NB: 1 international unit (IU) = 0.3 retinol equivalents (RE); 1 RE = 1 µg retinol = 0.0035 µmol retinol.
The current RDA for men is set at 900 RE (3,000 IU).
Acute toxicity occurs when adults and children ingest > 300,000 IU and > 60,000 IU over hours to days.
Chronic toxicity occurs with daily intakes of more than 100,000 IU for more than 6 months, or more than 25 000 IU for more than 6 years.
Rodahl (1949) reported on the acute and chronic effects of vitamin A in rats, mice, guinea pigs, rabbits, dogs and chickens. "Within a few hors after single doses in the range 50-100 I.U. vitaimn A/gm body weight [166-332 mg/kg], aymptoms of acute intoxication occurred in the form of general malady, changes in the pelts, drwosiness, muscular weakness and decreased activity."
Penniston KL, Tanumihardjo SA. The acute and chronic toxic effects of vitamin A. Am J Clin Nutr. 2006 Feb;83(2):191-201 PMID 16469975.
Institute of Medicine, Food and Nutrition Board. Dietary reference intakes for vitamin A, vitamin K, arsenic, boron, chromium, copper, iodine, iron, molybdenum, nickel, silicon, vanadium, and zinc. Washington, DC: National Academy Press; 2001:65-126.
Rodahl, K. Hypervitaminosis A and scurvy. Nature 164 (1949) 531.