From Vitamin A to Vitamin K, here is everything you need to know about what vitamins do, which foods have them, and why Americans are not getting enough.
Most Americans take at least one vitamin every day. But if you ask the average person what Vitamin K actually does, or why B12 matters more after age 50, you get a blank stare. We pop our supplements, drink our orange juice, and hope for the best.
That is not good enough.
Your body runs on vitamins. They power your immune system, keep your brain sharp, strengthen your bones, and help every single cell do its job. When you are low on one, things start going wrong. And because most deficiencies build up slowly, people spend years feeling tired, foggy, or run-down without ever connecting it to something as simple as a missing nutrient.
This guide walks you through every essential vitamin, from A all the way to K. No fluff, no filler. Just clear, practical information about what each one does, where to get it, and what happens when you do not get enough.
Why Vitamins Matter More Than Most People Think
Here is a sobering number: research suggests that more than 90 percent of Americans do not get enough of at least one essential vitamin from their diet alone. That is not a fringe group. That is nearly everyone.
Part of the problem is modern eating habits. Fast food, processed snacks, and on-the-go meals are convenient, but they are stripped of a lot of the nutrients your body is looking for. Part of the problem is also that most people simply do not know what they need or why they need it.
Before we get into the full breakdown, there is one key concept worth understanding.
Fat-soluble vitamins (A, D, E, and K) get stored in your body fat and liver. Your body holds onto them, which means you do not need them every single day, but it also means you can build up too much if you take very high doses over time.
Water-soluble vitamins (all eight B vitamins and Vitamin C) do not get stored the same way. Whatever your body does not use, it flushes out. That sounds reassuring, but it also means you need a steady supply from food or supplements to keep your levels where they should be.
With that foundation in place, here is your full vitamin alphabet.
Vitamin A: The Vision and Immunity Vitamin
Vitamin A is probably best known for eye health, and that reputation is well earned. It plays a direct role in producing the pigments your eyes need to function in low light. Consistent deficiency is one of the leading causes of preventable blindness worldwide.
But vision is just the beginning. Vitamin A is also critical for immune function. It helps maintain the linings of your respiratory tract, digestive system, and skin, which act as physical barriers against infection. Without enough of it, those barriers weaken.
It also supports cell growth and reproduction, which is why it shows up in so many skincare products.
Where to find it: Beef liver, sweet potatoes, carrots, spinach, and fortified dairy products. Your body can also convert beta-carotene from orange and yellow vegetables into Vitamin A.
Signs of deficiency: Night blindness is often the first noticeable symptom. Dry skin, frequent infections, and slow wound healing are also common.
American context: Most people in the U.S. get enough Vitamin A, but vegans and people with digestive conditions that affect fat absorption are at higher risk for low levels.
The B Vitamins: Eight Vitamins, One Big Family
The B vitamins are not a single nutrient. They are a family of eight distinct vitamins that share a general theme: they help your body convert food into energy and support the nervous system. Each one has its own specific role, and being low in one does not mean you are low in all of them.
Vitamin B1 (Thiamine): The Energy Starter
Thiamine helps your cells kick off the process of turning carbohydrates into usable energy. It is particularly important for the brain and nervous system, both of which rely heavily on glucose.
Where to find it: Whole grains, pork, legumes, and nuts.
Signs of deficiency: Fatigue, nerve tingling, and in severe cases, a condition called beriberi, which affects the heart and nervous system. Heavy alcohol use depletes thiamine rapidly, which is why it is often the first thing given to patients in alcohol recovery.
Vitamin B2 (Riboflavin): The Repair Vitamin
Riboflavin plays a role in energy production and also helps your body repair and maintain tissues. It works closely with other B vitamins to keep things running smoothly.
Where to find it: Eggs, dairy products, lean meats, and dark leafy greens.
Signs of deficiency: Cracked lips, sores at the corners of your mouth, a swollen or sore tongue, and sensitivity to light. These symptoms are uncomfortable enough that most people notice when riboflavin is low.
Vitamin B3 (Niacin): The Cholesterol Manager
Niacin does double duty. Like the other B vitamins, it supports energy metabolism. But it also plays a meaningful role in managing cholesterol levels. High-dose niacin has been used clinically to raise HDL (good) cholesterol, though it should only be used that way under medical supervision.
Where to find it: Chicken, tuna, pork, beef, and peanuts. Your body can also make a small amount from the amino acid tryptophan.
