Understanding Vitamins: Types, Needs, Deficiency and Excess

Overview

Vitamins are nutrients the body needs in small amounts to work properly. The NHS (UK) says most people should get all the nutrients they need from a varied and balanced diet, although some people may need extra supplements. [1] MedlinePlus (US National Library of Medicine) describes vitamins as substances needed for normal cell function, growth and development. [2]

This page is information only. It is not medical advice, and a clinician or pharmacist who knows your health history is the right person to ask about a specific vitamin, a medicine or a symptom.

The 13 essential vitamins

MedlinePlus lists 13 essential vitamins: A, C, D, E, K, and the B vitamins thiamin (B1), riboflavin (B2), niacin (B3), pantothenic acid (B5), B6, biotin (B7), folate (B9) and B12. [2] It also names choline and carnitine as vitamin-like nutrients that are not on that list. [2] A role-by-role table is on What Is a Vitamin? The 13 Essential Vitamins.

Fat-soluble and water-soluble

"Water-soluble" does not mean harmless. The full comparison is on Fat-Soluble vs Water-Soluble Vitamins.

Where vitamins come from

Food is the main source. Vitamin D is also made in the skin with sunlight, though the NHS says people in the UK do not make enough from sunlight between October and early March. [2][4] Gut bacteria make some vitamin K, and bowel bacteria can make vitamin B6 and biotin. [5][3] Vitamin B12 occurs naturally only in foods of animal origin, so fortified foods or supplements matter for people who eat no animal foods. [2] See Vitamin B12 for Vegetarians and Vegans.

Needs differ by country

Each country sets its own reference amounts, so one number is never universal. For vitamin C in adults:

An RDA is the average daily intake level enough to meet the needs of nearly all (97% to 98%) healthy people; an Adequate Intake (AI) is set when evidence is too limited for an RDA. [2] The Daily Value is the reference amount used to calculate %DV on US labels. [8] EFSA (EU) defines the Tolerable Upper Intake Level (UL) as the maximum level of total chronic daily intake from all dietary sources not expected to pose a risk. [9] Amounts used in studies are study amounts, not advice. See RDA, AI, DV and UL.

Deficiency versus low intake

MedlinePlus says a deficiency occurs when you do not get enough of a certain vitamin, and that it can cause health problems. [2] Examples include scurvy from lack of vitamin C [6] and anemia from too little B12. [3] Vitamin A deficiency is very rare in the United States, [10] and vitamin K deficiency is very rare, though it can follow long-term antibiotic treatment. [11] A tired feeling or a symptom on its own cannot confirm a shortage, and treating a suspected deficiency yourself is not advised: for example, folic acid above 1 mg a day can mask vitamin B12 deficiency, according to the NHS. [3] Blood tests have limits too: see Vitamin Deficiency Testing and Who Is at Risk of Vitamin Deficiency?.

Too much

Most harm from excess comes from supplements, not from food. Examples from the NHS: too many vitamin D supplements over a long time can cause too much calcium to build up; [4] the NHS says some research suggests an average of more than 1.5 mg of vitamin A a day over many years may affect bones, and many multivitamins and fish liver oils contain vitamin A; [12] high doses of nicotinic acid (a form of niacin) can cause flushing and, over a long time, liver damage; [3] and folic acid above 1 mg a day can mask vitamin B12 deficiency. [3] MedlinePlus adds that high-dose vitamin E supplements might raise the risk of bleeding in the brain. [13] See Vitamin Toxicity: Which Vitamins Have Upper Limits.

Urgent: if a child may have swallowed vitamins or supplements, call Poison Help at 1-800-222-1222 (US) or 911 if the person is unwell. Other countries: Poison Help by Country.

Fortified foods and supplements

Some foods have vitamins added. The NHS lists fat spreads and breakfast cereals fortified with vitamin D, [4] MedlinePlus lists fortified soymilk and cereals as B12 sources, [2] and notes that many US foods are now fortified with folic acid. [2] What is added differs by country: the NHS says UK cows' milk is generally not fortified with vitamin D "as it is in some other countries". [4] Fortified foods and supplements both count toward your total, so check labels. See Fortified and Enriched Foods.

MedlinePlus says supplements can be helpful during pregnancy and for special medical problems, and advises against taking more than 100% of the RDA without a provider's supervision. [2] Whether food is enough is covered on Food vs Supplement Sources and Are Vitamin Supplements Necessary?.

Explore the library

• Basics: What is a vitamin?, how the body absorbs and stores vitamins, how vitamins work, vitamin myths.
• Fat-soluble: A, D, E, K.
• Water-soluble: C, the B vitamins, choline.
• Forms: natural vs synthetic, liposomal.
• Deficiency and tests: symptoms, at-home test kits.
• Foods and life stages: cooking losses, life-stage overview, multivitamins.
• Evidence and safety: heart disease, cancer prevention, interactions, glossary.

The full list is at the Vitamins hub.