Vitamin Glossary: Terms From Absorption to Upper Level

How to use this glossary

Vitamin labels, news stories and clinic letters use a lot of short terms that look alike (RDA, AI, DV, UL, IU, RAE). This page defines them in everyday words, using what government health bodies and medical libraries say, and points to the page that goes deeper. It is information only. It cannot tell you what amount is right for you, which is a question for a clinician or pharmacist.

Reference amounts differ by country, age, sex, pregnancy and the form of the nutrient, so any number quoted here names its source. Where a term has no source we could read, we say so rather than guess.

A to B

• Absorption. The step where a nutrient moves from the gut into the body. Fat-soluble vitamins are absorbed more easily when there is dietary fat present. [1] See How the body absorbs and stores vitamins.
• Adequate Intake (AI). A level set when there is not enough research to set an RDA, chosen as one thought to ensure enough nutrition. [2] Vitamin K, pantothenic acid, biotin and choline have AIs in the Health Canada tables rather than RDAs. [3] See RDA, AI, DV and UL.
• Antioxidant. A substance that protects body tissue from damage caused by free radicals. [4] MedlinePlus lists vitamins C and E and beta-carotene as antioxidants. [1][2] A vitamin acting as an antioxidant in the body is not proof that a supplement prevents disease: MedlinePlus says beta-carotene supplements do not reduce cancer risk and may raise it in smokers and asbestos-exposed workers. [2] See Vitamin myths and misconceptions.
• Ascorbic acid. Another name for vitamin C. [5]
• B-complex. A name for products that combine several of the B vitamins. The NHS lists eight B vitamins: thiamin, riboflavin, niacin, pantothenic acid, B6, biotin, folate and B12. [6] See The B vitamins overview.
• Beta-carotene. A plant pigment the body can convert into vitamin A, so it is called provitamin A. [2][7] It is not preformed vitamin A. See Retinol vs beta-carotene.
• Bioavailability. How much of a nutrient the body can absorb and use from a food or supplement. Sources describe it case by case, for example that the vitamin B12 added to fortified foods is very easy to absorb. [8] Findings for one form do not automatically carry over to another.
• Biotin interference. The FDA says biotin, often found in dietary supplements, can significantly interfere with certain lab tests and cause incorrect results that may go undetected. [9] See Biotin and lab-test interference.

C to F

• Cobalamin. The chemical name behind vitamin B12. MedlinePlus lists vitamin B12 as cyanocobalamin, and the NHS says deficiency anaemia is usually treated with injections of hydroxocobalamin. [1][10] See Vitamin B12 forms compared.
• Coenzyme and cofactor. Coenzymes are small organic molecules that help enzymes do their work, and many vitamins act as coenzymes. [11][12] Some vitamins also have other roles, which is why one vitamin can matter for several body systems. See How vitamins work.
• Daily Value (DV) and % Daily Value. The FDA says DVs are the recommended amounts of nutrients to consume or not to exceed each day, and %DV shows how much one serving contributes. [13] On US labels, 5% DV or less per serving is considered low and 20% or more is considered high. [13] It is a label tool, not a personal requirement. See Reading a vitamin label.
• Deficiency. MedlinePlus describes a vitamin deficiency as not getting enough of a certain vitamin, which can cause health problems. [1] Symptoms alone cannot confirm one. See Vitamin deficiency symptoms and Vitamin deficiency testing.
• DFE (dietary folate equivalents). The unit the FDA uses for folate and folic acid on US labels, shown as mcg DFE. [13] See Vitamin units explained.
• Dietary Reference Intakes (DRIs). A set of reference intakes from the US Food and Nutrition Board used to plan and assess the intakes of healthy people, including the RDA and AI. [2]
• Dietary supplement. US federal law defines a supplement as a product taken by mouth, made to supplement the diet, containing a dietary ingredient such as a vitamin, mineral, herb or amino acid, and labeled as a supplement. [14] The FDA does not approve dietary supplements before they are marketed. [15]
• Fat-soluble vitamins. Vitamins A, D, E and K, which are stored in the liver, fatty tissue and muscles. [1] MedlinePlus warns that large amounts of fat-soluble vitamin supplements can build up in the body and cause harm. [1] See Fat-soluble vs water-soluble vitamins.
• Folate and folic acid. The NHS describes folate as a B vitamin found in many foods and folic acid as its manmade form. [6] See Folate vs folic acid vs methylfolate.
• Fortified food. A food with vitamins added. MedlinePlus notes that many foods are now fortified with folate as folic acid, and that fortified cereals are a source of B12 that vegetarians can use. [1][8] See Fortified and enriched foods.
• Free radicals. Substances that MedlinePlus says can harm cells, tissues and organs and are believed to play a role in certain conditions related to aging. [4]

