cholecalciferol

Vitamin D is really a prohormone. Once sunlight on skin (or diet) supplies it, the liver and kidneys convert it to calcitriol, which binds vitamin D receptors found in almost every tissue in the body. There it governs how much calcium the gut absorbs for bone mineralisation, and — because immune cells carry the same receptors — it helps fine-tune both the innate and adaptive immune response. It's the nutrient most people in northern latitudes run short of through the darker months.

Find it in food: Oily fish, egg yolk, fortified foods — and sunlight on skin.

ascorbic acid

Vitamin C is a water-soluble antioxidant and an essential cofactor for the enzymes that assemble collagen — the protein scaffold of skin, blood vessels, cartilage and connective tissue. It regenerates other antioxidants such as vitamin E, supports the function of white blood cells, and dramatically increases absorption of non-heme (plant) iron. Humans are one of the few animals that can't synthesise it, so it must come from the diet daily.

Find it in food: Citrus, peppers, berries, kiwi, broccoli, leafy greens.

retinol / beta-carotene

Vitamin A forms the light-sensitive pigment in the retina that makes low-light vision possible, and it maintains the epithelial barriers — skin and the mucosal linings of the gut and airways — that act as a first line of immune defence. It also directs the growth and differentiation of cells throughout the body. Plant foods supply it as beta-carotene, which the body converts to active vitamin A on demand.

Find it in food: Liver, dairy, eggs; orange and dark-green vegetables (beta-carotene).

menaquinone, MK-7

Vitamin K activates the proteins that put calcium where it belongs: it switches on osteocalcin, which binds calcium into the bone matrix, and matrix-Gla protein, which helps keep calcium out of artery walls. The K2 (MK-7) form circulates far longer than K1, so it reaches tissues beyond the liver. It works hand-in-hand with vitamin D in bone metabolism.

Find it in food: Natto and fermented foods, hard cheeses, egg yolk, organ meats.

pyridoxine, P5P

In its active form (PLP), B6 is a cofactor for more than 100 enzymes — most of them handling protein and amino-acid metabolism. It's required to manufacture neurotransmitters including serotonin, dopamine and GABA, to build haemoglobin for oxygen transport, and to keep homocysteine within a healthy range.

Find it in food: Poultry, fish, potatoes, chickpeas, bananas, fortified cereals.

vitamin B9 / 5-MTHF

Folate donates the single-carbon units the body uses to build and repair DNA and to recycle homocysteine into methionine — a hub of the methylation reactions that regulate genes and neurotransmitters. Demand is highest in rapidly dividing cells, which is why it's essential for red blood cell formation and in early pregnancy. The 5-MTHF form is already active, bypassing a conversion step that a common genetic variant slows down.

Find it in food: Leafy greens, legumes, asparagus, liver, fortified grains.

cobalamin

B12 partners with folate in DNA synthesis and red blood cell formation, and it's essential for maintaining myelin — the insulating sheath that lets nerves transmit signals quickly. It occurs naturally almost only in animal foods, so plant-based diets are a frequent shortfall, and absorption declines with age.

Find it in food: Meat, fish, shellfish, eggs, dairy; fortified foods for vegans.

Magnesium is a cofactor for more than 300 enzyme systems — including every reaction that uses ATP, the cell's energy currency. It governs the contraction and relaxation of muscle (the heart included), nerve signalling, blood-glucose control and blood pressure. Refining and processing strip much of it from food, and it's one of the most commonly under-consumed minerals in modern diets.

Find it in food: Nuts, seeds, whole grains, legumes, leafy greens, dark chocolate.

Zinc is a structural and catalytic part of hundreds of enzymes and of the 'zinc-finger' proteins that switch genes on and off. It's essential for the development and function of immune cells, for wound healing and cell division, and even for taste and smell. The body has no dedicated zinc store, so a steady daily intake matters.

Find it in food: Oysters and shellfish, meat, legumes, seeds, whole grains.

Selenium is built into selenoproteins such as glutathione peroxidase — enzymes that neutralise oxidative damage — and into the enzymes that activate thyroid hormone. Because the selenium content of crops depends on the soil they grew in, dietary intake varies widely from region to region.

Find it in food: Brazil nuts, fish, eggs, whole grains, poultry.

Iodine is the raw material the thyroid uses to make the hormones that set your whole-body metabolic rate — and, in early life, brain development. Intake has slipped in many regions as people use less iodised salt and eat less dairy and sea fish, so a mild shortfall has quietly become common again.

Find it in food: Sea fish, dairy, eggs, seaweed, iodised salt.

Potassium balances sodium and helps keep blood pressure healthy, and it's behind every nerve and muscle signal. Almost everyone on a modern, processed diet falls short — but it's a nutrient to fix on your plate, not in a capsule: the amounts that matter are far larger than any pill, so we point you to the food instead.

Find it in food: Potatoes, beans and lentils, leafy greens, bananas, tomatoes, yoghurt.

EPA and DHA are long-chain omega-3 fats the body can only make from plant ALA inefficiently. DHA is a major structural component of brain and retinal cell membranes, while EPA is the precursor to specialised 'resolving' molecules that help bring inflammation to a close. Together they support normal heart function, healthy blood triglyceride levels and blood pressure already in the normal range.

Find it in food: Oily fish (salmon, sardines, mackerel, herring), algae oil.

Choline is an essential nutrient the body can't make in sufficient amounts. It's the building block of acetylcholine — a key messenger for memory and muscle — and of the phospholipids in every cell membrane, and it helps the liver export fat. Most adults sit below the recommended intake.

Find it in food: Eggs, liver, fish, soybeans, cruciferous vegetables.

Fibre is the part of plants you don't digest — but your gut bacteria do, fermenting it into short-chain fatty acids that feed the gut lining. It also slows sugar absorption, feeds a diverse microbiome and keeps things moving. Most people eat barely half the fibre a day that research links to better health.

Find it in food: Whole grains, legumes, vegetables, fruit, nuts and seeds.