Vidyya Medical News Servicesm
Vidyya, from the Sanskrit "vaidya," a practitioner who has come to understand the science of life.

Volume 2 Published - 14:00 UTC    08:00 EST    18-January-2001      
Issue 18 Next Update - 14:00 UTC 08:00 EST    19-January-2001      

Vidyya Home  Vidyya

Home Of Our Sponsor, Vidyya.  Vidyya. Home

Vidyya Archives  Vidyya Archives

Search Vidyya  Search Vidyya

Visit Our Library  Ex Libris

Subscribe To Our News Service  Subscriptions

All About Us  About Vidyya




















Back To Vidyya Facts About Dietary Supplements:

Vitamin A And Carotenoids

Vitamin A: What is it?

What foods provide vitamin A?

What is the Recommended Dietary Allowance
for vitamin A for adults?

When can vitamin A deficiency occur?

 

 

 

What is the association between vitamin A, beta carotene and cancer?

What is the health risk of too much vitamin A?

What is the health risk of too many carotenoids?

Selected food sources of vitamin A

 


Vitamin A: What is it?

Vitamin A is a family of fat-soluble vitamins. Retinol is one of the most active, or usable, forms of vitamin A, and is found in animal foods such as liver and eggs. It can be converted to retinal and retinoic acid, other active forms of the vitamin A family. Some plant foods contain orange pigments called provitamin A carotenoids that the liver can convert to retinol. Beta-carotene is a provitamin A carotenoid found in many foods(1-3). Lycopene, lutein, and zeaxanthin are also carotenoids commonly found in food, but your body cannot convert them to vitamin A.

Vitamin A plays an important role in vision, bone growth, reproduction, cell division and cell differentiation, which is the process by which a cell decides what it is going to become (1, 4-7). It also maintains the surface linings of your eye and your respiratory, urinary, and intestinal tracts (8). When those linings break down, bacteria can enter your body and cause infection (8). Vitamin A also helps your body regulate its immune system (1, 4, 9). The immune system helps prevent or fight off infections by making white blood cells that destroy harmful bacteria and viruses. Vitamin A may help lymphocytes, a type of white blood cell that fights infections, function more effectively. Vitamin A also may help prevent bacteria and viruses from entering your body by maintaining the integrity of skin and mucous membranes (10, 11,12).

Some carotenoids, in addition to serving as a source of vitamin A, have been shown to function as antioxidants in laboratory tests. However, this role has not been consistently demonstrated in humans (13). Antioxidants protect cells from free radicals, which are potentially damaging by-products of the body's metabolism that may contribute to the development of some chronic diseases (2, 14-16).

What foods provide vitamin A?
Whole eggs, whole milk, and liver are among the few foods that naturally contain vitamin A. Vitamin A is present in the fat portion of whole milk, so it is not found in fat-free milk. Most fat-free milk and dried nonfat milk solids sold in the US are fortified with vitamin A (17). There are many other fortified foods such as breakfast cereals that also provide vitamin A. The tables of selected food sources of vitamin A suggest animal sources of vitamin A and plant sources of provitamin A carotenoids (18).

It is important for you to regularly eat foods that provide vitamin A or beta-carotene even though your body can store vitamin A in the liver (1). Stored vitamin A will help meet your needs when intake of provitamin A carotenoids or vitamin A is low (19, 20).

What is the Recommended Dietary Allowance for vitamin A for adults?
The Recommended Dietary Allowance (RDA) is the average daily dietary intake level that is sufficient to meet the nutrient requirements of nearly all (97-98%) healthy individuals in each life-stage and gender group (13). The 1989 RDAs (21) for vitamin A for adults and children are:

Life-Stage Children Men  Women  Pregnancy  Lactation
Ages 1-3 400 RE or 2000 IU*        
Ages 4-6 500 RE or 2500 IU        
Ages 7-10 700 RE or 3,500 IU   x x x
Ages 11-18   1000 RE or
5000 IU
800 RE or
4000 IU
800 RE or
4000 IU
800 RE or
4000 IU
an 19 +   1000 RE or
5000 IU
800 RE or
4000 IU
800 RE or
4000 IU
800 RE or
4000 IU
*Food labels list vitamin A in International Units (IU).

