"Vitamins" are organic* nutrients, necessary for health, that cannot be synthesized in sufficient quantities by an organism, and must be obtained from the diet. There are 13 vitamins in all, often divided into the water-soluble vitamins and fat-soluble vitamins. Water-soluble vitamins like vitamin C dissolve readily in water, which allows the body to easily dispose of any extra quantities. Fat-soluble vitamins are absorbed along with dietary fats, in fact, they require the presence of fat for absorption.
*An "organic compound" in this sense means the compound contains the element carbon, not to be confused with 'organic' as in farming and produce.
Each vitamin is defined by its biological activity, not its structure, so any given vitamin can exist in several distinct functional forms. What we call "vitamin E," for example, is not actually a single compound but a family of 8 related compounds all of which have vitamin E activity.
Humans can get most of the vitamins they need from a varied and balanced diet. But for a variety of reasons, obtaining all of the necessary vitamins in significant amounts on a day-to-day basis is difficult even with a good diet. This is a result of several factors; depletion of soil minerals, changes in manufacturing of food products, limited food choices, and fast-paced lifestyles. Individual foods can be good sources of many nutrients, and a good diet is still the best foundation for health. But the fact remains that most people will not receive significant amounts of all necessary vitamins from their diet alone; supplementing is essential for everyone. Given the importance of vitamins for health, the American Medical Association has recommended that all adults take a multivitamin daily.
How Vitamins Keep You Healthy
Vitamins serve a wide range of purposes in the body, all of which are equally important for overall health. Vitamins operate at the molecular level; without a good grasp of biochemistry in can be difficult to understand their roles in detail. Vitamins function as hormones, antioxidants, and enzyme cofactors that regulate cell chemistry, metabolism, and growth. The important thing to remember is that they're all equally important to health.
Vitamin Supplement Types
Vitamin supplements are sold singly and in various combinations. Among these are capsule, tablet, powder, liquid, chewable and even some topical products. Individual vitamins are taken for a wide-range of purposes. Vitamin C, for example, is often taken to support the immune system during colds or other illnesses. Vitamin B6 has helped many women with PMS and some types of neuropathy. Niacin or vitamin B3 is routinely used to lower cholesterol. Vitamin D is recommended as a preventative factor for bone health due to its role in calcium metabolism.
The most common type of vitamin combination is a multivitamin, a dietary supplement that usually contains all the necessary vitamins, and most of the minerals, in a range of potencies.