Signs of deficiency: Severe niacin deficiency causes pellagra, characterized by dermatitis, diarrhea, and cognitive decline. Mild deficiency can show up as fatigue and digestive issues.
Vitamin B5 (Pantothenic Acid): The Stress Support Vitamin
Pantothenic acid is involved in producing coenzyme A, which plays a central role in energy metabolism and the synthesis of hormones, including the stress hormones produced by your adrenal glands. It is sometimes called the anti-stress vitamin, though that label is a bit of an oversimplification.
Where to find it: It is found in almost every food, which is how it got its name. “Pantothenic” comes from the Greek word for “everywhere.” Chicken, beef, eggs, avocados, and whole grains are all solid sources.
Signs of deficiency: Rare, but can include fatigue, numbness, and irritability.
Vitamin B6 (Pyridoxine): The Brain Chemistry Vitamin
B6 is involved in over 100 enzyme reactions in the body, many of which affect brain function. It helps produce serotonin, dopamine, and other neurotransmitters that regulate mood, sleep, and focus. It also supports immune function and protein metabolism.
Where to find it: Poultry, fish, potatoes, and non-citrus fruits like bananas.
Signs of deficiency: Depression, confusion, weakened immune response, and a specific type of anemia. Older adults and people with kidney disease or alcohol use disorder are at higher risk.
Vitamin B7 (Biotin): The Hair and Metabolism Vitamin
Biotin is the one you see advertised heavily for hair and nail growth. The science on that is somewhat mixed, but biotin does play a genuine role in metabolizing fats, carbohydrates, and proteins. It is also essential during pregnancy for fetal development.
Where to find it: Eggs (especially the yolk), salmon, nuts, seeds, and sweet potatoes. Interestingly, eating large amounts of raw egg whites can block biotin absorption.
Signs of deficiency: Hair thinning, brittle nails, skin rashes, and fatigue. True deficiency is uncommon in people who eat a varied diet.
Vitamin B9 (Folate): The Cell Growth Vitamin
Folate is essential for DNA synthesis and cell division. This makes it particularly critical during periods of rapid growth, including pregnancy. Getting enough folate in early pregnancy significantly reduces the risk of neural tube defects in newborns.
Where to find it: Dark leafy greens, legumes, fortified cereals, and citrus fruit. The synthetic form, folic acid, is added to most enriched grain products in the U.S.
Signs of deficiency: Megaloblastic anemia, fatigue, mouth sores, and during pregnancy, increased risk of birth defects. Women who could become pregnant are typically advised to take folic acid supplements as a precaution.
Vitamin B12 (Cobalamin): The Nerve and Energy Vitamin
B12 is one of the most talked-about vitamins in the U.S., and for good reason. It keeps nerve cells healthy, supports red blood cell production, and plays a key role in DNA synthesis. It also works with folate to reduce levels of homocysteine, an amino acid linked to heart disease when elevated.
Where to find it: Almost exclusively in animal products: meat, fish, poultry, eggs, and dairy. This is why vegans and vegetarians need to pay close attention to their B12 intake.
Signs of deficiency: Fatigue, weakness, memory problems, numbness or tingling in the hands and feet, and in serious cases, irreversible nerve damage. B12 deficiency can develop slowly over years because the body stores a reserve. Adults over 50 are also at higher risk because stomach acid production declines with age, making it harder to absorb B12 from food.
Vitamin C: The Immune System Powerhouse
Vitamin C might be the most familiar vitamin on this list. It is the one people reach for when they feel a cold coming on, and there is real science behind that instinct. Vitamin C stimulates the production and function of white blood cells, which are your immune system’s front-line defenders.
It is also a powerful antioxidant, meaning it protects cells from damage caused by free radicals. And it plays a starring role in collagen production, which is essential for healthy skin, tendons, ligaments, and blood vessels.
Unlike most animals, humans cannot produce their own Vitamin C. We have to get it entirely from food or supplements.
Where to find it: Citrus fruits are the classic source, but bell peppers actually contain more Vitamin C than oranges. Strawberries, broccoli, kiwi, and Brussels sprouts are all excellent options.
Signs of deficiency: Severe, prolonged deficiency causes scurvy, which causes bleeding gums, joint pain, and skin problems. Milder deficiency shows up as slow wound healing, frequent illness, fatigue, and easy bruising.
American context: Smokers need significantly more Vitamin C than non-smokers because smoking depletes it rapidly. People who eat very few fruits and vegetables are also at risk.
