Vitamins are organic compounds your body generally can't make on its own (vitamin D is a partial exception, since skin can produce it from sunlight). They're classified by solubility — whether they dissolve in fat or water — and that single property determines a lot about how each vitamin behaves in your body.
Fat-soluble vitamins (A, D, E, K) are absorbed along with dietary fat and can be stored in your liver and fatty tissue for later use. That storage capacity is convenient, but it also means they can build up to harmful levels if you significantly over-supplement. Water-soluble vitamins (the B-vitamins and vitamin C) aren't meaningfully stored — your body uses what it needs and flushes the rest, which is why they generally need more frequent replenishment through diet.
Fat-Soluble Vitamins
Vitamin A (Retinol)
Essential for vision, cell growth, and immune function. Found as preformed vitamin A in animal products (liver, dairy, eggs) and as provitamin A (beta-carotene) in colorful fruits and vegetables. Adults generally need 700–900 mcg/day.
Vitamin D (Cholecalciferol)
Plays a central role in calcium absorption and bone health, and also supports muscle, immune, nervous system, and cardiovascular function. Your skin produces it from sun exposure; also found in fatty fish and fortified foods. Roughly half of people worldwide have suboptimal vitamin D levels, making supplementation common at various life stages.
Vitamin E (Tocopherol)
Functions primarily as an antioxidant, helping protect cell membranes. Found in nuts, seeds, soybeans, avocados, and leafy vegetables. The daily target for adults is about 15 mg.
Vitamin K (Phylloquinone)
Essential for normal blood clotting. Vitamin K1 comes mainly from leafy greens; vitamin K2 is produced by bacteria in your gut. Adults generally need 90–120 mcg/day. Newborns receive a vitamin K injection at birth because deficiency bleeding in infants can be serious.
Water-Soluble Vitamins
The B-vitamins and vitamin C make up this group. Most are involved in energy production, metabolism, and cell growth.
| Vitamin | Main Role | Good Sources | Adult Target |
|---|---|---|---|
| B1 (Thiamine) | Energy metabolism | Whole grains, nuts, poultry, soybeans | ~1.1–1.2 mg/day |
| B2 (Riboflavin) | Redox reactions, antioxidant support | Dairy, fortified grains | ~1.1–1.3 mg/day |
| B3 (Niacin) | Cellular energy reactions | Fish, meat, legumes, mushrooms | 14–16 mg/day |
| B5 (Pantothenic acid) | Energy production, hormone synthesis | Eggs, milk, whole grains | ~5 mg/day |
| B6 (Pyridoxine) | Protein metabolism, red blood cells | Chickpeas, poultry, fortified cereal | ~1.75 mg/day |
| B7 (Biotin) | Metabolizing protein, fat, carbs | Egg yolks, liver, oats | 5–35 mcg/day |
| B9 (Folate) | DNA synthesis, red blood cells | Leafy greens, broccoli, legumes | 400 mcg/day (more in pregnancy) |
| B12 (Cobalamin) | Nerve function, red blood cells | Meat, dairy, eggs (animal products only) | 2.4 mcg/day |
| Vitamin C | Collagen formation, immune function, antioxidant | Citrus, berries, peppers, tomatoes | ~40–120 mg/day |
Two B-Vitamins Worth Knowing in More Detail
Folate (B9) & B12
Folate (B9) is especially important before and during pregnancy — adequate intake significantly reduces the risk of neural tube defects. This is why supplementation is recommended for all women of reproductive age, not just those who are already pregnant.
B12 is found almost exclusively in animal products, which is why strict vegans and vegetarians are commonly advised to supplement. B12 is unusual among water-soluble vitamins in that your liver can store it for months to years, so deficiency typically develops slowly.