Vitamins: The Complete Evidence-Based Guide

Q: Which vitamins should most adults actually supplement?

Three have the strongest evidence for routine supplementation in adults eating a typical Western diet: vitamin D3 (most adults are sub-optimal in winter), vitamin B12 (especially for anyone over 50, vegetarian, or on metformin/PPIs), and folate (women planning pregnancy). Others are best obtained from food unless a clinician identifies a specific deficiency.

Q: What's the difference between fat-soluble and water-soluble vitamins?

Fat-soluble vitamins (A, D, E, K) are stored in the liver and adipose tissue, so daily intake isn't strictly required and overdose is possible. Water-soluble vitamins (C and the B-complex) are not stored long-term — excess is excreted in urine — so deficiencies show up faster but toxicity is rare.

Q: Should I take a multivitamin or individual vitamins?

For someone with no specific deficiency, a low-dose multivitamin can be reasonable insurance. For people with documented deficiencies (low D, low B12, MTHFR variants needing methylfolate), targeted single-vitamin supplementation is more effective and lets you control the form and dose.

Q: Can I overdose on vitamins?

Yes — primarily the fat-soluble ones. Vitamin A above 10,000 IU/day chronically can cause liver damage and birth defects; vitamin D above 10,000 IU/day for months can cause hypercalcemia; vitamin E above 1,000 mg/day may increase bleeding risk. Water-soluble vitamins are far harder to overdose on, though megadose B6 (>200 mg/day for years) can cause nerve damage.

Reviewed by Sarah Chen, MD · Last updated April 15, 2026

Thirteen organic compounds the human body cannot synthesize in adequate amounts and must obtain from diet or supplementation. This index covers every essential vitamin — what it does, who's commonly deficient, the best supplemental forms, and how much is too much.

⚡ Quick orientation

Fat-soluble (A, D, E, K) — stored in liver and fat; daily intake not strictly required; overdose possible at high chronic doses.

Water-soluble (C, B-complex) — not stored; excess excreted in urine; deficiencies appear quickly, toxicity is rare.

Strongest case for routine supplementation: Vitamin D3 (most adults sub-optimal in winter), B12 (older adults, vegetarians, people on metformin/PPIs), folate (women planning pregnancy).

Fat-soluble vitamins

The fat-soluble four (A, D, E, K) are absorbed alongside dietary fat and stored in the liver and adipose tissue. Take them with a meal that contains some fat — the absorption difference can be 2–4×.

Fat-soluble · Strong evidence

Vitamin A

Vision, immune function, epithelial integrity. Available as preformed retinol (animal sources) or beta-carotene (plants). RDA: 700–900 µg RAE/day.

Fat-soluble · Strong evidence

Vitamin D3

The "sunshine vitamin." Critical for calcium absorption, bone health, and immune modulation. The single most-justifiable supplement for most adults. RDA: 600–800 IU/day; common supplemental dose: 1,000–4,000 IU.

Fat-soluble · Moderate evidence

Vitamin E

A family of 8 fat-soluble antioxidants (4 tocopherols + 4 tocotrienols). Look for mixed tocopherols, not synthetic dl-alpha alone. RDA: 15 mg/day.

Fat-soluble · Moderate evidence

Vitamin K2 (MK-7)

Activates proteins that direct calcium into bone (and out of arteries). Often paired with D3. MK-7 form has a longer half-life than MK-4. Typical dose: 90–180 µg/day.

Water-soluble vitamins

Vitamin C and the eight B-complex vitamins dissolve in water and circulate freely. The body retains only a small amount, so consistent intake matters more than dose size — and toxicity is rare even at moderately high doses.

Water-soluble · Strong evidence

Vitamin C

Antioxidant, collagen synthesis cofactor, immune support. Absorption saturates above ~500 mg/dose; spread larger doses across the day. RDA: 75–90 mg/day.

B-complex · Moderate evidence

Vitamin B1 (Thiamine)

Energy metabolism and nerve function. Deficiency causes beriberi and Wernicke's encephalopathy — a real risk in heavy alcohol users. RDA: 1.1–1.2 mg/day.

B-complex · Moderate evidence

Vitamin B6 (Pyridoxine)

Cofactor in over 100 enzymatic reactions including neurotransmitter synthesis. The active form is P-5-P. Avoid chronic megadoses (>200 mg/day) — risk of peripheral neuropathy. RDA: 1.3–1.7 mg/day.

B-complex · Strong evidence

Vitamin B12 (Cobalamin)

Red-cell production, myelin sheath maintenance, methylation. Found only in animal foods; vegetarians, older adults, and people on metformin or PPIs are at highest risk. Forms: cyanocobalamin (cheap), methylcobalamin (active). RDA: 2.4 µg/day.

B-complex · Strong evidence

Folate / Folic Acid (B9)

DNA synthesis and red-cell formation. Critical pre-conception and in early pregnancy (prevents neural tube defects). Folic acid is synthetic; food folate and 5-MTHF are natural forms. RDA: 400 µg DFE/day; 600 µg in pregnancy.

