Vitamins are essential to human life. Among their many roles, they are involved in growth, the release of energy from foods, cellular reproduction, the immune system and vision. Without them, the body is unable to process food into energy or pass genetic information to new cells.

The first vitamins were not discovered by scientists until the early 20th century. They were found to be the components of certain foods known to help prevent or cure several diseases (such as limes, which help prevent scurvy). The commonly recognized vitamins include:

• Vitamin A
• B vitamins (e.g., B1, B2, B3, B9, B12)
• Vitamin C
• Vitamin D
• Vitamin E
• Vitamin K

Vitamins are classified by how they are absorbed by the body. Water-soluble vitamins (the B vitamins and vitamin C) can be absorbed directly into the blood from food passing through the alimentary canal. Fat soluble vitamins (vitamins A, D, E and K) need special molecules to escort them into the bile before they can move into the bloodstream and be used or stored in fat cells. The solubility also affects how they are stored in the body and which foods they can be found in.

Consuming a balanced diet will fulfill the body’s vitamin needs. This diet should include fruits and vegetables, whole grains, legumes, meats and dairy products. In addition, enriched foods and fortified foods have vitamins added and can replace many food groups that may be avoided due to medical conditions (e.g., milk intolerance) or ethical concerns (e.g., a vegan diet). When dietary sources are unable to provide sufficient vitamins, supplements are available. The use of supplements should only be done with the guidance of a physician, especially for people who belong to certain population groups, such as pregnant or breastfeeding women.

Vitamin deficiencies are rare in the United States and other industrialized nations. Most tend to occur in conjunction with other conditions (e.g., alcoholism). Resulting diseases include beriberi, scurvy and rickets.

Vitamins are a group of organic compounds necessary for the human body to function. Vitamins are called micronutrients because they are needed in much smaller quantities than macronutrients (e.g., carbohydrates, lipids, proteins).

Vitamins do not provide energy (calories) when broken down but are active throughout the body and function in different ways. Some are coenzymes, compounds that make chemical reactions within the body possible. Others act as hormones, signaling cells or tissues to perform a certain function. For instance, vitamin D signals the intestines to absorb more calcium to support bone growth. A few of the vitamins are antioxidants, which donate electrons to stabilize highly reactive chemicals called free radicals. If antioxidants are absent, free radicals take electrons from other chemicals in cells, reducing their ability to function and causing tissue damage.

Vitamins were first discovered in the early 20th century, when physicians and scientists began to recognize that chemicals in certain foods cured or alleviated what were then common medical conditions (e.g., beriberi, scurvy, rickets). Further study isolated the chemical compounds from the foods, which were named vitamins. The term vitamin is from “vita,” the Latin word for life, and “amine,” which is a chemical compound that includes nitrogen, an element in many of the first vitamins discovered. They were given a letter designation in order of discovery and since then many have also been given names that relate to their chemical formulas.

Vitamins are essential nutrients, meaning they are required for the body to function but must be obtained from dietary sources, or in some cases from supplements. The body makes some vitamins, but not enough to satisfy the body’s needs. The 14 compounds generally recognized as vitamins are the following:

• Vitamin A (retinol, retinal or retinoic acid)
  
  
• B vitamins. Due to similarity in their chemical structures, the B vitamins were once thought to be a single compound. They are now divided into nine commonly recognized vitamins that are grouped as B complex vitamins. These include:
  
  
• B1 (thiamin)
  
  
• B2 (riboflavin)
  
  
• B3 (niacin, nicotinic acid, nicotinamide or niacinamide)
  
  
• B5 (pantothenic acid)
  
  
• B6 (pyridoxine, pyridoxal or pyridoxamine)
  
  
• B9 (folate or folic acid)
  
  
• B12 (cobalamin)
  
  
• Biotin (sometimes called B7)
  
  
• Choline
  
  
• Vitamin C (ascorbic acid)
  
  
• vitamin D (calciferol)
  
  
• Vitamin E (tocopherol or tocotrienol)
  
  
• Vitamin K (phylloquinone, menaquinone, menadione or naphthoquinone)

Other compounds considered vitamins at one time have been reclassified. Some have been discovered to be nonessential (the body can make them). Others were not needed by the body or moved to other groups of chemicals, such as fatty acids.

The chemistry of vitamins is complex. Many are not one but actually several related chemical compounds. In addition, some are not obtained from food in a usable form. Instead, they are precursors or provitamins, which must undergo chemical reactions within the body before they can function. One example of this complexity is vitamin A. It includes three functioning forms (retinol, retinal and retinoic acid) and a storage form (retinyl ester). It can also be made from carotenoid precursors, such as beta carotene.

