The time required by your body to digest and absorb nutrients is the transit time. Before ultimately reaching the organic tissue, nutrients must move from the mouth to the stomach and then through the small intestine, where most absorption occurs. The remaining nutrients enter the large intestine, which includes the cecum, colon and rectum. Knowledge of the transit time is useful if you are trying to time your meals, for example, to align perfectly with the demands of a workout.
Studies have found two important factors that largely determine transit time. First, substances display a high amount of variability as they move through the digestive system. Second, materials leave segments of the digestive tubes in different orders than they arrive. For example, some nutrients are already entering the small and large intestines as other nutrients continue to move through the stomach. The true transit time occurs over a length of time rather than at any one specific moment.
In general, carbohydrate digests the quickest. Protein takes slightly longer and fat takes the longest to fully digest. The precise time depends upon the food that you ate, the complexity of the molecule, the nature of the nutrient and the order of breakdown that occurs in the digestive system. Further slowing digestion is by the presence of fiber, an indigestible portion of plant food that changes how the intestines absorb other nutrients and chemicals.
On average, 50 percent of the stomach content empties after 2.5 to 3 hours and completely empties after 4 to 5 hours. It requires another 2.5 to 3 hours before 50 percent of the contents in the small intestine empty. At this point, your digestive system has absorbed most of the nutrients. The remaining nutrients and components that continue on to the colon will take 30 to 40 more hours before the body fully excretes them.
Effects of Fiber
A study published in "The American Journal of Clinical Nutrition" by researchers from the University of Wisconsin tested the mouth-to-cecum transit time of the nutrients zinc and folate in a fiber-enhanced liquid meal versus a liquid meal with no fiber content. The average transit time for the fiber-enhanced drink was 210 minutes, with a total difference of plus or minus 52 minutes. For the regular drink, it was only 180 minutes, plus or minus 31 minutes. The fiber, derived from a soy polysaccharide, appeared to have a significant affect on the digestion and absorption of some, though not all, nutrients. It did not seem to affect glucose absorption.