Anatomy, Physiology and Pathophysiology Overview

For the body to grow and maintain itself, it needs the essential building blocks provided by carbohydrates, fats, and proteins. Vitamins and minerals are needed to efficiently utilize these nutrients. Vitamins are organic molecules needed in small quantities for normal metabolism and other biochemical functions, such as growth or repair of tissue. Equally important are minerals, inorganic elements found naturally in the earth. Enzymes are proteins secreted by cells; they act as catalysts to induce chemical changes in other substances. A coenzyme is a substance that enhances or is necessary for the action of enzymes. Many enzymes are useless without the appropriate vitamins and/or minerals that cause them to function properly. Both vitamins and minerals act primarily as coenzymes, binding to enzymes (or other organic molecules) to activate anabolic (tissue-building) processes in the body. For example, coenzyme A is an important carrier molecule associated with the citric acid cycle, one of the body’s major energy-producing metabolic reactions. However, it requires pantothenic acid (vitamin B₅) to complete its function in the citric acid cycle.

Vitamins and minerals are essential in our lives, whether or not we are conscientious in our food choices. Under most circumstances, daily requirements of vitamins and minerals are met by ingestion of fluids and balanced meals. Ingesting food maintains adequate stores of essential vitamins and minerals, serves to preserve intestinal structure, provides chemicals for hormones and enzymes, and prevents harmful overgrowth of bacteria.

Various illnesses can cause acute or chronic deficiencies of vitamins, minerals, electrolytes, and fluids. These conditions require replacement or supplementation of these nutrients. Common examples include extensive burn injuries and acquired immunodeficiency syndrome (AIDS). Excessive loss of vitamins and minerals may also be the result of poor dietary intake, an inability to swallow after cancer chemotherapy or radiation, or mental disorders such as anorexia nervosa. Poor dietary absorption can also be caused by various gastrointestinal (GI) malabsorption syndromes. In addition, drug and alcohol abuse are frequently associated with inadequate nutritional intake that warrants vitamin and mineral supplementation. Deficiencies in dietary protein, fat, and carbohydrates are also common. These nutrients are discussed in Chapter 55. Because of some of their distinct properties and functions in the body related to blood formation, iron and the vitamin folic acid (vitamin B₉) are discussed separately in Chapter 54.

Pharmacology Overview

The human body requires vitamins in specific minimum amounts on a daily basis, and these can be obtained from both plant and animal food sources. In some cases, the body synthesizes some of its own vitamin supply. Supplemental amounts of vitamin B complex and vitamin K are synthesized by normal bacterial flora in the GI tract. Vitamin D can be synthesized by the skin when the skin is exposed to sunlight.

In contrast with the aforementioned examples, there are some situations in which nutrient megadosing is known to be harmful. For example, any excess of one or more nutrients can result in deficiencies of other nutrients due to their chemical competition for sites of absorption in the intestinal mucosa. This is likely to be the case with megadosing of minerals, such as calcium, copper, iron, and zinc, and is less likely to result from vitamin megadosing. Vitamin megadosing can lead to toxic accumulations known as hypervitaminosis, especially with the fat-soluble vitamins A, D, and K. Vitamin E appears safer, however, even at doses 10 to 20 times the recommended DRI. Hypervitaminosis is less likely to occur with the water-soluble vitamins (B complex and C) because they are readily excreted through the urinary system. Nevertheless, it is known that megadosing with vitamin B₆ (pyridoxine) at 50 to 100 times the DRI can cause nerve damage.

Persons with an illness may be less tolerant of nutrient megadosing, although megadosing regimens are often prescribed for them. For example, megadosing may be more of a strain for a GI tract that is already weakened by illness. Megadosing can even interfere with chemotherapy drugs as well as radiation treatments, because these therapies work to destroy cancer cells through oxidation processes. Nutritional supplementation with antioxidants may impede such treatment mechanisms. Patients need to tell their health care providers any unusual nutritional regimens that they plan to try, especially if they have a serious illness.

Vitamin A

Drug Profile

There are three forms of vitamin A: retinol, retinyl palmitate, and retinyl acetate. Medications containing vitamin A may require a prescription, but many over-the-counter (OTC) products, such as vitamin A–containing multivitamins, are also available. All vitamin A products are classified as pregnancy category A.

Vitamin D

Vitamin D, also called the sunshine vitamin, is responsible for the proper utilization of calcium and phosphorus in the body. The two most important members of the vitamin D family are vitamin D₂ (ergocalciferol) and vitamin D₃ (cholecalciferol). They have different sites of origin but similar functions in the body. Ergocalciferol (vitamin D₂) is plant derived and is therefore obtained through dietary sources. The natural form of vitamin D produced in the skin by ultraviolet irradiation (sun) is chemically known as 7-dehydrocholesterol. It is more commonly referred to as cholecalciferol (vitamin D₃). This endogenous synthesis of vitamin D₃ usually produces sufficient amounts to meet daily requirements. Vitamin D is obtained through both endogenous synthesis and consumption of vitamin D₂–containing foods such as fish oils, salmon, sardines, and herring; fortified milk, bread, and cereals; and animal livers, tuna fish, eggs, and butter. Normal serum levels are 12 to 50 ng/mL.

Drug Profiles

There are four forms of vitamin D: calcifediol, calcitriol, dihydrotachysterol, and ergocalciferol. Vitamin D is available in OTC medications, such as multivitamin products, or by prescription. Although various pharmaceutical manufacturers may list their individual vitamin D products as pregnancy category C, these products are generally considered to be category A or B as long as the patient is not dosed at higher levels than recommended.

Vitamin E

Four biologically active chemicals called tocopherols (alpha, beta, gamma, and delta) make up the vitamin E compounds. Alpha tocopherol is the most biologically active natural form of vitamin E and can come from plant and animal sources.