The word "vitamin" is derived from the Latin word vita, meaning "life." Vitamins are organic substances that are indispensable for the maintenance of good health, despite the fact that they are present in minuscule quantities in food; 13 vitamins are considered to be essential. Although they are not a source of energy, vitamins play a crucial role in transforming fats and carbohydrates into a form of energy that can be used by the human body; they also facilitate growth and reproduction and help maintain bodily functions. Each vitamin plays a specific role, and they are not interchangeable because their structures are very different. A distinction is often made between water-soluble and fat-soluble vitamins.

Among the water-soluble vitamins are vitamin C and the B-complex vitamins, which include…

Water-soluble vitamins are not stored in significant quantities in the body; since they are soluble in water, they are eliminated in urine and sweat and must be replaced on a daily basis. Vitamin C is found mainly in fruits and vegetables, while B vitamins are found mainly in whole-grain cereals, meat, and dairy products.

Water-soluble vitamins play a role in a large number of biochemical reactions involved in the regeneration of skin, blood, and nerve cells. For example, vitamin C, which is also known as ascorbic acid, helps the body absorb the iron in food and is involved in the formation of collagen, a substance that enhances the resistance of skin, cartilage, bones, teeth, and blood vessels. B vitamins work together; if any one of them is lacking, the others cannot be used efficiently. They play a key role in the transformation of protein, carbohydrates, and fat into a form of energy that can be distributed throughout the body. They are also involved in the formation of antibodies and red blood cells, and ensure that the nervous and digestive systems function normally.

The fat-soluble vitamins include vitamins A, D, E, and K. Since they are soluble in fats, they can be stored in body fat and in the liver. They are eliminated very slowly in bile, and can thus be toxic if consumed in excessive quantities, especially in the form of supplements containing vitamin A or D.

Vitamin A, which is found mainly in dairy products, liver, and egg yolks, plays an important role in the enhancement of night vision and ensures that the immune system functions normally. This vitamin is also found in the form of precursors (such as carotene, a substance that promotes the formation of vitamins in the body) in yellow, green, and orange fruits and vegetables.

Vitamin D is often referred to as the "sun vitamin" because the skin contains a precursor that turns into vitamin D when exposed to the ultraviolet rays of the sun. This vitamin promotes the absorption of calcium and phosphorus, which are required for the formation of bone tissue. A deficiency in vitamin D during periods of growth can lead to rickets, or rachitis, a disease that has adverse effects on the development of bones. Although a few minutes of exposure to the sun provides all the vitamin D the body requires, it is commonly added to milk because many people are not exposed to sufficiently sunny conditions for long enough periods of time.

Vitamin D is thus found in milk as well as in eggs, liver, and fatty fish. Vitamin E is an antioxidant that prevents the formation of free radicals, substances that stimulate the growth of potentially cancerous cells. Vitamin E is found mainly in vegetable oils, wheat germ, and fish oils. Vitamin K is essential for the normal clotting of blood; in fact, its name is derived from the German word Coagulation. The bacterial flora in the intestine produces over half of the vitamin K required by the body; small amounts of vitamin K are also found in foods such as spinach, cabbage, milk, liver, and eggs.