What Are Vitamins? Types, Functions, and How Much You Need

What Are Vitamins, Really?

Vitamins are organic compounds your body needs in small amounts to function properly. Unlike macronutrients (protein, carbs, fat), vitamins don't provide energy directly. Instead, they act as catalysts, coenzymes, and regulators in hundreds of metabolic reactions — from converting food into ATP to repairing DNA.

There are 13 essential vitamins. Your body either can't make them at all or can't make enough, so they must come from food or supplements. This guide breaks down each one: what it does, how much you need, and where the evidence stands.

TL;DR

• 13 essential vitamins split into two groups: 4 fat-soluble (A, D, E, K) and 9 water-soluble (C + 8 B-vitamins)
• Fat-soluble vitamins are stored in body fat and liver; overdose is possible. Water-soluble ones are excreted daily; deficiency develops faster
• Vitamin D is the most common deficiency in Estonia — up to 90% of residents have suboptimal levels in winter (Cashman et al., 2016)
• A varied diet covers most vitamins, but D, B12 (for vegans), and folate (in pregnancy) often need supplementation
• More is not always better: megadosing fat-soluble vitamins carries real toxicity risks (Penniston & Tanumihardjo, 2006)

Two Families: Fat-Soluble vs Water-Soluble

This distinction matters for dosing and safety.

Fat-soluble vitamins (A, D, E, K) dissolve in fat, absorb better with meals, and accumulate in the body. You don't need them every day, but you can overdose if you take excessive amounts over time.

Water-soluble vitamins (C and the B-complex) dissolve in water, don't store well, and excess is excreted in urine. Deficiency can develop within weeks if intake drops — particularly with B1 and C (Lonsdale, 2006).

Practical rule: Take fat-soluble vitamins with your fattiest meal. Water-soluble ones can be taken any time, but B-vitamins are best in the morning since some people report they interfere with sleep.

The 13 Essential Vitamins at a Glance

How Vitamins Work in Your Body

Most vitamins function as coenzymes — helper molecules that enzymes need to catalyze reactions. Without adequate B-vitamins, for example, the citric acid cycle slows down and you feel fatigued not because you lack calories, but because you can't convert them efficiently (Kennedy, 2016).

Vitamin D is technically a prohormone. Your skin synthesizes it from UVB sunlight, and your liver and kidneys convert it into calcitriol — the active hormone that regulates over 200 genes involved in immunity, inflammation, and calcium metabolism (Holick, 2007).

Vitamin C is the classic antioxidant that also serves as a cofactor for collagen synthesis. Without it, collagen cross-links fail and you get scurvy — a disease historically devastating for sailors but rare today (Carr & Maggini, 2017).

Who Is at Risk of Deficiency?

Not everyone eating a "normal" diet gets enough of every vitamin. These groups face higher risk:

1. Nearly everyone in Estonia (vitamin D) — at latitude 59degN, UVB is too weak for skin synthesis from October through March. A 2016 study across 14 European countries found that roughly 40% of the population had serum 25(OH)D below 50 nmol/L, with northern countries hit hardest (Cashman et al., 2016)
2. Vegans and vegetarians (B12) — no plant food naturally contains B12; supplementation is mandatory, not optional (Pawlak et al., 2013)
3. Pregnant women (folate) — inadequate folate dramatically increases neural tube defect risk; 400 mcg/day supplementation is recommended starting before conception (De-Regil et al., 2015)
4. Older adults (B12, D) — B12 absorption drops with age due to declining stomach acid; D synthesis efficiency falls as skin thins
5. Athletes (B-complex, C, D) — increased metabolic demand, sweat losses, and often restrictive diets can deplete water-soluble vitamins faster

Common Mistakes People Make

1. Taking everything in a multivitamin and assuming they're covered — most multivitamins contain only 400-800 IU of vitamin D, which may be insufficient for someone who is already deficient. Get tested first
2. Megadosing vitamin C when sick — doses above 200 mg/day don't significantly increase blood levels due to absorption saturation (Padayatty et al., 2004). Consistent moderate intake matters more than crisis loading
3. Ignoring interactions — calcium blocks iron absorption; vitamin C enhances it. Vitamin D needs K2 to direct calcium to bones instead of arteries (Knapen et al., 2013)
4. Assuming "natural" means safe — vitamin A from liver can easily exceed the upper limit (3000 mcg/day). Chronic excess raises toxicity risk including liver damage (Penniston & Tanumihardjo, 2006)
5. Skipping fat with fat-soluble vitamins — taking D, A, E, or K on an empty stomach reduces absorption by up to 50%

How to Build Your Vitamin Strategy

Step 1: Start with food. A diet that includes fatty fish, eggs, leafy greens, berries, nuts, and whole grains covers most bases.

Step 2: Test vitamin D. A 25(OH)D blood test (available at Synlab, Medicumi, or your family doctor in Estonia; ~€15-25) tells you where you stand. Target: 75-125 nmol/L.

