Vitamins keep your body running by acting as helpers in thousands of chemical reactions, from converting food into energy to building bone and fighting infections. They don’t supply calories or fuel on their own. Instead, they enable the processes that turn what you eat into the structures and energy your cells need. Without adequate amounts of each vitamin, those processes slow down or stop entirely, leading to symptoms that range from fatigue to serious disease.
How Vitamins Work at the Cellular Level
Most vitamins function as coenzymes or cofactors, meaning they attach to enzymes and allow chemical reactions to happen. Think of an enzyme as a machine and a vitamin as the key that turns it on. All of the B vitamins work this way, providing cofactors for reactions involved in energy production, sugar metabolism, and the building of new molecules your cells need. Without the right vitamin present, the enzyme sits idle.
Some vitamins play a different role: they act as antioxidants, neutralizing unstable molecules (free radicals) that damage cells. Others function more like hormones, switching genes on or off to control how cells grow, mature, and specialize. Vitamin D, for instance, binds to receptors inside cells and directly influences which genes are active, a process that affects everything from calcium absorption to immune function.
Turning Food Into Energy
Your body breaks down carbohydrates, fats, and proteins through a series of chemical steps that ultimately produce ATP, the molecule your cells burn for energy. B vitamins are essential at nearly every stage of this process. Thiamine (B1), riboflavin (B2), and niacin (B3) serve as cofactors in the major energy-producing pathways, including the breakdown of sugars and fatty acids and the final steps where your cells generate the bulk of their ATP.
This is why a B-vitamin deficiency often shows up as persistent fatigue or weakness before any other symptom appears. Your cells literally cannot extract energy from food as efficiently. Vitamins cannot be converted into energy directly, but without them, the conversion machinery stalls. Eating enough calories means nothing if your body lacks the tools to process them.
Building and Protecting Your Bones
Vitamin D controls how much calcium your intestines absorb from food. When it’s activated in your body, it increases the number of calcium transporters lining your gut, raising the maximum amount of calcium you can pull in from a meal. In animal studies, vitamin D deficiency reduced calcium absorption efficiency by more than 75%. That’s a dramatic drop, and it explains why vitamin D deficiency leads to soft, weak bones even when calcium intake is adequate.
This process matters throughout life: during childhood for building strong bones, in adulthood for maintaining them, and in older age for preventing osteoporosis. Vitamin D needs to be activated by your kidneys before it can do this job, which is one reason kidney disease often leads to bone problems.
Supporting Your Immune System
Vitamin C does double duty in immune defense. It promotes the development and maturation of key immune cells, including T cells, B cells, and natural killer cells, the frontline fighters your body deploys against infections. It also helps balance the immune response, supporting strong reactions against pathogens while limiting excessive activation that could damage your own tissues.
Vitamin C is also a required cofactor for collagen synthesis, the process that builds the connective tissue in your skin, blood vessels, and organs. Collagen forms a physical barrier against infection, and it’s essential for wound healing. Without enough vitamin C, wounds heal slowly and old wounds can even reopen, which is the hallmark of scurvy.
Vitamin A plays a parallel role. Its acid form regulates cell growth and differentiation from embryonic development onward, and it maintains the integrity of epithelial tissues, the linings of your lungs, gut, and skin that serve as your body’s first defense against invaders.
Enabling Vision
Vitamin A is the foundation of your ability to see. Inside the light-sensing cells of your retina, a form of vitamin A called retinal is built into the pigment molecules that detect light. Every time light hits one of these pigments, the vitamin A molecule changes shape, triggering a nerve signal to your brain. The spent molecule is then recycled through a dedicated pathway called the visual cycle so it can be used again. This cycle runs continuously as long as you’re seeing, which is why vitamin A deficiency causes night blindness first: your eyes can’t regenerate those light-sensing pigments fast enough in dim conditions.
Clotting Your Blood
When you get a cut, your body launches a cascade of proteins that form a clot to stop the bleeding. Four of the core proteins in this cascade require vitamin K to function. Vitamin K enables a chemical modification that allows these clotting proteins to bind calcium, and calcium binding is what activates them. Without vitamin K, your blood cannot clot properly, which is why newborns (who have very little vitamin K) receive a dose at birth, and why people on certain blood-thinning medications need to keep their vitamin K intake consistent.
Protecting Cells From Damage
Vitamin E is the body’s primary fat-soluble antioxidant. It sits within the fatty membranes that surround every cell and intercepts chain reactions caused by free radicals before they can spread and destroy the membrane. This is particularly important in tissues with high metabolic activity, like your lungs, brain, and red blood cells, where free radical production is constant. Your cells also have built-in systems that recycle vitamin E after it neutralizes a free radical, so a relatively small amount can provide ongoing protection.
Fat-Soluble vs. Water-Soluble Vitamins
How your body handles a vitamin depends on whether it dissolves in fat or water, and this distinction affects everything from how you should eat them to how long they last in your body.
Fat-soluble vitamins (A, D, E, and K) are absorbed alongside the fats in your food, then stored in your liver and fat tissue for up to six months. Because your body can stockpile them, you don’t need to consume them every single day, but this storage capacity also means they can build up to toxic levels if you take excessive supplements.
Water-soluble vitamins (C and all eight B vitamins) enter your bloodstream directly and are not stored long-term. Whatever your body doesn’t use gets filtered out through your urine. This means you need a steady daily supply from food, but it also means toxicity from water-soluble vitamins is rare.
Food Pairings That Boost Absorption
Not all vitamins are absorbed equally well in every meal. Certain food combinations dramatically improve how much your body actually takes in.
- Spinach and citrus: The vitamin C in citrus fruits helps your body absorb the iron in spinach, which would otherwise be poorly absorbed from plant sources.
- Tomatoes and olive oil: Fat-soluble nutrients in tomatoes are absorbed much more effectively when eaten with healthy fats like olive oil or avocado.
- Cereal and milk: The vitamin D added to many cereals helps your intestines absorb more of the calcium in milk.
The same principle applies broadly: eating fat-soluble vitamins with a source of dietary fat improves their absorption. A salad with a fat-free dressing, for example, delivers less vitamin A and vitamin K to your body than the same salad with an oil-based dressing.
How Much You Need Each Day
Daily requirements vary by age and sex, but for healthy adults aged 19 to 30, the recommended intakes are:
- Vitamin A: 900 µg for men, 700 µg for women
- Vitamin C: 90 mg for men, 75 mg for women
- Vitamin D: 600 IU (15 µg)
- Vitamin E: 15 mg
- Vitamin K: 120 µg for men, 90 µg for women
- Folate: 400 µg
- Vitamin B12: 2.4 µg
Most people who eat a varied diet with plenty of fruits, vegetables, whole grains, and protein sources meet these targets without supplements. Deficiencies tend to show up in people with restricted diets, absorption disorders, or limited sun exposure (in the case of vitamin D). The symptoms of mild deficiency are often vague, things like fatigue, slow healing, or frequent illness, which is why they’re easy to miss and worth considering if those issues persist.