Beta-carotene is the pigment that gives carrots, sweet potatoes, and other yellow vegetables their characteristic coloring. This conjugated polyene also serves as a precursor that can be enzymatically converted into vitamin A in most animals and man. Although vitamin A is not present in any plants, the carotene precursor can be found in carrots, pumpkins, spinach, squash, watermelon, asparagus, broccoli, and cantaloupe. Liver is also a very good source for beta-carotene. Vitamin A (retinol) is an essential component for vision and it promotes bone growth, tooth development, and helps maintain healthy skin, hair, and mucous membranes. Deficiencies of vitamin A result in a number of maladies, including night blindness, dry skin, poor bone growth, weak tooth enamel, and weight loss.

Beta-carotene (provitamin A) is essential for vision, adequate growth, and tissue differentiation. Beta-carotene has excellent antioxidant properties that help neutralize free-radicals. Beta-carotene can quench singlet oxygen, a reactive molecule that is generated in the skin by exposure to ultraviolet light and which can induce pre-cancerous changes in the cells. Provitamin A is essential in the conversion of cholesterol into female estrogens and male androgens. Thyroxine, a thyroid hormone, stimulates the conversion of carotene into a usable nutrient and fat-splitting enzymes. Bile salts convert carotene. Individuals taking beta-carotene for extended periods of time should also supplement with vitamin E, as beta-carotene may reduce vitamin E levels. Factors that interfere with the absorption of provitamin A are strenuous physical activity performed within 4 hours of consumption, consumption of mineral oil, excessive consumption of alcohol, excessive consumption of iron, and use of cortisone and other drugs. Intake of polyunsaturated fatty acids with carotene result in rapid destruction of carotene unless antioxidants are present. Diabetics may not be able to convert carotene into provitamin A. An adequate supply of zinc is needed so that the liver can mobilize provitamin A out of its storage depots. Gastrointestinal and liver disorders, infections and any condition in which the bile duct is obstructed may limit the body's capacity to retain and use vitamin A. A low fat diet, resulting in little bile reaching the intestine, can cause vitamin A and carotene to be lost in the feces.

A deficiency of beta-carotene may result in night blindness; increased susceptibility to infections; loss of smell & appetite; frequent fatigue; lack of tearing; defective teeth & gums, retarded growth,brittle fingernails, cirrhosis of the liver, corneal ulcers, diarrhea, obstruction of the bile ducts, rough, dry, or prematurely aged skin, skin blemishes, softening of bones and teeth, sties in the eye, ulcerative colitis, xerosis.

50,000 IU's (provitamin A) daily may be toxic, 18,500 IU's (provitamin A) daily for 1-3 months are toxic for infants, 25,000 IU's (provitamin A) can produce liver damage. Large doses may lead to anemia, blurred vision, bone pain, diarrhea, fatigue, gout, hair loss, headaches, irregular periods, liver enlargement, nausea, vomiting. Prolonged excessive intake results in abnormalities in skin, eyes, and mucous membranes, blurred vision, bone fragility, deep bone pain, enlargement of liver and spleen, reduced thyroid activity, skin rashes, thickening of long bones. If toxicity is detected, the symptoms will disappear in a few days after the vitamin is withdrawn. (An International Unit (IU) of vitamin A is equivalent to 0.6 mcg of beta-carotene)