Step 3: Supplement the gaps. For most Estonians, a D3 supplement (2000-4000 IU/day in winter) is the single highest-impact addition. Add B12 if you're plant-based, folate if you're planning pregnancy.

Step 4: Don't stack blindly. Taking 10 different supplements without knowing your baseline is wasteful and potentially harmful. A good multivitamin fills minor gaps, while targeted supplements address confirmed deficiencies.

Frequently Asked Questions

Can I get all my vitamins from food alone?

In theory, yes — except vitamin D in northern countries. A Finnish study found that even people eating nutrient-dense diets in Nordic latitudes couldn't maintain adequate D levels without supplementation or fortified foods (Cashman et al., 2016). Everything else is achievable through a varied diet.

Which vitamins can be dangerous in high doses?

Vitamins A and D pose the greatest overdose risk among fat-soluble vitamins. Vitamin A above 3000 mcg/day can cause liver damage, and chronic D intake above 10,000 IU/day without monitoring can lead to hypercalcemia (Penniston & Tanumihardjo, 2006). Among water-soluble vitamins, B6 above 100 mg/day long-term can cause peripheral neuropathy.

Do athletes need more vitamins than average people?

Some, yes. B-vitamins (B1, B2, B3, B6) are consumed faster during intense training because they're needed for energy metabolism. Vitamin C turnover increases with heavy exercise. But the increase is moderate — roughly 1.5-2x the standard RDA — and usually covered by eating more food (Kennedy, 2016).

What's the difference between synthetic and natural vitamins?

For most vitamins, your body can't tell the difference. Synthetic folic acid is actually absorbed better than food folate. The major exception is vitamin E: natural d-alpha-tocopherol has about twice the bioactivity of synthetic dl-alpha-tocopherol (Traber, 2007). For other vitamins, form matters more than origin.

Should I take a multivitamin or individual vitamins?

A multivitamin is a reasonable insurance policy if your diet has gaps. But if you have a confirmed deficiency (say, vitamin D at 30 nmol/L), a multivitamin won't provide enough — you need a targeted high-dose supplement. Test, don't guess.

Estonia-Specific Notes

Estonia's latitude (58-59degN) makes vitamin D the top priority from October to March. The Estonian National Institute for Health Development recommends D supplementation for all age groups during the dark season. Beyond D, most Estonians eating a typical Nordic diet get adequate B-vitamins and C, though the shift toward processed convenience foods is eroding that advantage.

Local testing is accessible: Synlab and Medicumi labs offer vitamin D panels for around €15-25, often without a doctor's referral. MaxFit carries vitamin D supplements and multivitamins suitable for everyday use.

References

1. Cashman KD, Dowling KG, Skrabakova Z, et al. (2016). Vitamin D deficiency in Europe: pandemic? American Journal of Clinical Nutrition, 103(4), 1033-1044.
2. Penniston KL, Tanumihardjo SA. (2006). The acute and chronic toxic effects of vitamin A. American Journal of Clinical Nutrition, 83(2), 191-201.
3. Holick MF. (2007). Vitamin D deficiency. New England Journal of Medicine, 357(3), 266-281.
4. Kennedy DO. (2016). B Vitamins and the Brain: Mechanisms, Dose and Efficacy — A Review. Nutrients, 8(2), 68.
5. Carr AC, Maggini S. (2017). Vitamin C and Immune Function. Nutrients, 9(11), 1211.
6. Pawlak R, Lester SE, Babatunde T. (2013). The prevalence of cobalamin deficiency among vegetarians assessed by serum vitamin B12. European Journal of Clinical Nutrition, 68(5), 541-548.
7. De-Regil LM, Pena-Rosas JP, Fernandez-Gaxiola AC, Rayco-Solon P. (2015). Effects and safety of periconceptional oral folate supplementation for preventing birth defects. Cochrane Database of Systematic Reviews, 12, CD007950.
8. Padayatty SJ, Sun H, Wang Y, et al. (2004). Vitamin C pharmacokinetics: implications for oral and intravenous use. Annals of Internal Medicine, 140(7), 533-537.
9. Knapen MHJ, Drummen NE, Smit E, Vermeer C, Theuwissen E. (2013). Three-year low-dose menaquinone-7 supplementation helps decrease bone loss in healthy postmenopausal women. Osteoporosis International, 24(9), 2499-2507.
10. Lonsdale D. (2006). A review of the biochemistry, metabolism and clinical benefits of thiamin(e) and its derivatives. Evidence-Based Complementary and Alternative Medicine, 3(1), 49-59.

Explore vitamin D supplements, fat-soluble vs water-soluble vitamin differences, and beginner supplement stacks on MaxFit.

See also:

• Vitamiinid Küüntele: Complete Guide 2026
• Myvitamins Essential Omega-3: Budget-Friendly Omega-3 Review
• Omega-3 and Vitamin D: Two Deficiencies, One Solution