Vital Vitamin D: Supercharging Your Immune System

Featured Image Vitamin D.

Perhaps best known for its ability to shore up bone health, vitamin D also plays an important role in immune function and inflammation. Before advising clients on supplementation, learn about the intricacies of this hormone-like vitamin and how it affects key aspects of the immune system.

Vitamin D Deficiency Dangers

Vitamin D deficiencies abound worldwide, notably among individuals whose darker skin pigmentation makes absorbing the vitamin through sunlight all the more challenging. In addition, this D deficiency tends to affect older adults, obese individuals and those living in nursing homes— many of the same demographics hit hardest by COVID-19 infections, and with the most vulnerable immune systems.

Dr. David Meltzer, Chief of Hospital Medicine at the University of Chicago, has explored the potential impact of diminished vitamin D levels on contracting the novel COVID-19 virus. A serious COVID-19 infection often plunges one’s immune system into overdrive, resulting in rampant lung inflammation and concomitant shortness of breath, which can thwart or halt workouts.

A recent study analyzing over 200 patients hospitalized with a COVID-19 infection revealed that those with sufficient serum levels of vitamin D exhibited a reduction in the severity of the illness, lower levels of C-reactive protein (a marker for inflammation), an increase in the immune system’s lymphocytes, and a decreased risk of death. This combination strongly suggests an association between appropriate vitamin D stores and a properly functioning immune system.

If a client suffers from Inflammatory Bowel Syndrome (IBS), note that they may have impaired D signaling within the intestines, perhaps contributing to the host of comorbidities typically associated with IBS. Such a client might benefit from a reminder to get plenty of D, through food sources or regular sunshine exposure.

Dangerous Overdosing of D

Only a blood test can reveal genuine vitamin D deficiencies or rare excesses. While toxicologists feel that 4000 IU (International Units) of the sunshine vitamin in one’s bloodstream serves as a tolerable upper limit, this might only prove useful if one’s body already possesses adequate amounts of stored D.

The highly recognized Centers for Disease Control and Prevention (CDC) extolls the virtues of vitamin D in maintaining a healthy immune system, advocating a reasonable degree of supplementation along with vitamin C and zinc, also well-established infection fighters.

Many pro-supplement individuals prefer to leave no room for error, unnecessarily ingesting large excesses of many vitamins.

Such a practice often backfires, however, as “overdosing” on vitamin D can lead to a host of health problems. Hypercalcemia, the main consequence of D toxicity, results in a buildup of calcium in the blood. If left unchecked, such a situation leaves one prone to nausea, vomiting, frequent urination and an overall sense of weakness, further complicated by bone pain, kidney problems and the development of calcium-based kidney stones. Once health issues related to toxicity have occurred, reversing them takes time.

It’s also worth noting that a synergistic relationship has been identified between vitamins D and K. You’ll find that many supplements on the market today combine Vitamin D3 (a more absorbable form than D2) and K2.

Hearing Loss and Deficiency

Vitamin D deficiency can affect any bones in our bodies, including those tiny bones that make up the inner workings of the ear. This suggests a previously unrecognized association between D deficiency and cochlear deafness, commonly known as demineralization of the cochlea.

One research study aimed to assess hearing loss in patients with vitamin D deficiency and, perhaps more enlightening, a return to normal hearing upon increasing the vitamin. After adjusting for factors such as gender, age, and level of initial hearing loss, scientists found that upon receiving supplemental D, every participant no longer observed traces of poor hearing. The hearing loss of the control group (receiving no additional vitamin D) remained the same.

Mood, Memory, and Vitamin D

Recently the medical world noted a link between vitamin D deficiency and an increased risk for many mental health issues, among them depression, Alzheimer disease, epilepsy and generalized neurocognitive decline. Possibilities for drawing such conclusions may relate to vitamin D’s role in increasing serotonin levels in the brain, or perhaps the work of macrophages/phagocytosis of amyloid-β plaques in Alzheimer patients.


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Analysis of Intake

Data from the U.S. Preventive Services Task Force (USPSTF), published in The Annals of Internal Medicine, highlighted the controversy over what constitutes a diminished level of vitamin D. Some laboratories define deficiency as below 20 nanograms per milliliter (ng/mL), whereas others more generously list below 50 ng/mL as problematic. This gets further compounded by the fact that tests for vitamin D have poor reliability.

Dietitians and doctors typically cite the following yet confusing guidelines:

  • Recommended Dietary Allowance (RDA): Average daily level of intake sufficient to meet the nutrient requirements of 97%–98% of healthy individuals; often used to plan nutritionally adequate diets.
  • Adequate Intake (AI): Intake level assumed to ensure nutritional adequacy; sometimes established in lieu of RDA.
  • Estimated Average Requirement (EAR): Approximate daily intake level that meets the requirements of 50% of healthy individuals.
  • Tolerable Upper Intake Level (UL): Maximum daily intake unlikely to cause adverse health effects.

One major national study revealed that roughly 94% of Americans ingested less than the 4000 International Units (IU) EAR vitamin D from food/beverage sources or supplements.

Cell-Specific Autoimmune Response

In healthy populations, T cells play a crucial role in fighting infection. For individuals living with autoimmune diseases, however, T cells can attack the body’s own tissues.

A research team at the University of Edinburgh focused on how D affects a specific mechanism within the immune system – the ability of dendritic cells to activate T cells.

Professor Richard Mellanby, of the University of Edinburgh’s Centre for Inflammation Research, says that “Low vitamin D status has long been implicated as a significant risk factor for the development of several autoimmune diseases. Our study reveals one way in which vitamin D metabolites can dramatically influence the immune system.”

Since our scope of practice prohibits the dispensing of specific medical advice, suggesting food sources and sunshine, and referring your clients to a medical provider for advice on supplementation is the most prudent approach.



Cathleen Kronemer is an NFPT CEC writer and a member of the NFPT Certification Council Board. Cathleen is an AFAA-Certified Group Exercise Instructor, NSCA-Certified Personal Trainer, ACE-Certified Health Coach, former competitive bodybuilder and freelance writer. She is employed at the Jewish Community Center in St. Louis, MO. Cathleen has been involved in the fitness industry for over three decades. Feel free to contact her at [email protected] She welcomes your feedback and your comments!