Vitamins are organic compounds your body needs in small amounts to stay alive and function properly. There are 13 essential vitamins, and they do everything from converting food into energy to building bones, clotting blood, and defending against infection. Your body either can’t make these compounds at all or can’t make enough of them, so they have to come from food or supplements.
How Vitamins Power Your Cells
The most fundamental job of vitamins is helping your cells produce energy. When you eat carbohydrates, fats, or protein, your body breaks them down into smaller molecules and funnels them through a series of chemical reactions inside your cells’ mitochondria. B vitamins are essential players at nearly every step of this process. Thiamine (B1), riboflavin (B2), niacin (B3), and pantothenic acid (B5) all serve as co-enzymes, meaning they physically attach to enzymes and allow those reactions to happen. Without them, the whole system stalls.
This is why fatigue is one of the earliest signs of B vitamin deficiency. Your cells are literally producing less energy. Biotin (B7) and B12 also feed into this energy cycle by helping break down fatty acids and amino acids so they can enter the same pathway.
What Each Vitamin Does
The 13 essential vitamins split into two groups based on how your body handles them: four are fat-soluble (A, D, E, and K), and nine are water-soluble (vitamin C and the eight B vitamins). Fat-soluble vitamins dissolve in fat, get absorbed alongside dietary fats, and can be stored in your liver and fatty tissue. Water-soluble vitamins dissolve in water, and your body generally flushes out what it doesn’t use right away.
Here’s what each one does:
- Vitamin A: Maintains healthy skin, teeth, bones, and the tissue in the back of your eye that creates vision.
- Vitamin C: An antioxidant that promotes wound healing, helps your body absorb iron, and maintains healthy gums and connective tissue.
- Vitamin D: Helps your body absorb calcium for strong bones and teeth. Your skin can produce it from sunlight.
- Vitamin E: An antioxidant that protects cells from damage and helps form red blood cells.
- Vitamin K: Essential for blood clotting. It also activates proteins that direct calcium into bones and keep it out of artery walls.
- B1 (thiamine): Converts carbohydrates into energy. Important for heart function and healthy nerve cells.
- B2 (riboflavin): Supports body growth and red blood cell production.
- B3 (niacin): Maintains healthy skin and nerves. At higher doses, it can lower triglycerides.
- B5 (pantothenic acid): Central to food metabolism and hormone production.
- B6: Helps form red blood cells and supports brain function. The more protein you eat, the more B6 your body requires.
- B7 (biotin): Needed for metabolizing proteins and carbohydrates and producing hormones.
- B9 (folate): Works with B12 to form red blood cells and is critical for DNA production, which controls cell growth.
- B12: Supports the central and peripheral nervous systems, helps form red blood cells, and plays a role in metabolism.
Immune Defense
Several vitamins directly support your immune system, but vitamin C and vitamin D get the most attention. Vitamin C contributes to both your innate immune response (the general first line of defense) and your adaptive immune response (the targeted attack your body mounts against specific threats). It supports the function of various immune cells and has been shown to inhibit the replication of certain viruses, including influenza.
Vitamin D regulates the immune system in a different way. It helps maintain the integrity of your epithelial cells, which form the barrier between your body and the outside world. It also promotes the production of antimicrobial peptides, natural proteins that kill bacteria and viruses. During the COVID-19 pandemic, researchers found that vitamin D helps regulate the inflammatory response that can cause severe respiratory distress, reinforcing its role as an immune modulator rather than just a bone vitamin.
Bones, Blood, and Calcium
Vitamin D and vitamin K work as a team when it comes to bone health, though most people only hear about D. Vitamin D enables your body to absorb calcium from food. But once calcium enters your bloodstream, vitamin K2 determines where it goes. Bone-building cells produce a protein called osteocalcin that pulls calcium from the blood and binds it to bone. That protein is inactive until vitamin K2 switches it on.
Vitamin K2 also activates a separate protein in blood vessel walls that prevents calcium from accumulating there. When you don’t get enough K2, that protective protein stays inactive, and calcium can deposit in your arteries instead of your bones. This is why adequate vitamin K intake matters for cardiovascular health, not just bone density. Unfortunately, vitamin K2 is nearly absent from processed foods and low even in many otherwise healthy Western diets.
On the blood side, several vitamins are involved in producing red blood cells. B6, B9 (folate), and B12 all play direct roles, which is why deficiency in any of them can lead to anemia and fatigue.
What Deficiency Feels Like
Vitamin deficiencies rarely announce themselves with a single dramatic symptom. They tend to creep in as vague, overlapping problems that are easy to dismiss. Fatigue is the most common early sign across multiple deficiencies, particularly B6, B9, B12, and vitamin D. But the symptoms can get more specific as deficiency deepens.
B12 deficiency, for example, can cause numbness and tingling in your hands and feet, trouble with balance and coordination, memory loss, difficulty concentrating, and mood changes including depression. These happen because B12 is critical for your nervous system, and without it, nerve signaling deteriorates. Some people experience loss of bladder or bowel control, or even white spots on their skin.
Vitamin D deficiency weakens bones. In children, severe deficiency causes rickets, where bones bend and bow. In adults, it causes osteomalacia, a condition where bones become soft and fracture easily. Because vitamin D receptors exist throughout the body, low levels have also been linked to fatigue, muscle weakness, and impaired immune function.
How to Absorb Vitamins Effectively
Fat-soluble vitamins (A, D, E, and K) need dietary fat to be absorbed properly. Research shows that absorption drops significantly when your daily fat intake falls below about 5 grams. This is why taking a vitamin D supplement on an empty stomach or with a fat-free meal can reduce how much your body actually takes in. Eating these vitamins alongside foods that contain some fat, even a handful of nuts or a drizzle of olive oil, makes a meaningful difference.
The form of a vitamin also matters. Natural vitamin E (listed as d-alpha-tocopherol on labels) has significantly higher bioavailability than synthetic vitamin E (dl-alpha-tocopherol acetate). Studies show the natural form is retained in tissues more effectively and provides stronger antioxidant protection. This distinction applies to other vitamins too: folate from leafy greens behaves differently than folic acid in a supplement, and the B12 in meat is absorbed differently than the B12 in a multivitamin.
How Much You Need
The Dietary Guidelines for Americans set recommended daily amounts for each vitamin. A few key ones for adults: vitamin D is set at 600 IU per day for ages 19 to 70, increasing to 800 IU after age 70. Vitamin B12 is 2.4 micrograms per day for all adults. These numbers represent the intake sufficient for about 97% of healthy people, not the minimum to avoid disease.
Upper limits exist too, particularly for the fat-soluble vitamins your body stores rather than excretes. The tolerable upper intake for vitamin A (from preformed sources like liver or supplements, not from plant-based beta-carotene) is 3,000 micrograms per day for adults. For vitamin D, it’s 50 micrograms (2,000 IU) per day. Exceeding these levels consistently can cause toxicity. Vitamin A toxicity can damage your liver, while excessive vitamin D raises blood calcium to dangerous levels. Water-soluble vitamins are generally safer in high doses because your kidneys flush the excess, but that’s not a guarantee of zero risk.
For most people eating a varied diet with fruits, vegetables, whole grains, and some animal products, supplementation isn’t necessary for every vitamin. The exceptions tend to be vitamin D (especially if you live at a northern latitude or spend little time outdoors), B12 (especially for people over 50 or those eating a plant-based diet), and folate (for women who are pregnant or planning to become pregnant).