Commentary for Berakhot 70:17

And Rav Huna said: What is taught, "He is permitted water and salt, but forbidden any other food and drink," could not be referring to one who says "Food is forbidden to me" because "food " only refers to the five species of grain: wheat, barley, spelt, oats and rye. Rather it refers to one who says, "Everything nourishing is forbidden to me," and all things, granted they are not called "food" but they surely "nourish," except for water and salt. Therefore oil nourishes.

Viz. wheat, barley, rye, oats and spelt. Rab and Samuel would restrict the term "food," without further qualification, only to these and not, e.g., to oil.