Vitamins Explained: Types, Benefits, Dosage & How to Choose

If you’re asking what vitamins are, whether you need a multivitamin, or how to take them safely, you’re not alone. Vitamins are essential nutrients your body can’t make in sufficient amounts, and they’re required for energy metabolism, immunity, bones, eyesight, and more. This guide breaks down the types of vitamins, benefits supported by research, safe dosages, food sources, and how to choose supplements wisely.

What are vitamins? Why they matter

Vitamins are organic compounds needed in small amounts to support normal physiology—think enzymes that help you extract energy from food, repair tissues, and regulate hormones. There are 13 essential vitamins: A, C, D, E, K, and the eight B vitamins (B1, B2, B3, B5, B6, B7, B9/folate, B12).

• Without enough vitamins, deficiency symptoms can develop (for example, scurvy from lack of vitamin C or anemia from lack of folate/B12). Strong evidence underpins these relationships, established by decades of clinical observation and controlled trials.
• In holistic traditions, foods naturally rich in vitamins—like citrus (vitamin C), cod liver oil (vitamins A and D), and fermented soy natto (vitamin K2)—have been prized for vitality and resilience. These practices align with modern biochemistry, which now explains the mechanisms behind their benefits.

Fat‑soluble vs water‑soluble vitamins

Vitamins differ by how they’re absorbed, transported, and stored. Those properties affect how you take them and potential toxicity risks.

• Fat‑soluble (A, D, E, K)

  • Absorption: Enhanced when taken with dietary fat and bile.
  • Storage: Accumulate in liver and fatty tissues.
  • Implications: Deficiencies develop more slowly; toxicity is more likely with long‑term high doses. Malabsorption (e.g., celiac disease, Crohn’s, pancreatic or liver disorders) and fat‑blocking drugs (orlistat, cholestyramine) impair absorption.

• Water‑soluble (C and all B vitamins)

  • Absorption: Readily absorbed with water; excess is typically excreted in urine.
  • Storage: Minimal, except B12 which is stored in the liver.
  • Implications: Deficiencies can appear faster; toxicity is uncommon but possible at very high intakes (e.g., B6 neuropathy, niacin liver stress, vitamin C GI upset/kidney stones in some individuals).

Key vitamins: functions, deficiency signs, and what the evidence shows

Below are the essentials for each vitamin: what it does, how deficiency shows up, and what research says about supplementation for generally healthy adults. Evidence ratings are noted when applicable.

Vitamin A (retinol, carotenoids)

• What it does: Supports vision (11‑cis‑retinal in the retina), immune function, skin integrity, and gene expression via retinoic acid binding nuclear receptors.
• Deficiency signs: Night blindness, dry eyes/skin, higher infection risk. Severe deficiency is more common where diets lack animal foods or fortified products.
• Supplements: Strong evidence supports correcting deficiency and preventing blindness in deficient populations. For well‑nourished adults, routine high‑dose vitamin A is not advised; RCTs show beta‑carotene supplements increase lung cancer risk in smokers and asbestos‑exposed individuals (strong evidence). Prefer food carotenoids; avoid high‑dose retinol unless medically indicated.

B‑Complex (B1, B2, B3, B5, B6, B7, B9/folate, B12)

• Core roles:
  • B1 (thiamine): Carbohydrate metabolism (TPP coenzyme), nerve function. Deficiency causes beriberi/Wernicke‑Korsakoff (strong in alcohol use disorder, malnutrition).
  • B2 (riboflavin): FAD/FMN coenzymes for energy metabolism; deficiency causes mouth sores, dermatitis, light sensitivity.
  • B3 (niacin): Forms NAD/NADP for redox reactions; pharmacologic doses improve lipids but can cause flushing and liver toxicity; event reduction is uncertain in the statin era (mixed evidence).
  • B5 (pantothenic acid): CoA synthesis; deficiency is rare (fatigue, paresthesias).
  • B6 (pyridoxine/PLP): Amino acid metabolism, neurotransmitters; deficiency may cause anemia, dermatitis, neuropathy. Chronic high doses (>100 mg/day) can also cause neuropathy (strong evidence).
  • B7 (biotin): Carboxylase cofactor for fatty acid/glucose metabolism; deficiency is rare; high doses can interfere with lab tests (notably thyroid and troponin assays).
  • B9 (folate/folic acid): One‑carbon metabolism for DNA synthesis; deficiency causes megaloblastic anemia. Strong evidence: periconceptional folic acid prevents neural tube defects.
  • B12 (cobalamin): Methylation and myelin integrity; deficiency leads to anemia, neuropathy, cognitive changes. Risk is higher with vegan diets, metformin or PPI use, and in older adults (strong evidence).
• Supplement notes: Thiamine is essential in refeeding and alcohol use disorder (strong). Folate 400 mcg/day for those who may become pregnant (strong). B12 supplementation is often needed for vegans, after bariatric surgery, or with malabsorption (strong). Homocysteine‑lowering with B vitamins has not consistently reduced cardiovascular events (strong evidence of no benefit for primary prevention).

