Vitamins: Fat-Soluble vs Water-Soluble

Vitamins are needed in tiny amounts, milligrams or even micrograms a day, yet without them the body’s chemistry breaks down in dramatic, specific ways: bleeding gums, soft bones, night blindness, nerve damage. The word comes from “vital amine”, coined in 1912 by the biochemist Casimir Funk, who wrongly assumed they were all amines. The name stuck anyway. This guide explains what vitamins are chemically, why they split neatly into two groups, and what each one does.

What counts as a vitamin?

A vitamin is an organic compound that:

• is needed in small amounts for normal metabolism;
• cannot be made by the body at all, or not in sufficient amounts;
• therefore must come from the diet (or, in a couple of cases, from sunlight or gut bacteria).

Vitamins are not a chemical family. They’re a varied collection of molecules grouped by what our bodies need. A compound can be a vitamin for one species but not another: most animals make their own vitamin C, but humans, other apes, guinea pigs and fruit bats can’t.

The key chemical divide: polarity

Vitamins are classified by their solubility, which depends on their polarity (see polar vs non-polar molecules):

• Water-soluble vitamins have many polar groups (–OH, –COOH, –NH₂, charged groups) that form hydrogen bonds with water.
• Fat-soluble vitamins are mostly hydrocarbon chains and rings, with few polar groups, so they dissolve in lipids instead (see lipids explained).

This one chemical difference decides almost everything about how the body handles them.

Water-soluble vitamins: B group and C

Vitamin	Chemical name(s)	Main role	Deficiency
B1	Thiamine	Coenzyme (TPP) for breaking C–C bonds in carbohydrate metabolism	Beriberi
B2	Riboflavin	Part of FAD and FMN (electron carriers)	Cracked lips, sore tongue
B3	Niacin	Part of NAD⁺ and NADP⁺	Pellagra
B5	Pantothenic acid	Part of coenzyme A	Rare
B6	Pyridoxine	Coenzyme (PLP) for amino acid metabolism	Anaemia, nerve problems
B7	Biotin	Coenzyme for carboxylation (adding CO₂)	Rare; hair loss, rash
B9	Folate (folic acid)	One-carbon transfers; making DNA bases	Anaemia; neural tube defects in pregnancy
B12	Cobalamin	Methyl transfers; contains cobalt	Pernicious anaemia, nerve damage
C	Ascorbic acid	Antioxidant; collagen synthesis; iron absorption	Scurvy

Most B vitamins work as coenzymes: the body builds them into molecules that help enzymes carry electrons or chemical groups (see cofactors and coenzymes). Vitamin C is a reducing agent that keeps certain enzymes’ iron in the right oxidation state (see vitamin C chemistry).

Properties that follow from being water-soluble:

• They dissolve in blood plasma and travel freely.
• The body stores little of most of them (B12 is the notable exception, stored in the liver for years).
• Excess is usually excreted in urine, so toxicity is uncommon, though very high doses of some (such as B6) can cause harm.
• They need to be eaten regularly.
• They’re easily lost in cooking: they leach into cooking water, and some (especially C and folate) are destroyed by heat and oxygen. Steaming, microwaving and using the cooking water in sauces help preserve them.

Fat-soluble vitamins: A, D, E and K

Vitamin	Chemical name(s)	Main role	Deficiency
A	Retinol, retinal, retinoic acid; from β-carotene	Vision (retinal), cell growth, immune function	Night blindness; blindness in severe cases
D	Cholecalciferol (D3), ergocalciferol (D2)	Hormone controlling calcium absorption	Rickets (children), osteomalacia (adults)
E	Tocopherols	Antioxidant protecting membranes	Rare; nerve damage
K	Phylloquinone (K1), menaquinones (K2)	Activating blood-clotting proteins	Bleeding

Properties that follow from being fat-soluble:

• They’re absorbed with dietary fat, packaged into lipoproteins (bile is needed; see lipids explained). People who can’t absorb fat properly can become deficient.
• They’re stored in the liver and fatty tissue, sometimes for months.
• Because they’re stored, they don’t need to be eaten every day, but excessive intake can build up to toxic levels. Too much vitamin A (from supplements or large amounts of liver) can damage the liver and cause birth defects; too much vitamin D raises blood calcium dangerously.
• They’re more stable to cooking in water, though some are sensitive to light and oxidation.

A closer look at some vitamins

Vitamin A and vision

Vitamin A (retinol) is converted to retinal, which sits in light-sensitive proteins in the eye. When light strikes it, one C=C double bond flips from cis to trans, changing the molecule’s shape and triggering a nerve signal. That single chemical change is the first step in seeing. Plants provide β-carotene, an orange pigment that the body splits into two vitamin A molecules; that’s why carrots are linked with good eyesight.

Vitamin D: really a hormone

Your skin makes vitamin D when ultraviolet light breaks open one ring of a cholesterol derivative. The liver and kidneys then convert it into an active hormone that tells the gut to absorb calcium. See vitamin D chemistry.

Vitamin E: a membrane antioxidant

Vitamin E has a long hydrocarbon tail that anchors it in cell membranes and a phenol ring that can donate a hydrogen atom to a free radical, stopping a chain reaction that would otherwise damage the unsaturated fatty acids in membranes (see fatty acids).

Vitamin K and clotting

Several clotting proteins only work after an enzyme adds extra carboxyl groups to some of their glutamate side chains, letting them bind calcium. That enzyme needs vitamin K. The anticoagulant drug warfarin works by blocking the recycling of vitamin K, which is why patients on warfarin are advised to keep their vitamin K intake steady.

Vitamin B12: a vitamin with a metal

B12 has one of the most complex structures of any vitamin, with a cobalt ion at the centre of a ring similar to the porphyrin in haem. Its structure was determined by X-ray crystallography by Dorothy Hodgkin, who received the 1964 Nobel Prize in Chemistry. B12 is made only by microorganisms, so it’s found naturally only in animal foods; people on vegan diets need fortified foods or supplements.

How vitamins were discovered

Many vitamins were discovered through the diseases caused by their absence:

• In 1747, the Scottish naval surgeon James Lind showed in one of the first controlled trials that citrus fruit cured scurvy in sailors.
• In the late nineteenth century, Christiaan Eijkman found that chickens fed polished white rice developed a beriberi-like disease, cured by the rice bran; the missing factor was thiamine.
• Rickets was once common in industrial cities with smoky skies; cod liver oil and sunlight were found to prevent it.

Common misconceptions

• “More vitamins are always better.” Beyond meeting needs, extra water-soluble vitamins are mostly excreted, and extra fat-soluble ones can be toxic.
• “Natural vitamins are chemically different from synthetic ones.” A molecule is the same whatever its source; ascorbic acid from a factory is identical to that in an orange.
• “Vitamins give you energy.” They contain no usable energy. They help enzymes release energy from food.
• “All fat-soluble vitamins come from fatty foods only.” Vitamin D is made in skin, and vitamin K is found in green leafy vegetables and made by gut bacteria.

Key takeaways

• Vitamins are organic compounds needed in small amounts that the body can’t make (enough of).
• Water-soluble vitamins (B group, C) are polar, not stored much, excreted in urine, and easily lost in cooking; most B vitamins act as coenzymes.
• Fat-soluble vitamins (A, D, E, K) are non-polar, absorbed with fat, stored in the body, and can be toxic in excess.
• Each vitamin has specific jobs, and deficiencies cause specific diseases such as scurvy, rickets, beriberi and pellagra.
• For the minerals that work alongside vitamins, see elements in the human body.