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14 Signs of Vitamin D Deficiency & What You Can Do

Why Avoiding the Sun May Come With Side Effects

Key Takeaways:
  • Fatigue, depression, frequent colds, allergies, asthma, painful bones, and eczema are just a few of the 14 signs of vitamin D deficiency.
  • Nearly 66% of people in the U.S. have insufficient levels of vitamin D.
  • Unnecessary fear of the sun may be the most important (and fixable) contributing factor to low vitamin D levels.
  • Getting vitamin D from regular, healthy (non-burning) doses of sunshine may be the best way to get most of your vitamin D.
  • It’s easy and inexpensive to test your vitamin D level, and is essential when taking a vitamin D supplement.
  • An optimal vitamin D level is around  40–50 ng/mL for most people.
  • If your vitamin D level is low, you can bring it up with a combination of sun exposure, vitamin D-rich foods, and supplements.
  • Addressing an unhealthy gut is important for absorbing vitamin D from foods and supplements.

When you’re struggling with fatigue, depression, skin issues, and muscle pain, it’s easy to jump to the conclusion that there’s a serious problem. While these symptoms could indicate a larger issue, they’re also symptoms of vitamin D insufficiency and deficiency, which impacts about 66% of Americans [1]. Luckily, a simple, inexpensive blood test can help you quickly determine your vitamin D status. 

While labs don’t flag vitamin D deficiency until your level drops below 30 ng/mL, I see that most of my patients feel best in the 40–50 ng/mL range. If your vitamin D level is below this, simple strategies like safe sun exposure, improving your gut health, and (in some cases) supplementation, can help you boost it to a more desirable level and provide symptom relief.

In this article, I’ll share 14 signs of vitamin D deficiency and explain why so many Americans aren’t getting enough vitamin D. I’ll also discuss testing and what you can do to get your vitamin D level where it needs to be for optimal health. Before we jump in with the signs of vitamin D deficiency, let’s break down what vitamin D is, why it’s so important for your health, and who is most at risk for a deficiency. 

What is Vitamin D?

Vitamin D (cholecalciferol) is a steroid hormone that’s created in the body when ultraviolet B light (UVB) from the sun hits your bare skin. This sun-to-skin contact creates an inactive form of vitamin D that’s then converted to 25-hydroxy vitamin D (25(OH)D) in the liver. 25(OH)D is what travels in your bloodstream and is the form that’s most often measured on a vitamin D blood test. Once 25(OH)D makes its way to your kidneys, the biologically active form of vitamin D, known as calcitriol, is created [2]. 

While it’s probably most well-known for its bone health benefits, vitamin D helps with [3]:

  • Insulin production (and thus blood sugar regulation)
  • Heart muscle contractions
  • Immune system function
  • Inflammatory bowel disease (IBD) prevention

Having low levels of vitamin D (less than 30 ng/mL) is a known cause of rickets in children and osteomalacia in adults, both of which can cause weak bones (low bone density), muscle pain, and increase the risk of bone fractures [3]. Additionally, low levels of vitamin D are associated with many other health problems, but as I discuss in Healthy Gut, Healthy You, low levels are probably not a cause as much as they are a consequence of illness, like in the case of chronic inflammation.

Many people may be deficient or at least insufficient in their vitamin D levels. Worldwide it’s estimated that about half of the population, including 25% of U.S. adults are vitamin D deficient (less than 20 ng/mL). When we consider vitamin D insufficiency (20–30 ng/mL) the total grows to about 66%. This suggests that at any one time, only about one third of Americans have sufficient vitamin D levels [1]

Wintertime is a major factor predicting vitamin D deficiency, simply due to less sun exposure.  Groups who seem to be most at risk for vitamin D deficiency include [1, 4, 5]:

  • Women, including those who are pregnant
  • Non-Hispanic black populations
  • People aged 20-29
  • Older adults
  • Non-Western immigrants
  • Darker skin tones
  • Infants nourished only with breastfeeding
  • Those with low calcium intake
  • Those who are overweight or obese
  • Those who are immobile
  • Those with chronic kidney disease
  • Those with malabsorption syndromes (such as Crohn’s disease, celiac disease, cystic fibrosis, and severe liver disease)
  • Those taking drugs that decrease absorption (mineral oil, laxatives, orlistat, cholestyramine)

Now that you have a little background on vitamin D and how common low levels of vitamin D are, let’s get into the 14 signs of vitamin D deficiency.

