Vitamin A, a fat-soluble vitamin, is essential for growth, for the development and maintenance of epithelial tissue, and for vision, particularly in dim light.
Vitamin D3 (colecalciferol) compounds are fat-soluble sterols, which are essential for the proper regulation of calcium and phosphate homoeostasis and bone mineralisation.
Thiamine (Vitamin B1), a water-soluble vitamin, is an essential coenzyme for carbohydrate metabolism. Thiamine is used in the treatment and prevention of thiamine deficiency.
Riboflavin (Vitamin B2), a water-soluble vitamin, is essential for the utilisation of energy from food. The active, phosphorylated forms, flavine mononucleotide (FMN) and flavine adenine dinucleotide (FAD), are involved as coenzymes in oxidative/reductive metabolic reactions. Riboflavin is also necessary for the functioning of pyridoxine and nicotinic acid.
Nicotinic acid and nicotinamide (Vitamin B3), are water-soluble vitamin B substances which are converted to nicotinamide adenine dinucleotide and nicotinamide adenine dinucleotide phosphate (NADP). These coenzymes are involved in electron transfer reactions in the respiratory chain.
Ascorbic Acid (Vitamin C), a water-soluble vitamin, is essential for the synthesis of callogen and intercellular material.
Vitamin A is readily absorbed from the gastrointestinal tract. Vitamin A esters are hydrolysed by pancreatic enzymes to retinol which is then absorbed and re-esterified. Some retinol is stored in the liver. It is released from the liver bound to a specific a1-globulin (retinol-binding protein) in the blood. The retinol not stored in the liver undergoes glucuronide conjugation and subsequent oxidation to retinal and retinoic acid; these and other metabolites are excreted in urine and faeces.
Vitamin D substances are well absorbed from the gastrointestinal tract. The presence of bile is essential for adequate intestinal absorption; absorption may be decreased in patients with decreased fat absorption. Vitamin D compounds and their metabolites are excreted mainly in the bile and faeces with small amounts appearing in urine; there is some enterohepatic recycling but it is considered to have a negligible contribution to Vitamin D status. Certain vitamin D substances may be distributed into breast milk.