Vitamins and Minerals Explained: What Each One Does and Why You Need It

Vitamins and minerals are the micronutrients that keep your body running. They enable everything from energy production and immune defence to bone formation, cognitive function and red blood cell synthesis. Unlike macronutrients (protein, carbohydrates and fats), your body needs them in relatively small amounts, but the consequences of falling short can be significant.

This reference guide covers every essential vitamin and the most important dietary minerals. For each one, we explain what it does, how to recognise a deficiency, the best food sources, and when supplementation makes sense. UK Nutrient Reference Values (NRVs) are included throughout.

• The Vitamins: Fat-Soluble
• The Vitamins: Water-Soluble
• The Minerals: Macrominerals
• The Minerals: Trace Minerals
• When Supplementation Makes Sense
• The Bottom Line
• References

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The Vitamins: Fat-Soluble

Fat-soluble vitamins (A, D, E and K) are absorbed alongside dietary fat in the small intestine and stored in the liver and adipose tissue¹. Because the body retains them, they do not need to be consumed every single day, but chronic deficiency or excessive intake can both cause problems.

Vitamin A

Vitamin A supports immune function, vision (particularly night vision) and skin cell turnover. It is available as preformed retinol (from animal sources like liver, eggs and dairy) and as provitamin A carotenoids (from orange and dark green vegetables like carrots, sweet potatoes and spinach). The UK NRV is 800µg. Deficiency signs include dry skin, night blindness and increased susceptibility to infections. OneVit Complete Multivitamin provides vitamin A as beta-carotene.

Vitamin D

Vitamin D functions as both a vitamin and a hormone. It regulates calcium absorption, supports bone mineralisation, modulates immune function and influences mood regulation². The UK government recommends that everyone over five consider supplementing 10µg daily between October and March, and year-round for those with limited sun exposure. The UK NRV is 5µg, which reflects a lower threshold used for food labelling purposes and differs from the SACN population reference intake of 10µg/day.

Vitamin D exists in two supplemental forms: D2 (ergocalciferol, plant-derived) and D3 (cholecalciferol, animal-derived). D3 is generally considered more effective at raising and maintaining blood levels of 25(OH)D, the marker used to assess vitamin D status. NDNS data shows that 21-38% of UK adults fall below the 25 nmol/L deficiency threshold during winter months³. People with darker skin, those who cover their skin for cultural or religious reasons, night-shift workers and older adults are at particularly elevated risk.

OneVit Vitamin D3 & K2 provides cholecalciferol paired with vitamin K2 (menaquinone-7) for optimal calcium metabolism. Taking vitamin D with a meal containing dietary fat improves absorption, since it is a fat-soluble nutrient.

Vitamin E

Vitamin E is the body's primary fat-soluble antioxidant, protecting cell membranes from oxidative damage. It supports immune function, skin health and cardiovascular protection. Food sources include nuts, seeds, vegetable oils and leafy greens. The UK NRV is 12mg. True deficiency is rare in healthy adults but can occur in those with fat malabsorption conditions. OneVit Complete Multivitamin provides 100% of the NRV for vitamin E.

Vitamin K

Vitamin K comes in two main forms: K1 (phylloquinone, from leafy greens) and K2 (menaquinone, from fermented foods and animal products). K1 is essential for blood clotting, while K2 directs calcium towards bone tissue and away from arterial walls⁴. The UK NRV is 75µg. K2 is increasingly recognised for its synergistic relationship with vitamin D3 in bone health. OneVit Vitamin D3 & K2 includes 100µg of vitamin K2 (menaquinone-7).

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The Vitamins: Water-Soluble

Water-soluble vitamins dissolve in water, are absorbed in the small intestine and are excreted through urine when intake exceeds the body's needs¹. This means they need to be replenished more regularly through diet or supplementation.

Vitamin B1 (Thiamine)

Thiamine is essential for converting carbohydrates into energy and supports nerve function. Food sources include whole grains, pork and legumes. The UK NRV is 1.1mg. Deficiency is uncommon in the UK but can occur with chronic alcohol consumption.

