General information about Vitamins and Health

There are two types of vitamins: water soluble and fat soluble. A third category is what sunshine synthesizes as hormone D which in earlier times was labeled incorrectly as vitamin D.

Here are key points about vitamins:

• There are 12 known vitamins. [Vitamin D is not a vitamin; it is a hormone]
• Vitamins are either water-soluble [B, C] or fat-soluble [A, D, E, K].
• Fat-soluble vitamins are easier for the body to store than water soluble.
• Plant food is the best source of vitamins, but some people may be advised by a physician to use supplements.

Vitamin A

Most of the information comes from US NIH Fact sheet for vitamin A.

There are two forms of vitamin A in the human diet: Retinol and carotenoids, including beta carotene. Vitamin A is involved in immune function, vision, reproduction, maintaining functions of lungs, heart, kidneys, other organs and cellular communication. The body converts carotinoid pigments in vegetables into vitamin A.

Good sources include: Liver, cod liver oil, carrots, broccoli, sweet potato, butter, kale, spinach, pumpkin, collard greens, some cheeses, egg, apricot, cantaloupe melon, and milk.

Since vitamin A is fat soluble, it can be stored in the liver. Normal intake of vitamin A, 700 to 900 mcg retinol activity equivalents [RAE], is reported to be safe, but an excessive intake can be stored in the liver for long periods and can be harmful. Most persons in United States get an adequate amount of vitamin A from their diet.

Introduction

Vitamin A is the name of a group of fat-soluble retinoids, including retinol, retinal, and retinyl esters. Vitamin A is involved in immune function, vision, reproduction, and cellular communication. Vitamin A is critical for vision as an essential component of rhodopsin, a protein that absorbs light in the retinal receptors, and because it supports the normal differentiation and functioning of the conjunctival membranes and cornea [2-4]. Vitamin A also supports cell growth and differentiation, playing a critical role in the normal formation and maintenance of the heart, lungs, kidneys, and other organs [2].

Two forms of vitamin A are available in the human diet: preformed vitamin A (retinol and its esterified form, retinyl ester) and provitamin A carotenoids. Preformed vitamin A is found in foods from animal sources, including dairy products, fish, and meat (especially liver). By far the most important provitamin A carotenoid is beta-carotene; other provitamin A carotenoids are alpha-carotene and beta-cryptoxanthin. The body converts these plant pigments into vitamin A. Both provitamin A and preformed vitamin A must be metabolized intracellularly to retinal and retinoic acid, the active forms of vitamin A, to support the vitamin’s important biological functions. Other carotenoids found in food, such as lycopene, lutein, and zeaxanthin, are not converted into vitamin A.

The various forms of vitamin A are solubilized into micelles in the intestinal lumen and absorbed by duodenal mucosal cells. Both retinyl esters and provitamin A carotenoids are converted to retinol, which is oxidized to retinal and then to retinoic acid. Most of the body’s vitamin A is stored in the liver in the form of retinyl esters.

Retinol and carotenoid levels are typically measured in plasma, and plasma retinol levels are useful for assessing vitamin A inadequacy. However, their value for assessing marginal vitamin A status is limited because they do not decline until vitamin A levels in the liver are almost depleted. Liver vitamin A reserves can be measured indirectly through the relative dose-response test, in which plasma retinol levels are measured before and after the administration of a small amount of vitamin A. A plasma retinol level increase of at least 20% indicates an inadequate vitamin A level. For clinical practice purposes, plasma retinol levels alone are sufficient for documenting significant deficiency.

Recommended Intakes:

Intake recommendations for vitamin A and other nutrients are provided in the Dietary Reference Intakes (DRIs) developed by the Food and Nutrition Board (FNB) at the Institute of Medicine of the National Academies (formerly National Academy of Sciences). DRI is the general term for a set of reference values used for planning and assessing nutrient intakes of healthy people. These values, which vary by age and gender, include:

Recommended Dietary Allowance (RDA): Average daily level of intake sufficient to meet the nutrient requirements of nearly all (97%–98%) healthy individuals; often used to plan nutritionally adequate diets for individuals.

Adequate Intake (AI): Intake at this level is assumed to ensure nutritional adequacy; established when evidence is insufficient to develop an RDA.

Estimated Average Requirement (EAR): Average daily level of intake estimated to meet the requirements of 50% of healthy individuals; usually used to assess the nutrient intakes of groups of people and to plan nutritionally adequate diets for them; can also be used to assess the nutrient intakes of individuals.

Tolerable Upper Intake Level (UL): Maximum daily intake unlikely to cause adverse health effects.

RDAs for vitamin A are given as retinol activity equivalents (RAE) to account for the different bioactivities of retinol and provitamin A carotenoids, all of which are converted by the body into retinol (see Table 1). One mcg RAE is equivalent to 1 mcg retinol, 2 mcg supplemental beta-carotene, 12 mcg dietary beta-carotene, or 24 mcg dietary alpha-carotene or beta-cryptoxanthin.

