CHAPTER 81

Vitamins

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.

Basic considerations

Dietary reference intakes

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 1998 report—Dietary Reference Intakes for Thiamin, Riboflavin, Niacin, Vitamin B₆, Folate, Vitamin B₁₂, Pantothenic Acid, Biotin, and Choline—the Food and Nutrition Board defined four reference values: recommended dietary allowance (RDA), Adequate Intake (AI), Tolerable Upper Intake Level (UL), and Estimated Average Requirement (EAR). Collectively, these four values are referred to as Dietary Reference Intakes (DRIs).

Recommended dietary allowance.

The RDA is the average daily dietary intake sufficient to meet the nutrient requirements of nearly all (97% to 98%) healthy individuals. RDAs are based on extensive experimental data. Because RDAs represent average daily intakes, low intake on one day can be compensated for by high intake on another day. RDAs change as we grow older. In addition, they often differ for males and females, and typically increase for women who are pregnant or breast-feeding. You should appreciate that 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 are revised periodically as new information becomes available. Table 81–1 summarizes when the most recent revisions were made.

TABLE 81–1 

Where to Find Food and Nutrition Board Updates for Specific Vitamins*

Vitamin	Publication	Date
Biotin, folate, niacin, pantothenic acid, riboflavin, thiamin, and vitamins B6 and B12	Dietary Reference Intakes for Thiamin, Riboflavin, Niacin, Vitamin B6, Folate, Vitamin B12, Pantothenic Acid, Biotin, and Choline	1998
Vitamins C and E	Dietary Reference Intakes for Vitamin C, Vitamin E, Selenium, and Carotenoids	2000
Vitamins A and K	Dietary Reference Intakes for Vitamin A, Vitamin K, Arsenic, Boron, Chromium, Copper, Iodine, Iron, Manganese, Molybdenum, Nickel, Silicon, Vanadium, and Zinc	2002
Vitamin D	Dietary Reference Intakes for Calcium and Vitamin D	2010

^*All publications are from the Food and Nutrition Board, Institute of Medicine, and published by the National Academy Press, Washington, DC.

Adequate intake.

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 of adverse effects. Please note that the UL is not a recommended upper limit for intake. It is simply an index of safety. There is no known benefit to exceeding the RDA.

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 will 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.

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 (thiamin, 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 of 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 about $3.5 billion a year on multivitamin and multimineral supplements—with the hope of preventing cancer, heart disease, bone loss, and other chronic disorders. Is the money well spent? Maybe. Maybe not. According to an expert panel—convened by the Office of Dietary Supplements at the National Institutes of Health—there is insufficient evidence to recommend either for or against the use of multivitamins by Americans to prevent chronic disease. Hence, as one official put it, “If you’re taking a multivitamin, there’s no reason to stop—but if you’re not taking a multivitamin, there’s no reason to start, either.” The panel’s findings were published in a January 2007 supplement to the American Journal of Clinical Nutrition.

For people who do take a multivitamin supplement, the dosage should be moderate. Why? Because excessive doses can cause harm. For example, too much vitamin A increases the risk of osteoporosis in postmenopausal women, and can cause birth defects when taken early in pregnancy. In people who smoke, too much beta-carotene (a precursor of vitamin A), increases the risk of lung cancer. And in older people with chronic health problems, too much vitamin E increases the risk of 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—namely, 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 child-bearing age, and vitamin D (plus calcium) for postmenopausal women and other people at risk of fractures.

Fat-soluble vitamins

Vitamin A (retinol)

Actions.

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.

Sources.

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.

Units.

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). One IU is equal to 0.3 RAEs.

Requirements.

Current RDAs for vitamin A, set in 2002, are slightly lower than the previous RDAs. The new RDA 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 81–2.

TABLE 81–2 

Recommended Vitamin Intakes for Individuals

	Recommended Vitamin Intake Per Day
Life-Stage Group	Vitamin A (mcg)a	Vitamin C (mg)	Vitamin D (IU)b,c	Vitamin E (mg)d	Vitamin K (mcg)	Thiamin (mg)	Riboflavin (mg)	Niacin (mg)e	Vitamin B6 (mg)	Folate (mcg)f	Vitamin B12 (mcg)	Pantothenic Acid (mg)	Biotin (mcg)
Infants
0–6 mo	400*	40*	400*	4*	2*	0.2*	0.3*	2*	0.1*	65*	0.4*	1.7*	5*
7–12 mo	500*	50*	400*	5*	2.5*	0.3*	0.4*	4*	0.3*	80*	0.5*	1.8*	6*
Children
1–3 yr	300	15	600	6	30*	0.5	0.5	6	0.5	150	0.9	2*	8*
4–8 yr	400	25	600	7	55*	0.6	0.6	8	0.6	200	1.2	3*	12*
Male
9–13 yr	600	45	600	11	60*	0.9	0.9	12	1	300	1.8	4*	20*
14–18 yr	900	75	600	15	75*	1.2	1.3	16	1.3	400	2.4	5*	25*
19–30 yr	900	90	600	15	120*	1.2	1.3	16	1.3	400	2.4	5*	30*
31–50 yr	900	90	600	15	120*	1.2	1.3	16	1.3	400	2.4	5*	30*
51–70 yr	900	90	600	15	120*	1.2	1.3	16	1.7	400	2.4g	5*	30*
>70 yr	900	90	800	15	120*	1.2	1.3	16	1.7	400	2.4g	5*	30*
Female
9–13 yr	600	45	600	11	60*	0.9	0.9	12	1	300	1.8	4*	20*
14–18 yr	700	65	600	15	75*	1	1	14	1.2	400h	2.4	5*	25*
19–30 yr	700	75	600	15	90*	1.1	1.1	14	1.3	400h	2.4	5*	30*
31–50 yr	700	75	600	15	90*	1.1	1.1	14	1.3	400h	2.4	5*	30*
51–70 yr	700	75	600	15	90*	1.1	1.1	14	1.5	400	2.4g	5*	30*
>70 yr	700	75	800	15	90*	1.1	1.1	14	1.5	400	2.4g	5*	30*
Pregnancy
≤18 yr	750	80	600	15	75*	1.4	1.4	18	1.9	600i	2.6	6*	30*
19–30 yr	770	85	600	15	90*	1.4	1.4	18	1.9	600i	2.6	6*	30*
31–50 yr	770	85	600	15	90*	1.4	1.4	18	1.9	600i	2.6	6*	30*
Lactation
≤18 yr	1200	115	600	19	75*	1.4	1.6	17	2	500	2.8	7*	35*
19–30 yr	1300	120	600	19	90*	1.4	1.6	17	2	500	2.8	7*	35*
31–50 yr	1300	120	600	19	90*	1.4	1.6	17	2	500	2.8	7*	35*

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