Nefesh haChayim, by Rabbi Chaim of Volozhin זצ”ל, the principle disciple of the Vilna Gaon זצ”ל, states as follows:
Food for the body, if it is not good, and not acceptable to the body, does not sustain the body, but rather becomes waste and filth and excrement inside the body, and also weakens and tires the entire body, for because of this bad food, the energy of the spirit does not spread through the body properly, and sometimes the body becomes ill as a result. So too the actions that are not good or desirable, God forbid, become like the waste and filth of the spiritual worlds, strengthening the impure forces and kelipoth (‘coverings,’ like darkness shrouding spiritual light and goodness) — may the Merciful One save us from this.
Footnote 27 of the edition published by Rabbi Yissachar Dov Rubin ז”ל (the footnotes were written by him) adds:
Everything a man eats contains sustenance from above… Once the body is sustained by the life-giving elements of the food, the remainder which contains no sustenance at all is purged from the body. Therefore, the Generation of the Desert, who ate the manna, did not excrete any waste, for the manna could be completely metabolized with no leftover waste. But other foods, after the life-giving part is separated out to sustain the man, the waste is excreted devoid of any sustenance.