Ever heard of something called “the sunshine vitamin”? There’s only one vitamin it could be…

Vitamin D is arguably one of the most important vitamins out there, but oddly, it’s also one of the most common deficiencies. And with the sun being one of the best sources of vitamin D, how is it that so many people can be deficient?

If you’ve never tested your vitamin D levels or have and you’re low, we’re giving you some of the reasons behind why you might be deficient.

What Is Vitamin D?

Vitamin D is one of the four fat-soluble vitamins along with vitamins A, E, and K. While technically it’s classified as a vitamin, it’s actually a fat-soluble prohormone steroid with endocrine, paracrine, and autocrine functions 1.

It plays a role in nearly every body system and the vitamin D receptor (VDR) is found in most cells, which results in widespread actions on most physiologic processes—not just bone homeostasis 2.

The endocrine functions primarily target calcium and phosphorus levels, while the paracrine and autocrine functions differ based on what cell they’re being expressed in.

Why Vitamin D Is Important

While bone health is one of the most well-known roles of vitamin D in the human body, there are a few other important ones. Let's start with the most obvious, however:

Bone health

One of the biggest roles of vitamin D is regulating calcium and phosphate levels, which is critical for keeping bones healthy.

That’s because vitamin D is required for regulating the absorption of calcium and phosphate from the gut, which helps maintain bone tissue.

Without sufficient vitamin D, your body leaches calcium from bone to increase blood levels, which ultimately leads to less calcium in the bones and a higher risk of osteoporosis and fractures.


There’s a big connection between levels of vitamin D and mood. Take seasonal affective disorder, for example - a type of depression related to change of seasons and sunshine availability.

This is likely because there are vitamin D receptors in areas of the brain responsible for controlling mood 3. After supplementing vitamin D, studies find that people with depression show improvements in their symptoms 4.

Immune Function

Involved in both the adaptive and innate immune responses, adequate levels of vitamin D are required for keeping the immune system functioning properly 5.

A deficiency of vitamin D has been linked to an increased risk of autoimmune conditions like rheumatoid arthritis, MS, and diabetes, but also an increase in susceptibility to illness and infection 6.


As part of its role in the immune system, vitamin D also plays a part in regulating the inflammatory response through modulation of the production of inflammatory cytokines and inhibiting the proliferation of pro-inflammatory cells 7.

What Causes Vitamin D Deficiency?

Lack of sun

This one seems pretty obvious, but lack of sun exposure is the biggest reason for vitamin D deficiency. In fact, about 50% to 90% of vitamin D is absorbed through the skin via sunlight 8.

Because vitamin D3 is formed when UV rays hit the skin, insufficient sun exposure means insufficient production. And because vitamin D intake through diet is rarely sufficient to boost serum levels, sun exposure is essential.

However, it’s important to keep in mind that several factors can influence how much vitamin D your body produces 9:

  • Geography
  • Seasons
  • Clothing
  • Sunscreen use
  • Skin pigmentation


As you get older, your body’s ability to produce vitamin D decreases for a few reasons 10:

  • Decreased numbers of vitamin D receptors
  • Decreased renal production of vitamin D due to aging kidneys
  • Decreased skin production of vitamin D
  • Lack of vitamin D substrate (cholesterol)


It may not seem like an obvious link, but body weight may interfere with levels of vitamin D due to its role in modulating the activity of adipocytes 11. Studies suggest that low plasma vitamin D levels are associated with a higher BMI and waist circumference.

But it’s also related to distribution within the body. Because vitamin D is distributed into a larger volume in obese people, serum concentrations are lower 12. Specifically, vitamin D is found largely in serum, muscle, fat, and liver—all areas that are increased in obese people.

Poor diet

Cold-water fatty fish, organ meats, and dairy are some of the best food sources of vitamin D, and a diet lacking in these can contribute to lower serum levels.

This becomes more of a concern for vegans and vegetarians who limit animal products, so for people who follow a plant-based diet, supplementing vitamin D3 becomes essential to maintaining proper serum concentrations.

Additionally, people who consume a diet high in refined and processed foods with minimal whole foods and high-quality proteins are likely to consume less vitamin D.


People with malabsorption disorders are at a higher risk for vitamin D deficiency. That’s because vitamin D is a fat-soluble vitamin that’s normally absorbed in the small intestine alongside fat, so if your body isn’t absorbing fat properly, you’re also not absorbing your fat-soluble vitamins.

Final Thoughts

Vitamin D is kind of like the king of the vitamin world. It’s involved in many functions throughout the body and, despite how easy it is to get sufficient amounts through sun and diet, the vast majority of the world’s population doesn’t get enough.

Whether you’re not consuming enough through diet or don’t get much sun, supplementing with a high-quality vitamin D supplement becomes essential.

What better way to get your daily dose than with Testo Lab Pro! Containing a 2,000 IU dose of Vitamin D per serving, Testo Lab Pro not only improves total T, bioactive T, and free T, it also supports robust sexual function and overall health too!

What's not to love about that?!


  1. WZ Mostafa, RA Hegazy. Vitamin D and the skin: Focus on a complex relationship: A review. J Adv Res. 2015;6(6):793-804.
  2. D Bikle. Vitamin D: Production, Metabolism, and Mechanisms of Action. [Updated 2017 Aug 11]. In: Feingold KR, Anawalt B, Boyce A, et al., editors. Endotext [Internet]. South Dartmouth (MA): MDText.com, Inc.; 2000-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK278935/
  3. DW Eyles, S Smith, R Kinobe, M Hewison, JJ McGrath. Distribution of the vitamin D receptor and 1 alpha-hydroxylase in human brain. J Chem Neuroanat. 2005;29(1):21-30.
  4. R Jorde, M Sneve, Y Figenschau, J Svartberg, K Waterloo. Effects of vitamin D supplementation on symptoms of depression in overweight and obese subjects: randomized double blind trial. J Intern Med. 2008;264(6):599-609.
  5. M Hewison. Vitamin D and Innate and Adaptive Immunity. Vitamins & Hormones. 2011; 86:23-62.
  6. KL Munger, LI Levin, BW Hollis, NS Howard, A Ascherio. Serum 25-hydroxyvitamin D levels and risk of multiple sclerosis. JAMA. 2006;296(23):2832-2838.
  7. K Yin, DK Agrawal. Vitamin D and inflammatory diseases. J Inflamm Res. 2014;7:69-87.
  8. O Sizar, S Khare, A Goyal, et al. Vitamin D Deficiency. [Updated 2021 Jan 3]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2021 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK532266/
  9. Bikle D. Nonclassic actions of vitamin D. J Clin Endocrinol Metab. 2009;94(1):26-34. doi:10.1210/jc.2008-1454
  10. JC Gallagher. Vitamin D and aging. Endocrinol Metab Clin North Am. 2013;42(2):319-332.
  11. XM Mai, Y Chen, CA Camargo Jr, A Langhammer. Cross-sectional and prospective cohort study of serum 25-hydroxyvitamin D level and obesity in adults: the HUNT study. Am J Epidemiol. 2012;175(10):1029-1036.
  12. L Vranić, I Mikolašević, S Milić. Vitamin D Deficiency: Consequence or Cause of Obesity? Medicina (Kaunas). 2019;55(9):541.