Vitamin B2 (Riboflavin)

Riboflavin supports energy metabolism, skin health and red blood cell production. Sources include dairy products, eggs, lean meats and green vegetables. The UK NRV is 1.4mg. Signs of deficiency include cracked lips, sore throat and light sensitivity.

Vitamin B3 (Niacin)

Niacin contributes to energy metabolism, DNA repair and cellular signalling. Found in poultry, fish, peanuts and whole grains. The UK NRV is 16mg. The body can also synthesise niacin from the amino acid tryptophan.

Vitamin B5 (Pantothenic Acid)

Pantothenic acid is involved in the synthesis of coenzyme A, which is central to fatty acid metabolism and energy production. It is widely available in foods (the name derives from the Greek word "pantos," meaning everywhere). The UK NRV is 6mg. Deficiency is extremely rare.

Vitamin B6 (Pyridoxine)

Vitamin B6 is a cofactor in over 100 enzyme reactions, including amino acid metabolism, neurotransmitter synthesis (serotonin, dopamine, GABA) and haemoglobin formation. Sources include poultry, fish, potatoes and bananas. The UK NRV is 1.4mg. Marginal deficiency is more common than clinical deficiency and may contribute to fatigue, irritability and impaired immune function.

Vitamin B7 (Biotin)

Biotin supports the metabolism of fatty acids, amino acids and glucose. It is best known for its role in keratin production, the structural protein in hair, skin and nails. Food sources include eggs, nuts, seeds and organ meats. The UK NRV is 50µg. OneVit Biotin provides 10,000µg per serving to support hair, skin and nail health.

Vitamin B9 (Folate / Folic Acid)

Folate is critical for DNA synthesis, cell division and red blood cell formation. It is especially important during pregnancy, where the NHS recommends 400µg of folic acid daily before conception and through the first 12 weeks to help reduce the risk of neural tube defects⁵. Food sources include dark leafy greens, legumes and fortified cereals. The UK NRV is 200µg. NDNS data shows declining folate status across multiple age groups in the UK³.

Vitamin B12 (Cobalamin)

Vitamin B12 is essential for nerve function, red blood cell production and DNA synthesis. It is found almost exclusively in animal products (meat, fish, dairy, eggs), making supplementation critical for vegans and beneficial for vegetarians⁶. Absorption declines with age due to reduced stomach acid production. The UK NRV is 2.5µg. OneVit Vitamin B12 provides supplemental cobalamin in a bioavailable form.

Vitamin C (Ascorbic Acid)

Vitamin C is a powerful water-soluble antioxidant that supports immune function, collagen synthesis, iron absorption, wound healing and the protection of cells from oxidative stress. Humans are among the few mammals that cannot synthesise vitamin C endogenously, making dietary or supplemental intake essential.

The richest food sources include citrus fruits, strawberries, kiwi, bell peppers, broccoli and Brussels sprouts. The UK NRV is 80mg. While outright scurvy (severe vitamin C deficiency) is rare in the UK, suboptimal intake is more common than many people realise. Smokers have significantly higher vitamin C requirements due to increased oxidative stress, as do people under chronic physical or psychological stress and those recovering from illness or surgery.

Vitamin C plays a particularly important role in collagen production. It acts as a cofactor for the enzymes prolyl hydroxylase and lysyl hydroxylase, which are essential for stabilising collagen's triple-helix structure. Without adequate vitamin C, collagen synthesis is impaired, affecting skin, gums, blood vessels and connective tissue. OneVit Vitamin C provides targeted immune and collagen synthesis support.

OneVit B Complex delivers all eight essential B vitamins plus inositol in a single daily tablet, with NRVs ranging from 100% to over 1,300% to ensure meaningful physiological support.

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The Minerals: Macrominerals

Macrominerals are needed in larger quantities (typically over 100mg per day).

Calcium

Calcium is the most abundant mineral in the body, with 99% stored in bones and teeth. It is essential for bone structure, muscle contraction, nerve transmission, blood clotting and cellular signalling. Food sources include dairy products, fortified plant milks, tinned fish with bones (sardines, anchovies), tofu made with calcium sulphate and leafy greens like kale and bok choy.