Vitamin C (ascorbic acid)

• What it does: Antioxidant; cofactor for collagen hydroxylation (skin, gums, blood vessels), carnitine and neurotransmitter synthesis; enhances non‑heme iron absorption.
• Deficiency signs: Scurvy—fatigue, gum bleeding, bruising, poor wound healing (strong historical/clinical evidence).
• Supplements: Regular supplementation may modestly shorten duration of common colds but does not prevent them in the general population (moderate evidence). Smokers and those under intense physical stress may benefit more. High doses (>2,000 mg/day) can cause GI upset and may increase kidney stone risk in susceptible men (moderate evidence).

Vitamin D (D2 ergocalciferol, D3 cholecalciferol)

• What it does: Hormone‑like vitamin regulating calcium/phosphate; boosts intestinal calcium absorption; binds the vitamin D receptor affecting hundreds of genes.
• Deficiency signs: In children, rickets; in adults, osteomalacia, bone pain, muscle weakness. Low levels are common with limited sun exposure, darker skin, obesity, and aging.
• Supplements: Strong evidence supports vitamin D with calcium for fracture risk reduction in certain older adults and institutionalized populations. Effects on immunity, mood, and chronic disease are mixed; some RCTs show small or no benefits for major outcomes in the general population (moderate evidence). D3 raises levels more effectively than D2 (moderate evidence).

Vitamin E (alpha‑tocopherol and tocotrienols)

• What it does: Lipid‑soluble antioxidant protecting cell membranes.
• Deficiency signs: Rare, usually with fat malabsorption; can cause neuropathy, myopathy, hemolytic anemia.
• Supplements: Large RCTs have not shown cardiovascular or mortality benefits; high‑dose vitamin E may increase hemorrhagic stroke risk and interact with anticoagulants (strong evidence). The AREDS/AREDS2 formulas (antioxidants plus zinc/copper, sometimes vitamin E) may slow progression of intermediate age‑related macular degeneration (moderate to strong for that specific use).

Vitamin K (K1 phylloquinone, K2 menaquinones)

• What it does: Cofactor for gamma‑carboxylation of clotting factors (blood coagulation) and proteins like osteocalcin (bone health).
• Deficiency signs: Easy bruising, bleeding; more common in newborns and with malabsorption or long‑term antibiotic use.
• Supplements: Strong evidence supports vitamin K to correct deficiency and for newborn prophylaxis. For bone health, K2 (MK‑7) shows promise in some trials, but overall evidence for fracture reduction in the general population is mixed (emerging to moderate). Vitamin K intake interacts with warfarin—consistency is key.

Food sources: how to meet vitamin needs through diet

Research consistently supports getting most vitamins from varied, minimally processed foods (strong evidence). Focus on:

• Vitamin A: Liver (retinol, very potent), dairy, eggs; orange/dark green produce (carotenoids) like carrots, sweet potatoes, kale, spinach, mango.
• B vitamins: Whole grains, legumes, dairy, eggs, meat/poultry, fish, leafy greens, nuts/seeds; nutritional yeast (B12‑fortified for vegans). Many cereals and flours are fortified with folic acid and other B vitamins.
• Vitamin C: Citrus, berries, kiwi, bell peppers, broccoli, Brussels sprouts, tomatoes, amla/Indian gooseberry (traditional Ayurvedic source, very vitamin C‑rich).
• Vitamin D: Fatty fish (salmon, sardines), cod liver oil (traditional), egg yolks, UV‑exposed mushrooms; fortified milk/plant milks and cereals. Sunlight triggers D3 synthesis in skin, but needs vary by skin tone, latitude, season, and sunscreen use.
• Vitamin E: Nuts/seeds (almonds, sunflower seeds), wheat germ, vegetable oils (sunflower, safflower), avocado, spinach.
• Vitamin K: Leafy greens (K1: kale, spinach, collards), herbs; fermented foods like natto (K2, MK‑7; traditional Japanese food), some cheeses.

Eastern foodways emphasize “food as medicine”—seasonal produce, fermented foods, and nutrient‑dense animal products—many of which are naturally vitamin‑rich. These traditions align with modern guidance to prioritize whole foods.

Vegan and vegetarian nutrition and supplements guides can help you build a plan tailored to your eating style.

