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At-Home Vitamin D Testing and How to Improve Your Vitamin D Levels for a Strong Immune System

Have you had your Vitamin D levels checked recently? Vitamin D is one of the most important nutrients for a strong immune system, yet many people are Vitamin D deficient, especially during the winter.

Here's a primer on:

  • why Vitamin D levels matter for your immune system

  • why Vitamin D levels are lower in the winter

  • optimal Vitamin D levels

  • how to test your Vitamin D levels at home

  • how to naturally increase your Vitamin D to a healthy level

Low Vitamin D Levels = Increased Illnesses and Stronger Symptoms

Vitamin D deficiency is widespread and common. Although Vitamin D is mostly known as an important nutrient for bone health [1, 2], it is also crucial for a strong immune system.

Not only does Vitamin D regulate the immune system [3]; "epidemiological evidence indicates a significant association between vitamin D deficiency and an increased incidence of several infectious diseases [4]."

Low Vitamin D levels have been linked to increased risk of:​

  • contracting illnesses such as influenza and COVID-19 [5, 6, 7, 8]

  • severe illness and death from influenza and COVID-19 [9, 10, 11]

Therefore, making sure your Vitamin D levels are optimal is an important strategy for preventing illness and having milder symptoms in the event that you do catch an illness.

Vitamin D Deficiency in Winter and For People with Dark Skin

Sunshine is the natural mechanism by which our bodies can produce most of the needed Vitamin D. Our bodies naturally make Vitamin D when our skin is exposed to direct sunlight [12].

However, people tend to become Vitamin D deficient in the winter because the sunlight angle is reduced and there is less skin exposed to sunshine. People who live in northern latitudes are also more likely to suffer Vitamin D deficiency because of the reduced angle of the sun. Usage of sunscreen year-round also contributes to low Vitamin D levels.

Additionally, darker skin produces less Vitamin D from sun exposure. This is because "melanin in the skin [can block] the UVB solar radiation necessary for [Vitamin D] synthesis" [13].

I Aim for Vitamin D Levels of At Least 40ng/mL

The typical "normal" range for Vitamin D on blood tests is 30-80 ng/mL. In the studies of illness severity for COVID-19 and influenza, it was found that Vitamin D levels of at least 30-40 ng/mL were associated with reduced severity of illness. [9, 10, 11]

"Patients with vitamin D deficiency (<20 ng/mL) were 14 times more likely to have severe or critical disease than patients with 25(OH)D ≥40 ng/mL... Among hospitalized COVID-19 patients, pre-infection deficiency of vitamin D was associated with increased disease severity and mortality."

There is some evidence that having too high of Vitamin D levels can be harmful as well [14]. I personally aim for my Vitamin D levels to be in the range of ~40-70 ng/mL.

At-Home Vitamin D Testing

In order to know what your Vitamin D levels are, you'll need to have a blood test. You can request a Vitamin D test from your doctor, or you can actually test your Vitamin D levels at home. I have used the Everlywell Vitamin D test to check my own levels, and I love the convenience of being able to use this test whenever I want to without having to visit the doctor.

I found the EverlyWell Vitamin D test kit very simple to use. It comes with a lancet and blood collection card. Here is a video showing how to collect your blood sample.

Once you've collected your sample, just mail it in with the prepaid envelope. You will then receive your results by email. As an example, here are my recent results. I plan to test again towards the end of winter so I can assess whether my supplementation with Vitamin D3/K2 and extra-virgin cod liver oil has been sufficient to maintain optimal levels.

How to Increase Your Vitamin D Levels

Depending on the results of your Vitamin D test, you may want to work towards increasing your levels for optimal immune system function.

Three methods I use for maintaining good Vitamin D levels are:

  • sun exposure during the warm months

  • eating foods rich in Vitamin D

  • supplementation

Sunbathing for Vitamin D

Depending on what latitude you live at, sunbathing can be a natural and free way to boost Vitamin D levels.  Sunshine is also known to help promote healthy sleep patterns and reduce depression.

When I'm using sunshine to boost my Vitamin D levels, I aim to get sun exposure on a significant portion of my skin at least 10-20 minutes per day between the hours of 10am-2pm. If you don't sunbathe regularly, make sure to start with just a few minutes of exposure to prevent burning.

Using sunshine for Vitamin D can be less effective in the winter months because the angle of the sun is reduced. I live in southern New Mexico, and the angle of the sun in the winter is just barely enough to facilitate good Vitamin D production.

If you live above ~35 degrees latitude (which runs through southern Tennessee and Nevada, as well as Albuquerque NM), the angle of the sun is not strong enough through the winter months to obtain adequate Vitamin D from sunbathing alone.

Vitamin D Rich Foods

Organ meats, especially liver, are excellent food sources for Vitamin D. One easy way I have found to make sure my family is regularly eating at least a small amount of organ meats is to add finely chopped liver and heart (from the giblets) to each pot of chicken soup I make.

If your family doesn't enjoy eating organ meats, dessicated liver capsules may be a good way to meet this nutrient need. Although not quite as potent as organ meats, some other foods that are good sources of Vitamin D include egg yolks, clams and salmon.

Vitamin D Supplements

Studies have shown that supplementation is effective at raising Vitamin D levels [15]. Furthermore, a study of 663,000+ veterans found that supplementation with Vitamin D2 and D3 was associated with a 20-28% reduced chance of catching COVID-19 and a 25-33% reduction in mortality [16].

