As we slip into fall, modern fashion says it’s ok to keep wearing white, and modern science says it’s important to keep vitamin D levels up.

Warm weather, long hours of sunlight, and time off from work and school make it easier to maintain adequate vitamin D levels in the summer. As we head back indoors for work and school in the fall, many of us descend into a deficit of vitamin D. This puts our immune systems at a disadvantage as we also enter cold and flu season.

How does natural vitamin D synthesis work?

Of the 5 natural vitamin D analogs (D1-D5), vitamin D3, cholecalciferol, is the only one synthesized in the human body. UVB rays from the sun react with a substance in the skin called 7-dehydrocholesterol, breaking it down into pre-vitamin D3 [1]. With continued UV exposure, pre-D3 isomerizes into vitamin D3. D3 then binds a carrier protein to travel out of the skin to circulate the body and undergo further metabolism in the liver.

The takeaway here is that vitamin D is only synthesized when skin is exposed to sunlight (unprotected by clothing or SPF) for a certain amount of time daily. No sun on bare skin, no vitamin D3.

It is important to note that the more melanin pigment your skin makes, the more time you must spend in the sun before reaching the requirement for healthy vitamin D production. For example, someone with light skin, hair, and eyes may be able to get enough UV irradiation in 15 minutes of midday sun, while someone with dark skin, hair, and eyes may need closer to 40 minutes [2,3]. These numbers will change significantly of course depending on altitude, latitude, season, and surface area of exposed skin. 

Who is at risk of vitamin D deficiency?

It is estimated that about 50% of the global population suffers from vitamin D deficiency, but growth rates may be as high as 80% in melanated communities in the US [4,5]. This deficiency can result simply from living in areas where the weather is too cold to expose skin outdoors but is also likely due to western cultural norms that keep people indoors for work and school during daylight hours. 

On top of all this, education about the importance of sunscreen for skin health is more widespread than vitamin D education. Many people are protecting their skin from the sun, but neglecting to replace that missed vitamin D with a supplement. The solution can be that simple.

Aside from time in the sun and skin color, other factors affecting vitamin D levels include subcutaneous fat and fat malabsorption issues, as vitamin D is a fat-soluble vitamin. People with obesity tend to store more vitamin D in fat, and therefore have lower circulating vitamin D levels [4]. Those who have had gastric bypass surgery are also at risk for vitamin D absorption, as the upper section of the intestine, where dietary vitamin D is absorbed, is bypassed by the procedure [4]. 

The very young and very old tend to be vitamin D deficient at greater rates as well. Breast milk very rarely provides enough vitamin D for the needs of exclusively breastfed babies, unless the mother takes a potent daily supplement [4]. And, skin does slow production of vitamin D as we age, leaving the elderly unable to produce sufficient vitamin D without potentially damaging stretches of time in the sun [4].

All of these individuals would likely benefit from a vitamin D supplement of varying potencies.

Am I vitamin D deficient?

The only way to definitively determine whether or not you are vitamin D deficient is by a blood test. However, you can consider the requirements for vitamin D synthesis described above and estimate whether you spend enough time in the sun (without protective clothing or sunscreen) for your skin type. 

If you are not spending adequate time in the sun to generate healthy vitamin D internally, are you eating fortified food or drinks? Does your daily multivitamin contain a potent dose of vitamin D3? If the answer is “no” to all of these questions, you may want to consider getting tested for vitamin D deficiency and talking to your doctor about adding supplemental vitamin D to your daily health regimen. 

What does vitamin D do?

We typically associate vitamin D with bones because of its most well-known role in the body: allowing for the absorption of calcium. 

However, nearly every tissue in the human body expresses vitamin D receptors, a clear sign that vitamin D is important for the healthy function of every system of the body. 

Vitamin D deficiency is associated with the development of obesity, diabetes, hypertension, depression, fibromyalgia, chronic fatigue syndrome, osteoporosis, and neurodegenerative diseases including Alzheimer’s disease [6]. Vitamin D deficiency has been shown to play a role in seventeen varieties of different cancers as well as heart disease, stroke, autoimmune diseases, birth defects, and periodontal disease [6]. 

We should all be aware of, and concerned with, maintaining healthy vitamin D levels.

What is the link between vitamin D and immune system health?

Vitamin D deficiency is also associated with increased autoimmunity as well as increased susceptibility to infection. The mechanism behind this effect is likely due to vitamin D having the ability to modulate threats to the immune system on the cellular level. 

Three cell types within the immune system expressing the vitamin D receptor are B cells, T cells, and antigen-presenting cells [7]. These cell types are specifically involved in the recognition and memory of foreign microbial threats, for the quickest shut down of a potential infection. 

When it comes to protecting your body from colds and flu, these are key cell types you want to be working at full function. Data suggests vitamin D may be one of the most important nutrients for their ability to function swiftly and accurately.

How much supplemental vitamin D do I need?

Again, to know your ideal supplement dosage, a blood test and a doctor consultation is a great choice. 

The Endocrine Society suggests adults likely need at least 1,500–2,000 IU per day of supplemental vitamin D, and children and adolescents might need at least 1,000 IU per day to maintain consistent, healthy vitamin D levels in the circulation [8]. The tolerable upper intake level (or the maximum dose likely needed to provide benefit for a healthy individual) is 4,000 IU vitamin D per day for adults. 

If your sun exposure is minimal, or you have other health conditions affecting your ability to make or absorb vitamin D, your daily supplement dose may need to be much higher than the typical tolerable upper intake level. 

Vitamin D supplements are available at doses up to 25,000 IU through Metabolic Maintenance®, but a high dose like this should only be taken under the supervision of a physician. 


  1. Bikle, D. “Vitamin D: production, metabolism, and mechanisms of action. MDText. com.” (2017).
  2. Rhodes, Lesley E., et al. “Recommended summer sunlight exposure levels can produce sufficient (≥ 20 ng ml− 1) but not the proposed optimal (≥ 32 ng ml− 1) 25 (OH) D levels at UK latitudes.” Journal of Investigative Dermatology 130.5 (2010): 1411-1418.
  3. Amr, N., et al. “Vitamin D status in healthy Egyptian adolescent girls.” Georgian medical news 210 (2012): 65-71.
  4. Nair, Rathish, and Arun Maseeh. “Vitamin D: The “sunshine” vitamin.” Journal of pharmacology & pharmacotherapeutics 3.2 (2012): 118.
  5. Runestad, Todd. “Congress lauds vitamin D for COVID-19”. Natural Products Insider. Feb 25, 2021.
  6. Naeem, Zahid. “Vitamin d deficiency-an ignored epidemic.” International journal of health sciences 4.1 (2010): V.
  7. Aranow, Cynthia. “Vitamin D and the immune system.” Journal of investigative medicine 59.6 (2011): 881-886.
  8. NIH. “Vitamin D: Fact Sheet for Health Professionals”. NIH Office of Dietary Supplements. Accessed August 17, 2021.