14 Signs of Vitamin D Deficiency

Here are the top 14 signs of vitamin D deficiency:

  1. Depression or low mood [6]
  2. Autoimmunity [1, 7, 8, 9, 10]
  3. Inflammation [7, 8, 11]
  4. Eczema [12]
  5. Asthma [12]
  6. Seasonal or year-round allergies [12]
  7. Poor metabolism and weight gain [13]
  8. Frequent colds [1, 11]
  9. Fatigue [14, 15]
  10. Weak bones (osteoporosis) and more fractures [14]
  11. Muscle weakness [14]
  12. Bone pain, joint pain, and muscle pain [14]
  13. Calcium deficiency [16, 17, 18]
  14. Muscle twitching [14]

As you can see, many of these overlap with other health conditions, and there may be multiple contributing factors if you have these symptoms. But since checking your vitamin D level is relatively simple, it’s a good place to start. 

If your level is in the ideal 40–50 ng/mL range, then you can discuss other possible causes of your symptoms with your healthcare provider. If your vitamin D level is low, then you can easily bring it up with a combination of sun exposure, vitamin D-containing foods, and vitamin D supplements. So, let’s talk about vitamin D testing and what constitutes a deficiency.

Vitamin D Testing

If you’re struggling with the 14 signs of vitamin D deficiency, it’s tempting to just start taking a vitamin D supplement. But I want to caution you about doing this, number one: your symptoms could be related to something else. Number two: vitamin D is a fat-soluble vitamin, meaning it can be stored in the body. Since we can store vitamin D, there’s a potential for vitamin D toxicity, which is unlikely but nevertheless possible [3]. That being said, if you don’t have access to testing, following recommended dosing guidelines can help you prevent overdoing it.

Toxic blood levels of vitamin D can lead to very high blood levels of calcium, which can cause a spectrum of symptoms from fatigue, loss of appetite, and GI distress to confusion, heart arrhythmias, and coma [19]. It’s practically impossible to have toxic levels of vitamin D without supplementation or excessive amounts of fortified foods [3]. Even large amounts of sun exposure won’t reach toxic levels since the skin will destroy excess vitamin D [3, 20]. 

Since a vitamin D test is pretty inexpensive, it’s probably best to get your level checked before you start supplementing, and then check it periodically whether you’re supplementing or not. Unfortunately, there’s some controversy about what a healthy vitamin D level is, so let me walk you through the research and why I recommend a slightly higher level of vitamin D.

What Should Your Vitamin D Level Be?

This is where things get somewhat confusing. While a lot of evidence suggests that optimal vitamin D levels are different for everyone [21], many in the medical community agree that most people need to maintain a vitamin D blood level of at least 30 ng/mL (75 nmol/L) [1]. The current medical literature defines vitamin D status like this [1]: 

  • Severe vitamin D deficiency: <10 ng/mL (<25 nmol/L) 
  • Moderate vitamin D deficiency: 10-20 ng/mL (25–50 nmol/L)
  • Insufficiency: 20-30 ng/mL (50–75 nmol/L) 
  • Sufficiency: >30 ng/mL (>75 nmol/L) 
  • Toxicity: > 60 ng/mL (>150 nmol/L) [22]

In my experience in the clinic, most of my patients feel best with a vitamin D level in the range of 40–50 ng/mL. And researchers who traveled to East Africa to study hunter-gatherers, the Maasai and Hadza found the average vitamin D blood levels were about 46 ng/mL. Since these populations live in large amounts of sun (our greatest source of vitamin D), this may be a more appropriate level to aim for [23]. In addition, there are some other research studies that have concluded 48 ng/mL may be an ideal vitamin D level [24]. 

