Last update: 1.5.2017.
Vitamins are by definition neccessary for humans. They are by definition vital nutrients that we require and cannot synthesize in sufficient quantities. Thus the vitamins need to be obtained from diet.
Vitamins have diverse functions in the human body. Some have hormone-like functions, some function as antioxidants and some function as precursors for enzyme cofactors.
We must consume vitamins more or less regularly (depending on vitamin) to avoid deficiency. Vitamins are stored in the body. The amount of time we can live using our internal vitamin stores varies by vitamin and also by person. There is also the possibility of overdosing on vitamins which can even more dangerous than vitamin deficiency. Overdosing can easily happen when we are using supplments and do not know what we are doing.
Ideally we should consume enough vitamins but not too much. The big question is: What is enough for you?
FDA supplies daily values for vitamins and minerals
Recommended intakes of nutrients vary by age and gender and are known as Recommended Dietary Allowances (RDAs) and Adequate Intakes (AIs). However, one value for each nutrient, known as the Daily Value (DV), is selected for the labels of dietary supplements and foods. A DV is often, but not always, similar to one's RDA or AI for that nutrient. DVs were developed by the U.S. Food and Drug Administration (FDA) to help consumers determine the level of various nutrients in a standard serving of food in relation to their approximate requirement for it. The label actually provides the %DV so that you can see how much (what percentage) a serving of the product contributes to reaching the DV.