Results of the third National Health and Nutrition Examination survey (NHANES III 1988-91) (22) suggested that the dietary intake of some Americans does not meet recommended levels. The Continuing Survey of Food Intakes of Individuals (CSFII 1994) (23) suggested that diets of many Americans provide less than 75% of recommended intake levels. These surveys highlight the importance of encouraging Americans to include dietary sources of vitamin A in their daily diets.

There is no separate RDA for beta-carotene or other carotenoids. The Institute of Medicine (IOM) report suggests that consuming 3 to 6 mg of beta-carotene daily will maintain plasma B-carotene blood levels in the range associated with a lower risk of chronic diseases (13). This concentration can be achieved by a diet that provides five or more servings of fruits and vegetables per day.

When can vitamin A deficiency occur?
Vitamin A deficiency rarely occurs in the United States, but it is still a major public health problem in the developing world. It is most often associated with protein/calorie malnutrition and affects over 120 million children worldwide (8). It is also a leading cause of childhood blindness. In countries where immunization programs are not widespread and vitamin A deficiency is common, millions of children die each year from complications of infectious diseases such as measles. (8).

Signs of vitamin A deficiency include night blindness, dry skin, and decreased resistance to infections. In ancient Egypt it was known that night blindness could be cured after eating liver, which was later found to be a rich source of vitamin A (1). Vitamin A deficiency contributes to blindness by making the eye very dry, damaging the cornea of the eye (referred to as xerophthalmia), and promoting damage to the retina of the eye (24). Extremely dry skin, dry hair, sloughing off of skin, and broken fingernails are other common signs of vitamin A deficiency.

Vitamin A deficiency also decreases resistance to infections (1, 10). When there is not enough vitamin A, cells lining the lung lose their ability to remove disease-causing microorganisms. This may contribute to the pneumonia associated with vitamin A deficiency (9).

There is increased interest in subclinical forms of vitamin A deficiency, described as low storage levels of vitamin A that do not cause overt deficiency symptoms. This mild degree of vitamin A deficiency may increase children's risk of developing respiratory and diarrheal infections, decrease growth rate, slow bone development, and decrease likelihood of survival from serious illness (8, 25, 26).

Children living in the United States who are considered to be at increased risk for subclinical vitamin A deficiency include:

  • toddlers and preschool age children
  • children living at or below the poverty level
  • children with inadequate health care or immunizations
  • children living in areas with known nutritional deficiencies
  • recent immigrants or refugees from developing countries with high incidence of vitamin A deficiency or measles, and
  • children with diseases of the pancreas, liver, intestines, or with inadequate fat digestion/absorption (8).

As a result of the worldwide significance of vitamin A deficiency in children, the World Health Organization (WHO) and the United Nations International Children's Emergency Fund (UNICEF) issued joint statements about vitamin A and children's health. Both agencies recommend vitamin A administration for all children diagnosed with measles in communities where vitamin A deficiency is a serious problem and where death from measles is greater than 1%. In 1994, the American Academy of Pediatrics recommended vitamin A supplementation for children 6 to 24 months of age hospitalized with measles and for hospitalized children older than 6 months who are considered to be at high-risk for subclinical vitamin A deficiency (27).

Who may need extra vitamin A to prevent a deficiency?
Children with low body stores of vitamin A who have measles may require vitamin A supplementation, as discussed above. Also, individuals with chronic fat malabsorption also poorly absorb vitamin A. Fat malabsorption can occur with cystic fibrosis, sprue, pancreatic disorders, and after stomach surgery. Healthy adults usually have a one-year reserve of vitamin A stored in their livers and should not be at risk of deficiency during periods of temporary or short term fat malabsorption. Long term problems absorbing fat, however, can result in deficiency and may require vitamin A supplementation (8). Children may only have enough stores of vitamin A to last several weeks, so children with fat malabsorption may require immediate vitamin A supplementation (8).

What is the association between vitamin A, beta carotene and cancer?
Surveys suggest an association between diets rich in beta-carotene and vitamin A and a lower risk of many types of cancer (1, 28). There is evidence that higher intake of green and yellow vegetables or food sources of beta carotene and/or vitamin A decreased the risk of lung cancer (29). A number of studies have tested the role of beta-carotene supplements in cancer prevention(30). Unfortunately, recent intervention studies have not supported a protective role for beta carotene in cancer prevention. In a study of 29,000 men, incidence of lung cancer was greater in the group of smokers who took a daily supplement of beta carotene (31). The Carotene and Retinol Efficacy Trial, a lung cancer chemoprevention trial that provided randomized subjects with supplements of beta carotene and vitamin A, was stopped after researchers discovered that subjects receiving beta carotene had a 46% higher risk of dying from lung cancer (32). The IOM states that "B-carotene supplements are not advisable for the general population", although they also state that this advice "does not pertain to the possible use of supplemental B-carotene as a provitamin A source for the prevention of vitamin A deficiency in populations with inadequate vitamin A nutriture" (13).

