Study type: Blood collected in a gold-, red/gray-, green- [sodium or lithium heparin], light green-, green/green gray-, lavender- [EDTA], or pink- [K₂ EDTA] top tube for vitamin A and vitamin B₁; green-, green/green gray-, red-top tube protected from light at all times for vitamin B₆; gold-, red-, red/gray-, light green-, green/green gray-top tube protected from light at all times for vitamin B₁₂; green- [sodium or lithium heparin] top tube protected from light at all times for vitamin C; gold-, red-, red/gray-, green- [lithium heparin], or lavender- [EDTA] top tube for vitamin D 1,25 dihydroxy; gold- or red/gray-top tube for vitamin D 25 hydroxy; gold-, red/gray-, green- [sodium or lithium heparin], lavender- [EDTA], or pink- [K₂ EDTA] top tube for vitamin E; and gold-, red-, red/gray-, lavender- [EDTA], or pink- [K₂ EDTA] top tube protected from light at all times for vitamin K; related body system: vitamins contribute to multisystem effects.

Vitamin A, Vitamin B₁, Vitamin B₆, Vitamin C

Vitamin assays are used in the measurement of nutritional status. Low levels indicate inadequate oral intake, poor nutritional status, or malabsorption problems. High levels indicate excessive intake, vitamin intoxication, or absorption problems. Vitamin A is a fat-soluble nutrient that promotes normal vision and prevents night blindness; contributes to growth of bone, teeth, and soft tissues; supports thyroxine formation; maintains epithelial cell membranes, skin, and mucous membranes; and acts against infection. Vitamins B₁, B₆, and C are water soluble. Vitamin B₁ acts as an enzyme and plays an important role in the Krebs cycle of cellular metabolism. Vitamin B₆ is important in heme synthesis and functions as a coenzyme in amino acid metabolism and glycogenolysis. It includes pyridoxine, pyridoxal, and pyridoxamine. Vitamin C promotes collagen synthesis, maintains capillary strength, facilitates release of iron from ferritin to form hemoglobin, and functions in the stress response.

Vitamin B₁₂

Vitamin B₁₂ has a ringed crystalline structure that surrounds an atom of cobalt. It is essential in DNA synthesis, hematopoiesis, and central nervous system (CNS) integrity. It is derived solely from dietary intake. Animal products are the richest source of vitamin B₁₂. Its absorption depends on the presence of intrinsic factor. Circumstances that may result in a deficiency of this vitamin include the presence of stomach or intestinal disease as well as insufficient dietary intake of foods containing vitamin B₁₂. A significant increase in red blood cells (RBCs) means corpuscular volume may be an important indicator of vitamin B₁₂ deficiency.

Vitamin D

Vitamin D is a group of interrelated sterols that have hormonal activity in multiple organs and tissues of the body, including the intestines, kidneys, liver, skin, and bones. Ergocalciferol (vitamin D₂) is formed when ergosterol in plants is exposed to sunlight. Ergocalciferol is absorbed by the stomach and intestine when orally ingested. Cholecalciferol (vitamin D₃) is formed when the skin is exposed to sunlight or ultraviolet light. Vitamins D₂ and D₃ enter the bloodstream after absorption. Vitamin D₃ is converted to vitamin D 25-hydroxy by the liver and is the major circulating form of the vitamin. Vitamin D₂ is converted to vitamin D 1,25-dihydroxy (calcitriol) by the kidneys and is the more biologically active form. Vitamin D is transported throughout the body by albumin and vitamin D binding protein. Activated vitamin D or calcitriol carries out its biological functions by binding to a vitamin D receptor (VDR) in the target cells (bone, intestine, kidney, etc.). One of the better known functions of vitamin D includes regulation of mineral metabolism, mainly calcium and phosphorus. Three feedback loops involving vitamin D (and parathyroid hormone) promote:

• Intestinal absorption of calcium and phosphate (direct effect by activated vitamin D)
• Reabsorption of calcium and excretion of phosphate by the kidney (requires activation of vitamin D by PTH)
• Mobilization of calcium and phosphate stores from bone tissue (requires activation of vitamin D by PTH)

Current recommendations provided by the Choosing Wisely Campaign (an initiative of the American Board of Internal Medicine Foundation) advise the avoidance of population-based screening for vitamin D deficiency; deficiency of this vitamin is very common especially in individuals who avoid exposure to sunlight or in populations:

• living at high altitudes
• living in areas with limited sunlight (e.g., seasonally during winter)

Activated vitamin D is required by a number of important feedback loops in the body. Testing and monitoring are appropriate for high-risk groups with conditions that include:

• chronic kidney disease related to kidney feedback loop
• infections related to immune functions
• malabsorption related to intestinal feedback loop
• osteoporosis related to bone tissue feedback loop

Vitamin E

Vitamin E is a collection of powerful, fat-soluble antioxidants. Alpha-tocopherol appears to be the most plentiful and important form of eight vitamin E antioxidants; there are four tocopherols (alpha-, beta-, gamma-, and delta-) and four tocotrienols (alpha-, beta-, gamma-, and delta-). Antioxidants limit the production of free radicals by preventing the oxidation of unsaturated fatty acids. Free radicals are unstable chemical compounds that contain unshared electrons. They combine rapidly with oxygen during normal metabolic processes in the body when food is converted into energy or when they are taken into the body by environmental exposure from sources such as ultraviolet radiation, air pollution, or secondhand smoke. Vitamin E reserves in lung tissue provide a barrier against air pollution and protect RBC membrane integrity from oxidation. Oxidation of fatty acids in RBC membranes can result in irreversible membrane damage and hemolysis. For many years, scientists have been investigating whether vitamin E might play a role in the amelioration or prevention of chronic and degenerative diseases associated with damage caused by free radicals. Clinical trials, in general, have not provided consistent evidence to support the function of vitamin E as a defense against cardiovascular disease, cataracts, macular degeneration, cancer, and cognitive decline. Studies are currently in progress to further evaluate the potential protective properties of vitamin E. The use of vitamin E as a dietary supplement remains controversial. Current guidelines state that nutrition needs should be met through healthy dietary intake. Because vitamin E is found in a wide variety of foods, a deficiency secondary to inadequate dietary intake is rare. There is research to support the potential interaction between vitamin E and other medications, most notably anticoagulant and antiplatelet drugs. Overuse of supplementary vitamin E has been associated, in some studies, with increased risk of hemorrhagic stroke.

Vitamin K

Vitamin K is one of the fat-soluble vitamins. It is essential for the formation of prothrombin; factors VII, IX, and X; and proteins C and S. Vitamin K also works with vitamin D in synthesizing bone protein and regulating calcium levels. Vitamin K levels are not often requested, but vitamin K is often prescribed as a medication. Levels are considered if the patient has an abnormal INR and does not respond to vitamin K therapy. Approximately one-half of the body’s vitamin K is produced by intestinal bacteria; the other half is obtained from dietary sources. There are three forms of vitamin K: vitamin K₁, or phylloquinone, which is found in foods; vitamin K₂, or menaquinone, which is synthesized by intestinal bacteria; and vitamin K₃, or menadione, which is the synthetic, water-soluble, pharmaceutical form of the vitamin. Vitamin K₃ is two to three times more potent than the naturally occurring forms.