However, I have to admit that we really don’t know for sure whether all people and all skin types require the same levels of vitamin D [21]. One 2014 observational study found that people’s vitamin D levels vary based on how far their genetic origins are from the equator, and no clear harm is seen in populations with naturally lower levels of vitamin D [21]. 

There seems to be a complex link between skin color and vitamin D. Dark-skinned populations, which are typically lower in vitamin D than light-skinned ones, might have adaptations in vitamin D pathways that maintain normal health function when their levels are low [21]. So, while keeping your vitamin D within the 40–50 ng/mL range is probably a good place to start, it’s more important to listen to your body than it is to focus on a number, assuming you aren’t experiencing any signs of toxicity. 

At this point, you may be wondering why so many people have low vitamin D levels, so let’s unpack this a little more.

Why Are Low Vitamin D Levels Common?

While the lack of vitamin D is probably related to a variety of factors, unnecessary fear of the sun may be the most important contributing factor to the epidemic of low vitamin D levels we’re seeing. So let’s talk about the sun and its vital importance in maintaining vitamin D levels.

The Vitamin D-Sun Connection

Over the past several decades, we’ve been told to avoid the sun, or at least douse ourselves in sunscreen anytime we’re in it, to prevent skin cancer. This recommendation to avoid the sun may be a dangerous narrative. We actually get 50–90% of our vitamin D from the sun, so strict sun avoidance significantly reduces our ability to maintain adequate levels of vitamin D in the body, which likely has negative health effects [14].  

Observational research has found a significant association between more sun exposure and a lower risk of dying from all causes, even for people with the lightest skin [25, 26]. In addition, people with too little sun exposure may have similar all-cause death rates when compared to people who smoke but get a lot of sun [26]. While observational studies like these can’t prove cause and effect, this information reminds us that humans are probably meant to be in the sun, at least to some degree [20, 27, 28, 29]. 

Interestingly, the body self-regulates vitamin D levels. In other words, it will make more vitamin D coming out of winter when levels are at their lowest and then it slows down the production once levels become optimal [20].

Aside from creating vitamin D, getting the right amount of sun for your skin phototype may have benefits for:

Inadequate sun exposure is a major contributor to low vitamin D levels, but poor gut health is another factor.

Poor Gut Health

An unhealthy gut can decrease your absorption of vitamin D from food and supplements. So, if you’re spending enough time in the sun but seem to have a consistently low vitamin D level, it may be time to investigate poor gut health as a contributing factor. 

In the clinic, I prefer to address gut health before attempting to optimize micronutrient levels. After all, if you aren’t absorbing nutrients well due to a gut condition, adding more nutrients in supplement form won’t likely make much of an impact. Common conditions that can cause malabsorption of nutrients, including vitamin D, are:

  • Small intestine bacterial overgrowth (SIBO)
  • Irritable bowel syndrome (IBS)
  • Inflammatory bowel disease (IBD)
  • Celiac disease
  • Exocrine pancreatic insufficiency (EPI)
  • Gut pathogens/ infection
  • History of cholecystectomy (gallbladder removal)

Vitamin D is unique though, in that we can get it from the sun, so this makes sun exposure even more important for people who are dealing with a gut health issue and have difficulty absorbing it from supplements or food. 

Probiotics are one supplement I use routinely for improving gut health as they lower gut inflammation and help to correct dysbiosis [12, 20], which helps to improve nutrient absorption. And in fact, probiotics have been found to help improve vitamin D levels [22]. As your gut heals, you may be surprised to find vitamin D supplements are less necessary, especially if you’re eating a balanced diet and getting enough time in the sun.

In addition to insufficient sun exposure and poor gut health, here are some other factors to consider when it comes to vitamin D deficiency.

