Vitamin A is fat-soluble and, in its natural and synthetic forms, is referred to as retinoids. In the diet, these are pro-Vitamin As, from plants (carotinoids) and from animals (retinyl esters) and when absorbed are automatically bioconverted to it most active (reduced form), alpha-retinol. Alpha-Carotene, beta-carotene, and some xanthenes (cryptoxanthin etc.) either dietary or supplemental forms are transformed easily to alpha-retinol. There are rate-limiting steps that prevent the body from making too much of the active vitamin A, (alpha-retinol). Although, if too much of the precursors (carotenoids) are consumed, the benign carotenemia colors the individual. They can biologically use this excessive amount for months if they are cut off from this essential substance. Vitamin A plays a healthy role in the immune system (cellular and humeral), bones, vision, teeth, reproduction and improves tissue repair.
Clinical application of mega doses of Vitamin A or oral retinoids or carotenoids is used for prevention or treatment of Age Related Macular Degeneration, Night Blindness, Acne, Viral infections, and following trauma (surgery, strokes, heart attacks and infections) for enhanced healing. Although in 50 years of doctoring, neither I nor any of my colleagues have ever seen a case of an overdose, called hypervitaminosis A, many “real doctors” overly caution their patients of this possibility. Older age, chronic liver problems, protein malnutrition, and kidney disease may predispose one to this problem. A person who consumes too much alcohol, who has some liver problem and takes very large doses of beta-carotene may be also disposed in that there is an interaction between ethanol and this substance. Also, along with the Omega 3s, which are found in cod liver oil, large amounts of Vitamin A along with D also are present. A cheap source of Omega 3s and Vitamin D is Cod and other light flesh fish liver oils. Folks who consume large amounts of this may have toxicity. However, Vitamin A is not in other fish oil supplements in appreciable amounts. Many commercial foods are fortified with Vitamin A, but not enough to cause problems.
Toxicity includes laboratory abnormalities such as elevated liver enzymes, high serum calcium, and elevated lipids. Also GI symptoms are abdominal pain, loss of appetite, diarrhea, and liver failure. The bones become painful and brittle and the teeth fall out. Neurologically, headache, weakness, confusion and even coma have been described. Toxicity can affect the skin (dry, itching, pealing), hair (loss) and nails (brittle). Acute toxicity can occur in taking more than 500,000 IU one time. Daily intake of 25,000 IU for a month or, if predisposing factors are present, as little as 8,000IU a day can cause problems.
The RDA is 3,000 IU for males, and 2,310 for females. The official UL (Upper intake Level) is set at set at 10,000 IU daily for both sexes to prevent toxicity. Clinically, I recommend 100,000 IU for no longer than 8 days for acute infections and after surgery or trauma. For long-term treatment such as skin conditions (acne, psoriasis) 15,000 to 25,000 watching carefully for symptoms or lab abnormalities every month or so. Low serum levels (less than 45 mcg%) are associated with an increased incidence of cancer and Alzheimers as well as an increased mortality in AIDS. Vitamin A comes usually in 8,000 or 25,000 IU. One mcg equals 6 IU of alpha or beta-carotene, 10 of plant carotenoids, 4.1 from dairy product and 3.33 in animal and other fortified foods. Absorption is slightly better if taken with other fats and not inhibited when combined with other supplements or foods.