B-complex · Weak evidence

Biotin (B7)

Cofactor for fat, carbohydrate, and amino-acid metabolism. Deficiency is rare. The hair/nail/skin marketing claims are largely unsupported in non-deficient adults. AI: 30 µg/day.

B-complex-adjacent · Moderate evidence

Choline

Technically not a vitamin but functions like one. Required for cell membranes (phosphatidylcholine), acetylcholine synthesis, and methylation. Most adults under-consume. AI: 425–550 mg/day.

Methylated forms

People with common MTHFR gene variants (C677T, A1298C) convert folic acid and cyanocobalamin to their active forms less efficiently. Methylated supplements bypass this bottleneck.

Methylated · Moderate evidence

Methylfolate (5-MTHF)

The bioactive form of folate. Crosses the blood-brain barrier. Preferred over folic acid for anyone with MTHFR variants or treatment-resistant depression where folate is implicated. Typical dose: 400–1,000 µg/day.

Methylated · Moderate evidence

Methylcobalamin (Methyl-B12)

The bioactive coenzyme form of B12. Used directly without conversion. Slightly better retention vs. cyanocobalamin, but cyano is fine for most. Typical dose: 500–1,000 µg/day sublingual.

RDA reference table (adults)

Recommended Dietary Allowances from the U.S. National Academies / NIH ODS. RDAs cover ~97% of healthy adults; "Adequate Intake" (AI) is used where an RDA hasn't been established.

Vitamin	Adult men	Adult women	Upper limit (UL)	Type
Vitamin A	900 µg RAE	700 µg RAE	3,000 µg RAE	Fat-soluble
Vitamin D	600–800 IU	600–800 IU	4,000 IU	Fat-soluble
Vitamin E	15 mg	15 mg	1,000 mg	Fat-soluble
Vitamin K	120 µg (AI)	90 µg (AI)	Not established	Fat-soluble
Vitamin C	90 mg	75 mg	2,000 mg	Water-soluble
B1 Thiamine	1.2 mg	1.1 mg	Not established	Water-soluble
B6 Pyridoxine	1.3–1.7 mg	1.3–1.5 mg	100 mg	Water-soluble
B9 Folate	400 µg DFE	400 µg DFE	1,000 µg (folic acid)	Water-soluble
B12 Cobalamin	2.4 µg	2.4 µg	Not established	Water-soluble
B7 Biotin	30 µg (AI)	30 µg (AI)	Not established	Water-soluble
Choline	550 mg (AI)	425 mg (AI)	3,500 mg	Water-soluble

How to choose a vitamin supplement

Most marketing in the vitamin aisle is noise. A few decision rules cut through it:

Test before you supplement. A serum 25(OH)D, B12, and folate panel costs less than a year of guessing. Treat what's actually low.
Match the form to the goal. D3 outperforms D2; methylfolate outperforms folic acid for MTHFR variants; mixed tocopherols outperform synthetic dl-alpha-tocopherol.
Mind the elemental amount. "1,000 mg of [compound]" is not the same as "1,000 mg of [vitamin]." Check the supplement-facts panel.
Take fat-solubles with fat. A, D, E, and K absorb 2–4× better with a meal containing fat.
Avoid the megadose trap. More is not better — and for A, D, E, B6, it's actively worse at chronic high doses.
Look for third-party testing. USP, NSF, or ConsumerLab seals indicate the bottle contains what the label claims.

Frequently asked questions

Which vitamins should most adults actually supplement?

Three have the strongest case: vitamin D3 (most adults are sub-optimal in winter), B12 (especially over 50, vegetarian, or on metformin/PPIs), and folate (women planning pregnancy). Others are best obtained from food unless a deficiency is identified.

What's the difference between fat-soluble and water-soluble vitamins?

Fat-soluble (A, D, E, K) are stored in liver and fat — daily intake not strictly required, but overdose is possible. Water-soluble (C and the B-complex) are not stored — excess is excreted in urine, so deficiencies appear faster but toxicity is rare.

Should I take a multivitamin or individual vitamins?

A low-dose multi can be reasonable insurance for someone with no specific deficiency. For documented deficiencies (low D, low B12, MTHFR variants), targeted single-vitamin supplementation is more effective and lets you control form and dose.

Can I overdose on vitamins?

Yes — primarily the fat-soluble ones. Vitamin A >10,000 IU/day chronically can cause liver damage and birth defects; vitamin D >10,000 IU/day for months can cause hypercalcemia; vitamin E >1,000 mg/day may increase bleeding risk. Water-soluble vitamins are far harder to overdose, though megadose B6 (>200 mg/day for years) can cause nerve damage.

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How we organize this category: Each vitamin entry is research-graded (Strong / Moderate / Weak / Insufficient) based on the quality and quantity of human RCTs. RDAs sourced from the U.S. National Academies and the NIH Office of Dietary Supplements. Read our editorial policy.

Last reviewed: April 15, 2026 · Next review: July 2026

Disclaimer: This information is for educational purposes only and should not replace medical advice. Always consult a qualified healthcare provider before starting any supplement, especially if you have a medical condition or take prescription medications. These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease.