Vitamins are organic compounds that your body needs in small amounts to function properly. Unlike macronutrients (carbohydrates, proteins, and fats), which provide energy, vitamins regulate metabolic processes and support immune function, bone health, blood clotting, and numerous other essential functions. The body cannot synthesize most vitamins in sufficient quantities, making dietary intake necessary.
Vitamins are divided into two categories based on how they are stored in the body. Fat-soluble vitamins (A, D, E, and K) are stored in the liver and fatty tissues and can accumulate to potentially toxic levels with excessive supplementation. Water-soluble vitamins (C and the B-complex) dissolve in water and are not stored to any significant degree—excess amounts are excreted in urine, making toxicity from food sources nearly impossible, though high-dose supplements can cause problems.
Vitamin A
Vitamin A is essential for vision, immune function, cell growth, and reproductive health. It exists in two forms: preformed vitamin A (retinoids) found in animal products, and provitamin A (carotenoids) found in plant foods. Retinol from animal sources is readily absorbed and used by the body, while carotenoids from plants must be converted to active vitamin A—a process that varies based on genetics and individual differences.
Good food sources include liver, egg yolks, dairy products, and colorful vegetables like sweet potatoes, carrots, spinach, and kale. Vitamin A deficiency causes night blindness and compromises immune function. However, excessive preformed vitamin A from supplements can cause liver damage and birth defects, making it one of the vitamins most likely to cause toxicity from over-supplementation.
B-Complex Vitamins
The B vitamins work as coenzymes, helping enzymes carry out critical chemical reactions in every cell. They include B1 (thiamine), B2 (riboflavin), B3 (niacin), B5 (pantothenic acid), B6 (pyridoxine), B7 (biotin), B9 (folate), and B12 (cobalamin). Each has distinct functions, though they often work together. Deficiencies in any one B vitamin can disrupt the entire system.
Vitamin B12
B12 is particularly important and deserves special attention because deficiency is common, especially in older adults. B12 is required for nerve function, red blood cell formation, and DNA synthesis. It is found almost exclusively in animal products, making vegans and strict vegetarians at risk unless they supplement or consume fortified foods. Older adults often lack sufficient stomach acid to extract B12 from food, making supplementation advisable after age 50.
Folate (B9)
Folate is essential for cell division, DNA synthesis, and preventing neural tube defects in developing embryos. The synthetic form, folic acid, added to fortified grains and available in supplements, is more bioavailable than the natural folate in foods. All women of childbearing age should consume adequate folate, making folate one of the rare nutrients where supplementation is clearly indicated for a large population group.
Most people eating a varied diet with plenty of fruits, vegetables, whole grains, and lean proteins get all the vitamins they need from food. Supplements fill specific gaps, not general deficiencies.
Vitamin C
Vitamin C (ascorbic acid) is an antioxidant that protects cells from damage, is essential for collagen synthesis, enhances iron absorption, and supports immune function. While it has not been proven to prevent colds, some evidence suggests it may reduce their duration and severity. The Recommended Dietary Allowance is 75mg for women and 90mg for men—easily obtained from citrus fruits, bell peppers, strawberries, broccoli, and other produce.
Scurvy, the classic vitamin C deficiency disease causing bleeding gums, joint pain, and poor wound healing, is rare today but can occur in people with severely restricted diets. Megadoses of vitamin C (thousands of milligrams daily) are not beneficial and can cause digestive upset and increase risk of kidney stones in susceptible individuals.
Vitamin D
Vitamin D functions as a hormone in the body, regulating calcium absorption, supporting bone health, modulating immune function, and influencing cell growth. Unlike other vitamins, vitamin D can be synthesized in the skin upon exposure to UVB radiation from sunlight. However, many factors limit this synthesis: winter season, northern latitudes, darker skin pigmentation, aging, sunscreen use, and indoor lifestyles.
Dietary sources of vitamin D are limited—fatty fish, egg yolks, and fortified foods provide some. However, most people in northern climates cannot meet their vitamin D needs from diet and sun alone, making supplementation widely recommended. The recommended intake is 600 to 800 IU daily, though many experts consider this conservative, with 1000 to 2000 IU daily commonly suggested for adults.
Calculate Your Vitamin Needs
Use our nutrition calculator to assess whether your diet provides adequate vitamins or if supplementation might be beneficial.
Assess Vitamins →Vitamin E
Vitamin E is an antioxidant that protects cell membranes from oxidative damage. It is found in nuts, seeds, vegetable oils, and green leafy vegetables. Deficiency is rare and typically occurs only in people with fat absorption disorders or genetic conditions. Vitamin E supplementation has not been shown to prevent chronic diseases, and high-dose supplementation may actually be harmful, increasing bleeding risk and mortality in some studies.
Vitamin K
Vitamin K is essential for blood clotting and bone health. It exists in two forms: K1 (phylloquinone) found in green leafy vegetables, and K2 (menaquinone) found in fermented foods and produced by gut bacteria. Newborns receive vitamin K injections at birth to prevent hemorrhagic disease, as this vitamin is poorly transferred across the placenta and breast milk is low in vitamin K.
Vitamin K works synergistically with calcium and vitamin D to support bone health. Some research suggests that vitamin K2 supplementation may reduce fracture risk and support cardiovascular health by preventing calcium deposition in arteries, though more research is needed to confirm these findings.