The world has a flock of chickens to thank for the discovery of vitamins and, with time, vitamin supplements. Back in the 1880s, a scientist named Christiaan Eijkman studied chickens because he found they could develop a beriberi-like condition. Beriberi is a disease with two types: wet and dry. Wet beriberi affects the heart and circulatory system and can cause heart failure. Dry beriberi damages nerves, resulting in the loss of feeling in the legs.

Christiaan discovered a flock of chickens that had recovered from beriberi. He found out the chickens had been fed leftover processed rice from a military hospital. Once the chickens resumed a diet of unprocessed rice, their symptoms resolved. Christiaan realized something vital in the outer layer of rice was stripped away during processing, what scientists later identified as vitamin B1, or thiamine. It was Polish biochemist Casimir Funk who called that something “vital amine,” or vitamin. Over the next century, vitamins became a booming, multi-billion-dollar business.

Thirteen can claim the name vitamin: A, B1, B2, B3, B5, B6, B7, B9, B12, C, D, E, and K. Vitamins are defined as nutrients essential for human diet and growth that must be obtained through diet or supplement because they are not naturally manufactured by the body. Once identified as vitamins B4, B8, B10, and B11, the following lost their vitamin designation because they did not meet this definition: adenine, inositol, para-aminobenzoic acid, and salicylic acid.  Similar explanations exist for vitamins F-J; they either do not meet the definition or their properties were absorbed into other, recognized vitamins. Each vitamin has a special function to fulfill within the body. Vitamins A, D, E, and K are fat-soluble, meaning that they can be stored in the body’s fatty tissues and in the liver for days to months. Water-soluble vitamins B (all of them) and C cannot be stored and must be replenished daily. Ideally, one obtains vitamins from food. However, in countries where a variety of food is not available or for a person with a poor diet, supplementing is necessary.

One stop vitamin shopping, in the form of multivitamins, is a big part of the vitamin and supplement industry. Formulas abound for every age group — for the pregnant, for athletes, for men, for women, and for seniors. Scientists agree that multivitamins are not necessarily necessary. Here in the United States, a wide variety of food is available as a vitamin source, so in theory supplementing vitamins should not be required.

However, individual supplements are available. B vitamins should be taken in complex form, since excess amounts in one B vitamin can adversely affect the level or effectiveness of others. For those with health conditions, the elderly, and those with particular medical conditions, certain supplements could be dangerous. When in doubt about vitamin supplementing, consult a physician, pharmacist, or nutritional specialist. It is always a good idea to inform your pharmacist and physician of which vitamins you are taking in addition to any over the counter medications so appropriate monitoring of drug interactions can be performed.

Vitamin A (retinol, beta carotene)

Vitamin A is really vitamin “see.” That is, it is necessary for good eye health. It is also necessary for cell recognition, immune function, and reproduction. As a super achiever, it helps the heart, lungs, kidneys, and other organs function properly. It is important for tissue growth, including skin and hair, and helps produce sebum, the oil that maintains skin and hair moisture.

Luckily, this vitamin is readily available in a wide variety of foods, including liver, carrots, broccoli, sweet potatoes, butter, kale, spinach, pumpkin, collards, eggs, cantaloupe, and milk. While vitamin A deficiency is rare in the United States, those who have it may suffer from night blindness; infections of the throat, chest, and abdomen; infertility; and delayed growth in children. Too much vitamin A can be toxic, so it is wise to seek professional advice prior to supplementing.

Vitamin B1 (thiamine)

Vitamin B1 produces enzymes that help break down blood sugar. It is instrumental in preventing complications of the nervous system, brain, muscles, heart, stomach, and intestines. Electrolyte flow also benefits from vitamin B1. It is believed to help canker sores, vision problems, diabetic pain, stress, and motion sickness. Diets including pork, cereal grains, brown rice, asparagus, potatoes, cauliflower, oranges, and eggs are rich in this vitamin. Deficiency of vitamin B1, as previously mentioned, can cause beriberi as well as the neurodegenerative disorder Wernicke-Korsakoff syndrome. Unlike vitamin A, experts do not believe excess vitamin B1 is harmful.

