What Are Vitamins
Vitamins are essential organic compounds required in small amounts for normal growth, metabolic function, and disease prevention — categorized as either fat-soluble (A, D, E, K) or water-soluble (C and the eight B vitamins).
Vitamins are organic compounds that the body requires in small amounts to function normally. Unlike macronutrients (which provide energy), vitamins serve as regulators and cofactors — enabling biological processes that otherwise couldn't occur. The word "vitamin" comes from "vital amine" — historically associated with life-sustaining properties.
Most vitamins cannot be synthesized in adequate quantities by the human body and must be obtained from food or, in some cases, supplements.
Fat-Soluble vs. Water-Soluble Vitamins
Vitamins are classified by how they are absorbed and stored:
Fat-Soluble Vitamins: A, D, E, K
- Absorbed with dietary fat and stored in the liver and fatty tissues
- Can accumulate to toxic levels if over-supplemented (particularly vitamins A and D)
- Don't need to be consumed daily — the body draws on stored reserves
Water-Soluble Vitamins: Vitamin C and the 8 B Vitamins
- Dissolve in water and are not significantly stored (excess is excreted in urine)
- Need regular dietary replenishment
- Toxicity from food is rare; high-dose supplements can still cause issues (e.g., high-dose B6 causing nerve damage)
Essential Vitamins and Their Functions
| Vitamin | Key Functions | Deficiency Consequences | Sources |
|---|---|---|---|
| Vitamin A | Vision, immune function, growth | Night blindness, immune deficiency | Liver, sweet potato, spinach |
| Vitamin D | Calcium absorption, bone health, immune function | Rickets (children), osteomalacia, immune impairment | Sunlight, fatty fish, fortified milk |
| Vitamin E | Antioxidant, cell membrane protection | Nerve damage, anemia | Nuts, seeds, vegetable oils |
| Vitamin K | Blood clotting, bone metabolism | Impaired clotting, bone loss | Leafy greens, fermented foods |
| Vitamin C | Collagen synthesis, antioxidant, immune function | Scurvy | Citrus, bell peppers, strawberries |
| Thiamine (B1) | Energy metabolism, nerve function | Beriberi, Wernicke's encephalopathy | Whole grains, legumes |
| Riboflavin (B2) | Energy metabolism, growth | Sores, impaired growth | Dairy, lean meat, spinach |
| Niacin (B3) | Energy metabolism, DNA repair | Pellagra | Meat, fish, peanuts |
| Pantothenic Acid (B5) | Coenzyme A synthesis | Rare; fatigue, irritability | Widespread in food |
| Vitamin B6 | Protein metabolism, neurotransmitter synthesis | Anemia, depression | Poultry, fish, potatoes |
| Biotin (B7) | Fatty acid synthesis, carbohydrate metabolism | Rare; hair loss, skin rash | Eggs, nuts, legumes (raw egg whites can deplete) |
| Folate (B9) | DNA synthesis, cell division | Neural tube defects (pregnancy), megaloblastic anemia | Leafy greens, beans, fortified grains |
| Vitamin B12 | Nerve function, red blood cell production | Pernicious anemia, neurological damage | Meat, fish, dairy, eggs; vegans must supplement |
Most Common Vitamin Deficiencies
Despite living in food-rich environments, vitamin deficiencies remain prevalent:
- Vitamin D: Extremely common, especially in people with limited sun exposure, darker skin, or living at northern latitudes. Subclinical deficiency affects an estimated 40–50% of the U.S. population. Linked to bone disease, immune dysfunction, and depression.
- Vitamin B12: At risk: vegans, vegetarians, older adults (decreased absorption), and those taking metformin (reduces B12 absorption). B12 deficiency causes irreversible nerve damage if untreated.
- Folate: Critically important during early pregnancy (first 4 weeks, often before pregnancy is confirmed) — deficiency causes neural tube defects. All women of childbearing age are advised to take a folic acid supplement (400mcg/day).
Vitamin Supplements vs. Whole Foods
The evidence overwhelmingly supports getting vitamins from a varied, whole-food diet rather than supplements:
- Vitamins in whole foods are accompanied by fiber, cofactors, and phytonutrients that interact synergistically
- Several large trials of isolated vitamin supplements (E, A, beta-carotene) have shown neutral or even harmful outcomes
- Fat-soluble vitamins can accumulate to toxic levels from supplements (especially vitamin A and D in high doses)
When supplements are appropriate:
- Confirmed deficiency
- Specific life circumstances (prenatal folate; vegan B12; vitamin D for most adults in winter months or with limited sun exposure)
- Absorption issues (older adults with B12; malabsorptive conditions)
Getting Adequate Vitamins Through Food
A dietary pattern that consistently provides all essential vitamins:
- Varied vegetables and fruit (vitamins C, K, folate, A, carotenoids)
- Whole grains (B vitamins)
- Legumes (folate, B6, thiamine)
- Eggs and dairy (B12, A, D, riboflavin)
- Fortified foods (plant milks, cereals — often fortified with D, B12, folate)
- Fatty fish (D, B12, omega-3s)
- Nuts and seeds (E, B vitamins)
- Sun exposure (the most efficient source of vitamin D)
The single most effective dietary instruction for vitamin adequacy: eat a wide variety of whole foods, with an emphasis on vegetables, fruit, legumes, and whole grains. Variety ensures that gaps from any one food are covered by others.