Jacqueline Rosenjack Burchum DNSc, FNP-BC, CNE
Vitamins have the following defining characteristics: (1) they are organic compounds, (2) they are required in minute amounts for growth and maintenance of health, and (3) they do not serve as a source of energy (in contrast to fats, carbohydrates, and proteins), but rather are essential for energy transformation and regulation of metabolic processes. Several vitamins are inactive in their native form and must be converted to active compounds in the body.
Reference values on dietary vitamin intake, as set by the Food and Nutrition Board of the Institute of Medicine of the National Academy of Sciences, were established to provide a standard for good nutrition. In a 2006 report—Dietary Reference Intakes: The Essential Guide to Nutrient Requirements—the Food and Nutrition Board defined five reference values: Recommended Dietary Allowance (RDA), Adequate Intake (AI), Tolerable Upper Intake Level (UL), Estimated Average Requirement (EAR), and Acceptable Macronutrient Distribution Range (AMDR). Collectively, these five values are referred to as Dietary Reference Intakes (DRIs). Of these, the RDA, AI, UL, and EAR apply to vitamins. (The AMDR is used for macronutrients such as fats and carbohydrates.)
Recommended Dietary Allowance
The RDA is the average daily dietary intake sufficient to meet the nutrient requirements of nearly all (97%–98%) healthy individuals. These figures are not absolutes. RDAs change as we grow older. In addition, they often differ for males and females and typically increase for women who are pregnant or breastfeeding. Furthermore, RDAs apply only to individuals in good health. Vitamin requirements can be increased by illness, and therefore published RDA values may not be appropriate for sick people. RDAs, which are based on extensive experimental data, are revised periodically as new information becomes available. Current values are available at fnic.nal.usda.gov/dietary-guidance/dietary-reference-intakes/dri-reports.
The AI is an estimate of the average daily intake required to meet nutritional needs. AIs are employed when experimental evidence is not strong enough to establish an RDA. AIs are set with the expectation that they will meet the needs of all individuals. However, because AIs are only estimates, there is no guarantee they are adequate.
Tolerable Upper Intake Level
The UL is the highest average daily intake that can be consumed by nearly everyone without a significant risk for adverse effects. Please note that the UL is not a recommended upper limit for intake. It is simply an index of safety.
Estimated Average Requirement
The EAR is the level of intake that will meet nutrition requirements for 50% of the healthy individuals in any life-stage or gender group. By definition, the EAR may be insufficient for the other 50%. The EAR for a vitamin is based on extensive experimental data and serves as the basis for establishing an RDA. If there is not enough information to establish an EAR, no RDA can be set. Instead, an AI is assigned, using the limited data on hand.
Acceptable Macronutrient Distribution Range
The AMDR is a range for macronutrients (e.g., proteins, carbohydrates, fats) associated with optimal health. Intake of a nutrient below the established range for that nutrient increases the risk for malnourishment. Intake of a nutrient above the established range for that nutrient increases the risk for chronic diseases.
Classification of Vitamins
The vitamins are divided into two major groups: fat-soluble vitamins and water-soluble vitamins. In the fat-soluble group are vitamins A, D, E, and K. The water-soluble group consists of vitamin C and members of the vitamin B complex (thiamine, riboflavin, niacin, pyridoxine, pantothenic acid, biotin, folic acid, and cyanocobalamin). Except for vitamin B12, water-soluble vitamins undergo minimal storage in the body, and hence frequent ingestion is needed to replenish supplies. In contrast, fat-soluble vitamins can be stored in massive amounts, which is good news and bad news. The good news is that extensive storage minimizes the risk for deficiency. The bad news is that extensive storage greatly increases the potential for toxicity if intake is excessive.
Should We Take Multivitamin Supplements?
In the United States we spend billions each year on multivitamin and multimineral supplements. Is the money well spent? Maybe. Maybe not. An expert panel—convened by the Office of Dietary Supplements at the National Institutes of Health—has spoken out on this issue. They report that there is insufficient evidence to recommend either for or against the use of multivitamins by Americans to prevent chronic disease.
