In her book "Gulp: Adventures on the Alimentary Canal," Mary Roach discusses how peanuts, unlike many other foods, often pass through the digestive system largely undigested. This phenomenon is well-known and can be observed by anyone who examines their stool, leading to the humorous notion that they provide a clear visual indicator of digestion. This characteristic is not unique to peanuts, as nuts and corn kernels also exhibit similar behavior.

Due to their unique resistance to digestion, peanuts are employed as "marker foods" in informal tests to measure bowel transit time. This refers to the duration from eating the food to its elimination from the body. Roach highlights these aspects to illustrate the complexity of the digestive process and the varied ways different foods are processed within our systems.