Vitamin B2 (riboflavin)

This vitamin breaks down carbohydrates, fats, and proteins. With its partner, vitamin A, it maintains the digestive system, keeps the liver, eyes, nerves, muscles, and skin healthy, and helps prevent cataracts. It is important for fetal development. Food sources are fish, poultry, meat, artichokes, avocado, currants, lima beans, mushrooms, nuts, cruciferous vegetables, and whole grain breads. Symptoms of vitamin B2 deficiency include mouth ulcers, dry skin, sore throat, and light sensitivity, to name just a few. This is a vitamin that can interfere with other medications, so consultation with an expert prior to supplementing is advised.

Vitamin B3 (niacin)

Vitamin B3, which is used to convert food into energy, assists with the body’s use of protein and fat and maintains a healthy nervous system, hair, and skin. Food sources include tuna, salmon, milk, tomatoes, leafy vegetables, broccoli, tofu, and lentils. Fatigue, constipation, circulatory problems, depression, memory loss, rough skin, and rashes on skin exposed to the sun are all indicators of insufficient vitamin B3. Excess vitamin B3 can cause itchy skin, nausea, vomiting, rash, and dizziness. It can trigger gout, affect eyesight, lower blood pressure, and lead to gastrointestinal problems.

Vitamin B5 (pantothenic acid)

This vitamin assists with the metabolism of fats and proteins and, like other B vitamins, helps keep hair, skin, and eyes healthy. It helps form sex hormones as well as red blood cells. Those deficient in this vitamin may experience apathy, depression, sleep disorders, vomiting, muscle cramps, and upper respiratory infections.

Vitamin B5 can be sourced from pork, poultry including duck, beef, organ meat, salmon, lobster, whole grain breads and cereals, yogurt, lentils, mushrooms, avocado, cauliflower, and tomatoes. This vitamin is another that should be supplemented carefully and with professional advice. It can adversely react with tetracycline, some Alzheimer’s Disease drugs, and blood thinners.

Vitamin B6 (pyridoxine)

More than 100 enzyme reactions benefit from vitamin B6. Its main role is in helping the body metabolize proteins, fats, and carbohydrates for energy. It is instrumental in creating neurotransmitters, including serotonin and dopamine. It also creates hemoglobin. Food sources are chicken, beef liver, tuna, chickpeas, banana, raisins, bulgur, spinach, rice, and watermelon. Signs of vitamin B6 deficiency include anemia, confusion, seizures, depression, and peripheral neuropathy with accompanying tingling and pain in the hands and feet.

Fortunately, deficiency is rare in the United States. Those most likely to experience deficiency in this vitamin are the obese, those who consume excess alcohol, and those who are pregnant or breastfeeding. While it is nearly impossible to consume excess vitamin B6 through one’s diet, excess via supplement can, over time, cause severe sensory neuropathy.

Vitamin B7 (biotin)

Like its vitamin B cousins, this vitamin helps metabolize protein, carbohydrates, and fats. It is important for healthy pregnancies and for nails, hair, and skin. Good sources of this vitamin are wheat bran, whole eggs, oysters, liver, spinach, and cheese. While deficiency is rare, it can cause hair loss; a scaly rash around the eyes, nose, and mouth; lethargy; and hallucinations.

Smokers, pregnant women, and patients receiving prolonged intravenous nutrition are at risk for deficiency of vitamin B7. Alone, excess amounts of this vitamin are not harmful. However, it may cause adverse reactions with drugs like Haldol and Zyprexa, herbal supplements, and other vitamins, including B5.

Vitamin B9 (folic acid)

Vitamin B9 is a must-use supplement for those hoping to become pregnant, those who are pregnant, and those who are breastfeeding. It is essential for making DNA and RNA and for making red and white blood cells in bone marrow. Sources include legumes, beef liver, sunflower seeds, leafy vegetables, and bananas.