Vitamin D: The Sunshine Vitamin Americans Are Missing
Vitamin D might be the single most important vitamin to pay attention to right now. Studies consistently show that roughly 42 percent of American adults are deficient. Some estimates go higher, particularly among people with darker skin, those who live in northern states with less year-round sunlight, and older adults who spend more time indoors.
Your skin produces Vitamin D when exposed to sunlight, specifically UV-B radiation. But between indoor lifestyles, sunscreen use, and long winters, many Americans simply do not get enough sun exposure to maintain healthy levels.
Vitamin D is essential for calcium absorption, which makes it a cornerstone of bone health. But its role goes far beyond bones. Vitamin D receptors are found throughout the body, including in the brain, heart, and immune cells. Research has linked low Vitamin D levels to increased risk of depression, autoimmune conditions, and certain cancers, though more research is ongoing.
Where to find it: Fatty fish like salmon and mackerel, fortified milk and orange juice, egg yolks, and beef liver. Many Americans also need to supplement, especially during winter months.
Signs of deficiency: Fatigue, bone pain, muscle weakness, and mood changes. Because these symptoms are vague and develop slowly, many people do not realize they are deficient without a blood test.
Vitamin E: The Antioxidant Protector
Vitamin E is a fat-soluble antioxidant that protects cell membranes from oxidative damage. It also supports immune function and helps widen blood vessels to prevent clotting.
It is often marketed aggressively as a skin care ingredient, and it does have legitimate benefits for skin health, both when eaten and applied topically.
Where to find it: Nuts and seeds, particularly almonds and sunflower seeds. Vegetable oils, spinach, and broccoli are also good sources.
Signs of deficiency: True deficiency is uncommon in healthy adults, but people with fat malabsorption conditions are vulnerable. Symptoms include nerve damage, muscle weakness, and vision problems.
Vitamin K: The Clotting and Bone Vitamin
Vitamin K is essential for blood clotting. Without it, even a small cut could become a serious problem. It activates the proteins that allow your blood to clot properly.
Less well known is Vitamin K’s role in bone health. It activates a protein called osteocalcin that binds calcium to bone matrix, which is why some researchers believe Vitamin K2 (a specific form found in fermented foods and animal products) may work hand in hand with Vitamin D to support bone density.
Where to find it: Vitamin K1 comes mainly from leafy greens like kale, spinach, and broccoli. Vitamin K2 is found in fermented foods like natto, aged cheeses, and some animal products.
Signs of deficiency: Easy bruising, excessive bleeding from cuts, and in severe cases, internal bleeding. Newborns are typically given a Vitamin K injection at birth because they are born with very low levels.
Important note: People taking blood thinners like warfarin need to monitor their Vitamin K intake carefully, since it directly affects how the medication works.
The Four Vitamins Most Americans Are Not Getting Enough Of
Based on national nutrition surveys, these are the vitamins most commonly lacking in the American diet:
Vitamin D tops the list by a wide margin. If you live north of Atlanta and spend most of your time indoors, there is a good chance your levels are low.
Vitamin B12 is the second most common deficiency, particularly among vegans, vegetarians, and adults over 50.
Vitamin C is often low in people who eat few fresh fruits and vegetables, as well as in smokers.
Folate (B9) is a concern especially for women of childbearing age, though fortification of grain products has helped close the gap in the general population.
A Few Things Worth Knowing Before You Supplement
More is not always better. Fat-soluble vitamins, particularly Vitamins A and D, can reach toxic levels if you take very high doses over a long period. Always follow dosage guidelines and talk to your doctor before starting any new supplement routine.
Food first is still the best approach. Whole foods provide vitamins in forms your body absorbs and uses well, along with fiber, minerals, and other compounds that work together. Supplements fill gaps, but they are not a substitute for a balanced diet.
A simple blood panel can tell you exactly where you stand. If you are curious whether you are deficient in something, ask your doctor for a nutrient panel at your next checkup. It takes the guesswork out of supplementing.
The Bottom Line
Vitamins are not complicated in theory. They are nutrients your body cannot make on its own (or cannot make in sufficient quantities), so you need to get them from food or supplements. Each one has a specific role. Each one matters.
The challenge is that modern life makes it genuinely hard to get everything you need from diet alone, which is why understanding what each vitamin does and where it comes from is worth your time.
Bookmark this guide. The next time you feel run-down, foggy, or just off, it might give you a useful place to start.
This article is for informational purposes only and is not intended as medical advice. Always consult with a qualified healthcare provider before making changes to your supplement routine or diet.