What is the health risk of too much vitamin A?
Hypervitaminosis A refers to high storage levels of vitamin A in the body that can lead to toxic symptoms. Toxicity can result in dry, itchy skin, headache, fatigue, hair loss, loss of appetite, vomiting, and liver damage. When toxic symptoms arise suddenly, which can happen after consuming very large amounts of vitamin A over a short period of time, signs of toxicity include dizziness, blurred vision, and muscular uncoordination (6-8, 33, 34).

Although hypervitaminosis A can occur when very large amounts of liver are regularly consumed, most cases of vitamin A toxicity result from an excess intake of vitamin A in supplements. A generally recognized safe upper limit of intake for vitamin A from diet and supplements is 1,600 to 2,000 RE (8,000 to 10,000 IU) per day (33, 35). The Institute of Medicine is currently reviewing the scientific literature on vitamin A. They are considering revising the RDAs and establishing an Upper Limit (UL) of safe intake for vitamin A.

Vitamin A toxicity also can cause severe birth defects. Women of child-bearing age are advised to limit their total daily intake of vitamin A (retinol) from foods and supplements combined to no more than 1,600 RE (8,000 IU) per day.

Retinoids are compounds that are chemically similar to vitamin A. Over the past 15 years, synthetic retinoids have been prescribed for acne, psoriasis, and other skin disorders (36). Isotretinoin (Roaccutane® or Accutane®) is considered an effective anti-acne therapy. At very high doses, however, it can be toxic, which is why this medication is usually saved for the most severe forms of acne (37). The most serious consequence of this medication is birth defects.

It is extremely important for sexually active females who may become pregnant and who take these medications to use an effective method of birth control. Women of childbearing age who take these medications are advised to undergo monthly pregnancy tests to make sure they are not pregnant.

What is the health risk of too many carotenoids?
Nutrient toxicity traditionally refers to adverse health effects from a high intake of a particular vitamin or mineral. For example, large amounts of the active form of vitamin A (naturally found in animal foods such as liver but also available in dietary supplements) can cause birth defects.

Provitamin A carotenoids such as beta-carotene are generally considered safe because they are not traditionally associated with specific adverse health effects. The conversion of provitamin A carotenoids to vitamin A decreases when body stores are full, which naturally limits further increases in storage levels. A high intake of provitamin A carotenoids can turn the skin yellow, but this is not considered dangerous to health (38).

Recent clinical trials that suggested a greater incidence of lung cancer and total mortality (death) in current smokers who supplemented their diet with 20 mg of beta-carotene per day have raised concern about the safety of beta-carotene supplements. However, conflicting studies make it difficult to interpret the health risk. For example, the Physicians' Health Study compared the effects of taking 50 mg beta-carotene every other day to a placebo (sugar pill) in over 22,000 male physicians and found no adverse health effects (39). Also, a trial that tested the ability of four different nutrient combinations to inhibit the development of esophageal and gastric cancers in 30,000 men and women in China suggested that after 5 years those participants who took a combination of beta-carotene, selenium and vitamin E had a 13% reduction in cancer deaths (40). One point to consider is that there may be a relationship between alcohol and beta-carotene because "only those men who consumed more than 11 g per day of alcohol (approximately one drink per day) showed an adverse response to B-carotene supplementation" in the lung cancer trial (13).

The Institute of Medicine did not set a Tolerable Upper Intake Level (UL), the highest level of daily nutrient intake that is likely to pose no risk of adverse health effects, for B-carotene or carotenoids. Instead, they concluded that B-carotene supplements are not advisable for the general population. As stated earlier, however, they may be appropriate as a provitamin A source or for the prevention of vitamin A deficiency in specific populations (13).