Other Causes of Vitamin D Deficiency

While it’s probably different for everyone, research suggests that most people who don’t get adequate sun exposure need to be taking in at least 400 international units (IU) of vitamin D in their food and/or from dietary supplements daily [34]. But it’s estimated that only 5% of people in the U.S. consume this amount on a routine basis, so insufficient intake is another contributing factor to inadequate vitamin D levels [35]. Other causes include [3, 14]:

  • Insufficient absorption, possibly related to poor gut health, chronic inflammation and obesity
  • Decreased vitamin D production in the body from liver or kidney disease or hyperparathyroidism
  • Use of certain medications (anticonvulsants, rifampicin, thiazides, and corticosteroids)
  • End-organ resistance where organs don’t respond to vitamin D. (This is usually hereditary.)

So while lack of sun exposure is probably the main factor, you may have a variety of other contributing factors to address. So what do you do if your vitamin D level is below the recommended range?  

Improving Your Vitamin D Level

If you’ve determined that you have a low vitamin D level, you’ll need to replenish your vitamin D with sun exposure, food, and/or supplements. 

If you opt for supplements, I’d recommend you work with a healthcare provider to determine the best dose for you. If you choose to replete your vitamin D level on your own, here’s a possible  dosing regimen for vitamin D3, which is the more effective form when compared to vitamin D2 [36, 37]:

  • Close to normal level: try starting with 2,000 IUs per day 
  • Insufficient level: try 2,000-4,000 IUs per day
  • Deficient level: try 8,000-10,000 IUs per day

If you’re insufficient or deficient, it’s important to retest your level so you can adjust your vitamin D dosage (you probably won’t need a higher dose for more than a few months). If you choose to not retest but you have a confirmed deficiency, you may want to stick with a daily dose of 4,000 IUs of vitamin D3 per day rather than going for a higher dose. 

Again, it’s best to work with a healthcare provider who can tailor your recommendations as you may need a higher amount of vitamin D in order to get back into the optimal range [38], especially if you aren’t getting out in the sun. But I’d caution you from taking too much vitamin D in supplement form or for too long without testing due to the higher risk of toxicity.

When choosing a vitamin D supplement, it doesn’t matter whether it comes in capsules, liquid drops, oral spray, or tablets, as long as it’s vitamin D3 in oil, and you can take it with or without meals [39, 40, 41, 42]. 

The one caveat here is if you have a fat malabsorption disorder, like EPI, or have had your gallbladder removed. In this case, you may want to opt for tablets without oil. Vitamin D3 without oil should contain some type of stearic acid (usually magnesium stearate) [43].

If you’d like to use sun exposure to correct a vitamin D deficiency, I provide the following guidance in Healthy Gut, Healthy You:

  • Full body sun exposure in a bathing suit where skin gets slightly pink produces roughly 17,500 IUs of vitamin D, so you would need 3 sessions per week.
  • If arms and legs are exposed to the sun and your skin is slightly pink, you produce roughly 6,000 IUs of vitamin D, so you would need 7 sessions per week.

Depending on where you live and your personal schedule, you may need to use a combined approach to optimize your vitamin D level. Here’s a chart comparing the pros and cons of using sunshine, food, and vitamin D supplements: 

Source of Vitamin D Pro Con
  • Can replenish vitamin D levels well [14, 20]
  • Improves immune system function [30]
  • Lowers blood pressure [25
  • Reduces risk of chronic disease [20, 25
  • Improves gut microbiome [33, 44, 45]
  • No risk of vitamin D toxicity [20]
  • Burning can increase skin cancer risk and accelerate skin aging [46]
  • Little risk of vitamin D toxicity [35]
  • Very few good food sources of vitamin D (fatty fish, egg yolks, and shiitake mushrooms) [3, 34, (47]
Vitamin D supplement
  • Can replenish vitamin D as well as sunshine  [48]
  • May replenish vitamin D better for people with darker skin [49].
  • Risk for vitamin D toxicity with high doses [50]
  • Financial cost

Sunshine and supplementation are by far the best ways to improve your vitamin D level. There are very few food sources of vitamin D, and the foods that do contain vitamin D don’t contain enough to help replenish a low vitamin D level. 

