The length of time the human body takes to absorb vitamins primarily depends upon the specific vitamin and the individual's gastrointestinal health. Most vitamins are absorbed within minutes in the small intestine, but they must first travel through the stomach, where they are likely to undergo processing for about 3 1/2 hours, according to NutriStrategy.
Vitamins are absorbed during the process of digestion. The National Digestive Diseases Information Clearinghouse explains that digestion works by moving food through the gastrointestinal tract, which begins in the mouth and ends with the large intestine. Digestion begins with chewing. The chewed food passes through the esophagus into the stomach, mixing with digestive juices that break down large molecules of food into smaller molecules. The smaller molecules are absorbed through the walls of the small intestine, making their way into the bloodstream either directly or indirectly.
Water-soluble vitamins, which include all the B vitamins and vitamin C, pass directly into the bloodstream. Fat-soluble vitamins, however, require further processing by the bile released into the small intestine, and they must enter the lymph vessels before entering the bloodstream, according to Helpguide. Four vitamins are fat-soluble: A, D, E and K.
Helpguide explains that absorption of some vitamins is affected by the presence of other vitamins or minerals and vice versa. For example, vitamin C helps the body absorb iron.