What components of a tossed salad will pass through the human digestive tract and be digested the least?

Cellulose (in the lettuce)

Explanation:

A tossed salad usually consists of many vegetables such as lettuce, onions, tomatoes, cucumbers, etc. and then, these vegetables have added dressings which can be of different types. When a person eats a tossed salad then the lettuce will be least digested as it is composed mainly of cellulose and the human body is incapable of digesting cellulose. The inability to digest cellulose by human beings is because there are beta acetal linkages present in the cellulose and human beings lack the enzymes that can break the linkage. However, the undigested cellulose aids in the process of digestion as it acts like fiber and thus, the process of digestion can take place smoothly. Thus, the intake of cellulose-rich foods is also essential.