The UK NRV is 800mg. Calcium absorption is highly dependent on vitamin D status: without adequate vitamin D, the body cannot efficiently absorb calcium from the gut, regardless of how much calcium you consume. This is one of the key reasons why vitamin D and calcium are almost always discussed together in the context of bone health. Absorption is also affected by the presence of other dietary factors: oxalates (in spinach) and phytates (in whole grains) can reduce calcium uptake, while adequate dietary protein supports it.

Magnesium

Magnesium participates in over 300 enzymatic reactions, including energy production, muscle and nerve function, bone formation, blood sugar regulation and protein synthesis. Despite its importance, NDNS data indicates that a significant proportion of UK adults have magnesium intakes below recommended levels³. Food sources include nuts, seeds, whole grains and dark leafy greens. The UK NRV is 375mg.

Magnesium comes in various supplemental forms, and the form matters for absorption. Magnesium oxide is cheap but poorly absorbed, while chelated forms (citrate, glycinate, bisglycinate) offer significantly better bioavailability. Symptoms of magnesium insufficiency can be subtle: muscle cramps, poor sleep, irritability, fatigue and headaches. Because magnesium is involved in so many processes, even a marginal deficit can have widespread effects.

OneVit Magnesium provides a bioavailable form of supplemental magnesium designed for optimal absorption.

Potassium

Potassium helps regulate fluid balance, nerve signals and muscle contractions (including heartbeat). It counterbalances sodium in blood pressure regulation. Sources include bananas, potatoes, avocados, beans and spinach. The UK NRV is 2,000mg. Most adults benefit from increasing dietary potassium rather than supplementing, as high-dose potassium supplements can affect heart rhythm.

Phosphorus

Phosphorus works alongside calcium in bone and tooth formation and is involved in energy metabolism (as a component of ATP). It is widely available in protein-rich foods. The UK NRV is 700mg. Deficiency is uncommon in Western diets.

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The Minerals: Trace Minerals

Trace minerals are needed in smaller quantities but are no less important.

Iron

Iron is essential for oxygen transport (as a component of haemoglobin), energy metabolism and immune function. It comes in two dietary forms: haem iron (from animal sources such as red meat, poultry and fish, which is more readily absorbed) and non-haem iron (from plant sources such as lentils, chickpeas, spinach and fortified cereals, whose absorption is significantly enhanced by consuming vitamin C alongside it).

The UK NRV is 14mg. Women of childbearing age are the group most commonly affected by low iron status, with NDNS data showing that 25% of women aged 19 to 64 have intakes below the LRNI⁷. Vegetarians and vegans, frequent blood donors and endurance athletes are also at elevated risk. Symptoms of iron deficiency include persistent fatigue, pale skin, shortness of breath during exertion, brittle nails and difficulty concentrating. OneVit Iron provides supplemental iron that is best paired with OneVit Vitamin C for enhanced absorption. Iron should not be taken at the same time as calcium-rich foods or supplements, as calcium inhibits non-haem iron absorption.

Zinc

Zinc supports immune function, wound healing, taste perception, reproductive health and DNA synthesis. It is found in red meat, shellfish (particularly oysters), seeds and legumes. The UK NRV is 10mg. Zinc deficiency can impair immune function; in men with frank zinc deficiency, low zinc status has been associated with reduced testosterone levels⁸.

Selenium

Selenium is a component of selenoproteins that protect against oxidative damage and support thyroid function. Brazil nuts are the richest source, with smaller amounts in fish, eggs and whole grains. The UK NRV is 55µg.

Iodine

Iodine is essential for thyroid hormone synthesis, which regulates metabolic rate, growth and development. Sources include dairy, seafood and iodised salt. The UK NRV is 150µg. Iodine insufficiency has been identified as a concern in some UK population groups, particularly young women⁹.