H to P

• Hypercalcemia. Too much calcium in the blood. The NHS says taking too many vitamin D supplements over a long period can cause it, which can weaken bones and damage the kidneys and heart. [16] See Vitamin D toxicity.
• International Unit (IU). A unit used for some vitamins, mainly A, D and E. The NHS says the conversion to milligrams or micrograms depends on the type of vitamin, and for vitamin D 1 microgram equals 40 IU. [17][16] Never convert other vitamins by that rule.
• Intrinsic factor. A protein made in the stomach that is needed to absorb vitamin B12 efficiently. In pernicious anemia the body destroys the cells that make it. [8] See Pernicious anemia and intrinsic factor.
• mg and mcg. The NHS explains that a milligram is one thousandth of a gram and a microgram is one millionth of a gram, so 1,000 micrograms equal 1 milligram. [17] Mixing them up is a thousand-fold error.
• NE (niacin equivalents). The unit for niacin on US labels, shown as mg NE. [13]
• Neural tube defect. A problem with the baby's brain or spine, or both, such as spina bifida. The NHS recommends folic acid before and in early pregnancy to reduce the chance of one. [18] See Folic acid in pregnancy.
• Peripheral neuropathy. Loss of feeling in the arms and legs. The NHS says 200 mg or more of vitamin B6 a day can cause it and that in a few cases it has been permanent. [6] See Vitamin B6 toxicity.
• Pernicious anemia. An autoimmune condition in which the body destroys the stomach cells that make intrinsic factor. [8]
• Preformed vitamin A. Vitamin A found in animal products such as meat, fish, poultry and dairy, as opposed to provitamin A from plants. [2] Upper limits in the EFSA overview apply to preformed vitamin A. [19]
• Provitamin. A compound the body converts into a vitamin, such as beta-carotene into vitamin A. [2]
• Pyridoxine. Another name for vitamin B6. [6]

R to W

• RAE (retinol activity equivalents). The unit for vitamin A on US labels, shown as mcg RAE. [13] The NHS uses retinol equivalents (RE). [7] Check a published conversion table before comparing them.
• Recommended Dietary Allowance (RDA). The average daily level of intake that is enough to meet the nutrient needs of nearly all (97% to 98%) healthy people. [2] Population, sex, age and country matter. See Vitamin intake references compared.
• Retinol. The preformed, active form of vitamin A, found in animal liver, whole milk and some fortified foods. [2]
• Supplement Facts panel. The nutrition box on a US supplement label. The FDA says it must list the same nutrients as the Nutrition Facts label when they are present in an amount greater than zero. [13]
• Tocopherol. The family name for vitamin E. MedlinePlus calls vitamin E "an antioxidant also known as tocopherol." [1]
• Tolerable Upper Intake Level (UL). EFSA defines the UL as the maximum level of total chronic daily intake of a nutrient from all dietary sources which is not expected to pose a risk of adverse health effects. [19] It is a ceiling, not a target. Several vitamins have no UL because of a lack of data, which is not the same as being safe in any amount. [3][19] See Which vitamins have upper limits.
• Vitamin. MedlinePlus describes vitamins as substances needed for normal cell function, growth and development, and lists 13 essential ones. [1] See What is a vitamin?.
• Vitamin-like nutrients. Nutrients the body needs that are not counted among the 13, such as choline and carnitine. [1] See Vitamin-like compounds.
• Water-soluble vitamins. Vitamin C and all the B vitamins. MedlinePlus says they are not stored in the body, extra amounts leave in the urine, and the exception is vitamin B12, which can be stored in the liver for many years. [1] "Leaves in the urine" does not make high amounts harmless. See Vitamin B6 toxicity.

Common names for the 13 vitamins

Units that look alike

What this glossary cannot tell you

Proper evidence not found: in the NHS, MedlinePlus, FDA, EFSA, Health Canada and NCCIH pages read for this glossary, there was no single agreed definition of "bioavailability" or "active form" that applies across all vitamins. Treat marketing claims built on those words with caution; see Natural vs synthetic vitamins. Pages from the US NIH Office of Dietary Supplements could not be read in this session, so US National Academies figures are not quoted from that site.

For the ideas behind these words, start with Understanding vitamins, or go back to the Vitamins hub.