How Are Vitamins Absorbed?

Vitamins are essential nutrients made up of carbon and hydrogen, meaning they are organic compounds. The human body requires 13 essential vitamins to support normal daily functions necessary for growth and development. Doctors recommend you meet your daily recommended intake of each vitamin through eating a healthy diet, but in some cases a supplement can help avoid the onset of a deficiency that can lead to adverse health conditions. Your body absorbs vitamins through the digestive system, and many factors affect this absorption.

Digestive System

The body absorbs nearly all ingested nutrients through the lining of the small intestine, according to Colorado State University. To reach the small intestine the food, or supplement, must travel through the upper digestive system, which includes the mouth, esophagus and stomach. For food this is vital, as the enzymes and acid in the stomach begin to break down food, which releases the vitamins for absorption. For supplements, getting exposed to the stomach acid can affect the integrity of the vitamin prior to getting to the small intestine, which may in turn reduce the amount of vitamin available for absorption.

Water-Soluble Vitamins

Scientists classify vitamins into two groups based on how the body absorbs them. Water-soluble vitamins dissolve in water prior to absorption. The vitamins then get absorbed through the lining of the small intestines with water and electrolytes -- salts that help regulate the balance of water in the body. The small intestines receive approximately 7 to 9 L of fluid per day and absorb about 80 percent of it. The water absorbs through the process of osmosis, in which water moves across a membrane, in this case the cell membrane of the intestinal lining. The absorption of water with the vitamins depends upon the concentration of solute, or salts, so as the body absorbs salt it creates a gradient that pulls in additional water and vitamins. The water-soluble vitamins include all the B vitamins and vitamin C.

Fat-Soluble Vitamins

Fat-soluble vitamins attach to fat molecules prior to absorption. Absorption of fat and fat-soluble vitamins, which include vitamins A, K, D and E, requires the aid of bile acids -- derivatives of cholesterol produced by liver cells. The bile acids emulsify the fat, meaning they break it apart into smaller droplets. This process is vital to promote the transport of fat-soluble vitamins across the lining of the small intestine. The fat-soluble vitamins then enter the lymph system before getting transported into the bloodstream.

Factors

Many factors affect the absorption of vitamins. The source of the vitamin makes a difference. Vitamins from natural food sources get absorbed more efficiently than vitamins in supplements. An enteric coating helps to protect the supplement from getting broken down by the acids in the stomach and can increase the absorption. Conditions that affect the lining of the small intestine can also affect vitamin absorption. Crohn's disease and irritable bowel syndrome both cause chronic inflammation of the small intestine, which can reduce vitamin absorption. Celiac disease, a condition that occurs when the body cannot tolerate the protein gluten, damages the villi in the intestines, which reduces the absorption of vitamins.