Vitamins are molecules required by the body in small amounts for a variety of essential processes in the body. They are classified as micronutrients because they are normally required in small amounts: usually a few milligrams (mg) or micrograms (μg) per day. Most vitamins cannot be synthesised by the body so must be obtained by the diet. An exception is vitamin D which can be synthesised by the action of sunlight on the skin. Small amounts of niacin (a B vitamin) can be made from the amino acid, tryptophan.
Vitamins have a diverse range of functions in the body, including:
Co-factors in enzyme activity
Antioxidants (prevent damage from free radicals)
Pro-hormone (only vitamin D)
If insufficient amounts of vitamins are available to the body because of a poor diet or some medical condition, such as malabsorption disorders or inborn errors of metabolism, a deficiency disease can develop. Vitamin deficiency diseases are rare in the UK but still occur in some parts of the world.
Vitamins have been grouped into two categories: fat soluble vitamins and water soluble vitamins. Originally vitamins were given letters (A, B, C etc.) but are now more commonly referred to by their names, e.g. folate, riboflavin.
Requirements and recommended dietary intakes
The body requires different amounts of each vitamin because each vitamin has a different set of functions. Requirements vary according to age, sex and physiological state (for example pregnancy). They may also be influenced by state of health. The Department of Health has drawn up recommendations in the form of Dietary Reference Values (DRVs) for vitamins for different groups of healthy people. The Reference Nutrient Intake (RNI) is the amount of a nutrient that will satisfy the needs of practically all the population (97.5%); in other words it is usually not necessary to exceed the RNI. Lower Reference Nutrient Intakes have also been established. These are levels to be sufficient for only 2.5% of a given population, everyone else will require more. So if, say, 10% of a population group have intakes of a nutrient below the LRNI for that nutrient, it is highly likely that the majority of these people are having insufficient intakes for their needs.
Tables showing the mean intakes of vitamins from food sources as a percentage of Lower Reference Nutrient Intake (LRNI), by age and sex, are available in the attached pdf file on page 13. These tables give an indicator of where intakes of vitamins are of concern.
Vitamins are nutrients required by the body in small amounts, for a variety of essential processes.
Most vitamins cannot be made by the body, so need to be provided in the diet.
Vitamin D can be made by the body in the skin when it is exposed to sunlight.
Vitamins are grouped into fat-soluble vitamins and water-soluble vitamins.
Requirements for vitamins change across life stages.
Fat Soluble Vitamins
Water Soluble Vitamins
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