is turned into magical energy.This is not explained anywhere in the VN, it's not canon in any sense of the word...
Servants don't need to eat, and the amount of mana they replenish from eating is extremely minimal. When servants eat, its simply out of courtesy or because they find food comforting. Saber might be a special case though, since she is not a true servant and still has a living body. She's basically a dimension-hopping corpse on the brink of death. However, considering the fact that she initially rejects Shirou when he offers food, the implication is that even as a fake servant Saber fully sustains herself on mana.