He explains that when we eat something, it is transformed into a part of us. In this way we raise up the plants and the animals to the level of m'daber, by transforming its very matter into part of us, part of a speaking person.
This is what it means when the Torah says that the Land of Israel devours those that dwell in it, we are consumed by the land and are elevated to become of the same nature as the Land itself.
The nature of the Land of Israel is Emunah, and so when we are consumed by the land, we become pure Emunah, for this reason, someone who walks four amoth in the land of Israel is said to be without sin.
Similarly, the Tzaddik is intimately connected with the Land of Israel, until he himself is called the Land, [similar to how a Tzaddik is called Shabbath, it would seem] and so being close to the Tzaddik means that he too consumes you and you become elevated to the status and nature of the Tzaddik.
This is all only possible if we have a true desire to connect to HaShem, just as food that cannot be digested will be rejected by the body, so too, has v'shalom, someone who has no desire to connect to HaShem will be rejected by the Land.
If we do have a true desire to connect to HaShem, then we must feed our body to our soul, so that our soul consumes our body, as we explained above, and our body is elevated to the level of our soul.
Even further, we must feed ourselves to our Emunah, so that we may become Emunah incarnate and connect to the Tzaddikim, and dwell in the Land of Israel.
These are very novel concepts but I don't think it is so distant from us to accomplish, or so difficult to digest, pun intended. When we speak about feeding ourselves to our soul, or our Emunah, we are talking about whether we are exhausting our Emunah and our Soul to meet the needs of our body, or we are exhausting our body to meet the needs of our Soul and our Emunah.
As is taught many places, (I saw it first in the Tanya, I believe) our body is the wick, upon which our soul burns. If our body is bent to the service of our soul, then the body becomes the clothing, and the chariot for the desires of our soul. If, has v'shalom, the opposite is true, then since the soul is what animates the body, the soul is forced to serve as the lowly servant of the body, helping it to obtain its desires. The deeper lesson of Rebbe Nachman is that through this service, the lowly body can be digested and transfigured into the holy stuff of the soul itself.