By Michael Ormsbee, PhD, Florida State University

Edited by Kate Findley and proofread by Angela Shoemaker, The Great Courses Daily

Vitamins serve many important functions in our bodies and are involved in digestion, absorption, energy production, antioxidant purposes, and growth. Professor Ormsbee breaks down the different vitamins and explains how to avoid deficiency and toxicity.

Function of Vitamins

Vitamins, a micronutrient, function as regulators of numerous metabolic reactions that release energy from food or help with energy transfer to produce ATP, the energy currency of our bodies. Vitamins are termed organic compounds because they contain carbon just like all three of the macronutrients—carbohydrates, fats, and proteins.

Vitamins also act as cofactors and coenzymes, which are required for various interactions in our metabolic pathways. For example, aerobic metabolism requires the shuttling of electrons into the electron transport chain, which is how ATP is made. This shuttling mechanism requires riboflavin and niacin—two water-soluble vitamins—to function properly. 

Almost all textbooks will define vitamins as natural components of foods usually present in small amounts, essential for normal physiological function, and, when absent from the diet, will cause a specific deficiency in that particular vitamin. This means that you must get vitamins from the food you eat.

Classifying Vitamins

Vitamins are classified as either fat-soluble or water-soluble, depending on if they dissolve in fat or water. Fat-soluble vitamins include A, D, E, and K. The body absorbs them from the gastrointestinal tract and stores them in the liver, fatty tissues, and parts of your cells that contain fat such as the cell membrane. 

Fat-soluble vitamins moving in your blood bind to dietary fat. This means that if fat is not part of your diet, you will likely have suboptimal fat absorption and may be set up for a fat-soluble vitamin deficiency. Fat-soluble vitamins are not readily excreted and are stored in your fat cells and liver.

Water-soluble vitamins include B vitamins and vitamin C. These vitamins are also absorbed from the gastrointestinal tract and must be transported in the blood with what are called carrier proteins. 

Water-soluble vitamins are not stored in large amounts in the body because body water turnover is so constant. They are also easily excreted through urine and a bit through your sweat.

Because they are so easily excreted and not stored, a daily intake of less than 50% of any of the water-soluble vitamins can lead to deficiency in about four weeks. That means that if you are missing an entire food group like carbohydrates or fats—which can easily happen if you follow a fad diet—you could be deficient in some vitamins.

B vitamins are often called the B complex of vitamins because they comprise many vitamins. B₆ and B₁₂ tend to get more press.

However, the B vitamins also include B₁, which is thiamine, B₂ riboflavin, B₃ niacin, B₅ pantothenic acid, B₆ pyridoxine, B₇ biotin, B₉ folic acid, and B₁₂ or cobalamin. Together, these make up the B complex, but each serves a specific role in our overall functioning.

Deficiencies and Toxicities

Both deficiencies and toxicities are possible with vitamins. A deficiency is not enough; a toxicity is too much. 

Toxicity is not likely to occur from food alone. It’s more common when people are taking supplements. Thus, it’s important to talk to a professional about what you are taking and how much.

Deficiencies and toxicities are more likely with certain vitamins and minerals than others. Fat-soluble vitamins are stored more efficiently in the body—there is less fat turnover compared to water turnover. 

Vitamin A toxicity is the most common form of a fat-soluble vitamin toxicity because it can occur when intake exceeds just two times the needed recommendations. When vitamin A accumulates in the liver, you can have both acute and chronic outcomes. 

Headache, rashes, visual changes, bone pain, skin cracking, mouth ulcers, yellowing of the skin, nausea, and vomiting are acute outcomes that can occur. Chronic toxicity can lead to liver damage.

Vitamin A also goes by some other names like retinol or retinoic acid. In fact, vitamin A can come in two different forms. 

Preformed vitamin A includes retinol and is found in many animal sources like dairy, fish, and meat. The other form, provitamin A, is often referred to as carotenoid and the primary source is from beta-carotene, which is converted to vitamin A in your body. 

You get provitamin A from foods like leafy green vegetables, orange and yellow vegetables, and tomato products. Don’t be afraid to consume these foods, but use caution if you are eating a lot of these foods combined with daily supplementation.

Water-soluble vitamins, on the other hand, are much less likely to lead to toxicity, but it could occur from overconsumption of niacin, B₆, and vitamin C. Again, toxicities are far less common for whole-food choices than from supplements such as multivitamins.

Getting the Most Out of Vitamins

A number of things can impact our overall vitamin needs such as exercise, stress levels, aging, gender, and medications. Micronutrient needs will vary from person to person so a one-size-fits-all approach is not possible.

Another interesting note about vitamins is that their status in foods can change just by how you store and cook food. Poor storage and preparation of food can result in a decreased vitamin content of your food. 

This includes excessive cooking; exposure to light, heat, air, and water can all lower the vitamin content of foods. Generally, it is best to consume food soon after harvest, or to buy foods that have been frozen quickly after harvest. The sooner a fresh fruit or vegetable is frozen, a greater vitamin content will be preserved.

This article was edited by Kate Findley, Writer for The Great Courses Daily, and proofread by Angela Shoemaker, Proofreader and Copy Editor for The Great Courses Daily.