To help consumers and medical professionals determine if they are getting enough nutrients to satisfy the needs of the body, the U.S. National Academy of Sciences established dietary reference intakes (DRIs). They are based on age and gender (including whether women are pregnant or breastfeeding). Adequate intakes (AI) are estimated for some vitamins (B5, biotin, choline, D and K). This is the amount of each vitamin believed to be necessary to maintain health, but there is some scientific uncertainty. For infants (0 to 12 months old), AI expresses the average intake of healthy, breastfed children. Many of the vitamins also have tolerable upper intake levels (UL). This is the maximum daily intake of vitamins a patient may consume without risking damage to his or her health. These recommendations are measured in micrograms per day (mcg/day) or milligrams per day (mg/day). The following table provides the DRIs for adult men and women.

DRIs for Adults (19-50 years)

In addition to milligrams and micrograms, some nutritional labels may show the amount of some vitamins in international units (IU). IUs are different for each vitamin and do not convert consistently to other units of measure. For example, one IU equals:

• 0.3 micrograms of retinol (a form of vitamin A)
  
  
• 0.025 micrograms of cholecalciferol (a form of vitamin D)
  
  
• 0.667 milligrams of alpha-tocopherol (a form of vitamin E)

The 14 recognized vitamins are classified as either fat soluble or water soluble. This is largely based on how they are absorbed into the body. Fat soluble vitamins require specific carrier molecules to be transported through the body for storage in fat cells and the liver. Water soluble vitamins are able to move freely across cell membranes, allowing them to be absorbed directly into the blood. Other general differences include:

The fat soluble vitamins include:

• Vitamin A (retinol, retinal, retinoic acid). A group of nutrients essential for growth, vision and immune response. It is active in the eye, where it helps convert light to nerve signals in the retina (the back of the eye) and helps maintain the health of the cornea (the outer membrane). Vitamin A is active in the skin and cells that line internal surfaces (epithelial cells) and also acts as an antioxidant, which helps protect cells from damage caused by unstable molecules called free radicals. Vitamin A, along with other vitamins and minerals, also has a role in the remodeling of bone.
  
  
• Vitamin D (calciferol). Works with calcium and phosphorous to build and maintain strong bones. When exposed to sunlight, skin cells make vitamin D and in some cases dietary consumption may be unnecessary.
  
  
• Vitamin E (tocopherol, tocotrienol). An antioxidant that is also necessary for the proper function of the reproductive, nervous and muscular systems.
  
  
• Vitamin K (phylloquinone, menaquinone, menadione or naphthoquinone). Required for blood to clot. Without vitamin K, even minor injuries could cause massive blood loss, leading to death. It is also involved in bone formation.

The water-soluble vitamins include:

• B vitamins. In general, each of the B vitamins is involved in growth and the metabolism of carbohydrates, lipids and proteins. Other functions include:
  
  
• Vitamin B1 (thiamine). Found on the membrane of nerve cells, where it aids communication between nerves and cells.
  
  
• Vitamin B2 (riboflavin). Necessary for cell development and function and may be involved in blood cell production. It is also involved in breaking down fats and proteins.
  
  
• Vitamin B3 (niacin, nicotinic acid, nicotinamide, niacinamide). Used in the production of new cells, as well as fatty acids and steroids. Large doses may be prescribed to treat high cholesterol. B3 can also be made from tryptophan (an amino acid).
  

  

• Vitamin B5 (pantothenic acid). An essential component in the processes that make lipids, neurotransmitters and hemoglobin. The body produces some B5 itself, but not enough to meet its needs.
  
  
• Vitamin B6 (pyridoxine, pyridoxal, pyridoxamine). Required in the process to make amino acids and maintain blood glucose levels. Like B5, vitamin B6 is also needed to synthesize neurotransmitters and to make hemoglobin.
  
  
• Vitamin B9 (folate, folic acid). Involved in the synthesis of proteins and DNA, the genetic blueprint for the body. B9 is essential in the earliest stages of pregnancy to prevent neural tube defects in the fetus.
  
  
• Vitamin B12 (cobalamin). Helps maintain the health of the nervous system. It also has roles in the breakdown of fatty acids and amino acids, and the creation of red blood cells and DNA. B12 is the only water-soluble vitamin that requires a specific chemical (intrinsic factor [IF]) to be absorbed into the bloodstream.
  
