Vitamin D is, in fact, a nutrient.
Too often we read articles that begin by saying “vitamin D is actually a hormone,” or that “few people view their vitamin D supplement as hormone replacement therapy,” or the opening of many scientific papers “vitamin D is a secosteroid hormone,” but this is simply misleading.
This is an important distinction, and supplementing with vitamin D is definitely NOT hormone replacement therapy. Beyond bone health, vitamin D as 25(OH)D is used by nearly every cell in the body as a signaling molecule. That means that most actions of vitamin D, its regulatory effects on immune cells for example, are not as a hormone but as a signaling molecule not under regulatory control. Today we will explain why most of the time vitamin D is actually a nutrient.
What is a Vitamin?
Vitamins are nutrients that we must get from outside the body, usually in our diet, because they cannot be made within the body, and are necessary for normal bodily functions and overall health. For this reason, vitamins are called essential nutrients. Vitamins are required in small quantities, and are distinct from other biologically important compounds such as proteins, carbohydrates, and lipids which are required in larger quantities. In general, vitamins play important roles in enzyme function and regulation, facilitating or controlling vital chemical reactions in the body’s cells. If a vitamin is not present in adequate levels in the body, a specific deficiency disease may develop. “Vitamin D” meets this definition. As humans, our major source of vitamin D is the sun. UVB rays in sunshine convert a precursor in the skin to vitamin D3 or cholecalciferol. However, when we do not get enough sunlight or we are unable to make vitamin D because of the angle of the sun, we must get vitamin D from our diet and thus it becomes a vitamin. Cholecalciferol is found in a few dietary sources (such as fatty fish and fortified milk and margarine), or it can be taken as a supplement. Extreme vitamin D deficiency results in rickets in children and osteoporosis in adults.
What is a Hormone?
A hormone is a substance produced in one part of the body and released into the blood stream to carry out its regulatory functions on another part of the body and is itself under regulatory control. Hormones are important in maintaining balance or “homeostasis.” Hormones in very small quantities carry out their functions by producing a response from a specific organ or tissue.
“Vitamin D” in its role in maintaining normal calcium levels is also a hormone and levels of 1,25(OH)2D are regulated by parathyroid hormone which respond to calcium levels. Calcium levels must be kept in a very narrow range because of its importance in muscle contraction, nerve conduction, and blood clotting – these are functions that are vital to life. Calcium is also important for building bones, which also act as a calcium storage depot.
Vitamin D is Mostly a Nutrient
The diagram above demonstrates how vitamin D is handled in the body.
Whether from sun, food or supplement, the liver converts the majority of vitamin D3 to 25(OH)D. This is the form that we measure in the blood because it is present in the highest quantities and lasts for the longest period of time (for 2-3 months). The conversion of 25(OH)D to 1,25(OH)2D in the kidney produces the hormonal form of vitamin D that circulates in the blood.
Most other cells and tissues in the body have the enzyme needed to make 1,25(OH)2D and the receptor needed to use 1,25(OH)2D. However, this form of 1,25(OH)2D does not meet the requirements of a “hormone” because it is a signaling molecule that modulates the cell making it or nearby cells.
Circulating 25(OH)D may be thought of as a “pre-hormone,” but only in respect of the small amount that is converted in the kidneys to circulating 1,25(OH)2D, which then acts as a hormone. In the blood, 1,25(OH)2D has a concentration about 1/1000th that of circulating 25(OH)D (assuming a healthy level of 25(OH)D of 40-60 ng/ml) and is broken down very rapidly (within about 14 hours).
Research over the past several decades has shown that vitamin D is important to many biological pathways in the body. Of importance recently are the roles vitamin D plays in regulating immune cells. Other cells and tissues in which vitamin D plays regulatory roles include the brain, bone, heart, liver, lung, gut, muscles, skin, and teeth. In fact, low levels of vitamin D have been associated with many different diseases of these systems including autoimmune diseases, cancer, diabetes, heart disease, dementia and cognitive decline, respiratory infections and COVID-19, among others.
What is the take-home message? It is more accurate to refer to vitamin D as a nutrient, not a hormone!
REFERENCES:
Cooper GM. The Cell: A Molecular Approach. 2nd edition. Sunderland (MA): Sinauer Associates; 2000. Signaling Molecules and Their Receptors.
Vieth R. Vitamin D supplementation: cholecalciferol, calcifediol, and calcitriol. European Journal of Clinical Nutrition. 2020; 74: 1493–1497.
