Vitamin D Demystified: Understanding the 5 Key Factors that Affect Your Levels

Vitamin D

The nickname for vitamin D is the “sunshine vitamin” – for good reason. The main source of vitamin D for people who don’t take a supplement is exposure to sunlight.  Still, approximately 42% of the U.S. population is vitamin D deficient, with 90% of Americans of color having levels too low. Severe vitamin D deficiency, defined as 25(OH)D <30 nmol/L (or 12 ng/ml), has a prevalence rate of 5.9% in the United States.

This essential nutrient is critical for bone health, immune function, and many other bodily processes. But what are the risk factors for vitamin D deficiency? Let’s look at five key factors that can negatively impact your vitamin D level and what you can do to ensure you get enough of this vital nutrient.

But what are the risk factors for vitamin D deficiency? Let’s look at five key factors that can negatively impact your vitamin D level and what you can do to ensure you get enough of this vital nutrient.

#1 Location

The amount of vitamin D you can synthesize from sun exposure depends on several factors, including where you live. If you live at a high latitude, such as in the northern part of the United States or Canada, you receive less direct sunlight during the fall and winter months. According to Harvard Health, the skin makes little if any vitamin D from the sun at latitudes above 37 degrees north or below 37 degrees south of the equator, except during the summer.

If you live closer to the equator, you will receive more direct sunlight and are less likely to be deficient in vitamin D. Furthermore, individuals who live in areas with frequent overcast skies are more prone to vitamin D deficiency because they get less sunlight.

That’s why it’s important to get outside most days of the week and expose your skin to sunlight. But even if you do, it may not be enough to boost your vitamin D into a healthy range.  For example, older adults may have a reduced capacity to synthesize vitamin D from sunlight, while individuals with darker skin may require more sunlight exposure to produce the same amount of vitamin D as those with lighter skin. So, you can’t always depend on sun exposure to meet your vitamin D needs.

#2 Skin Pigment

The pigment that gives your skin and hair color, melanin, also impacts your vitamin D levels. Melanin has a strong affinity for the ultraviolet rays that stimulate vitamin D synthesis in the skin, meaning it can block vitamin D synthesis at the level of your skin. So, if you have a lot of melanin in your skin, you need more sun exposure than someone with lighter skin to produce the same amount of vitamin D.

If you have dark skin, it may not be practical to count on sun exposure to provide you with adequate vitamin D. Therefore, those with dark skin must supplement their diets with other sources of vitamin D, including dietary sources such as fish, eggs, and fortified dairy products, as well as supplements. Since food is not a reliable source of vitamin D, a supplement may be necessary.

#3 Sunscreen Use

Sunscreen can play a role in reducing vitamin D synthesis, as it blocks ultraviolet light, the primary source of vitamin D precursors. However, the amount of ultraviolet light that sunscreen blocks depend on the specific product, the amount you use, and how much you apply. Most people don’t apply enough sunscreen to completely block the ultraviolet rays that boost vitamin D production.

The downside of sun exposure is that it increases your risk of skin cancer, which is why so many people wear sunscreen when they step outside. Yet sunscreen also reduces your body’s ability to produce vitamin D from sun exposure. Balancing the risks of skin cancer and premature aging with the risks of vitamin D deficiency can be challenging, which is why many people choose to take a vitamin D supplement.

#4 Body Weight

Studies have linked being overweight or obese with low vitamin D levels, but the relationship between weight and vitamin D is complex, and scientists don’t fully understand it. Some research suggests being overweight may lead to a drop in vitamin D levels, while other studies indicate that low vitamin D levels contribute to weight gain. Regardless of the cause-and-effect relationship, people with higher body weights are at a higher risk of vitamin D deficiency.

#5 Age

As we age, our skin becomes less efficient at synthesizing vitamin D, and the kidneys become less efficient at converting the precursor form of vitamin D into the active form. According to a 2013 study, aging reduces vitamin D production in the skin by 50%, resulting in a decrease in the concentration of 7-dehydrocholesterol (a precursor to vitamin D) in the outer layer of the skin.

In addition, older adults are often less physically active and spend less time outdoors, both of which can further contribute to vitamin D deficiency. This is why it’s important that older adults get enough vitamin D through their diet, as well as spending some time outdoors and in the sun each day.


Vitamin D is an essential nutrient that is vital in various bodily processes. Understanding the five key factors that can negatively impact your vitamin D level is the first step in ensuring you get enough of this important nutrient. Whether you choose to get more sun exposure, take a vitamin D supplement, or amend your diet, there are many ways to boost your vitamin D levels and improve your health.

Talk to your doctor about whether you need a supplement and how much to take. Vitamin D helps with calcium absorption and supports healthy bones, as well as various other health benefits. A doctor will consider your diet, lifestyle, and other factors to determine if a vitamin D supplement is right for you and how much to take. They may also suggest checking your vitamin D level through a blood test to see where your level currently is.


  • Barkley, C. (2021). The power of vitamin D: What experts already know (and are still learning) about the “sunshine vitamin” – UTHealth News – UTHealth. Uth.edu. uth.edu/news/story.htm?id=0520d178-ab7a-49af-858e-a7adeec0b30e#:~:text=Vitamin%20D%20deficiency%20is%20already,National%20Institutes%20of%20Health%20database.
  • Amrein, K., Scherkl, M., Hoffmann, M., Neuwersch-Sommeregger, S., Köstenberger, M., Tmava Berisha, A., Martucci, G., Pilz, S., & Malle, O. (2020). Vitamin D deficiency 2.0: an update on the current status worldwide. European Journal of Clinical Nutrition, 74(11), 1498-1513. doi.org/10.1038/s41430-020-0558-y
  • Gallagher JC. Vitamin D and aging. Endocrinol Metab Clin North Am. 2013 Jun;42(2):319-32. doi: 10.1016/j.ecl.2013.02.004. Epub 2013 Apr 9. PMID: 23702404; PMCID: PMC3782116.
  • Time for more vitamin D (2023). Available at: https://www.health.harvard.edu/staying-healthy/time-for-more-vitamin-d (Accessed: 27 March 2023).

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