  
• Biotin (sometimes called B7). Involved in the process that creates the blood sugar glucose. Some biotin is made by bacteria in the alimentary canal, but it must also be consumed with the diet.
  
  
• Choline. Necessary to transport fats through the body and the synthesis of several neurotransmitters.
  
  
• Vitamin C (ascorbic acid). Involved in the production of the connective protein collagen, neurotransmitters, hormones and hemoglobin. Like vitamins A and E, vitamin C also acts as an antioxidant, protecting cells from highly reactive chemicals.

While each vitamin has a specific function, they are also involved in complex interdependent relationships. Without B6, the tryptophan can not be converted to B3, potentially leading to a deficiency. Vitamin A is needed to remodel bone to allow for growth, but too much can counteract the bone formation of vitamins D and K.

The vitamins are involved in major processes throughout the body, making life possible. Without them, food could not be converted to energy, there would be no vision, and wounds would not heal. In addition to these functions, physicians have found that supplements of certain vitamins may help prevent or treat a variety of conditions. Some examples of these uses include:

• Birth defects. Folic acid (B9) is recommended for women of child-bearing age to reduce the risk of neural tube defects, such as spina bifida. During pregnancy, women should not take any vitamin supplements without the advice of a physician due to the possibility of resulting birth defects.
  
  
• Cholesterol. B3 supplements (niacin) are frequently prescribed to reduce levels of low-density lipoproteins (“bad cholesterol”). They also raise the amount of high-density lipoproteins (“good cholesterol”) in the blood, but with a less significant effect.
  
  
• Anemia. Several of the B vitamins are involved in making red blood cells and hemoglobin. Deficiencies of these vitamins may cause several types of anemia, which can be treated with supplements. Supplements may also be used to treat other types of the condition, such as B6 for hereditary sideroblastic anemia.

In addition to these conditions, some vitamins may be recommended to reduce the impact of free radicals. Free radicals will take electrons from other molecules, creating more free radicals. If this process continues, cell and tissues can be damaged, which contributes to some types of cancer and cardiovascular disease. Antioxidants, such as vitamins A, C and E, contribute electrons to free radicals, but remain stable. They can also accept electrons from other sources, allowing them to function repeatedly as antioxidants. This stops the destructive cycle and prevents cell damage and resulting diseases.

Links have been found between lower incidences of some diseases and increased vitamin intake through supplements or diet. Research continues on the mechanisms responsible. Topics of study include:

• Cardiovascular disease and dementia. Vitamins B6, folic acid and B12 are being investigated for their ability to lower homocysteine levels. This amino acid is found in high levels in people suffering from heart disease, blood clots, strokes, and Alzheimer's disease. To date, reducing the amount of homocysteine has not been proven to reduce health risks. Research continues into the use of vitamin C, which so far has yielded mixed results. Mortality rates due to cardiovascular disease actually increase among smokers who use vitamin A, especially in the form of beta carotene supplements.
  
  
• Hypertension (high blood pressure). Supplements of vitamins B9 and D are being studied for possible links to reduced hypertension. Results have been mixed, with some populations showing benefits while others have not.
  
  
• Cancer. Folic acid supplements have been associated in some cases with reduced incidence of colon and breast cancers, especially for moderate users of alcohol. High levels of vitamin D are also associated with reduced risk of developing cancer, but scientists are unsure what amount of supplementation produces this effect.
  
  
• Osteoporosis and bone fractures. Supplements of vitamins folic acid (B9), B12 and D may help reduce bone decay and the incidence of fractures. However, dietary sources of calcium along with weight bearing exercise have consistently been shown to reduce the risk of osteoporosis.

In the United States and other industrialized nations, eating a balanced diet provides sufficient vitamins for most people. This diet should follow the recommendations of the food guide pyramid and include servings of:

• Fruits and vegetables
• Whole grains
• Legumes (e.g., peas, beans and nuts)
• Animal products (e.g., meats, eggs, dairy products)
• Enriched foods and fortified foods (e.g., cereals, pastas, fruit juices)

While these food groups contain significant amounts of vitamins, many factors may reduce the amount the body can use. The bioavailability of vitamins can be changed by how the food is processed or cooked, its freshness and other foods consumed at the same time.

While most people are able to get sufficient vitamins from dietary sources, supplements are commonly used. A study by the U.S. Centers for Disease Control and Prevention (CDC) found they are taken by approximately 40 percent of Americans. While this behavior may be unnecessary and potentially harmful for some people, supplements are likely to improve the health of some groups. People who might benefit from supplement use include:

• Elderly. The body’s ability to absorb many vitamins diminishes with age. This problem can be exacerbated by poor diets that may be consumed by elderly people.