Vitamins: what the research says

• Deficiency correction saves lives and prevents disease (strong): Vitamin A in deficiency, folic acid to prevent neural tube defects, vitamin D for rickets/osteomalacia, thiamine in refeeding and alcohol use disorder, and B12 for pernicious anemia.
• Routine high‑dose antioxidant supplementation is not broadly beneficial (strong to moderate): Trials of vitamin E and beta‑carotene do not reduce cardiovascular events; beta‑carotene increases lung cancer risk in smokers.
• Vitamin D helps bone outcomes in selected groups; broad extra‑skeletal benefits are mixed (moderate): Pairing with calcium is often necessary for fracture benefit.
• Vitamin C modestly reduces cold duration but not incidence in the general population (moderate); higher needs in smokers.
• B‑vitamin therapy lowers homocysteine but hasn’t consistently lowered heart disease or stroke events in primary prevention (strong).
• Food‑first patterns correlate with better outcomes than reliance on supplements alone (strong, supported by cohort data and dietary trials like Mediterranean‑style patterns).

When supplements may be helpful

Targeted vitamin supplementation is valuable when diet alone can’t reliably meet needs or when clinical conditions increase requirements. Examples include:

• Pregnancy and preconception: 400–800 mcg/day folic acid at least 1 month before conception through the first trimester (strong). Prenatal formulas often include iron, iodine, choline, and DHA. See prenatal vitamins.
• Infants and children: Vitamin D (often 400 IU/day for breastfed infants per pediatric guidance) and vitamin K at birth (strong). Follow your pediatrician’s plan.
• Older adults: Vitamin D (limited sun), B12 (reduced absorption), possibly calcium with D for bone health (strong/moderate depending on risk profile).
• Restricted diets: Vegans/strict vegetarians (B12; possibly D, and fortified foods for folate); low‑calorie or medically supervised diets.
• Malabsorption or surgeries: Celiac disease, inflammatory bowel disease, pancreatic insufficiency, liver disease, bariatric surgery (fat‑soluble vitamins and B12 often needed; strong need for medical supervision).
• Medications: Metformin and PPIs (B12), isoniazid (B6), orlistat/cholestyramine (fat‑soluble vitamins), certain anticonvulsants (vitamin D, K, folate). Always review with your clinician.
• Lifestyle or exposures: Limited sun (D), smokers (higher vitamin C needs), heavy alcohol use (thiamine), darker skin at high latitudes (D).

For general wellness in healthy adults with varied diets, multivitamins can fill small gaps, but large RCTs have not shown clear reductions in major chronic disease or mortality (moderate/strong). See our overview of multivitamins.

Safety, interactions, toxicity risks, and recommended intake ranges

Most people can meet needs from food; if supplementing, avoid “more is better.” Stay within recommended ranges unless advised by a clinician.

• General adult RDAs/AIs (typical ranges):

  • Vitamin A: 700–900 mcg RAE/day; Upper Limit (UL): 3,000 mcg RAE/day for preformed A. Beta‑carotene supplements are not recommended for smokers.
  • Vitamin D: 600–800 IU (15–20 mcg)/day; common maintenance 1,000–2,000 IU/day; UL: 4,000 IU (100 mcg)/day without medical supervision. Target 25(OH)D blood levels often cited at ~20–50 ng/mL; individualized.
  • Vitamin E (alpha‑tocopherol): 15 mg/day; UL: 1,000 mg/day (approx. 1,500 IU natural). Caution with anticoagulants.
  • Vitamin K: 90–120 mcg/day (AI); no established UL. Keep intake consistent on warfarin.
  • Vitamin C: 75–90 mg/day; smokers +35 mg/day; UL: 2,000 mg/day.
  • Thiamine (B1): ~1.1–1.2 mg/day.
  • Riboflavin (B2): ~1.1–1.3 mg/day.
  • Niacin (B3): 14–16 mg/day (NE); UL for supplements: 35 mg/day (flushing threshold), higher prescription doses under supervision only.
  • Pantothenic acid (B5): 5 mg/day (AI).
  • Pyridoxine (B6): 1.3–1.7 mg/day; UL: 100 mg/day (lower limits used in some regions).
  • Biotin (B7): 30 mcg/day (AI). Warn labs about biotin use to avoid test interference.
  • Folate (B9): 400 mcg DFE/day; UL for synthetic folic acid from supplements/fortification: 1,000 mcg/day (to avoid masking B12 deficiency).
  • Cobalamin (B12): 2.4 mcg/day; no UL; high‑dose oral or injections may be needed in deficiency.

