Vitamins are essential compounds that keep your body running by acting as helpers in hundreds of chemical reactions. They don’t supply energy on their own, but without them, your body can’t convert food into fuel, build new tissue, fight off infections, or repair damage. There are 13 essential vitamins, and each one has a specific job. Most of them work as coenzymes, meaning they attach to enzymes and activate the chemical reactions that sustain life.
How Vitamins Work Inside Your Body
Think of vitamins as keys that unlock your body’s machinery. Enzymes do the heavy lifting of metabolism, but many of them can’t function until a vitamin plugs into them and switches them on. This is why vitamins are involved in such a wide range of processes: energy production, wound healing, immune defense, brain signaling, and protecting cells from damage.
Your body can’t manufacture most vitamins in sufficient quantities, so you need a steady supply from food or, in some cases, sunlight. What happens to a vitamin after you consume it depends on whether it dissolves in fat or water, a distinction that shapes everything from how it’s absorbed to how long it stays in your system.
Fat-Soluble vs. Water-Soluble Vitamins
The 13 vitamins split into two groups based on how your body handles them.
Fat-soluble vitamins (A, D, E, and K) are absorbed along with dietary fats and stored in your liver and fat tissue for up to six months. Because your body holds onto them, you don’t need to consume them every single day, but this storage ability also means they can build up to harmful levels if you take very high doses over time.
Water-soluble vitamins (the eight B vitamins and vitamin C) dissolve in water and enter your bloodstream directly. Your body doesn’t store them. Whatever it doesn’t need gets filtered out through urine, which is why you need a fresh supply most days. The tradeoff is that toxicity from water-soluble vitamins is rare since excess amounts leave the body quickly.
B Vitamins and Energy Production
The B vitamins are the engine room of your metabolism. Their main collective job is helping turn carbohydrates, fats, and proteins from food into ATP, the molecule your cells use as fuel. Each B vitamin handles a different part of this process.
Thiamin (B1) kicks off the reactions that convert glucose into ATP. Riboflavin (B2) is a component of two coenzymes needed for energy production and cellular metabolism. Niacin (B3) helps transfer the energy found in food into usable form. Biotin (B7) assists the enzymes that break down fats, carbs, and proteins. Without adequate B vitamins, this entire energy pipeline slows down, which is why fatigue is one of the earliest signs of deficiency.
Several B vitamins also play critical roles in your nervous system. B1 supports nerve cell function. B6 contributes to brain health and immune function. B12 is essential for organ and brain function, and a deficiency can cause irreversible brain damage. Folate (B9) is critical for the proper formation of the brain and spinal cord in a developing fetus, which is why it’s so heavily emphasized during pregnancy.
The recommended daily intake for B12 is 2.4 micrograms for adults. That’s a tiny amount, but people who eat little or no animal products, older adults, and those with digestive conditions often fall short.
What Vitamin C Does
Vitamin C is best known for immune support, but its core function is helping your body build and maintain connective tissue. It’s required for producing collagen, the protein that gives structure to your skin, blood vessels, tendons, and bones. It also works as an antioxidant, neutralizing unstable molecules called free radicals that damage cells over time.
Adult men need about 90 mg daily, and adult women need about 75 mg. A single orange or cup of strawberries covers most of that. Because vitamin C is water-soluble, your body doesn’t stockpile it, so consistent intake matters.
What Each Fat-Soluble Vitamin Does
Vitamin A
Vitamin A is essential for vision, particularly your ability to adjust from bright light to darkness. It also supports immune function, skin health, and cell growth. In its beta-carotene form (found in orange and dark green vegetables), it acts as a strong antioxidant that helps protect against cellular damage. When vitamin A runs low, impaired night vision is one of the first symptoms. Severe deficiency can lead to corneal drying and clouding, which can cause blindness.
Vitamin D
Vitamin D promotes calcium absorption and manages calcium and phosphorus levels in your blood, which allows for normal bone growth and development. Your skin produces it when exposed to sunlight, but many people don’t get enough this way, especially in northern climates or during winter months. The recommended daily intake is 600 IU for adults up to age 70 and 800 IU for those over 71. Deficiency leads to soft, weakened bones. In children, this shows up as bone deformities. In adults, it causes bone tenderness and increased fracture risk.
Vitamin E
Vitamin E is the body’s primary fat-soluble antioxidant. It protects cell membranes from damage caused by free radicals, both the ones your body produces naturally during metabolism and those from external sources like air pollution, cigarette smoke, and ultraviolet light. Good sources include nuts, seeds, and vegetable oils.
Vitamin K
Vitamin K is essential for producing fibrin, the protein that forms blood clots. Without enough of it, even minor cuts or injuries could lead to uncontrolled bleeding. It’s also necessary for building and maintaining bone tissue. Leafy green vegetables are the richest dietary source.
What Happens When You Don’t Get Enough
Because each vitamin has a specific role, deficiency symptoms tend to be distinctive. Low vitamin A impairs night vision. Low vitamin D weakens bones. Low B12 causes cognitive and sensory problems, and in severe cases, dementia. These symptoms usually develop gradually over weeks or months, not overnight, which makes them easy to dismiss early on.
Certain groups face higher deficiency risk. Older adults absorb B12 less efficiently. People with limited sun exposure often run low on vitamin D. Those on highly restrictive diets may miss several vitamins at once. Pregnancy increases the need for folate, iron, and several other nutrients simultaneously.
Food Sources vs. Supplements
For most people, a varied diet provides all the vitamins the body needs. But when supplements enter the picture, absorption isn’t always identical to what you’d get from food. Synthetic folate, for example, is actually more easily absorbed than the natural form found in food. For vitamin C, studies have found no significant difference between natural and synthetic forms. Synthetic vitamin E, on the other hand, has lower biological activity than its natural counterpart, meaning you’d need a higher dose to get the same effect.
One practical detail that makes a real difference with fat-soluble vitamins: take them with a meal that contains some fat. Since they’re absorbed alongside dietary fats, taking them on an empty stomach reduces how much your body actually picks up. Water-soluble vitamins don’t have this requirement and can be taken at any time.