Vitamins: Classification and Overview

Think about a factory that builds hundreds of products every day. The machines do the heavy lifting, but without a specific set of small tools, none of those machines can run. Vitamins work the same way inside your body. They are organic compounds that the body needs in tiny amounts to carry out essential chemical reactions, yet it cannot manufacture most of them on its own. That means your diet is the only reliable supply line.

This topic introduces the full lineup of vitamins, sorts them into two major families, and gives you a ready-reference table connecting each vitamin to what it does, where you find it, and what goes wrong when you do not get enough of it.

What Are Vitamins?

Vitamins are organic molecules (containing carbon) that the body requires in very small quantities for normal growth, metabolism, and overall health. Unlike macronutrients such as carbohydrates, fats, and proteins that supply energy in bulk, vitamins act mainly as coenzymes (helper molecules that assist enzymes in carrying out biochemical reactions) or as antioxidants (substances that neutralise harmful reactive molecules).

Two key points to remember:

• The body cannot synthesise most vitamins, so they must come from food. The notable exception is Vitamin D, which the skin can produce with the help of sunlight.
• A missing vitamin does not just slow things down; it causes a specific disease. Each vitamin deficiency has a signature condition, from scurvy (Vitamin C) to beri-beri (Vitamin $B_1$), which makes vitamins a favourite topic in competitive examinations.

Two Families: Water-Soluble and Fat-Soluble

Vitamins split neatly into two groups based on whether they dissolve in water or in fat. This single property affects how the body absorbs, transports, stores, and excretes them.

Water-Soluble Vitamins

These dissolve in water and travel freely through the bloodstream. The body does not store them in large amounts, so you need a regular daily intake. Any excess is simply flushed out through urine. This group includes:

• Vitamin C (ascorbic acid)
• The B-complex family, a set of eight distinct vitamins that share a common numbering system:
  • $B_1$ : Thiamine
  • $B_2$ : Riboflavin
  • $B_3$ : Niacin
  • $B_5$ : Pantothenic acid
  • $B_6$ : Pyridoxine
  • $B_7$ : Biotin
  • $B_9$ : Folic acid
  • $B_{12}$ : Cyanocobalamin

Fat-Soluble Vitamins

These dissolve in fats and oils. The body absorbs them along with dietary fat in the intestine and can store them in the liver and fatty tissues for weeks or even months. Because they accumulate, both deficiency and excess can cause problems. The four fat-soluble vitamins are:

• Vitamin A : Retinol
• Vitamin D : Calciferol (with two sub-forms, cholecalciferol $D_3$ and ergocalciferol $D_2$)
• Vitamin E : Tocopherols
• Vitamin K : Phylloquinone (from plants) and menaquinones (from bacteria)

A handy way to remember the fat-soluble set: A, D, E, K.

The Master Vitamin Table

The table below is your single consolidated reference. It lists every vitamin alongside its chemical name, where to find it in food, what role it plays in the body, and which disease appears when it is missing.

Vitamin	Chemical Name	Key Dietary Sources	Biological Function	Deficiency Disease
A	Retinol	Fish liver oil, carrots, spinach, milk, papaya, mango	Supports vision and growth	Night blindness, xerophthalmia (dry eyes), keratinisation of skin
$B_1$	Thiamine	Yeast, milk, cereals, green vegetables, liver, pork	Coenzyme as TPP (thiamine pyrophosphate) in glycolysis	Beri-beri (peripheral nerve damage)
$B_2$	Riboflavin	Soybean, green vegetables, yeast, egg white, milk, liver, kidney	Coenzyme as FMN and FAD in redox reactions	Cheilosis (cracks and lesions at corners of mouth, lips, and tongue)
$B_3$	Niacin	Cereals, green leafy vegetables, liver, kidney	Coenzyme as $NAD$ and $NADP^+$ in redox reactions	Pellagra (photosensitive dermatitis)
$B_5$	Pantothenic acid	Mushrooms, avocado, egg yolk, sunflower oil	Part of coenzyme A; involved in carbohydrate, protein, and fat metabolism	Inadequate growth
$B_6$	Pyridoxine	Meat, cereals, milk, whole grains, eggs	Coenzyme in amino acid metabolism; helps form haeme in haemoglobin	Convulsions
$B_7$	Biotin	Liver, kidney, milk, egg yolk, vegetables, grains	Coenzyme in fatty acid biosynthesis	Depression, hair loss, muscle pain
$B_9$	Folic acid	Eggs, meat, beetroot, leafy vegetables, cereals, yeast	Nucleic acid synthesis; maturation of red blood cells	Megaloblastic anaemia
$B_{12}$	Cobalamin	Eggs, meat, fish	Coenzyme in amino acid metabolism; red blood cell maturation	Pernicious anaemia
C	Ascorbic acid	Citrus fruits (orange, lemon), tomato, amla, leafy vegetables	Antioxidant; essential for collagen synthesis	Scurvy (bleeding gums)
D	Cholecalciferol ($D_3$), Ergocalciferol ($D_2$)	Fish liver oil, milk, egg yolk; also synthesised via sunlight exposure	Promotes absorption and deposition of calcium in bones	Rickets (children), osteomalacia (adults)
E	Tocopherols	Cottonseed oil, sunflower oil, wheat germ oil, vegetable oils	Antioxidant; protects cell membranes from oxidative damage	Muscular dystrophy (muscular weakness) and neurological dysfunction
K	Phylloquinone and menaquinones	Green leafy vegetables, soybean oil, tomato	Essential for blood clotting	Increased clotting time, haemorrhagic diseases

Understanding the Key Terms

• Coenzyme : a small molecule that binds to an enzyme and helps it carry out a specific reaction. Many B vitamins work this way.
• TPP (thiamine pyrophosphate) : the active coenzyme form of $B_1$, used during glycolysis (the breakdown of glucose for energy).
• FMN (flavin mononucleotide) and FAD (flavin adenine dinucleotide) : the two coenzyme forms of $B_2$, both involved in oxidation-reduction (redox) reactions that transfer electrons.
• $NAD$ (nicotinamide adenine dinucleotide) and $NADP^+$ : coenzyme forms of $B_3$; they accept and donate electrons in metabolic pathways.
• Coenzyme A (CoA) : formed with the help of $B_5$; it is central to the metabolism of fats, carbohydrates, and proteins.
• Redox reactions : chemical reactions where one substance loses electrons (oxidation) and another gains them (reduction). Many energy-producing pathways in the body are redox reactions.
• Antioxidant : a molecule that neutralises free radicals (unstable, reactive molecules) and prevents them from damaging cells.
• Xerophthalmia : abnormal dryness of the eye surface (conjunctiva and cornea), caused by Vitamin A deficiency.
• Keratinisation : a process where skin cells become tough and hardened; excessive keratinisation from Vitamin A deficiency makes skin rough and dry.
• Cheilosis : painful cracking and inflammation at the corners of the mouth and on the lips, linked to riboflavin deficiency.
• Pellagra : a disease caused by niacin deficiency, named from the Italian words for “rough skin.”
• Megaloblastic anaemia : a blood disorder where the bone marrow produces abnormally large, immature red blood cells that cannot carry oxygen effectively.
• Pernicious anaemia : a severe form of anaemia caused by the body’s inability to absorb Vitamin $B_{12}$, often due to a lack of intrinsic factor in the stomach.
• Haemorrhagic diseases : conditions involving excessive or uncontrolled bleeding, resulting from impaired blood clotting.