• Notable interactions and cautions:

  • Warfarin and vitamin K: Keep vitamin K intake stable; discuss any supplement changes with your clinician.
  • Retinoids (isotretinoin, acitretin) and vitamin A: Avoid additional vitamin A supplements.
  • High‑dose vitamin E with anticoagulants/antiplatelets: May increase bleeding risk.
  • Niacin with alcohol or certain lipid‑lowering drugs: Increased liver and muscle risks at pharmacologic doses.
  • Vitamin C enhances non‑heme iron absorption—can be helpful in iron deficiency but caution in iron overload.
  • Long‑term antibiotics may lower vitamin K–producing gut bacteria; clinical significance varies.
  • Biotin can cause false lab results (notably thyroid and cardiac troponin immunoassays). Stop 48–72 hours before testing, per lab guidance.

This list is not exhaustive. For personal dosing, medication checks, and lab targets, consult your clinician.

How to choose a vitamin supplement: labels, quality marks, dosing tips, and FAQs

Choosing among thousands of products can be confusing. A careful read of the label helps.

• Look for third‑party testing: USP Verified, NSF, or other reputable programs. These marks indicate the product contains what it claims and is free of certain contaminants.
• Check forms and amounts:
  • Vitamin D3 (cholecalciferol) typically raises levels more effectively than D2.
  • Vitamin A: Prefer beta‑carotene for general use; avoid high‑dose preformed retinol unless indicated.
  • Folate: Folic acid works well for most; 5‑MTHF (methylfolate) is an alternative for those with MTHFR variants or folate metabolism issues.
  • B12: Cyanocobalamin is stable and effective; methylcobalamin is also used—choose based on clinician advice and cost/availability.
  • Vitamin E: Natural d‑alpha‑tocopherol is more bioactive than synthetic dl‑alpha; mixed tocopherols are sometimes included, but clinical advantages are unclear.
  • Vitamin K: K1 for general intake; K2 MK‑7 has a longer half‑life; bone outcomes evidence is still emerging.
• Dose sensibly: Aim near RDA unless correcting a deficiency or following medical advice. Avoid exceeding ULs.
• Timing and absorption:
  • Take fat‑soluble vitamins with a meal containing fat.
  • Split water‑soluble doses (C and B complex) if higher amounts cause GI upset.
  • Separate certain minerals from other meds if applicable (e.g., calcium/iron can affect absorption of some drugs)—check our supplements guide.
• Gummies vs tablets: Gummies can be easier to take but often contain fewer nutrients (especially minerals) and added sugars; verify amounts.
• “Natural” vs “synthetic”: The body typically recognizes the vitamin molecule itself; quality and dose matter more than the marketing term. There are exceptions (e.g., natural vs synthetic vitamin E potency differs).

FAQs

• Do most people need a multivitamin?
  • For generally healthy adults with varied diets, multivitamins can cover small gaps but haven’t consistently shown major disease‑prevention benefits (moderate/strong). They may help specific groups (older adults, restricted diets). See multivitamins.
• Should I test my levels?
  • Testing is useful when deficiency risk is higher (e.g., vitamin D, B12) or symptoms suggest a problem. Work with your clinician; see lab testing.
• Can I take vitamins together?
  • Usually yes. Take fat‑soluble vitamins with food; split high doses of C/B if needed. If you’re on medications, ask about interactions.
• How long until I feel a difference?
  • Correcting deficiencies can improve symptoms within weeks (e.g., B12‑related fatigue/neuropathy, iron with vitamin C for absorption, vitamin D for muscle aches). For general wellness without deficiency, effects may be subtle or not noticeable.
• Are “whole‑food” vitamins better?
  • Foods are best sources overall. “Whole‑food” supplements may include concentrates of fruits/vegetables or yeast, but evidence that they outperform standard vitamins for clinical outcomes is limited (emerging). Prioritize diet quality.

Practical takeaways

• Build a food‑first foundation: Vegetables, fruits, whole grains, legumes, nuts/seeds, quality proteins, and fermented foods naturally supply vitamins.
• Know your risk: Pregnancy plans, vegan diet, limited sun, certain medications, digestive disorders, older age—these raise odds of vitamin gaps.
• Supplement with purpose: Choose targeted vitamins at evidence‑based doses; avoid megadoses unless prescribed.
• Check quality: Prefer third‑party tested products and read labels carefully.
• Coordinate care: Share your supplement list with your healthcare team, especially if you take prescription meds or will have lab testing.

Disclaimer

This guide is for educational purposes and should not replace personalized medical advice. Talk with a qualified healthcare professional before starting, stopping, or changing any vitamin supplement, especially if you are pregnant, nursing, managing a medical condition, or taking medications.