I want to point out here that there are definitely health benefits that you’d miss out on if you avoid the sun and only take a vitamin D supplement, lke improved mood and reduced stress. Additionally, unlike with supplements, there’s virtually no risk of vitamin D toxicity when you’re getting your vitamin D from the sun [20]. If you can swing it, sunshine is the way to go for getting your vitamin D. 

Once you get your vitamin D level into the optimal range of 40–50 ng/mL, you’ve got to work to keep it there. Let’s discuss how you can maintain your vitamin D level.

Maintaining a Healthy Vitamin D Level

Getting vitamin D from regular, healthy (non-burning) doses of sunshine may be the best way to get most of your vitamin D. The key here is to practice safe sun exposure. Being in the sun seems to have important health benefits but you can get too much of a good thing. 

Of course, excessive sun exposure even without burning can age your skin, but it may also increase your risk of non-melanoma skin cancers. And sunburns should definitely be avoided because they may raise the risk of all types of skin cancers, including melanoma [46]. 

If you want to find the right amount of sun for you, check out this chart that breaks it down based on skin type and the UV Index. 

Each skin phototype (lightest to darkest skin), response to UV, cancer risk, SED (amount of UV required to burn) and time in the sun at each UV Index level before burning [30, 51, 52, 53].

Skin phototype Skin response to UV Green
(Very high)
I – Extremely sensitive Always burns 15 min– 1.5 hr 18–30 min 12–15 min 9–12 min 7.5 min
II – Very sensitive Burns easily 1 hr 15 min–2.5 hr  30–50 min 20–25 min 15–20 min 12.5 min
III – Sensitive Burns moderately 1.5 hr–3 hr 36 min–1 hr 24–30 min 18–24 min 15 min
IV – Mildly sensitive Burns minimally 2–4 hr 48 min–1 hr 18 min 32–40 min 24–32 min 20 min
V – Resistant  Rarely burns  3–6 hr 1 hr 12 min–3 hr 48 min–1 hr 36–48 min 30 min
VI – Very resistant Almost never burns 4.5–9 hr 1.8–3 hr 1 hr 12 min–1.5 hr 54 min–1 hr 12 min 40 min

This chart can give you a good estimate of the right amount of sun exposure to prevent burning, but always pay attention to your body. If the chart recommends a certain amount of time but you’re already feeling the burn, then please take a break from the sun. 

During periods when you have less sun exposure, you may need to focus on food and supplements to maintain your vitamin D level. Here’s a chart detailing the average amount of vitamin D in a multivitamin versus a single vitamin D supplement and the best food sources of vitamin D:

Required daily amount [34]
Amount in common 1-a-day multivitamins for nonpregnant adults [54] Amount in common single vitamins for nonpregnant adults [54] Vitamin D-rich in foods
Women Men
10–15 micrograms(400–600 IU) 10–15 micrograms(400–600 IU) 25 micrograms (1,000 IU) 1–125 micrograms (400–5,000 IU) Fatty fish (trout, salmon, tuna, mackerel), fortified beverages, breads, and cereals, egg yolks, shiitake mushrooms [3, 34, 47]

As you can see, the amount of vitamin D contained in multivitamins and individual vitamins can vary widely, so it’s important to read labels and speak with your doctor. If you are deficient in vitamin D, the 1,000 IU per day found in a multivitamin is probably not enough to raise your levels.

Maintain a Healthy Vitamin D Level with Sunshine and Supplements 

Vitamin D is a steroid hormone important for bone health, immune system function, and blood sugar regulation. Many Americans are either deficient in or have an insufficient vitamin D level. This is largely due to an unnecessary fear of the sun based on the current narrative that tells us to avoid it at all costs in order to avoid skin cancer. The truth is, the sun is our major source of vitamin D, and practicing safe sun exposure is an excellent way to not only increase our vitamin D levels but to also improve other health parameters. 

Although controversy exists, I recommend a vitamin D blood level of 40-50 ng/mL. If you’re experiencing signs of vitamin D deficiency, I recommend having an inexpensive blood test to determine your level before beginning a supplement. If your vitamin D is below the recommended range, work with a healthcare provider to bring it up with sunshine, supplements, and/or food. And remember, if you’re getting out in the sun but struggle with a low vitamin D level consistently, you may need to take a look at the health of your gut. 

