Vitamin D: Why It Functions More Like a Hormone Than a Vitamin
Most people think of vitamin D as the “bone vitamin.”
They know it helps with calcium absorption and preventing osteoporosis, but what many people do not realize is that vitamin D may be one of the most important signaling molecules in the human body.
In fact, many scientists argue that vitamin D should not be thought of like a traditional vitamin at all. From a biological perspective, vitamin D functions much more like a hormone.
Unlike traditional vitamins that must be obtained entirely through diet, vitamin D is produced by the body in response to sunlight and then undergoes multiple metabolic conversions before becoming active.
Once activated, it influences gene expression and affects nearly every major organ system.
As a physician focused on preventative medicine, metabolic health, and longevity, I routinely evaluate vitamin D status because its effects extend far beyond bone health.
Emerging research continues to reveal connections between vitamin D and immune function, cardiovascular health, mood regulation, insulin sensitivity, inflammation, muscle function, and healthy aging.
The question is no longer whether vitamin D matters. The question is how much it matters.
Why Vitamin D Is Technically More Like a Hormone
Most vitamins function as nutrients the body cannot adequately produce on its own.
Vitamin D is different.
When ultraviolet B, or UVB, rays from sunlight strike the skin, cholesterol-derived compounds are converted into vitamin D3.
The vitamin D then undergoes processing in two major organs:
- The liver
- The kidneys
This produces the biologically active form known as calcitriol.
Calcitriol acts as a hormone by binding to vitamin D receptors throughout the body.
Researchers have identified vitamin D receptors in:
- Immune cells
- Brain tissue
- Skeletal muscle
- Pancreatic cells
- Fat tissue
- Cardiovascular tissue
- Reproductive organs
This widespread receptor distribution helps explain why vitamin D affects so many different biological processes.
Vitamin D and Gene Expression
One of the most fascinating aspects of vitamin D biology is its influence on gene regulation.
Activated vitamin D interacts directly with DNA through the vitamin D receptor system.
Researchers estimate that vitamin D may influence the expression of hundreds to thousands of genes involved in:
- Cellular growth
- Immune regulation
- Inflammation
- Metabolism
- Cell differentiation
This is one reason vitamin D deficiency has been associated with such a wide range of health conditions.
Rather than affecting a single pathway, vitamin D functions as a regulator across multiple biological systems.
Vitamin D and the Immune System
The immune system is one of the areas where vitamin D appears to play a particularly important role.
Vitamin D helps regulate both:
Innate Immunity
Your body’s first line of defense against infections.
Adaptive Immunity
The specialized immune response that develops over time.
Researchers have found that vitamin D helps support immune balance rather than simply “boosting” immunity.
This distinction is important.
Doctor’s Note: A healthy immune system does not need to be constantly activated. It needs to be regulated. Vitamin D appears to help support that balance.
Emerging evidence suggests that vitamin D deficiency may be associated with increased susceptibility to certain infections and immune dysregulation.
Vitamin D and Mood
Many patients are surprised to learn that vitamin D receptors are found throughout the brain.
Research has identified vitamin D activity in regions involved in:
- Mood regulation
- Emotional processing
- Cognitive function
- Neurotransmitter production
Low vitamin D levels have been associated with higher rates of depressive symptoms in some populations.
Vitamin D should not be viewed as a standalone treatment for depression, but maintaining adequate levels may be an important part of supporting overall brain health.
This relationship may help explain why some individuals experience seasonal mood changes during periods of reduced sunlight exposure.
Vitamin D and Metabolic Health
One of the most exciting areas of current research involves vitamin D’s relationship to metabolic health.
As we have discussed in previous articles on obesity, inflammation, insulin resistance, and GLP-1 medications, metabolic health extends far beyond weight alone.
Vitamin D receptors are present in pancreatic beta cells responsible for insulin production.
Research suggests vitamin D may influence:
- Insulin secretion
- Insulin sensitivity
- Glucose metabolism
- Inflammatory pathways
Although studies have produced mixed results regarding supplementation outcomes, low vitamin D levels are frequently observed in individuals with obesity, prediabetes, type 2 diabetes, and metabolic syndrome.
Researchers continue to investigate whether optimizing vitamin D status may improve metabolic outcomes in certain patient populations.
Vitamin D and Inflammation
Chronic inflammation is increasingly recognized as a major contributor to modern disease.
Inflammation plays a role in cardiovascular disease, diabetes, obesity, neurodegenerative disease, autoimmune conditions, and many other chronic conditions.
Vitamin D appears to influence multiple inflammatory pathways.
Research suggests vitamin D may help regulate inflammatory cytokines and support immune balance.
While vitamin D is not an anti-inflammatory medication, maintaining adequate levels may contribute to a healthier inflammatory environment within the body.
Vitamin D and Cardiovascular Health
Cardiovascular disease remains the leading cause of death in the United States.
Researchers have explored links between vitamin D deficiency and hypertension, coronary artery disease, heart failure, and stroke risk.
Although supplementation studies have produced mixed findings regarding cardiovascular outcomes, vitamin D deficiency has consistently been associated with poorer cardiovascular health markers.
