There isn't a true antonym for "protein" in the same way there might be for words like "hot" and "cold." "Protein" is a specific type of molecule, and there's no single word that directly signifies the opposite of its chemical composition or biological function.

However, depending on the context, you might consider some related terms that could be interpreted as opposites:

* Carbohydrate: This is another major type of organic molecule found in food and essential for energy. While not a direct opposite, it contrasts with protein in its primary function.

* Fat/Lipid: These are another category of organic molecules, also crucial for energy and various biological processes, but with a distinct structure and function from protein.

* Non-protein: This is a general term that encompasses everything that isn't protein, but it's not a specific alternative.

Ultimately, the best "opposite" of protein depends on the specific aspect you want to emphasize. If you're talking about its role in building muscle, "fat" might be a better contrast than "carbohydrate." If you're talking about its molecular structure, then "non-protein" might be more appropriate.