Vitamin is an umbrella term for a number of unrelated organic substances necessary in trace amounts for normal metabolic functioning of the body. Vitamins are involved in many biochemical reactions inside of us. When sufficient amounts of a particular vitamin are lacking in a persons diet, the processes that require them will either proceed incorrectly, or not at all. When this balance is upset, disease develops.

Vitamins are found in many types of food. Different foods also contain a wide variation in the percentages of each vitamin present. Some foods may contain insufficient amounts (if any) of one vitamin, while containing an abundance of another particular vitamin. This is the premise of a proper diet (combining different foods to ensure the necessary amounts of each nutrient. Vitamins are also produced by our bacterial flora residing in the gut. Bacteria occupying our gut aid digestion by breaking down undigested food. They also form a protective barrier, attempting to prevent more virulent bacteria, and other microbes from establishing residency in our bodies. Through metabolism during food breakdown, different species of bacteria may produce small amounts of vitamin K, niacin, riboflavin, vitamin B₁₂, and folic acid. However, the amounts of these vitamins produced by bacterial flora in the gut are usually insufficient and exogenous supply is required through food and vitamin supplements.

A minimum amount of each vitamin is needed in order for the biochemical systems of the body to function both correctly and efficiently. To simplify dose of each vitamin, we have come up with the term, recommended daily allowance (RDA). The recommended daily allowance of a vitamin, is an amount of a particular vitamin needed on a daily basis to function normally. We must not forget that each RDA for a particular vitamin is based on averages in the population. These are not always correct for each individual, but can be tailored accordingly. The RDA is only a guideline, and with good intentions. The RDA keeps things simple, assuming that we are not all well-versed in the sciences behind more accurate calculation of need. But as we learn, we shall see that RDA can vary considerably depending on body weight, sex (male, female), metabolism, lifestyle, genetics, and disease states. Strong variations in these variables can cause deficiencies leading to disease as will cover.

Each vitamin has a particular function in the body. Some vitamins perform functions such as helping us to obtain energy from the sugars we consume. Other vitamins help to properly build connective tissues. Some vitamins act as cofactors, binding to enzymes and hormones, enabling them to work more efficiently (a catalyst, in a sense). Other vitamins work as intermediaries in reactions during metabolism. We shall discuss each vitamin in detail as we move along. The important thing to keep in mind as we continue is that each vitamin is essential to a particular reaction or set of reactions in the body. As you will discover, every vitamin has a "vital" role in our health. Because of the "vital" role these substances play in our health, the word "vitamin" was coined.