Vitamins are essential, non-caloric nutrients that are needed for the human metabolism. They cannot be produced by the human body, but have to be obtained from the daily diet. Their principal function is as cofactors for enzymatic reactions. They also play a role in various other body functions, including skin regeneration, eyesight, functioning of the nervous system and the immune system and blood clotting.

The body needs different amounts of different vitamins. Different people also have different needs. Children, old people, people who suffer from diseases as well as pregnant women need higher amounts of certain vitamins in their daily diet.

There are two classes of vitamins: the fat-soluble vitamins (A, D, E, and K) and the water-soluble vitamins (B and C). Fat-soluble vitamins taken up in excess can be stored in the human body, whereas excess amounts of water-soluble vitamins are excreted. This is why an excess uptake of fat-soluble vitamins can occasionally cause symptoms of toxicity, which rarely occurs for water-soluble vitamins. On the contrary, deficiency symptoms are more likely to occur for water-soluble vitamins, as they cannot be stored in tissues. These deficiency symptoms range from minor problems, such as headache, skin problems or loss of appetite to severe diseases such as beri-beri, which is caused by a long-term vitamin B1 deficiency or scurvy, which is caused by a long term vitamin C deficiency. However, severe deficiencies are hardly found in developed countries. Nevertheless sub-optimal intakes do occur for many sub-groups of the population.

Vitamins are found in all kinds of food, fruit, vegetables, cereals, meat, fish and milk products. Their levels depend on the type of food itself, as well as on how it is stored and processed. Long storage and processing reduce vitamin levels in food.