Vitamins

Vitamins are classified by solubility, and that single property governs how the body stores and excretes them. Vitamin C (ascorbic acid) carries several hydroxyl groups and is highly polar, making it water-soluble; the body cannot store appreciable amounts, so any excess is filtered by the kidneys and lost in urine, which is why it must be supplied regularly in the diet. Vitamin A (retinol) has a long hydrophobic isoprenoid chain and is fat-soluble, so it dissolves in body lipids and is stored in tissues such as the liver, allowing harmful accumulation when intake is excessive. This solubility contrast directly explains the two fates. Calling vitamin C a protein and vitamin A a carbohydrate misidentifies their chemical nature. The acid-base description is misleading; although vitamin C is mildly acidic, that is not the basis of its rapid excretion. Both vitamins must be obtained from diet, so endogenous synthesis is not the distinction. This is the standard NCERT classification of fat-soluble and water-soluble vitamins. As a check, the other water-soluble B-group vitamins are likewise not stored, while fat-soluble D, E and K can also accumulate, consistent with the solubility rule.