Understanding the Role of Vitamins

Vitamins are widely recognized as being essential to a healthy diet. Yet, the functions they perform in the body are sometimes misunderstood.


The basic function of vitamins and minerals is to aid enzyme function. For this reason, vitamins are sometimes referred to as co-enzymes, or cofactors. Vitamins do not have any calories unto themselves in the way they are used by the body, and so do not produce any energy on their own, contrary to what some marketing messages may advocate. Many vitamins and minerals work synergistically, that is, they each contribute a unique feature that aids the function of another. In general, a living organism must obtain the necessary vitamins and minerals through food and drink. Today, however, it’s become common for many people to rely on supplements for an adequate supply of vitamins and minerals as a result of poor dietary choices.

One of the unique things about as a class of nutrient is that they all have different chemical structures and all are essential. As discussed, an “essential” nutrient is one that is not synthesized by the body or not in quantities necessary to sustain life across time. There are some circumstances, such as Vitamin D- known as 1,25 dihydro-cholcalciferol in its active form, which requires the skin to be in contract with the sun in order to be activated. While Vitamin K and some B vitamins are produced by bacterial flora, they are not present in quantities sufficient to sustain most people.

Classifying Vitamins

In general, vitamins serve multiple functions and many serve very unique functions. Vitamins are known as micronutrients because they are not required in large amounts. Based on their chemical structure, vitamins are organic compounds because they include the element carbon in their structure. However, unlike some other nutrients, vitamins are classified according to the biological and chemical activity they perform, rather than their chemical structure. Vitamins have been designated by letters and in the case of “B” vitamins; they are further divided into letters and number (B6, B12, etc.).

Vitamins are classified as to their solubility in either fat or water. This means they will either combine with either hydrophilic, or water-based environments, or hydrophobic, or fat-based environments. Vitamins A, D, E, and K are fat-soluble, while the B-complex and C vitamins are water-soluble. Water-soluble vitamins will not persist as long in the body as fat soluble vitamins, and so the risk of toxicity or overdose from vitamins in the class are rare.

Vitamins & Metabolism

The B vitamins are necessary for the proper function of metabolism in humans. Each B vitamin serves a key role as a cofactor in the body’s metabolic pathways. For example, thiamine-B1, niacin- B3, and pyridoxine-B6, aid in carbohydrate metabolism; riboflavin- B2, in fat metabolism, and thiamin, pantothenic acid, and biotin- in protein metabolism. Having too little of these vitamins can contribute to fatigue during exercise. A common assumption is that an abundance of these vitamins provide energy; this is not the case, however.

Other vitamins play major roles in human health and normal function. Vitamins A, C, and E, for instance, have known anti-oxidant qualities. Folic acid, and cobalamine-B12 are necessary to the formation of red and white blood cells, while Vitamins D and C contribute to the formation of bone, cartilage, and connective tissues. Vitamin K is necessary for sugar residues and proteins combine to form glycoproteins, which include several clotting factors. Thus, when someone is taking anti-coagulant medication, he or she should be mindful of foods containing high amounts of Vitamin K.

Recommended Vitamin Intake

Some vitamins are still under study to determine their dosage for “optimum” function; however, it is widely accepted that an excess intake of any vitamin or mineral does not enhance function. Just as a container can hold only so much before an excess amount spills out, too much of some vitamins can cause toxicities.

In recent years, the United States Department of Agriculture has formulated new guidelines on the amounts of vitamins needed for optimal function, and the amount needed by 97% of the population. The Estimated Average Requirement, or EAR, is the amount necessary to meet the need plus a margin of safety. The Recommended Daily Intake (RDI) or Recommended Dietary Allowance (RDA) represents the amount of a vitamin needed to avoid disease or deficiencies in 97.5% of the population. For some vitamins, not enough scientific evidence has been discovered to give an EAR, so the Adequate Intake (AI) is given as the recommended amount.


1. http://fnic.nal.usda.gov/dietary-guidance/dietary-reference-intakes/dri-tables

2. The National Federation of Professional Trainers. Sports Nutrition Manual. 2nd Ed. Lafayette, IN: NFPT, 2006.


These resources are for the purpose of personal trainer growth and development through Continuing Education which advances the knowledge of fitness professionals. This article is written for NFPT Certified Personal Trainers to receive Continuing Education Credit (CEC). Please contact NFPT at 800.729.6378 or [email protected] with questions or for more information.