Selected Food Sources of vitamin A

As the 2000 Dietary Guidelines for Americans state, "Different foods contain different nutrients. No single food can supply all the nutrients in the amounts you need" (41). The following tables suggest dietary sources of vitamin A and provitamin A carotenoids. As the tables indicate, liver, eggs and whole milk are good animal sources of vitamin A. Many orange fruits and green vegetables are good sources of provitamin A carotenoids. Including these foods in your daily diet will help you meet your daily need for vitamin A. In addition, food manufacturers fortify a wide range of products with vitamin A. Breakfast cereals, pastries, breads, crackers, cereal grain bars and other foods may be fortified with 10% to 15% of the DV for vitamin A. It is important to read the nutrition facts panel of the food label to determine whether a food provides vitamin A.

If you want more information about building a healthful diet, refer to the Dietary Guidelines for Americans and the Food Guide Pyramid.

 

Table of Selected Animal Sources of vitamin A (18)

 Food IU/International Unit %DV *
Liver, beef, cooked, 3 oz 30,325 610
Liver, chicken, cooked, 3 oz 13,920 280
Egg substitute, fortified, 1/4 c 1355 25
Fat free milk, fortified w/ vitamin A, 1 c 500

10

Cheese pizza, 1/8 of a 12" diameter 380 8
Milk, whole, 3.25% fat, 1 c 305 6
Cheddar cheese, 1 oz 300 6
Whole egg, 1 medium 280 6
Swiss cheese, 1 oz 240 4
Margarine, soft, corn oil, 1 tsp 165 4
Yogurt, fruit flavored, low fat, 1 c 120 2

* DV = Daily Value. DVs are reference numbers based on the Recommended Dietary Allowance (RDA). They were developed to help consumers determine if a food contains a lot or a little of a specific nutrient. The DV for vitamin A is 5,000 IU (1,000 RE). The percent DV (%DV) listed on the nutrition facts panel of food labels tells adults what percentage of the DV is provided by one serving. Percent DVs are based on a 2,000-calorie diet. Your Daily Values may be higher or lower depending on your calorie needs. Foods that provide lower percentages of the DV will contribute to a healthful diet.

Table of Selected Plant Sources of vitamin A (from beta-carotene) (18)

Plant sources such as beta carotene are not as well absorbed as animal sources of vitamin A.

 Food IU/International Unit
%DV *

Carrot, 1 raw (7 1/2")

20,250

410

Carrots, boiled, 1/2 c slices

19,150

380

Carrot juice, canned, 1/2 c

12,915

260

Mango, raw, without refuse, 1 fruit

8,050

160

Sweet potatoes, 1/2 c Junior mashed

7,430

150

Spinach, boiled, 1/2 c

7,370

150

Cantaloupe, raw, 1 c cubes

5,160

100

Kale, boiled, 1/2 c

4,810

100

Vegetable soup, prepared with equal volume water, 1 c

3,005

60

Pepper, sweet, red, raw, 1/2 c sliced

2,620

50

Apricots, without skin, canned in water, 1/2 c halves

2,055

40

Spinach, raw, 1 cup

2,015

40

Broccoli, frozen, chopped, boiled, 1/2 c

1,740

35

Apricot nectar, canned, 1/2 c

1,650

30

Oatmeal, instant, fortified, low sodium, dry, 1 packet

1,050

20

Tomato juice, canned, 6 oz

1,010

20

Ready-to-eat cereal, fortified, 1 oz (15% fortification)

750

15

Peaches, canned, water pack, 1/2 c halves or slices

650

15

Peach, raw, 1 medium

525

10

Papaya, raw, 1 small

430

10

Orange, raw, 1 large

375

8

Asparagus, boiled, 4 spears

325

6

Tomato, red, ripe, raw, 1/2" thick slice

170

2

DV = Daily Value. DVs are reference numbers based on the Recommended Dietary Allowance (RDA). They were developed to help consumers determine if a food contains a lot or a little of a specific nutrient. The DV for vitamin A is 5,000 IU (1,000 RE). The percent DV (%DV) listed on the nutrition facts panel of food labels tells adults what percentage of the DV is provided by one serving. Percent DVs are based on a 2,000-calorie diet. Your Daily Values may be higher or lower depending on your calorie needs. Foods that provide lower percentages of the DV will contribute to a healthful diet.


Vidyya. Home |  Ex Libris |  Vidyya  | 
Subscription Information |  About Vidyya |  Vidyya Archives | 

Editor: Susan K. Boyer, RN
© Vidyya. All rights reserved.