· Manaloahealth
Here is a fact that surprises most people: vitamin D is not, in the strict biological sense, a vitamin at all. A vitamin is something your body cannot make and must get from food.
Vitamin D breaks that rule. Your skin manufactures it when sunlight hits it, and once inside the body it is converted into a powerful hormone that acts on cells throughout your entire system.
Scientists have known this for a long time. The authoritative dietary reference report on the nutrient plainly describes vitamin D as a prohormone rather than a true vitamin, precisely because the body can synthesize it through the action of sunlight (Institute of Medicine, NCBI Bookshelf). Let's unpack what that actually means and why it matters.
What makes it a hormone
A hormone is a signaling molecule made in one part of the body that travels to act on cells elsewhere. Vitamin D fits this definition once it goes through a two-step activation process.
Vitamin D is technically a secosteroid, a steroid-like molecule that, like all steroid hormones, is built from cholesterol (Stokes and Lammert, 2016).
The version you get from sun, food, or a supplement is biologically inert to start. It has to be activated:
First, the liver converts it into a storage form called 25-hydroxyvitamin D, or calcidiol. This is the form doctors measure in a blood test to check your levels.
Second, the kidneys convert that into the active hormone, 1,25-dihydroxyvitamin D, better known as calcitriol. This final step is tightly controlled by parathyroid hormone and your blood calcium levels (Latic and Erben, 2022).
Calcitriol is the hormone. It works by binding to the vitamin D receptor, or VDR, which acts as a switch that turns genes on and off. Through this mechanism, active vitamin D influences the expression of hundreds of genes, with some research pointing to nearly a thousand (Abdrabbo et al., 2022). Crucially, the vitamin D receptor is found on nearly every cell type in the body, including immune cells, which is why its reach extends so far beyond bone (Fabbri et al., 2022).
What vitamin D is responsible for
Calcium, phosphate, and bone. This is the classic and best-established job. Active vitamin D raises blood calcium by increasing calcium absorption in the gut, reabsorption in the kidneys, and release from bone when needed (Fabbri et al., 2022). Without enough of it, the body cannot properly mineralize the skeleton, which historically led to rickets in children and soft, weakened bones in adults. Bone health remains the core, non-controversial reason vitamin D matters.
The immune system. Because the vitamin D receptor sits on immune cells, calcitriol also acts locally within the immune system, where it helps regulate inflammatory responses. In immune cells, vitamin D can be activated on site to exert nearby, non-hormonal effects on the body's defenses (Latic and Erben, 2022).
Muscle function. Vitamin D receptors are present in skeletal muscle, and adequate vitamin D status is linked to muscle health, while deficiency has been associated with muscle weakness (Faraldi et al., 2024).
Cell growth and other systems. Beyond these roles, research has explored vitamin D's involvement in cell proliferation and a wide range of organ systems. Here an honest note is important. While laboratory and observational studies suggest broad effects, large randomized clinical trials on outcomes like cancer, cardiovascular disease, and cognition have produced mixed and often inconclusive results (Marino et al., 2025). The bone and calcium role is rock solid; many of the headline-grabbing "extra" benefits are still being sorted out.
How to actually get vitamin D
There are three practical routes: sunlight, food, and fortified drinks. Most people need a combination.
1. Sunlight
This is the route that makes vitamin D a hormone in the first place. When UVB rays hit bare skin, they convert a cholesterol precursor into vitamin D3. Most people worldwide meet at least some of their needs this way (NIH Office of Dietary Supplements).
But sun synthesis is highly variable. Season, time of day, latitude, cloud cover, and air pollution all change how much UVB reaches you. So do personal factors: more melanin (darker skin) reduces production, older skin makes less, and sunscreen blocks the UVB involved. One detail people often miss is that sunlight through a window does not work, because glass filters out the UVB (NIH Office of Dietary Supplements, Consumer Fact Sheet). This is also why sun exposure alone is unreliable for many people, especially in winter or at northern latitudes.