Deficiency during pregnancy can affect the fetus’s nervous system. While there are no dietary restrictions on this vitamin, high supplement intake can mask vitamin B12 deficiency, a condition that can cause permanent nerve damage and paralysis.

Vitamin B12 (cyanocobalamin)

The last of the B vitamins is required for brain function, nerve tissue health, and red blood cell production. It also helps create and regulate DNA. Dietary sources include ham, poultry, lamb, fish, dairy products, and eggs. Those following a vegan diet should consider using a vitamin B12 supplement. Even minor deficiency can trigger symptoms like fatigue, memory problems, confusion, and depression.

It can also cause constipation and loss of appetite. Left uncorrected, these symptoms can lead to numbness and tingling of the hands and feet. Certain medications can deplete B12 so talking to a doctor about which of those can cause this and who needs supplementation and regular monitoring is a smart decision. This becomes even more serious, with irreversible results such as permanent nerve and brain damage. Those with high levels of deficiency in vitamin B12 are at risk of developing psychosis, mania, and dementia.

Vitamin C (ascorbic acid)

Mothers back in the day always encouraged drinking orange juice so kids could get their vitamin C. Not only is it an antioxidant, this vitamin also helps form and maintain bones, blood vessels, and skin. It boosts the immune system, aids wound healing, and helps produce collagen. Spinach, tomatoes, green peas, grapefruit, strawberries, oranges, kiwi, and red and green peppers all are great sources of vitamin C. They should be consumed raw, however, because heat and cooking in water can destroy vitamin content. Iron intake can help increase absorption of vitamin C.

Limited diets, health conditions involving intestinal malabsorption, and smoking or exposure to secondhand smoke can result in vitamin C deficiency. Too much of this vitamin can result in kidney stones and cardiovascular problems.

Vitamin D (ergocalciferol)

This vitamin supports lung function; supports brain, nervous, and immune system health; and regulates insulin levels. Despite its designation as a “vitamin,” vitamin D does not follow the typical definition — in fact, it is considered to be a prohormone — because the body can create it through sun exposure.

Dietary sources include mushrooms, beef liver, cheese, fatty fish, egg yolks, fortified milk, and fortified cereals. When it comes to sun exposure, those with pale skin produce more vitamin D in the sun than those with darker skin. The more skin exposed, the more vitamin D the body will make. Midday is the best time for vitamin producing sun exposure.

Indicators of deficiency are fatigue, hair loss, muscle pain, low mood, and impaired wound healing. Excessive levels of vitamin D often occur from taking too many supplements. High level supplements should only be taken upon a medical recommendation following testing. D levels should be rechecked periodically when high doses of the supplement are being taken.

Vitamin E (tocopherol)

A boosted immune system, widened blood vessels, healthy cell interaction, and antioxidants are all benefits of vitamin E. It is found in broccoli, spinach, nuts and seeds, and in vegetable, sunflower, and safflower oils. Deficiency is rare, usually linked to diseases like cystic fibrosis and Crohn’s disease. While foods containing this vitamin do not need to be limited, excessive vitamin E increases the risk of bleeding and stroke.

Vitamin K (phylloquinone)

Vitamin K is necessary for blood clotting and bone metabolism. Leafy greens, figs, parsley, soybean oil, and grapes are all sources of this vitamin. The traditional Japanese breakfast food natto is particularly rich in vitamin K. Deficiency in this vitamin can lead to excessive bleeding. Those who take blood thinning medications should consult with their physician prior to supplementing vitamin K.

It is easy to understand why some might find the world of vitamins and vitamin supplements overwhelming. A healthy diet is always a best choice. However, if the need to supplement arises, it pays to read labels carefully and, especially for those taking other medications, consult with an expert prior to supplementing. Christiaan Eijkman would be surprised to see how vast the world of vitamins and their supplements is today — all courtesy of a flock of chickens.

Editor’s note: This article was reviewed by Sarah Arnold, PharmD candidate, USC College of Pharmacy, and Maddie Dean, PharmD candidate, USC College of Pharmacy, and Brandon Bookstaver, PharmD, FCCP, FIDSA, BCPS.