Copper

Copper is involved in iron metabolism, connective tissue formation and antioxidant defence. Sources include shellfish, nuts, seeds and organ meats. The UK NRV is 1mg. Excessive zinc supplementation can impair copper absorption, which is why the ratio between the two matters.

Manganese, Chromium and Molybdenum

These trace minerals participate in various enzymatic processes including bone formation (manganese), blood sugar regulation (chromium) and sulphite metabolism (molybdenum). They are typically obtained in adequate amounts through a varied diet and are included in comprehensive multivitamin formulations like OneVit Complete Multivitamin.

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When Supplementation Makes Sense

A balanced, varied diet should always be the primary source of vitamins and minerals. However, supplementation is warranted when dietary intake consistently falls short, when physiological demands are elevated, or when specific risk factors make deficiency more likely.

The nutrients where UK supplementation is most commonly beneficial include vitamin D (particularly October to March), iron (for women of childbearing age and those on restricted diets), vitamin B12 (for vegans and older adults), folate (for women planning pregnancy), omega-3 (for those not eating two portions of oily fish weekly) and magnesium (widely under-consumed relative to requirements).

It is worth noting that more is not always better. Fat-soluble vitamins accumulate in body tissues, and chronic high-dose supplementation of vitamins A and D can cause toxicity. Water-soluble vitamins are generally safer at higher intakes because the body excretes excess amounts, but even these have upper tolerable limits. Always stay within recommended dosing unless directed otherwise by a healthcare professional.

Nutrient interactions also matter. Calcium and iron compete for absorption. Vitamin C enhances non-haem iron uptake. Vitamin D requires vitamin K2 for optimal calcium metabolism. Fat-soluble vitamins need dietary fat for absorption. Understanding these relationships helps you get the most from your supplements.

A quality multivitamin like OneVit Complete Multivitamin provides a convenient baseline that covers the broadest range of potential gaps in a single daily tablet. From there, targeted individual supplements address specific goals or diagnosed deficiencies. Blood testing through your GP can identify specific shortfalls and guide your routine with precision rather than guesswork.

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The Bottom Line

Vitamins and minerals are not optional extras. They are fundamental to every system in the body. Understanding what each one does helps you make informed choices about your diet and supplement routine, prioritising the nutrients that your body is most likely to need based on your lifestyle, age, sex and health goals.

When in doubt, start with the basics: a quality multivitamin, vitamin D (especially in the UK) and any nutrients flagged by blood tests or a healthcare professional. Build from there based on your individual circumstances.

Disclaimer: This article is for informational purposes only and is not intended to replace professional medical advice. If you suspect a nutrient deficiency, please speak with a healthcare professional who can arrange appropriate testing.

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References

1. Maqbool MA, Aslam M, Akbar W, Iqbal Z. Fat-Soluble Vitamins A, D, E, and K: Review of the Literature and Points of Interest for the Clinician. J Clin Med. 2024;13(13):3641. MDPI

2. Scientific Advisory Committee on Nutrition (SACN). Vitamin D and Health. 2016. GOV.UK

3. Office for Health Improvement and Disparities. National Diet and Nutrition Survey 2019 to 2023: Report. 2025. GOV.UK

4. Maresz K. Proper Calcium Use: Vitamin K2 as a Promoter of Bone and Cardiovascular Health. Integr Med (Encinitas). 2015;14(1):34-39. PMC

5. NHS. Vitamins, supplements and nutrition in pregnancy. NHS

6. Allen LH. How common is vitamin B-12 deficiency? Am J Clin Nutr. 2009;89(2):693S-696S. PubMed

7. Public Health England. National Diet and Nutrition Survey: Results from Years 9 to 11 (combined) – Statistical Summary. 2020. GOV.UK

8. Prasad AS, Mantzoros CS, Beck FW, Hess JW, Brewer GJ. Zinc status and serum testosterone levels of healthy adults. Nutrition. 1996;12(5):344-348. PubMed

9. Sherwood RA. Iodine deficiency in the UK: a public health issue? Ann Clin Biochem. 2022;59(5):319-326. PubMed