Why can't forbidden pleasures be elevated to holiness? The Tanya's eighth chapter confronts this question head-on.
The answer lies in the three completely impure kelipot (קליפות)—the shells of absolute spiritual darkness. Unlike kelipat nogah, which is translucent and can swing toward holiness, these three shells are sealed shut. They correspond to the 365 prohibitions of the Torah. When a person eats non-kosher food, even accidentally, even with the intention of using the energy for a mitzvah, the vitality of that food cannot ascend. It is held captive within the impure shell.
Rabbi Schneur Zalman goes further. The distinction between permitted and forbidden desires reflects two fundamentally different types of evil inclination. The craving for forbidden things—non-kosher food, prohibited relationships, theft—is what the Tanya calls a "non-Jewish demon." It draws from the three sealed kelipot and has no capacity for elevation. The craving for permitted pleasures—eating a kosher meal for the taste alone, pursuing comfort—is a "Jewish demon." It is still a demon. But it is a demon that can be redeemed.
This distinction has practical consequences for the body. Since every piece of food becomes flesh and blood, the Tanya says the body itself absorbs the spiritual quality of what it consumes. Even permitted food consumed without sacred intention leaves a residue of kelipah (a shell of impurity) in the flesh. This is why, according to the Tanya, the body undergoes chibut hakever (חיבוט הקבר), the purification of the grave—a post-mortem process of cleansing the physical body from the accumulated residue of worldly pleasure.
The only person exempt from this purification? Rabbi Yehuda HaNasi, the editor of the Mishnah (the earliest code of rabbinic law), who reportedly declared on his deathbed that he had never derived personal pleasure from this world—not even from his little finger. For everyone else, the body keeps a record of every pleasure consumed, and the account must eventually be settled.