'Learn from me, my son, what are hunger and thirst. When a man is thus said to be hungry, water is carrying away (digests) what has been eaten by him. Therefore as they speak of a cow-leader (go-nâya), a horse-leader (asva-nâya), a man-leader (purusha-nâya), so they call water (which digests food and causes hunger) food-leader (asa-nâya). Thus (by food digested &c), my son, know this offshoot (the body) to be brought forth, for this (body) could not be without a root (cause).

In plain words: The father explains hunger: when we are hungry, it is because water has digested and carried away the food we ate. So water is called the 'leader of food.' He says the body grows from food, and the body must have a deeper cause, like a sprout has a root.