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.
1 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. 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. 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.