Which of the following is NOT a fundamental digestive substance?

Proteins are essential nutrients that play a crucial role in building and repairing tissues, as well as in other bodily functions, but they are not considered a fundamental digestive substance. In the context of digestion, fundamental substances typically refer to those that facilitate the breakdown and absorption of nutrients rather than the nutrients themselves.

Hydrochloric acid (HCl) is important for creating an acidic environment in the stomach that aids in digesting food and activating digestive enzymes. Digestive enzymes are specialized proteins that help break down carbohydrates, fats, and proteins into smaller molecules that can be absorbed. Bile, produced by the liver, is essential for the emulsification of fats, improving their digestion and absorption in the intestine.

While proteins must be digested to be utilized by the body, they do not serve the primary role of breaking down food, which is characteristic of the other substances listed. Thus, proteins are the outlier in this context.