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7.1: Vitamins: Basic Concepts

Skills to Develop

  • Define vitamin
  • Know the difference between fat soluble and water soluble vitamins
  • Recognize the names of each of the 13 vitamins
  • For each vitamin, know what the vitamin does and what happens to the human body if: the vitamin is too low in the diet for too long (deficiency); the vitamin is too high in the diet for too long (toxicity) and be able to list some food sources that are rich in each of the vitamins.  

Vitamins are organic compounds (meaning they contain Carbon) and they are vital nutrients that organisms require in limited amounts. Just like other essential nutrients, vitamins must be obtained from the diet since the organism can not make them (or can not make enough of them) for survival. Interestingly, not all organisms have the same vitamin requirements. For example, ascorbic acid (one form of vitamin C) is a vitamin for humans, because the human body can not make it for us, but most all other animals have the ability to make this vitamin for themselves.  

Thirteen vitamins are universally recognized. Some researchers believe there are more vitamins that have yet to be discovered. Vitamins are classified as either: fat soluble (will dissolve in oil) or water soluble (will dissolve in water). Only four of the vitamins: A, D, E and K are fat soluble. The other 9 vitamins are water soluble, including: vitamin C and all of the B vitamins (Riboflavin, Niacin, Thiamin, B6, Folate, B12, Pantothenic Acid and Biotin). It is also important to note that, the term "vitamin" can refer to a number of compounds that all show the biological activity associated with a particular vitamin. For example, "vitamin A", includes the compounds: retinal, retinol, retinoic acid and four known carotenoids. All of these compounds can be converted into active vitamin A in the body and are therefore considered to be precursors to vitamin A.  

Vitamins have diverse biochemical functions. Most of the vitamins have several important functions, not just one. Some vitamins, such as vitamin D and A, have hormone-like functions where they travel through the blood and tell certain cell types what to do. Other vitamins function as antioxidants (vitamin E and vitamin C for example). The largest number of vitamins, the B complex vitamins, function primarily as "enzyme helpers" called coenzymes. Coenzymes are needed for enzymes to function.

Vitamins are needed in very small quantities. When you look at Table \(\PageIndex{1}\), at the amounts needed, you'll see that the vitamins are measured in microgram (µg) or milligram (mg) amounts. Remember that there are 1,000 mg in 1 gram and 1,000 micrograms in 1 milligram. Those are tiny quantities and yet we can not live without them! As you read about each of the vitamins and their important roles in human health, keep in mind there can be too much of a good thing. These days, people sometimes choose to take large doses of vitamin supplements. Large doses of vitamin supplements can make it easy to get to toxic amounts of vitamins. I encourage you to think of vitamins as being on a continuum where you can be deficient (too low), just right, or toxic (too high).  If you are just a little deficient you may only be able to detect the deficiency with a blood test but if you are deficient long enough and severe enough, systems of the body will eventually begin to lose function and you will see signs and symptoms overtly.  At that stage it is called a "deficiency disease". If a deficiency disease is caught early enough it can usually be reversed by giving the person the vitamin they were deficient in but in some cases the damage that has been done is too severe and can not be reversed.  


Please click on the vitamin name below and you will be linked to a Wikipedia article to learn about that vitamin. Keep in mind, for quiz/exam purposes, you're looking for: function (what does the vitamin do?), deficiency symptoms (what happens if I'm too low for too long), toxicity symptoms (what happens if I have too much for too long?) and food sources (what can I eat to get more of this vitamin) for each of the 13 vitamins.

Table \(\PageIndex{1}\): List of vitamins
Vitamin generic
descriptor name
Vitamin chemical name(s) Solubility
(fat or water)
Recommended dietary allowances
(male, age 19–70)
Deficiency disease Upper Intake Level
Overdose disease Food sources
Vitamin A Retinol, retinal, and
four carotenoids
including beta carotene
Fat 900 µg Night blindness, hyperkeratosis, and keratomalacia 3,000 µg Hypervitaminosis A Liver, orange, ripe yellow fruits, leafy vegetables, carrots, pumpkin, squash, spinach, fish, soya milk, milk
Vitamin B1 Thiamine Water 1.2 mg Beriberi, Wernicke-Korsakoff syndrome N/D[10] Drowsiness or muscle relaxation with large doses. Pork, oatmeal, brown rice, vegetables, potatoes, liver, eggs
Vitamin B2 Riboflavin Water 1.3 mg Ariboflavinosis, glossitis, angular stomatitis N/D   Dairy products, bananas, popcorn, green beans, asparagus
Vitamin B3 Niacin, niacinamide Water 16.0 mg Pellagra 35.0 mg Liver damage (doses > 2g/day) and other problems Meat, fish, eggs, many vegetables, mushrooms, tree nuts
Vitamin B5 Pantothenic acid Water 5.0 mg[13] Paresthesia N/D Diarrhea; possibly nausea and heartburn. Meat, broccoli, avocados
Vitamin B6 Pyridoxine, pyridoxamine, pyridoxal Water 1.3–1.7 mg Anemia[15] peripheral neuropathy 100 mg Impairment of proprioception, nerve damage (doses > 100 mg/day) Meat, vegetables, tree nuts, bananas
Vitamin B7 Biotin Water 30.0 µg Dermatitis, enteritis N/D   Raw egg yolk, liver, peanuts, leafy green vegetables
Vitamin B9 Folic acid, folate, folinic acid Water 400 µg Megaloblastic anemia and deficiency during pregnancy is associated with birth defects, such as neural tube defects 1,000 µg May mask symptoms of vitamin B12 deficiency; other effects. Leafy vegetables, pasta, bread, cereal, liver
Vitamin B12 Cyanocobalamin, hydroxocobalamin, methylcobalamin Water 2.4 µg Megaloblastic anemia[16] N/D Acne-like rash [causality is not conclusively established]. Meat and other animal products
Vitamin C Ascorbic acid Water 90.0 mg Scurvy 2,000 mg Vitamin C megadosage Many fruits and vegetables, liver
Vitamin D Cholecalciferol (D3), Ergocalciferol (D2) Fat 10 µg[17] Rickets and osteomalacia 50 µg Hypervitaminosis D Fish, eggs, liver, mushrooms
Vitamin E Tocopherols, tocotrienols Fat 15.0 mg Deficiency is very rare; sterility in males and abortions in females, mild hemolytic anemia in newborn infants[18] 1,000 mg Increased congestive heart failure seen in one large randomized study. Many fruits and vegetables, nuts and seeds
Vitamin K phylloquinone, menaquinones Fat 120 µg Bleeding diathesis N/D Increases coagulation in patients taking warfarin. Leafy green vegetables such as spinach, egg yolks,

