Vitamins are nutrients required in small amounts to maintain life. Most vitamins are called essential because they cannot be made by the human body. They must be obtained from foods or vitamin supplements. Exceptions are vitamin D, which can be manufactured in the skin if exposed to sunlight, and vitamin K, which can, to some extent, be synthesized by bacteria in the gut.
Vitamins are organic compounds. Small amounts of vitamins are found in all the organic components of diets. This includes everything eaten from plant or animal sources.
Vitamins are important to many vital functions in the body. One of their main roles is to act as enzymes or enzyme helpers. Enzymes act like keys that turn on reactions in the body. Every reaction has a distinct enzyme to start the process.
Small amounts of vitamins are essential for life. However, vitamin levels that are too high can cause serious side effects. Dietary Reference Intakes (DRIs) are used to guide a person's intake of vitamins. DRIs for a particular vitamin are broken down by life-stage and gender groups. These groups reflect a person's age and sex.
Four reference values are included in the DRIs: Estimated Average Requirement (EAR). This is the daily intake needed to meet the requirements of half the people in a particular group.Recommended Daily Allowance (RDA). This is the daily intake that is needed to meet the needs of most individuals in a group.Adequate Intake (AI). This value is used when the EAR cannot be determined. It is an estimate of the average daily intake needed for a group.Tolerable Upper Intake Level (UL). This is the highest daily intake that a particular group can have without side effects.
Vitamins are divided into two groups: fat soluble and water soluble. Water-soluble vitamins dissolve in water-based fluids. These include the eight B vitamins: B1, B2, B3, B6, B12, folate, biotin, and pantothenic acid. Vitamin C also is a water-soluble vitamin.
The fat-soluble vitamins are A, D, E, and K. These vitamins dissolve in fat. They are transported by fat through the body, and are stored in fat in the body. They are found in fatty foods. Sources include dairy foods, fat, oils, and the fat-containing parts of grains and vegetables. There is considerably more risk of toxicity following excessive intake, with fat-soluble vitamins than with water-soluble vitamins, because fat-soluble vitamins accumulate in the tissues rather than being excreted in the urine.