  

• Pregnant and breastfeeding women. Expecting and lactating women need additional nutrients to support the developing fetus and growing newborn. Still overdoses of some vitamins are known to cause a number of birth defects and infant development, so using any supplement should be discussed with a physician.
  
  
• Vegetarians and vegans. People who do not consume any animal products have limited or no natural sources for some vitamins (e.g., B12). Non-animal options to meet the body’s needs include supplements and fortified and enriched foods.

While they may be useful for some people, there are risks associated with supplement use. Most cases of vitamin overdoses, especially among children, are related to supplement use. They also do not provide the carbohydrates, fiber and other beneficial nutrients found in vitamin-rich foods, which can lead to malnutrition. Due to the potential risks associated with vitamin overdoses, supplements should only be used with the guidance of a physician.

Vitamin supplements also may not always contain the ingredients advertised. One recent study of various multivitamin products found that more than half of the supplements evaluated did not contain ingredients in the stated amounts. In addition, many of these products contained lead or failed to break apart properly once inside the digestive tract.

Vitamins are necessary for the human body to function. Too much or too little stresses cells and tissues. This can lead to serious, life-threatening diseases.

Overdoses of vitamins are most likely to occur due to supplement abuse, but there are rare reports of cases resulting from excessive dietary intake. For most water soluble vitamins, overdoses cause only minor symptoms because the kidneys excrete the excess in urine. An exception is B6, which can cause severe nerve damage when taken in large doses. Fat soluble vitamins are stored in body fat cells, which makes them more likely to cause a toxic reaction when too much is consumed over time. Examples include liver and bone damage caused by vitamin A and interference in blood clotting resulting from excess vitamin E.

Vitamin deficiencies have become rare in the United States and most other industrialized nations. When they do occur, they are often in conjunction with other factors. These risk factors include:

• Alcohol abuse. Alcohol can affect the body’s ability to use vitamins in many ways. It can limit absorption, increase excretion and make vitamins unusable. Alcoholics also tend to replace foods with alcohol, leading to general malnutrition.
  
  
• Smoking. Smoking produces free radicals that use up antioxidants, including vitamins A, C and E. Without increasing intake, smokers are more likely to suffer deficiency of one or all of these vitamins.
  
  
• Limited fat absorption. Any condition (e.g., inflammatory bowel disease) that reduces the body’s ability to absorb fat has the potential to also cause a deficiency of fat-soluble vitamins.
  
  
• Genetic deficiencies or absorption disorders. Despite dietary intake, some genetic conditions may block chemical reactions necessary to absorb or use individual vitamins, leading to deficiencies.

Prolonged vitamin deficiencies may cause severe disease. A few examples include:

• Beriberi and Wernicke-Korsakoff syndrome. Deficiencies in vitamin B1 (thiamin) can cause neurological and cardiovascular symptoms, possibly leading to paralysis, heart failure and death.
  
  
• Pellagra. Symptoms of niacin (B3) deficiency include diarrhea, skin conditions and dementia.
  
  
• Scurvy. Deficiencies of vitamin C reflect its role in the production of the connective protein collagen. Symptoms of vitamin C deficiencies include bleeding under the skin, bone pain and loose teeth.
  
  
• Rickets and osteomalacia. Both of these diseases are characterized by soft and weak bones, resulting in skeletal abnormalities. Vitamin D deficiency in children causes rickets and osteomalacia in adults.

The following table provides some signs and symptoms of vitamin overdose and deficiency:

Preparing questions in advance can help patients to have more meaningful discussions with health professionals regarding their conditions. Patients may wish to ask their doctor or registered dietitian the following questions related to vitamins:

1. Am I or my family members at risk for vitamin deficiency or toxicity?
   
   
2. How can I learn more about sources of vitamins and how much I need each day?
   
   
3. Should I consult a registered dietitian to reduce any risk of vitamin deficiency or overdose? Can you recommend one?
   
   
4. What early signs and symptoms of vitamin deficiency should I be on the lookout for?
   
   
5. What are the treatment options for my vitamin deficiency?
   
   
6. Should I use supplements to prevent vitamin deficiency?
   
   
7. Should I eat more animal or plant sources to increase my vitamin intake?
   
   
8. What are the risks of consuming excess vitamins?
   
   
9. As I age, how should I change my vitamin consumption?
   
   
10. Should I be concerned about how my lifestyle impacts my body’s ability to use vitamins?