While there’s practically no risk of vitamin D toxicity with food and sunshine, excessive doses of vitamin D from supplements can lead to toxic levels in the body, so be conservative if you choose to supplement on your own. For help with boosting your levels by healing your gut, contact us at the Ruscio Institute for Functional Health for a personalized plan.

The Ruscio Institute has developed a range of high-quality formulations to help our patients and audience. If you’re interested in learning more about these products, please click here. Note that there are many other options available, and we encourage you to research which products may be right for you.

➕ References

  1. Cui A, Xiao P, Ma Y, Fan Z, Zhou F, Zheng J, et al. Prevalence, trend, and predictor analyses of vitamin D deficiency in the US population, 2001-2018. Front Nutr. 2022 Oct 3;9:965376. DOI: 10.3389/fnut.2022.965376. PMID: 36263304. PMCID: PMC9573946.
  2. Bizzaro G, Antico A, Fortunato A, Bizzaro N. Vitamin D and autoimmune diseases: is vitamin D receptor (VDR) polymorphism the culprit? Isr Med Assoc J. 2017 Jul;19(7):438–43. PMID: 28786260.
  3. Chauhan K, Shahrokhi M, Huecker MR. Vitamin D. In: StatPearls. Treasure Island (FL): StatPearls Publishing; 2022. PMID: 28722941.
  4. van Schoor N, Lips P. Global overview of vitamin D status. Endocrinol Metab Clin North Am. 2017;46(4):845–70. DOI: 10.1016/j.ecl.2017.07.002. PMID: 29080639.
  5. Gani LU, How CH. PILL Series. Vitamin D deficiency. Singapore Med J. 2015 Aug;56(8):433–6; quiz 437. DOI: 10.11622/smedj.2015119. PMID: 26311908. PMCID: PMC4545131.
  6. Rajabi-Naeeni M, Dolatian M, Qorbani M, Vaezi AA. Effect of omega-3 and vitamin D co-supplementation on psychological distress in reproductive-aged women with pre-diabetes and hypovitaminosis D: A randomized controlled trial. Brain Behav. 2021 Nov;11(11):e2342. DOI: 10.1002/brb3.2342. PMID: 34473420. PMCID: PMC8613419.
  7. Piantoni S, Andreoli L, Scarsi M, Zanola A, Dall’Ara F, Pizzorni C, et al. Phenotype modifications of T-cells and their shift toward a Th2 response in patients with systemic lupus erythematosus supplemented with different monthly regimens of vitamin D. Lupus. 2015 Apr;24(4–5):490–8. DOI: 10.1177/0961203314559090. PMID: 25801892.
  8. El Amrousy D, El Ashry H, Hodeib H, Hassan S. Vitamin D in children with inflammatory bowel disease: A randomized controlled clinical trial. J Clin Gastroenterol. 2021 Oct 1;55(9):815–20. DOI: 10.1097/MCG.0000000000001443. PMID: 33060436.
  9. Manson JE, Cook NR, Lee I-M, Christen W, Bassuk SS, Mora S, et al. Vitamin D supplements and prevention of cancer and cardiovascular disease. N Engl J Med. 2019 Jan 3;380(1):33–44. DOI: 10.1056/NEJMoa1809944. PMID: 30415629. PMCID: PMC6425757.
  10. Zhang J, Chen Y, Li H, Li H. Effects of vitamin D on thyroid autoimmunity markers in Hashimoto’s thyroiditis: systematic review and meta-analysis. J Int Med Res. 2021 Dec;49(12):3000605211060675. DOI: 10.1177/03000605211060675. PMID: 34871506. PMCID: PMC8711703.