Health Effects

Vitamins are essential for the normal growth and development of a multicellular organism. Using the genetic blueprint inherited from its parents, a fetus begins to develop, at the moment of conception, from the nutrients it absorbs. It requires certain vitamins and minerals to be present at certain times. These nutrients facilitate the chemical reactions that produce among other things, skin, bone, and muscle. If there is serious deficiency in one or more of these nutrients, a child may develop a deficiency disease. Even minor deficiencies may cause permanent damage.[21]

For the most part, vitamins are obtained with food, but a few are obtained by other means. For example, microorganisms in the intestine — commonly known as "gut flora" — produce vitamin K and biotin, while one form of vitamin D is synthesized in the skin with the help of the natural ultraviolet wavelength of sunlight. Humans can produce some vitamins from precursors they consume. Examples include vitamin A, produced from beta carotene, and niacin, from the amino acid tryptophan.

Once growth and development are completed, vitamins remain essential nutrients for the healthy maintenance of the cells, tissues, and organs that make up a multicellular organism; they also enable a multicellular life form to efficiently use chemical energy provided by food it eats, and to help process the proteins, carbohydrates, and fats required for respiration.


Humans must consume vitamins periodically but with differing schedules, to avoid deficiency. The human body's stores for different vitamins vary widely; vitamins A, D, and B12 are stored in significant amounts in the human body, mainly in the liver, and an adult human's diet may be deficient in vitamins A and D for many months and B12 in some cases for years, before developing a deficiency condition. However, vitamin B3 (niacin and niacinamide) is not stored in the human body in significant amounts, so stores may last only a couple of weeks. For vitamin C, the first symptoms of scurvy in experimental studies of complete vitamin C deprivation in humans have varied widely, from a month to more than six months, depending on previous dietary history that determined body stores.

Deficiencies of vitamins are classified as either primary or secondary. A primary deficiency occurs when an organism does not get enough of the vitamin in its food. A secondary deficiency may be due to an underlying disorder that prevents or limits the absorption or use of the vitamin, due to a "lifestyle factor", such as smoking, excessive alcohol consumption, or the use of medications that interfere with the absorption or use of the vitamin. People who eat a varied diet are unlikely to develop a severe primary vitamin deficiency. In contrast, restrictive diets have the potential to cause prolonged vitamin deficits, which may result in often painful and potentially deadly diseases.

Well-known human vitamin deficiencies involve thiamine (beriberi), niacin (pellagra), vitamin C (scurvy), and vitamin D (rickets). In much of the developed world, such deficiencies are rare; this is due to (1) an adequate supply of food and (2) the addition of vitamins and minerals to common foods, often called fortification. In addition to these classical vitamin deficiency diseases, some evidence has also suggested links between vitamin deficiency and a number of different disorders.


In large doses, some vitamins have documented side-effects that tend to be more severe with a larger dosage. The likelihood of consuming too much of any vitamin from food is remote, but overdosing (vitamin poisoning) from vitamin supplementation does occur. At high enough dosages, some vitamins cause side-effects such as nausea, diarrhea, and vomiting. When side-effects emerge, recovery is often accomplished by reducing the dosage. The doses of vitamins differ because individual tolerances can vary widely and appear to be related to age and state of health. In 2008, overdose exposure to all formulations of vitamins and multivitamin-mineral formulations was reported by 68,911 individuals to the American Association of Poison Control Centers (nearly 80% of these exposures were in children under the age of 6), leading to 8 "major" life-threatening outcomes, but no deaths.

Governmental regulation

Most countries place dietary supplements in a special category under the general umbrella of foods, not drugs. As a result, the manufacturer, and not the government, has the responsibility of ensuring that its dietary supplement products are safe before they are marketed. Regulation of supplements varies widely by country. In the United States, a dietary supplement is defined under the Dietary Supplement Health and Education Act of 1994.[55] There is no FDA approval process for dietary supplements, and no requirement that manufacturers prove the safety or efficacy of supplements introduced before 1994.[32][33] The Food and Drug Administration must rely on its Adverse Event Reporting System to monitor adverse events that occur with supplements.[56] In 2007, the US Code of Federal Regulations (CFR) Title 21, part III took effect, regulating GMP practices in the manufacturing, packaging, labeling, or holding operations for dietary supplements. Even though product registration is not required, these regulations mandate production and quality control standards (including testing for identity, purity and adulterations) for dietary supplements.[57] In the European Union, the Food Supplements Directive requires that only those supplements that have been proven safe can be sold without a prescription.[58] For most vitamins, pharmacopoeial standards have been established. In the United States, the United States Pharmacopeia (USP) sets standards for the most commonly used vitamins and preparations thereof. Likewise, monographs of the European Pharmacopoeia (Ph.Eur.) regulate aspects of identity and purity for vitamins on the European market.


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