Vitamins: Fat-Soluble & Water-Soluble Types, Deficiencies

Vitamins are organic compounds needed in tiny amounts that the body cannot make (or cannot make enough of), so they must come from food. They split into two families: fat-soluble vitamins (A, D, E, K), which dissolve in fat, are stored in the liver and adipose tissue, and can accumulate to toxic levels; and water-soluble vitamins (the B-complex and C), which are not stored and need regular replenishment.

This matters because deficiencies remain widespread — vitamin D deficiency affects a large share of Indians despite abundant sunshine, B12 deficiency is common in vegetarian diets, and vitamin A deficiency still threatens children's eyesight. Each vitamin's deficiency produces a textbook disease, and recognising the pattern is classic clinical medicine.

Key Concepts

• Vitamin A (retinol): needed for vision, immunity, and epithelia. Deficiency causes night blindness, then xerophthalmia and corneal damage. Excess causes liver damage and birth defects.
• Vitamin D: made in skin from sunlight, activated by liver then kidney; controls calcium absorption. Deficiency causes rickets in children and osteomalacia in adults.
• Vitamin E: a fat-soluble antioxidant protecting membranes; rare deficiency causes nerve damage.
• Vitamin K: activates clotting factors II, VII, IX, and X; deficiency causes bleeding. Newborns get a vitamin K shot because their gut bacteria are not yet making it.
• B1 (thiamine): cofactor for carbohydrate-metabolism enzymes; deficiency causes beriberi and Wernicke's encephalopathy.
• B3 (niacin): precursor of NAD⁺ and NADP⁺; deficiency causes pellagra — the 4 Ds: diarrhoea, dermatitis, dementia, death.
• B9 (folate) and B12 (cobalamin): both needed for DNA synthesis; deficiency causes megaloblastic anaemia. B12 needs intrinsic factor for absorption, and its deficiency uniquely damages the spinal cord.
• Vitamin C: needed for collagen synthesis and iron absorption; deficiency causes scurvy — bleeding gums, poor wound healing. Humans cannot synthesise it.

Formulas

Vitamin D unit conversion: 1 microgram (µg) of vitamin D = 40 IU (international units). So: dose in µg = dose in IU ÷ 40. Assumptions: vitamin D₃ (cholecalciferol) labelling; limits: IU definitions differ between vitamins, so this conversion applies only to vitamin D.

Worked Example

Decode a supplement label. The adult RDA is 600 IU per day. Step 1: 600 ÷ 40 = 15 µg. A common supplement provides 1000 IU. Step 2: 1000 ÷ 40 = 25 µg. Step 3: 1000 ÷ 600 ≈ 1.67 — about 1.7 times the daily requirement. A safe maintenance dose (toxicity needs sustained intakes far above this), but it shows why IU versus µg matters.

Common Misconceptions

• More vitamins are always better. Fat-soluble vitamins accumulate — excess A damages the liver and excess D dangerously raises blood calcium. Megadose B6 can damage nerves.
• Vitamin C prevents colds. Large trials show little prevention benefit in the general population; it may slightly shorten colds, which is not the same as preventing them.
• Only vegetarians get B12 deficiency. Pernicious anaemia (no intrinsic factor), stomach surgery, and long-term antacid use also block absorption regardless of diet.

Real-World Applications

• Antenatal folic acid prevents neural tube defects — the highest-impact vitamin intervention in medicine.
• Vitamin A mega-dose campaigns twice yearly protect millions of children from blindness.
• Vitamin D plus calcium treats rickets and postmenopausal bone loss.
• Newborn vitamin K injection prevents haemorrhagic disease of the newborn.

Vitamins divide into storable fat-soluble (A, D, E, K — effective but potentially toxic in excess) and non-stored water-soluble (B-complex, C — needing steady intake). Each deficiency draws a classic clinical picture, from night blindness to pellagra's 4 Ds to megaloblastic anaemia — and the right supplement at the right time prevents real disease.