Vitamins Explained: What Each One Does
From vitamin A to K, a plain-language guide to what vitamins do, where to find them, and why balance beats megadoses.
What Are Vitamins?
Vitamins are organic compounds — meaning they contain carbon and come originally from living things — that the body requires in small amounts to function normally. Unlike carbohydrates, proteins, and fats, vitamins do not provide energy directly. Instead, they act as essential helpers in biochemical reactions: enabling enzymes to work, building tissues, protecting cells, and coordinating processes throughout the body.
There are 13 vitamins currently recognised as essential for human health. The body cannot synthesise most of them in sufficient quantities, so they must be obtained through food. A few exceptions exist — vitamin D can be made in the skin using sunlight, and the gut microbiome produces some vitamin K — but for most vitamins, diet is the primary source.
Fat-Soluble vs Water-Soluble Vitamins
Vitamins are categorised into two groups based on how they dissolve and how the body handles them:
- Fat-soluble vitamins (A, D, E, K): Absorbed along with dietary fat in the small intestine and transported in the lymphatic system before reaching the bloodstream. They can be stored in the liver and fat tissue for weeks or months. This storage is useful — it provides a buffer against temporary dietary shortfalls — but it also means excessive intake can lead to toxic accumulation.
- Water-soluble vitamins (vitamin C and the eight B vitamins): Absorbed directly into the bloodstream. The body uses what it needs and excretes any surplus in urine relatively quickly, which is why toxicity from water-soluble vitamins is rare (though not impossible at very high doses). The flip side is that regular dietary intake matters more, as these vitamins are not stored to the same extent.
The Fat-Soluble Vitamins
Vitamin A
Vitamin A is essential for vision (particularly in low light), immune function, and cell growth. The active form in the body is retinol, found in animal foods like liver, dairy, and eggs. Plants provide beta-carotene and other carotenoids, which the body converts to retinol as needed. Deficiency causes night blindness and, in severe cases, total vision loss — it remains a leading cause of preventable blindness in some parts of the world.
Vitamin D
Vitamin D is unique in that the body can produce it when skin is exposed to UVB sunlight. It is essential for calcium absorption and bone mineralisation — without adequate vitamin D, calcium from food cannot be properly absorbed, leading to soft or fragile bones. Vitamin D also has roles in immune function and muscle strength. Food sources include oily fish, egg yolks, and fortified foods, but for many people at higher latitudes, sunlight exposure and/or supplements are important. This is a topic worth discussing with a doctor or dietitian if you have concerns.
Vitamin E
Vitamin E is a group of related antioxidant compounds, the most active of which is alpha-tocopherol. Antioxidants neutralise free radicals — reactive molecules that can damage cell membranes, proteins, and DNA. Vitamin E is found in vegetable oils, nuts, seeds, and green leafy vegetables. Deficiency is rare in healthy adults but can cause nerve and muscle damage in people with fat absorption disorders.
Vitamin K
Vitamin K is essential for blood clotting — several clotting factors cannot be activated without it. It also plays a role in bone metabolism. There are two main dietary forms: K1 (found in leafy green vegetables) and K2 (found in fermented foods and some animal products). Deficiency leads to impaired clotting, which can cause excessive bleeding.
The Water-Soluble Vitamins
| Vitamin | Main Role(s) | Rich Food Sources |
|---|---|---|
| Vitamin C | Antioxidant; collagen synthesis; immune support | Citrus fruit, berries, peppers, kiwi, broccoli |
| B1 (Thiamine) | Energy metabolism (carbohydrates) | Wholegrains, pork, legumes, nuts |
| B2 (Riboflavin) | Energy metabolism; cell growth | Dairy, eggs, lean meat, leafy greens |
| B3 (Niacin) | Energy metabolism; DNA repair | Meat, fish, poultry, wholegrains |
| B5 (Pantothenic acid) | Synthesises coenzyme A (energy and fat metabolism) | Widespread; meat, avocado, wholegrains |
| B6 (Pyridoxine) | Protein metabolism; neurotransmitter synthesis | Poultry, fish, potatoes, bananas |
| B7 (Biotin) | Fat and carbohydrate metabolism | Eggs, nuts, seeds, liver |
| B9 (Folate) | DNA synthesis; cell division; foetal development | Leafy greens, legumes, fortified grains |
| B12 (Cobalamin) | Nerve function; red blood cell formation; DNA synthesis | Meat, fish, dairy, eggs; fortified foods for vegans |
Vitamin C
Vitamin C (ascorbic acid) is essential for synthesising collagen — the structural protein found in skin, tendons, blood vessels, and connective tissue throughout the body. It is also a potent antioxidant and supports immune function. Severe deficiency causes scurvy, historically significant among sailors on long voyages without fresh food. The body cannot make vitamin C at all; it must come from food. Most fruits and vegetables are good sources.
The B Vitamins
The B vitamins are a group of eight chemically distinct compounds that share a general role in cell metabolism — they help convert food into usable energy by acting as coenzymes. Several have additional specific roles. Folate (B9) is critical in early pregnancy for neural tube development. Vitamin B12 is essential for the myelin sheaths that protect nerve fibres and for producing healthy red blood cells; it is found almost exclusively in animal products, making it a particular consideration for those eating a vegan diet.
Our mineral functions post covers the dietary minerals that work alongside vitamins in these metabolic pathways.
Vitamin Deficiency: Signs and Causes
Most people in countries with access to a varied diet do not develop serious vitamin deficiency. However, some groups have higher needs or reduced ability to obtain or absorb specific vitamins:
- Older adults may absorb B12 less efficiently due to reduced stomach acid production.
- People with limited sun exposure are at higher risk of vitamin D insufficiency.
- Those eating plant-exclusive diets need to plan carefully for B12, and potentially for D, iodine, and omega-3 fatty acids.
- People with conditions affecting fat absorption (such as coeliac disease or Crohn's disease) may struggle to absorb fat-soluble vitamins.
- Pregnancy significantly increases needs for folate, iron, and vitamin D among others.
If you are concerned about vitamin deficiency for any reason, a blood test can often give a clear picture and a healthcare professional or registered dietitian can advise on appropriate action. Do not attempt to self-diagnose or self-treat significant suspected deficiencies with supplements without guidance.
Do You Need Supplements?
For most healthy adults eating a reasonably varied diet, supplements are not necessary. Food provides vitamins in a natural matrix of other nutrients that support absorption and use; the same benefit does not always apply to isolated supplements.
There are some situations where supplementation is evidence-supported: vitamin D for those with limited sun exposure, folate (B9) for those who could become pregnant, B12 for those eating a vegan diet, and iron for those diagnosed with deficiency. High-dose supplements of fat-soluble vitamins — particularly A and D — can cause toxicity if taken in excess, so megadosing without medical oversight carries real risks.
The best source of detailed, personalised nutritional guidance is a registered dietitian. For general vitamin reference information, you can also use our vitamin reference tool or explore nutrition topics further in our vitamins and minerals guide.
Getting Vitamins Through Food
A diet centred on varied vegetables, fruits, wholegrains, legumes, nuts, and moderate amounts of lean protein and dairy (or fortified plant alternatives) covers the needs of most vitamins for most healthy people. No single food contains all vitamins in adequate amounts — variety is genuinely important here.
Cooking methods affect vitamin content. Water-soluble vitamins (especially C and some B vitamins) can be lost when food is boiled in water. Steaming, roasting, or eating foods raw where appropriate helps preserve these vitamins. Fat-soluble vitamins are generally more stable to heat. Our nutrition basics guide covers broader dietary principles for all essential nutrients.