Estimated mean requirement and safe level of intake for vitamin A, in Retinol Equivalents (RE) (1 RE = 1 mcg. of retinolor 6 mcg. of beta-carotene):

• Adolescents, 10–18 years: 330–400 (mcg RE/day, mean requirement), 600 (mcg RE/day, recommended safe intake)
• Adult females, 19–65 years: 270 (mcg RE/day, mean requirement), 500 (mcg RE/day, recommended safe intake)
• Adult females, 65+ years: 300 (mcg RE/day, mean requirement), 600 (mcg RE/day, recommended safe intake)
• Males: 300 (mcg RE/day, mean requirement), 600 (mcg RE/day, recommended safe intake)

Food sources, examples (RE per 100 grams).

Fresh fruits and raw and cooked fruit vegetables:

• Mango - 523
• Apricots - 260 (dry - 730)
• Cantaloup 322
• Squash, cooked 714
• Red pepper, raw - 580

Root (tubers) and green vegetables, flowers:

• Carrots, raw - 2574
• Sweet potato, cooked - 2180
• Dandelion, raw - 1400
• Spinach, raw - 674

Seaweed:

• Spirulina - 28333
• Nori - 4895

The most frequently occurring clinical indicator ofvitamin A deficiency is night-blindness, which is the earliest manifestation of xerophthalmia. 

In its mild form it is generally noticeable after stress from a bright light that bleaches the rhodopsin (visual purple) found in the retina. Vitamin A deficiency prolongs the time to regenerate rhodopsin, and thus delays adaptation time in dark environments.

Night-blind young children tend to stumble when going from bright to dimly-lit areas and they, as well as night-blind mothers, tend to remain inactive at dusk and at night.

Provitamin A carotenoids are found in green leafy vegetables (e.g. spinach, amaranth, and young leaves from various sources), yellow vegetables (e.g. pumpkins, squash, and carrots), and yellow and orange non-citrus fruits (e.g. mangoes, apricots, and papayas).

Red palm oil produced in several countries worldwide is especially rich in provitamin A.

Some other indigenous plants also may be unusually rich sources of provitamin A: 

• palm fruit known in Brazil as burití, found in areas along the Amazon River (as well as elsewhere in Latin America), 
• fruit known as gac in Viet Nam (used to colour rice). 

Vitamins are a group of substances that are needed for normal cell function, growth, and development. There are 13 essential vitamins:

• Vitamin A (retinol, retinal, 4 carotenoids)
• Vitamin C (ascorbic acid)
• Vitamin D (D3 - Cholecalciferol, D2 - Ergocalciferol)
• Vitamin E (tocopherols, tocotrienols)
• Vitamin K (phylloquinone, menaquinones)
• Vitamin B1 (thiamine)
• Vitamin B2 (riboflavin)
• Vitamin B3 (niacin)
• Vitamin B5 (pantothenic acid)
• Vitamin B6 (pyridoxine, pyridoxal, pyridoxamine)
• Vitamin B7 (biotin)
• Vitamin B9 (folic acid)
• Vitamin B12 (cyanocobalamin)

Fat-soluble vitamins are stored in the body's fatty tissue: vitamins A, D, E, K. 

Water-soluble vitamins - the body must use almost all water-soluble vitamins right away - vitamins B1, B2, B3, B5, B6, B7, B9 and C.
Vitamin B12 is the only water-soluble vitamin that can be stored in the liver for many years.

Retinoids retinol, retinal, and retinoic acid - 3 active forms of vitamin A - "preformed" vitamin A.

Beta carotene can easily be converted to vitamin A by the human body. 

Large amounts of supplemental vitamin A (but not beta carotene) can be harmful to bones.

Vitamin A keeps tissues and skin healthy, plays an important role in bone growth. Diets rich in the carotenoids alpha carotene and lycopene seem to lower lung cancer risk. Carotenoids act as antioxidants. Foods rich in the carotenoids lutein and zeaxanthin may protect against cataracts. Essential for vision lycopene may lower prostate cancer risk.

Recommended daily amount: 700 mcg - 900 mcg or 3 mg - 6 mg beta-carotene (~ 1 cup of raw cantaloupe or sweet red peppers, or 2 mangoes, or 1/5 of one baked sweet potato). 

Because the body converts all dietary sources of vitamin A into retinol, 1 mcg of physiologically available retinol is equivalent to the following amounts from dietary sources: 1 mcg of retinol, 12 mcg of beta-carotene, and 24 mcg of alpha-carotene or beta-cryptoxanthin. From dietary supplements, the body converts 2 mcg of beta-carotene to 1 mcg of retinol.

Carotenoids are a class of more than 750 pigments synthesized by plants, algae, and photosynthetic bacteria. These richly colored molecules are the sources of the yellow, orange, and red colors of many plants. Fruit and vegetables provide most of the 40 to 50 carotenoid phytonutrients found in the human diet.

The most common carotenoids in North American diets are α-carotene, β-carotene, β-cryptoxanthin, lutein, zeaxanthin, and lycopene. 

Provitamin A carotenoids - α-carotene, β-carotene, and β-cryptoxanthin - can be converted by the body to retinol (vitamin A), but not lutein, zeaxanthin, and lycopene. 

Dietary lutein and zeaxanthin help maintain optimal visual function - they absorb damaging blue light that enters the eye.

The results of observational studies suggest that diets high in carotenoid-rich fruit and vegetables are associated with reduced risks of cardiovascular disease and some cancers. But high-dose β-carotene supplements did not. 

Plant foods have an important preventive influence in a population at high risk for lung cancer. However, persons who use beta-carotene supplements do not benefit from the protective compounds in plant foods.