Vitamins are organic compounds your body needs in small amounts to run hundreds of essential chemical reactions, from converting food into energy to building bone and fighting infections. Your cells can’t make most of them on their own, so you rely on food (or supplements) to keep a steady supply. Despite being needed only in tiny quantities, measured in milligrams or micrograms, a shortfall in even one vitamin can disrupt processes throughout the body.
How Vitamins Work Inside Your Cells
Most vitamins function as helpers for enzymes, the proteins that carry out chemical reactions in your cells. Without the right vitamin attached, many enzymes can’t do their jobs. The B vitamins, for example, all serve as these molecular helpers (called cofactors) for reactions involved in energy production, sugar metabolism, and building DNA.
Because each cell only needs a small amount of these helpers, animals lost the ability to produce most vitamins over the course of evolution. It was more efficient to absorb them from food than to maintain the biological machinery to manufacture them. That evolutionary shortcut is the reason vitamins are “essential” in your diet: your body literally cannot make them.
Fat-Soluble vs. Water-Soluble Vitamins
Vitamins split into two broad categories based on how your body stores and eliminates them, and this distinction matters for both absorption and safety.
Water-soluble vitamins (the B vitamins and vitamin C) dissolve in water and are not stored in the body to a significant extent. Any excess is filtered out through urine, which means you need a relatively consistent daily intake. The upside is that toxicity from water-soluble vitamins is rare.
Fat-soluble vitamins (A, D, E, and K) dissolve in fat and are stored in your liver and fatty tissues. This creates a reservoir your body can draw on when intake drops, but it also means excessive supplementation can build up over time and cause toxicity. Eating some dietary fat alongside these vitamins improves their absorption, which is why a drizzle of olive oil on a salad helps you get more vitamin A from the carrots in it.
Turning Food Into Energy
The B vitamins are the main players in energy metabolism. They don’t provide energy themselves (that comes from calories in food), but without them your cells can’t efficiently extract energy from carbohydrates, fats, and proteins.
Thiamine (B1) is critical for breaking down glucose through the main energy-producing cycle inside your cells. Riboflavin (B2) helps metabolize carbohydrates, proteins, and fats into usable fuel. Niacin (B3) is converted into two key molecules that shuttle electrons during energy production, essentially keeping the power grid of your cells running. B12 supports the synthesis of DNA, hormones, and proteins. A deficiency in any single B vitamin can negatively affect how your cells process glucose, amino acids, and fatty acids.
This is why fatigue is one of the earliest and most common signs of B vitamin deficiency. Your cells are still receiving fuel, but they can’t convert it efficiently.
Building Skin, Bones, and Connective Tissue
Vitamin C is essential for producing collagen, the most abundant protein in your body. Collagen provides structural support to skin, tendons, ligaments, and blood vessels. Vitamin C activates the enzymes that fold collagen into its stable triple-helix shape. Without enough vitamin C, collagen production falters, leading to the weakened gums, slow wound healing, and skin changes seen in scurvy.
Vitamin C also works as a powerful antioxidant, neutralizing harmful molecules that damage cells during inflammation. This dual role in tissue building and cellular protection makes it important both for recovery from injuries and for everyday maintenance.
Vitamin D controls how much calcium your intestines absorb from food. When vitamin D levels are adequate, your body can pull calcium into the bloodstream and direct it to bones and teeth. Without it, even a calcium-rich diet may not be enough because the mineral passes through your gut without being absorbed efficiently. Vitamin K complements this process: its K1 form is transported to the liver, where it regulates the production of proteins needed for blood clotting. Vitamin K also helps direct calcium into bones rather than into arteries, which is why researchers describe D and K as working together for both bone and cardiovascular health.
Vision and Cell Growth
Vitamin A plays a unique role in your eyes. In the retina, a form of vitamin A serves as the light-sensitive molecule inside photoreceptor cells. When a photon of light hits this molecule, it changes shape, triggering an electrical signal that your brain interprets as vision. After each flash, enzymes in the eye recycle the molecule back to its original form so it can detect light again. This continuous recycling cycle is why even a mild vitamin A deficiency can cause night blindness: there simply isn’t enough of the molecule to keep the cycle running smoothly in low light.
