How the Body Absorbs, Stores and Uses Vitamins

In brief

Vitamins are released from food during digestion, absorbed mostly in the small intestine, carried by the blood or lymph, and then used or stored. NIDDK (US) says your digestive system breaks nutrients, including vitamins, into parts small enough to absorb, and the small intestine absorbs most of the nutrients in your food. [1] What happens next depends on the vitamin. This page describes the process; it does not tell anyone when or how to take a supplement.

Step by step

1. Breaking food down. Chewing, stomach acid, bile and enzymes break food apart. NIDDK says the liver makes bile, which helps digest fats and some vitamins, and the pancreas adds digestive juice. [1]
2. Crossing the gut wall. The walls of the small intestine take digested nutrients into the bloodstream; special cells help them cross the lining. [1]
3. Two routes. Blood carries simple sugars, amino acids and some vitamins to the liver, while the lymph system absorbs fatty acids and vitamins. [1] This is consistent with MedlinePlus saying fat-soluble vitamins (A, D, E, K) are absorbed more easily with dietary fat. [2]
4. Gut bacteria. Bacteria in the large intestine help break down remaining nutrients and make vitamin K. [1] The NHS says bowel bacteria can also make vitamin B6 and biotin. [3]

Vitamin B12 needs a stomach protein

B12 is a well-documented special case. A protein called intrinsic factor, released by cells in the stomach, binds B12 so the intestine can absorb it. When the stomach does not make enough, B12 is not absorbed properly, which is the cause of pernicious anemia. [4] MedlinePlus also lists weight-loss surgery, celiac disease and Crohn disease among situations where deficiency is more common. [5] See Pernicious Anemia and Intrinsic Factor and Vitamin B12.

Where vitamins are stored

The NHS says unused vitamin A is stored, so it is not needed every day, [6] and the same is said for vitamin E and vitamin K (stored in the liver). [7][8] Storage is why long-term high intakes of fat-soluble supplements can build up: MedlinePlus says A, D, E and K supplements can accumulate and cause harm. [2] See Fat-Soluble vs Water-Soluble Vitamins.

What can interfere with absorption

• Conditions of the small intestine or pancreas. MedlinePlus lists celiac disease, Crohn disease, pancreatic exocrine insufficiency, bacterial overgrowth, parasites and surgery that removes part of the small intestine; it notes pancreatic enzymes help absorb fats, so too few makes fats and some nutrients harder to absorb. [9]
• Stomach changes and weight-loss surgery can reduce B12 absorption. [4][5] See Vitamins After Bariatric Surgery and Malabsorption, Celiac Disease, IBD and Vitamin Deficiency.
• Age. The NHS says it becomes more difficult to absorb B12 as you get older. [3]
• Medicines. MedlinePlus says long-term antibiotic treatment can lead to vitamin K deficiency. [10] For other medicines see Vitamin and Medicine Interactions.

Bioavailability and forms

"Bioavailability" means how much of a nutrient the body actually absorbs and can use. Proper evidence not found for a general ranking of forms across vitamins in the pages read for this article (MedlinePlus, NHS and NIDDK). Forms differ vitamin by vitamin and findings do not transfer between them. Two documented examples: MedlinePlus says the B12 used to fortify foods is very easy to absorb, [5] and the NHS describes folic acid as the manmade form of folate. [3] Form-specific questions are covered on Natural vs Synthetic Vitamins and Taking Vitamins With Food, With Fat and at Different Times.