My kids and I use the following two supplements for maintaining good Vitamin D levels in the winter months:

  1. Rosita's extra virgin cod liver oil (EVCLO) - This whole-food supplement is an excellent natural source of Vitamin D. Whole food supplements have greater absorption than taking specific vitamins alone. EVCLO's relatively mild flavor and superb nutrition have made it a win-win in our household. With so many rancid and overly-processed cod liver oils on the market, Rosita's EVCLO is the only cod liver oil that I trust. EVCLO is superior because the ancient-Viking harvesting process uses no heat, pressure, or chemicals. This minimal processing means that EVCLO is rich in naturally-occurring Vitamins A and D. We use both liquid and capsules of EVCLO.

  2. Adapt Naturals Vitamin D3/K2 oil - This flavorless oil comes in a dropper bottle that makes it very easy to take Vitamin D regularly. Studies have shown that Vitamin D works best in conjunction with Vitamin K2 [17], so I like the convenience of having these two nutrients in one easy-to-take supplement. Adapt Naturals is Chris Kresser's supplement company. Chris is a functional medical practitioner and I've trusted his common sense healthy diet and lifestyle advice for over a decade. We've been using his Vitamin D3/K2 oil for about a year, and it really is flavorless! If you want to give it a try, here is a coupon code that will give you 15% off your first order: NOURISHED15

I hope you find this information helpful in optimizing your immune systems, for fewer illnesses and milder symptoms!

Disclaimer and Affiliate Disclosure

I am not a doctor or licensed healthcare professional. I am a homeopathic practitioner whose services are considered complementary and alternative by the state of New Mexico. The information described herein is provided for educational use only and is not a substitute for medical treatment.

Links to Amazon, Rosita, and Adapt Naturals are affiliate links. If you use these links, your price remains the same and I may earn a small commission. Thanks for supporting this site!


[1] Segheto, Kátia Josiany et al. “Vitamin D and bone health in adults: a systematic review and meta-analysis.” Ciencia & saude coletiva vol. 26,8 (2021): 3221-3244.

[2] Holick, M F. “Vitamin D and bone health.” The Journal of nutrition vol. 126,4 Suppl (1996): 1159S-64S.

[3] Vanherwegen AS, Gysemans C, Mathieu C. "Regulation of Immune Function by Vitamin D and Its Use in Diseases of Immunity", Endocrinology and Metabolism Clinics of North America.  2017 Dec;46(4):1061-1094.

[4] Kikuta J, Ishii M. "Current Topics on Vitamin D. The Effects of Vitamin D on the Immune System", Clinical Calcium. 2015 Mar;25(3):359-65.

[5] Meltzer DO, Best TJ, Zhang H, Vokes T, Arora V, Solway J. Association of Vitamin D Status and Other Clinical Characteristics With COVID-19 Test Results. JAMA Netw Open. 2020;3(9):e2019722.

[6] Kaufman, Harvey W et al. “SARS-CoV-2 positivity rates associated with circulating 25-hydroxyvitamin D levels.” PloS one vol. 15,9 e0239252. 17 Sep. 2020, doi:10.1371/journal.pone.0239252.

[7] Zhu, Zhixin et al. “Association Between Vitamin D and Influenza: Meta-Analysis and Systematic Review of Randomized Controlled Trials.” Frontiers in nutrition vol. 8 799709. 7 Jan. 2022.

[8] Grant, William B et al. “Evidence that Vitamin D Supplementation Could Reduce Risk of Influenza and COVID-19 Infections and Deaths.” Nutrients vol. 12,4 988. 2 Apr. 2020.

[9] Dror, Amiel A et al. “Pre-infection 25-hydroxyvitamin D3 levels and association with severity of COVID-19 illness.” PloS one vol. 17,2 e0263069. 3 Feb. 2022.

[10] Charoenngam, Nipith et al. “Association of Vitamin D Status With Hospital Morbidity and Mortality in Adult Hospitalized Patients With COVID-19.” Endocrine practice : official journal of the American College of Endocrinology and the American Association of Clinical Endocrinologists vol. 27,4 (2021): 271-278.

[11] Hurst, Emma A et al. “Vitamin D insufficiency in COVID-19 and influenza A, and critical illness survivors: a cross-sectional study.” BMJ open vol. 11,10 e055435. 22 Oct. 2021.

[12] Wacker, Matthias, and Michael F Holick. “Sunlight and Vitamin D: A global perspective for health.” Dermato-endocrinology vol. 5,1 (2013): 51-108.

[13] Ames, Bruce N et al. “Does the High Prevalence of Vitamin D Deficiency in African Americans Contribute to Health Disparities?.” Nutrients vol. 13,2 499. 3 Feb. 2021.

[14] Kaur, Parjeet et al. “Vitamin D toxicity resulting from overzealous correction of vitamin D deficiency.” Clinical endocrinology vol. 83,3 (2015): 327-31.

[15] Joh, Hee-Kyung et al. “Effect of sun exposure versus oral vitamin D supplementation on serum 25-hydroxyvitamin D concentrations in young adults: A randomized clinical trial.” Clinical nutrition (Edinburgh, Scotland) vol. 39,3 (2020): 727-736.

[16] Gibbons, Jason B et al. “Association between vitamin D supplementation and COVID-19 infection and mortality.” Scientific reports vol. 12,1 19397. 12 Nov 2022. 

[17] van Ballegooijen, A. J., Pilz, S., Tomaschitz, A., Grübler, M. R., & Verheyen, N. (2017). The Synergistic Interplay between Vitamins D and K for Bone and Cardiovascular Health: A Narrative Review. International journal of endocrinology, 2017, 7454376.

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