For people who do take a multivitamin supplement, the dosage should be moderate because excessive doses can cause harm. For example, too much vitamin A increases the risk for osteoporosis in postmenopausal women and can cause birth defects when taken early in pregnancy. In older people with chronic health problems, too much vitamin E increases the risk for death. Because of these and other concerns, high-dose multivitamin supplements should be avoided. Instead, supplements that supply 100% or less of the RDA should be used.
Although research supporting the use of multivitamin supplements is inconclusive, we do have solid data supporting the use of three individual vitamins—vitamin B12, folic acid, and vitamin D. Who should take these vitamins? Nutrition experts recommend vitamin B12 for all people over age 50, folic acid for all women of childbearing age, and vitamin D (plus calcium) for postmenopausal women and other people at risk for fractures.
What About Protective Antioxidant Effects?
Dietary antioxidants are defined as substances present in food that can significantly decrease cellular and tissue injury caused by highly reactive forms of oxygen and nitrogen, known as free radicals. These free radicals, which are normal byproducts of metabolism, readily react with other molecules. The result is tissue injury known as oxidative stress. Antioxidants help reduce oxidative stress by neutralizing free radicals before they can cause harm.
Although high doses of antioxidant supplements have been touted for their ability to prevent chronic diseases such as cardiovascular disease and cancer, much of this is information carried over from assumptions made a quarter century ago. Despite plausible theories and observational studies that provided support for protective effects of antioxidant supplements, more recent and more rigorous trials have failed to show protection against heart disease, cancer, or any other long-term illness. The National Center for Complementary and Alternative Medicine examined well-designed experimental studies that included more than 100,000 subjects and concluded that most studies failed to demonstrate a role for antioxidant-related reduction in disease development. Further, they identified that high doses of certain antioxidants might actually increase the risk for disease. For example, high doses of beta-carotene were associated with an increase of lung cancer in people who smoked, and high doses of vitamin E were associated with an increase of prostate cancer and stroke. Additionally, some antioxidant supplements were responsible for significant drug interactions.
What’s the bottom line? The National Academy of Sciences recommends limiting intake of antioxidant supplements to amounts that will prevent nutritional deficiency and avoiding doses that are potentially harmful. Of course, people should continue to obtain antioxidants as part of a healthy diet.
Vitamin A, also known as retinol, has multiple functions. In the eye, vitamin A plays an important role in adaptation to dim light. The vitamin also has a role in embryogenesis, spermatogenesis, immunity, growth, and maintaining the structural and functional integrity of the skin and mucous membranes.
Requirements for vitamin A can be met by (1) consuming foods that contain preformed vitamin A (retinol) and (2) consuming foods that contain provitamin A carotenoids (beta-carotene, alpha-carotene, beta-cryptoxanthin), which are converted to retinol by cells of the intestinal mucosa. Preformed vitamin A is present only in foods of animal origin. Good sources are dairy products, meat, fish oil, and fish. Provitamin A carotenoids are found in darkly colored, carotene-rich fruits and vegetables. Especially rich sources are carrots, cantaloupe, mangoes, spinach, tomatoes, pumpkins, and sweet potatoes.
The unit employed to measure vitamin A activity is called the retinol activity equivalent (RAE). By definition, 1 RAE equals 1 mcg of retinol, 12 mcg of beta-carotene, 24 mcg of alpha-carotene, or 24 mcg of beta-cryptoxanthin. Why are the RAEs for the provitamin A carotenoids 12 to 24 times higher than the RAE for retinol? Because dietary carotenoids are poorly absorbed and incompletely converted into retinol. Hence, to produce the nutritional equivalent of retinol, we need to ingest much higher amounts of the carotenoids. In the past, vitamin A activity was measured in international units (IU). This IU designation is still commonly used on product labels.
The current RDA for vitamin A for adult males is 900 RAEs, and the RDA for adult females is 700 RAEs. RDAs for individuals in other life-stage groups are shown in Table 65.1.
Recommended Vitamin Intakes for Individuals
|Life-Stage Group||Recommended Vitamin Intake Per Day|
|Vitamin A (mcg)a||Vitamin C (mg)||Vitamin D (IU)b,c||Vitamin E (mg)d||Vitamin K (mcg)||Thiamine (mg)||Riboflavin (mg)||Niacin (mg)e||Vitamin B6 (mg)||Folate (mcg)f||Vitamin B12 (mcg)||Pantothenic Acid (mg)||Biotin (mcg)|