  11. Mangin M, Sinha R, Fincher K. Inflammation and vitamin D: the infection connection. Inflamm Res. 2014 Oct;63(10):803–19. DOI: 10.1007/s00011-014-0755-z. PMID: 25048990. PMCID: PMC4160567.
  12. Li Q, Zhou Q, Zhang G, Tian X, Li Y, Wang Z, et al. Vitamin D Supplementation and Allergic Diseases during Childhood: A Systematic Review and Meta-Analysis. Nutrients. 2022 Sep 23;14(19). DOI: 10.3390/nu14193947. PMID: 36235600. PMCID: PMC9571357.
  13. Ong MW, Tan CH, Cheng AKS. Prevalence and Determinants of Vitamin D Deficiency Among the Overweight and Obese Singaporeans Seeking Weight Management Including Bariatric Surgery: a Relationship with Bone Health. Obes Surg. 2018 Aug;28(8):2305–12. DOI: 10.1007/s11695-018-3142-y. PMID: 29453514.
  14. Sizar O, Givler A. Vitamin D Deficiency. In: StatPearls. Treasure Island (FL): StatPearls Publishing; 2018. PMID: 30335299.
  15. Lima GL, Paupitz J, Aikawa NE, Takayama L, Bonfa E, Pereira RMR. Vitamin D Supplementation in Adolescents and Young Adults With Juvenile Systemic Lupus Erythematosus for Improvement in Disease Activity and Fatigue Scores: A Randomized, Double-Blind, Placebo-Controlled Trial. Arthritis Care Res (Hoboken). 2016 Jan;68(1):91–8. DOI: 10.1002/acr.22621. PMID: 25988278.
  16. Hypocalcemia: Causes, Symptoms & Treatment [Internet]. [cited 2022 Dec 5]. Available from:
  17. Christakos S, Dhawan P, Porta A, Mady LJ, Seth T. Vitamin D and intestinal calcium absorption. Mol Cell Endocrinol. 2011 Dec 5;347(1–2):25–9. DOI: 10.1016/j.mce.2011.05.038. PMID: 21664413. PMCID: PMC3405161.
  18. Morelli MB, Santulli G, Gambardella J. Calcium supplements: Good for the bone, bad for the heart? A systematic updated appraisal. Atherosclerosis. 2020 Mar;296:68–73. DOI: 10.1016/j.atherosclerosis.2020.01.008. PMID: 32033778. PMCID: PMC7276095.
  19. Asif A, Farooq N. Vitamin D Toxicity. In: StatPearls. Treasure Island (FL): StatPearls Publishing; 2023. PMID: 32491799.
  20. Grigalavicius M, Moan J, Dahlback A, Juzeniene A. Vitamin D and ultraviolet phototherapy in Caucasians. J Photochem Photobiol B, Biol. 2015 Jun;147:69–74. DOI: 10.1016/j.jphotobiol.2015.03.009. PMID: 25846579.
  21. Durazo-Arvizu RA, Camacho P, Bovet P, Forrester T, Lambert EV, Plange-Rhule J, et al. 25-Hydroxyvitamin D in African-origin populations at varying latitudes challenges the construct of a physiologic norm. Am J Clin Nutr. 2014 Sep;100(3):908–14. DOI: 10.3945/ajcn.113.066605. PMID: 25008852. PMCID: PMC4135499.
  22. Pilz S, Zittermann A, Trummer C, Theiler-Schwetz V, Lerchbaum E, Keppel MH, et al. Vitamin D testing and treatment: a narrative review of current evidence. Endocr Connect. 2019 Feb 1;8(2):R27–43. DOI: 10.1530/EC-18-0432. PMID: 30650061. PMCID: PMC6365669.
  23. Luxwolda MF, Kuipers RS, Kema IP, Dijck-Brouwer DAJ, Muskiet FAJ. Traditionally living populations in East Africa have a mean serum 25-hydroxyvitamin D concentration of 115 nmol/l. Br J Nutr. 2012 Nov 14;108(9):1557–61. DOI: 10.1017/S0007114511007161. PMID: 22264449.
  24. Baggerly CA, Cuomo RE, French CB, Garland CF, Gorham ED, Grant WB, et al. Sunlight and Vitamin D: Necessary for Public Health. J Am Coll Nutr. 2015 Jun 22;34(4):359–65. DOI: 10.1080/07315724.2015.1039866. PMID: 26098394. PMCID: PMC4536937.