2. Food
Very few foods contain vitamin D naturally, which is a big part of why deficiency is common (NIH Office of Dietary Supplements).
The best natural sources are:
Fatty fish, which lead the pack. Trout, salmon, tuna, mackerel, and sardines are among the richest natural sources, along with fish liver oils like cod liver oil (NIH News in Health, 2024).
Smaller amounts show up in egg yolks, beef liver, and cheese (NIH Office of Dietary Supplements, Consumer Fact Sheet).
Mushrooms are the notable plant source, but with a catch: only mushrooms exposed to ultraviolet light contain meaningful amounts. Ones grown in the dark have very little (NIH Office of Dietary Supplements, Consumer Fact Sheet).
3. Drinks and fortified foods
Because natural food sources are so limited, fortification does most of the heavy lifting in the modern diet. Fortified foods provide most of the vitamin D that people in the United States actually consume (NIH Office of Dietary Supplements, Consumer Fact Sheet). Good beverage and fortified options include:
Cow's milk, nearly all of which is fortified in the US, at roughly 120 IU per cup (NIH Office of Dietary Supplements, Consumer Fact Sheet).
Plant-based milks such as soy, almond, and oat, many of which are fortified to similar levels. Check the label, since amounts vary by brand.
Some brands of orange juice, along with many breakfast cereals, some yogurts, and certain margarines, which have vitamin D added (NIH News in Health, 2024).
A useful habit: check the Nutrition Facts label, because whether a given carton of juice or plant milk contains vitamin D depends entirely on whether the manufacturer chose to fortify it.
The bottom line
Vitamin D earns its "hormone" label honestly. Your body can make it from sunlight, activate it through your liver and kidneys, and use it as a genetic switch that influences bone, muscle, and immune function across nearly every tissue. Its role in calcium balance and bone health is beyond dispute, while its broader effects remain an active and sometimes uncertain area of research.
For getting enough, the realistic answer is a mix: some sensible sun exposure, fatty fish and eggs where you can, and fortified drinks and foods to fill the gap. And because both deficiency and excess carry risks, with too much vitamin D able to push blood calcium dangerously high (Marino et al., 2025), it is worth talking to a healthcare provider before starting high-dose supplements or if you suspect your levels are low.
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Patient safety is Mana Loa Health's top priority. The information discussed on this blog is not intended to recommend the self management of health problems or wellness. It is not intended to endorse or recommend any particular type of medical treatment or advice. The information provided on this website is for informational purposes and not a substitute for professional medical advice, diagnosis, or treatment. If you have questions or concerns about your health, please talk to your healthcare provider. No information contained on this blog should be used by any reader to disregard medical and/or health related advice or provide a basis to delay consultation with a physician or a qualified healthcare provider.
References
NIH Office of Dietary Supplements. Vitamin D: Fact Sheet for Consumers. https://ods.od.nih.gov/factsheets/VitaminD-Consumer/
NIH Office of Dietary Supplements. Vitamin D: Fact Sheet for Health Professionals. https://ods.od.nih.gov/factsheets/VitaminD-HealthProfessional/
NIH News in Health. Digging Into Vitamin D. 2024. https://newsinhealth.nih.gov/2023/04/digging-into-vitamin-d
Institute of Medicine, Food and Nutrition Board. Overview of Vitamin D, in Dietary Reference Intakes for Calcium and Vitamin D. NCBI Bookshelf. https://www.ncbi.nlm.nih.gov/books/NBK56061/
Stokes CS, Lammert F. Vitamin D supplementation: less controversy, more guidance needed. F1000Research, 2016 (open access). https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4991525/
Latic N, Erben RG. Rapid Nontranscriptional Effects of Calcifediol and Calcitriol. PMC, 2022. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8951353/
Calcifediol: Mechanisms of Action. PMC, 2023. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC10610216/
Cholecalciferol (vitamin D3): efficacy, safety, and implications in public health. PMC, 2025. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC12183072/