*Adequate Intake (AI), equivalent to the mean intake of vitamin A in healthy, breastfed infants.

International Units and mcg RAE

Vitamin A is now measured in mcg RAE, but it was previously measured in International Units (IUs). To convert IU to mcg RAE, use the following:

• 1 IU retinol = 0.3 mcg RAE
• 1 IU supplemental beta-carotene = 0.3 mcg RAE
• 1 IU dietary beta-carotene = 0.05 mcg RAE
• 1 IU dietary alpha-carotene or beta-cryptoxanthin = 0.025 mcg RAE

RAE can only be directly converted into IUs if the source or sources of vitamin A are known. For example, the RDA of 900 mcg RAE for adolescent and adult men is equivalent to 3,000 IU if the food or supplement source is preformed vitamin A (retinol) or if the supplement source is beta-carotene. This RDA is also equivalent to 18,000 IU beta-carotene from food or to 36,000 IU alpha-carotene or beta-cryptoxanthin from food. Therefore, a mixed diet containing 900 mcg RAE provides between 3,000 and 36,000 IU vitamin A, depending on the foods consumed.

Sources of Vitamin A:

Food: Concentrations of preformed vitamin A are highest in liver and fish oils [2]. Other sources of preformed vitamin A are milk and eggs, which also include some provitamin A [2]. Most dietary provitamin A comes from leafy green vegetables, orange and yellow vegetables, tomato products, fruits, and some vegetable oils. The top food sources of vitamin A in the U.S. diet include dairy products, liver, fish, and fortified cereals; the top sources of provitamin A include carrots, broccoli, cantaloupe, and squash.

Table 2 suggests many dietary sources of vitamin A. The foods from animal sources in Table 2 contain primarily preformed vitamin A, the plant-based foods have provitamin A, and the foods with a mixture of ingredients from animals and plants contain both preformed vitamin A and provitamin A.

*DV = Daily Value. FDA developed DVs to help consumers compare the nutrient contents of foods and dietary supplements within the context of a total diet. The DV for vitamin A is 900 mcg RAE for adults and children age 4 years and older [8], where 1 mcg RAE = 1 mcg retinol, 2 mcg beta-carotene from supplements, 12 mcg beta-carotene from foods, 24 mcg alpha-carotene, or 24 mcg beta-cryptoxanthin. FDA does not require food labels to list vitamin A content unless vitamin A has been added to the food. Foods providing 20% or more of the DV are considered to be high sources of a nutrient, but foods providing lower percentages of the DV also contribute to a healthful diet.

Dietary supplements:

Vitamin A is available in multivitamins and as a stand-alone supplement, often in the form of retinyl acetate or retinyl palmitate. A portion of the vitamin A in some supplements is in the form of beta-carotene and the remainder is preformed vitamin A; others contain only preformed vitamin A or only beta-carotene. Supplement labels usually indicate the percentage of each form of the vitamin. The amounts of vitamin A in stand-alone supplements range widely. Multivitamin supplements typically contain 750–3,000 mcg RAE (2,500–10,000 IU) vitamin A, often in the form of both retinol and beta-carotene. About 28%–37% of the general population uses supplements containing vitamin A. Adults aged 71 years or older and children younger than 9 are more likely than members of other age groups to take supplements containing vitamin A.

Vitamin A Intakes and Status:

According to an analysis of data from the 2007–2008 National Health and Nutrition Examination Survey (NHANES), the average daily dietary vitamin A intake in Americans aged 2 years and older is 607 mcg RAE. Adult men have slightly higher intakes (649 mcg RAE) than adult women (580 mcg RAE). Although these intakes are lower than the RDAs for individual men and women, these intake levels are considered to be adequate for population groups. Data from NHANES III, conducted in 1988–1994, found that approximately 26% of the vitamin A in RAEs consumed by men and 34% of that consumed by women in the United States comes from provitamin A carotenoids, with the remainder coming from preformed vitamin A, mostly in the form of retinyl esters. The adequacy of vitamin A intake decreases with age in children. Furthermore, girls and African-American children have a higher risk of consuming less than two-thirds of the vitamin A RDA than other children.

Vitamin A Deficiency:

Vitamin A deficiency is rare in the United States. However, vitamin A deficiency is common in many developing countries, often because residents have limited access to foods containing preformed vitamin A from animal-based food sources and they do not commonly consume available foods containing beta-carotene due to poverty.

Groups at Risk of Vitamin A Inadequacy: The following groups are among those most likely to have inadequate intakes of vitamin A:

Premature Infants: In developed countries, clinical vitamin A deficiency is rare in infants and occurs only in those with malabsorption disorders. However, preterm infants do not have adequate liver stores of vitamin A at birth and their plasma concentrations of retinol often remain low throughout the first year of life. Preterm infants with vitamin A deficiency have an increased risk of eye, chronic lung, and gastrointestinal diseases.

