Final answer:
In a study on fiber digestibility, lignin would be used as an internal marker due to its nature as a major source of dietary insoluble fiber that goes undigested through the gastrointestinal tract.
Step-by-step explanation:
If you're a researcher conducting a study on the digestibility of various fibers, you would use lignin as an internal marker. Lignin is a natural dietary component that is a major source of dietary insoluble fiber. Unlike other dietary fibers, such as cellulose, enzymes do not easily break down lignin. This characteristic allows lignin to pass through the gastrointestinal tract unchanged, making it a reliable internal marker for digestive studies. It can help in assessing the extent to which other nutrients are digested and absorbed in the presence of various fibers. Additionally, lignin may alter the rate and metabolism of soluble fibers, thus having an effect on how other nutrients and chemicals are absorbed within the gastrointestinal system.