Encyclopedia of Diets - A Guide to Health and Nutrition

body cavities, and are more susceptible to infection.

People who get too much vitamin A have weaken

bones that tend to break easily and have a chronic

feeling of illness, including headache, nausea, irrita-

bility, fatigue, and muscle and joint pain. Women who

get too much vitamin A may have disrupted menstrual

cycles. Excess vitamin A can also cause birth defects in

a developing fetus.

Description

Vitamin A was the first fat-soluble vitamin to be

discovered. In 1913, two groups of American scientists

experimenting with animal feed almost simultaneously

discovered a substance essential to health that was

present in whole milk but absent in fat-free milk.

They called this ‘‘fat-soluble factor A,’’ later renamed

vitamin A. Today scientists know that vitamin A is

found in food that comes from both animal and

plants, is used by many systems in the body besides

vision, and comes in several different forms.

Vitamin A from animal sources

Vitamin A found in food that comes from animals

is in the form of a compound called retinol or pre-

formed vitamin A. Sometimes retinol is called ‘‘true’’

vitamin A because it can be used by the body without

any chemical changes. It can also be converted into

retinoic acid, a compound involved in the control of

gene expression. About 80% of the retinol in an indi-

vidual’s diet is absorbed by the body.

Good sources of retinol include beef or chicken

liver, whole eggs, whole milk, and cheese made with

whole milk. Some manufactured foods such as break-

fast cereals and fat-free milk are fortified with vitamin

A in the form of retinol.Dietary supplementsof vita-

min A and multivitamin tablets or capsules also con-

tain this form of vitamin A. Americans who eat meat

get about 70% of the vitamin A in their diet from

animal sources.

Vitamin A from plant sources

Vitamin A found in plants is called provitamin A

carotenoid. Provitamins cannot be directly used by the

body but can be chemically convert into usablevita-

mins.Carotenoidsare a family of more than 560 com-

pounds, some of which can be converted into retinol.

The carotenoids that can be converted into retinol by

humans are mainly bets-carotene, alpha-carotene, and

beta-cryptoxanthin. Of these, beta-carotene is con-

verted twice as efficiently as alpha-carotene or beta-

cryptoxanthin. However it takes 12 micrograms (mcg)

of beta-carotene to equal the activity of 1 mcg of

retinol. Carotenoids are found in yellow and orange

vegetables and in some deep green vegetables where

their orange color is not noticeable. Good sources of

provitamin A carotenoid include carrots, cantaloupe,

apricots, mango, papaya, spinach, and kale. Vegans

(people who do not eat any animal products) must be

especially careful to get enough of these vegetables.

Vitamin A’s role in health

Almost everyone living in the developed world

gets enough vitamin A to maintain health from their

normal diet. The same is not true in the developing

world where famine and limited food choices often

prevent individuals, especially children, from getting

enough vitamin A and other nutrients. When too little

Vitamin A

Recommended Tolerable Upper

Age Dietary Allowance Intake Level

Children 0–6 mos. 1,330 IU 400 RAE 2,000 IU 600 RAE

Children 7–12 mos. 1,670 IU 500 RAE 2,000 IU 600 RAE

Children 1–3 yrs. 1,000 IU 300 RAE 2,000 IU 600 RAE

Children 4–8 yrs. 1,330 IU 400 RAE 3,000 IU 900 RAE

Children 9–13 yrs. 2,000 IU 600 RAE 5,610 IU 1,700 RAE

Boys 14–18 yrs. 3,000 IU 900 RAE 9,240 IU 2,800 RAE

Girls 14–18 yrs. 2,310 IU 700 RAE 9,240 IU 2,800 RAE

Men 19 yrs. 3,000 IU 900 RAE 10,000 IU 3,000 RAE

Women 19 yrs. 2,310 IU 700 RAE 10,000 IU 3,000 RAE

Pregnant women

19  yrs. 2,500 IU 750 RAE 10,000 IU 3,000 RAE

Breastfeeding women

19  yrs. 4,300 IU 1,300 RAE 10,000 IU 3,000 RAE

Vitamin A

Food (retinol)

Beef liver, cooked, 3 oz. 27,185 IU

Chicken liver, cooked, 3 oz. 12,325 IU

Skim milk, vitamin A fortified, 1 cup 500 IU

Butter, 1 tbsp. 325 IU

Egg, 1 whole 300 IU

Whole milk cheddar cheese, 1 oz. 280 IU

Whole milk, 1 cup 250 IU

Vitamin A

Food (provitamin A carotenoid)

Spinach, cooked, ½ cup 11,460 IU

Kale, cooked, ½ cup 9,560 IU

Carrot, raw, unpeeled, 1 whole (7.5”) 8,670 IU

Cantaloupe, 1 cup 5,410 IU

Spinach, raw, 1 cup 2,800 IU

Papaya, 1 cup 1,530 IU

Carrot, raw, peeled, sliced, ½ cup 1,285 IU

Mango, 1 cup 1,260 IU

Tomato juice, 6 oz. 820 IU

Cereal, vitamin A fortified, 1 serving 500–770 IU

IU  International Unit

RAE  retinol activity equivalents

(Illustration by GGS Information Services/Thomson Gale.)

Vitamin A