Infants and Young Children in Developing Countries: In developed countries, the amounts of vitamin A in breast milk are sufficient to meet infants’ needs for the first 6 months of life. But in women with vitamin A deficiency, breast milk volume and vitamin A content are suboptimal and not sufficient to maintain adequate vitamin A stores in infants who are exclusively breastfed. The prevalence of vitamin A deficiency in developing countries begins to increase in young children just after they stop breastfeeding. The most common and readily recognized symptom of vitamin A deficiency in infants and children is xerophthalmia.

Pregnant and Lactating Women in Developing Countries: Pregnant women need extra vitamin A for fetal growth and tissue maintenance and for supporting their own metabolism. The World Health Organization estimates that 9.8 million pregnant women around the world have xerophthalmia as a result of vitamin A deficiency. Other effects of vitamin A deficiency in pregnant and lactating women include increased maternal and infant morbidity and mortality, increased anemia risk, and slower infant growth and development. People with Cystic Fibrosis: Most people with cystic fibrosis have pancreatic insufficiency, increasing their risk of vitamin A deficiency due to difficulty absorbing fat. Several cross-sectional studies found that 15%–40% of patients with cystic fibrosis have vitamin A deficiency. However, improved pancreatic replacement treatments, better nutrition, and caloric supplements have helped most patients with cystic fibrosis become vitamin A sufficient. Several studies have shown that oral supplementation can correct low serum beta-carotene levels in people with cystic fibrosis, but no controlled studies have examined the effects of vitamin A supplementation on clinical outcomes in patients with cystic fibrosis.

Vitamin A and Health: This section focuses on three diseases and disorders in which vitamin A might play a role: cancer, age-related macular degeneration (AMD), and measles.

Cancer: Because of the role vitamin A plays in regulating cell growth and differentiation, several studies have examined the association between vitamin A and various types of cancer. However, the relationship between serum vitamin A levels or vitamin A supplementation and cancer risk is unclear.

Measles: Measles is a major cause of morbidity and mortality in children in developing countries. About half of all measles deaths happen in Africa, but the disease is not limited to low-income countries. Vitamin A deficiency is a known risk factor for severe measles. Vitamin A also reduced the incidence of croup but not pneumonia or diarrhea, although the mean duration of fever, pneumonia, and diarrhea was shorter in children who received vitamin A supplements.

Blindness: The body needs vitamin A to maintain the corneas and other epithelial surfaces, so the lower serum concentrations of vitamin A associated with measles, especially in people with protein-calorie malnutrition, can lead to blindness.

Health Risks from Excessive Vitamin A: Because vitamin A is fat soluble, the body stores excess amounts, primarily in the liver, and these levels can accumulate. Although excess preformed vitamin A can have significant toxicity (known as hypervitaminosis A), large amounts of beta-carotene and other provitamin A carotenoids are not associated with major adverse effects. The manifestations of hypervitaminosis A depend on the size and rapidity of the excess intake. The symptoms of hypervitaminosis A following sudden, massive intakes of vitamin A, as with Arctic explorers who ate polar bear liver, are acute. Chronic intakes of excess vitamin A lead to increased intracranial pressure (pseudotumor cerebri), dizziness, nausea, headaches, skin irritation, pain in joints and bones, coma, and even death. Although hypervitaminosis A can be due to excessive dietary intakes, the condition is usually a result of consuming too much preformed vitamin A from supplements or therapeutic retinoids. When people consume too much vitamin A, their tissue levels take a long time to fall after they discontinue their intake, and the resulting liver damage is not always reversible.

Vitamin A and Healthful Diets: The federal government’s 2020-2025 Dietary Guidelines for Americans notes that “Because foods provide an array of nutrients and other components that have benefits for health, nutritional needs should be met primarily through foods. … In some cases, fortified foods and dietary supplements are useful when it is not possible otherwise to meet needs for one or more nutrients (e.g., during specific life stages such as pregnancy).”

The Dietary Guidelines for Americans describes a healthy dietary pattern as one that: Includes a variety of vegetables; fruits; grains (at least half whole grains); fat-free and low-fat milk, yogurt, and cheese; and oils. Many fruits, vegetables, and dairy products are good sources of vitamin A. Some ready-to-eat breakfast cereals are fortified with vitamin A. Includes a variety of protein foods such as lean meats; poultry; eggs; seafood; beans, peas, and lentils; nuts and seeds; and soy products. Beef liver contains high amounts of vitamin A. Other sources of the nutrient include eggs and some fish. Limits foods and beverages higher in added sugars, saturated fat, and sodium. Limits alcoholic beverages. Stays within your daily calorie needs.