This is one reason many preventative medicine specialists include vitamin D assessment as part of a broader cardiovascular and metabolic risk evaluation.
What Is an Optimal Vitamin D Level?
This remains one of the most debated topics in medicine.
Most laboratories define vitamin D status approximately as follows:
- Deficient: Less than 20 ng/mL
- Insufficient: 20 to 29 ng/mL
- Generally adequate: 30 to 50 ng/mL
Some longevity and preventative medicine practitioners aim for levels somewhat higher than the minimum adequate range.
However, more is not always better.
Vitamin D optimization is not the same as megadosing. Excessive supplementation can lead to toxicity, elevated calcium levels, kidney stones, and other complications.
The goal is optimization—not excess.
Who Is at Higher Risk for Vitamin D Deficiency?
Several groups face increased risk.
Individuals with Darker Skin Tones
Melanin reduces UVB penetration and vitamin D production.
Individuals with Obesity
Vitamin D can become sequestered within adipose tissue.
Older Adults
The skin becomes less efficient at producing vitamin D with age.
Individuals with Limited Sun Exposure
Indoor occupations, limited outdoor time, seasonal changes, and consistent sun avoidance can reduce vitamin D production.
Patients with Certain Gastrointestinal Conditions
Malabsorption disorders may impair vitamin D absorption.
Should Everyone Take a Vitamin D Supplement?
Not necessarily.
The best approach is individualized.
Factors to consider include:
- Baseline vitamin D level
- Geographic location
- Sun exposure
- Diet
- Medical history
- Body composition
- Medications
For patients with documented deficiency, supplementation is often appropriate. For others, lifestyle factors and periodic monitoring may be sufficient.
Practical Strategies for Maintaining Healthy Vitamin D Levels
Get Safe Sun Exposure
Moderate sun exposure remains the most natural source of vitamin D production.
Eat Vitamin D-Rich Foods
Examples include fatty fish, egg yolks, fortified dairy products, and fortified plant-based milks.
Monitor High-Risk Individuals
Testing may be appropriate for individuals with risk factors for deficiency.
Use Evidence-Based Supplementation
Supplementation should be individualized based on laboratory results and medical history.
What Recent Research Is Revealing
The vitamin D story continues to evolve.
Researchers are now exploring connections between vitamin D and:
- Healthy aging
- Muscle preservation
- Frailty prevention
- Immune resilience
- Cognitive health
- Metabolic disease
- Inflammation
What has become increasingly clear is that vitamin D’s role extends far beyond bones.
Its influence reaches multiple systems that affect long-term health and disease prevention.
Final Thoughts
Vitamin D may be labeled a vitamin, but biologically it behaves much more like a hormone.
Its effects extend into gene expression, immune regulation, brain function, metabolism, inflammation, and cardiovascular health.
While vitamin D is not a cure-all, maintaining healthy levels represents one of the simplest and most cost-effective strategies for supporting overall wellness.
Final Thought from Dr. Co: As with most aspects of preventative medicine, the goal is not chasing perfection. The goal is creating an environment where the body can function at its best. Sometimes that starts with something as simple as understanding the power of vitamin D.
References
- Holick MF. Vitamin D Deficiency. New England Journal of Medicine. 2007;357(3):266-281.
- Bouillon R, Marcocci C, Carmeliet G, et al. Skeletal and Extraskeletal Actions of Vitamin D. The Lancet Diabetes & Endocrinology. 2019;7(8):641-653.
- Pilz S, Zittermann A, Trummer C, et al. Vitamin D Testing and Treatment: A Narrative Review of Current Evidence. Endocrine Connections. 2019;8(2):R27-R43.
- Carlberg C, Velleuer E. Vitamin D and the Regulation of Gene Expression. Current Opinion in Clinical Nutrition & Metabolic Care. 2020;23(6):462-469.
- Charoenngam N, Holick MF. Immunologic Effects of Vitamin D on Human Health and Disease. Nutrients. 2020;12(7):2097.
- Berridge MJ. Vitamin D Deficiency and Diabetes. Biochemical Journal. 2017;474(8):1321-1332.
- Manson JE, Cook NR, Lee IM, et al. Vitamin D Supplements and Prevention of Cancer and Cardiovascular Disease. New England Journal of Medicine. 2019;380:33-44.
- Martineau AR, Jolliffe DA, Hooper RL, et al. Vitamin D Supplementation to Prevent Acute Respiratory Infections. BMJ. 2017;356:i6583.
- Eyles DW, Burne THJ, McGrath JJ. Vitamin D, Effects on Brain Development, Adult Brain Function and the Links Between Low Levels of Vitamin D and Neuropsychiatric Disease. Frontiers in Neuroendocrinology. 2013;34(1):47-64.
- Institute of Medicine. Dietary Reference Intakes for Calcium and Vitamin D. National Academies Press. 2011.
Disclaimer: This article is for educational purposes only and does not replace personalized medical advice. Always speak with a qualified healthcare professional before starting, stopping, or changing any supplement, especially if you have kidney disease, elevated calcium levels, sarcoidosis, parathyroid disease, are pregnant, or take prescription medications.