Beyond vision, a derivative of vitamin A binds to receptors inside the nucleus of cells and directly controls the activity of roughly 500 genes in the human genome. These genes govern cellular differentiation, the process by which unspecialized cells become specific cell types during growth, immune response, and tissue repair. This gene-regulating role makes vitamin A particularly important during pregnancy and childhood development.
What Affects How Well You Absorb Vitamins
Getting enough vitamins in your diet is only part of the equation. Several factors determine how much actually makes it into your bloodstream.
- Dietary fat: Fat-soluble vitamins (A, D, E, K) need fat present in the gut to be absorbed. Taking a vitamin D supplement with a fat-free meal reduces how much you absorb.
- Plant-based food structure: Vitamins in plant foods can be trapped inside cell walls or bound by compounds like phytate and fiber, reducing bioavailability compared to animal-based sources or supplements.
- Gut health: A healthy gut microbiome increases absorption of vitamins and minerals. Conversely, bacterial overgrowth or gut inflammation can reduce the availability of several vitamins.
- Age: Older adults have a reduced ability to absorb certain vitamins, particularly B12, which requires a specific stomach protein for absorption that declines with age.
- Medications: Several common medications reduce vitamin absorption and status, including certain antacids, diabetes drugs, and anti-seizure medications.
- Life stage: Pregnancy and lactation increase your body’s absorptive capacity for many nutrients, a built-in adaptation to meet higher demands.
What Happens When You Get Too Little
Vitamin B12 deficiency is one of the most common deficiencies worldwide, especially in populations with low intake of animal products (meat, dairy, eggs). Because B12 is involved in nerve function and DNA synthesis, the symptoms go well beyond fatigue. Neurological signs include tingling and numbness in the hands and feet, difficulty walking, dizziness, memory problems, and difficulty concentrating. In more severe cases, personality changes, vision blurring, and even seizures can occur. Importantly, these neurological symptoms can appear before any changes show up on a standard blood count, so a normal-looking blood test doesn’t rule out deficiency.
Vitamin D deficiency is widespread in northern latitudes and among people who spend most of their time indoors, since sunlight triggers vitamin D production in the skin. Low levels weaken bones over time because calcium absorption drops. Vitamin A deficiency remains a leading cause of preventable blindness in developing countries and impairs immune function.
What Happens When You Get Too Much
Because fat-soluble vitamins accumulate in the body, overdose is a real concern with high-dose supplements. The tolerable upper limit for vitamin A in adults is 3,000 micrograms per day. Beyond that, acute toxicity can cause nausea, vomiting, headache, dizziness, blurred vision, and peeling skin on the palms and soles. Chronic excess over weeks or months leads to more serious effects: hair loss, brittle nails, bone pain, joint pain, and liver enlargement. In severe cases, vitamin A toxicity causes a dangerous increase in pressure inside the skull.
Water-soluble vitamins are generally safer in excess because your kidneys flush out what you don’t need. Still, extremely high doses of certain B vitamins or vitamin C from supplements can cause digestive upset or, rarely, nerve damage. The risk of toxicity from food alone is essentially zero for all vitamins. Problems arise almost exclusively from supplements.
Daily Amounts Your Body Needs
The FDA’s current daily values for adults provide a useful benchmark. These are the amounts that meet the needs of most healthy people:
- Vitamin A: 900 micrograms
- Vitamin C: 90 milligrams
- Vitamin D: 20 micrograms (800 IU)
- Vitamin E: 15 milligrams
- Thiamine (B1): 1.2 milligrams
- Riboflavin (B2): 1.3 milligrams
- Niacin (B3): 16 milligrams
- Vitamin B6: 1.7 milligrams
- Vitamin B12: 2.4 micrograms
- Folate: 400 micrograms
Most people eating a varied diet that includes vegetables, fruits, whole grains, and some animal products will meet these targets without supplements. The groups most likely to fall short are older adults (B12, D), people on restrictive diets (B12, iron, D), pregnant women (folate, iron), and people with limited sun exposure (D).