  25. Scragg R, Rahman J, Thornley S. Association of sun and UV exposure with blood pressure and cardiovascular disease: A systematic review. J Steroid Biochem Mol Biol. 2019 Mar;187:68–75. DOI: 10.1016/j.jsbmb.2018.11.002. PMID: 30412763.
  26. Lindqvist PG, Epstein E, Landin-Olsson M. Sun Exposure – Hazards and Benefits. Anticancer Res. 2022 Apr;42(4):1671–7. DOI: 10.21873/anticanres.15644. PMID: 35346986.
  27. Jager N, Schöpe J, Wagenpfeil S, Bocionek P, Saternus R, Vogt T, et al. The Impact of UV-dose, Body Surface Area Exposed and Other Factors on Cutaneous Vitamin D Synthesis Measured as Serum 25(OH)D Concentration: Systematic Review and Meta-analysis. Anticancer Res. 2018 Feb;38(2):1165–71. DOI: 10.21873/anticanres.12336. PMID: 29374754.
  28. Micić I, Jeon I-H, Park S-H, Hwa S-S, Chun J-M, Stojiljković P. The effect of short-term low-energy ultraviolet B irradiation on bone mineral density and bone turnover markers in postmenopausal women with osteoporosis: a randomized single-blinded controlled clinical trial. Srp Arh Celok Lek. 2013;141(9–10):615–22. DOI: 10.2298/sarh1310615m. PMID: 24364223.
  29. Felton SJ, Cooke MS, Kift R, Berry JL, Webb AR, Lam PMW, et al. Concurrent beneficial (vitamin D production) and hazardous (cutaneous DNA damage) impact of repeated low-level summer sunlight exposures. Br J Dermatol. 2016 Dec;175(6):1320–8. DOI: 10.1111/bjd.14863. PMID: 27411377. PMCID: PMC5215649.
  30. Lucas RM, Neale RE, Madronich S, McKenzie RL. Are current guidelines for sun protection optimal for health? Exploring the evidence. Photochem Photobiol Sci. 2018 Dec 5;17(12):1956–63. DOI: 10.1039/c7pp00374a. PMID: 29904757.
  31. Shore-Lorenti C, Brennan SL, Sanders KM, Neale RE, Lucas RM, Ebeling PR. Shining the light on Sunshine: a systematic review of the influence of sun exposure on type 2 diabetes mellitus-related outcomes. Clin Endocrinol (Oxf). 2014 Dec;81(6):799–811. DOI: 10.1111/cen.12567. PMID: 25066830.
  32. Zhou Z, Zhou R, Zhang Z, Li K. The Association Between Vitamin D Status, Vitamin D Supplementation, Sunlight Exposure, and Parkinson’s Disease: A Systematic Review and Meta-Analysis. Med Sci Monit. 2019 Jan 23;25:666–74. DOI: 10.12659/MSM.912840. PMID: 30672512. PMCID: PMC6352758.
  33. Bosman ES, Albert AY, Lui H, Dutz JP, Vallance BA. Skin exposure to narrow band ultraviolet (UVB) light modulates the human intestinal microbiome. Front Microbiol. 2019 Oct 24;10:2410. DOI: 10.3389/fmicb.2019.02410. PMID: 31708890. PMCID: PMC6821880.
  34. Morris AL, Mohiuddin SS. Biochemistry, Nutrients. In: StatPearls. Treasure Island (FL): StatPearls Publishing; 2022. PMID: 32119432.
  35. Reider CA, Chung R-Y, Devarshi PP, Grant RW, Hazels Mitmesser S. Inadequacy of Immune Health Nutrients: Intakes in US Adults, the 2005-2016 NHANES. Nutrients. 2020 Jun 10;12(6). DOI: 10.3390/nu12061735. PMID: 32531972. PMCID: PMC7352522.
  36. Balachandar R, Pullakhandam R, Kulkarni B, Sachdev HS. Relative Efficacy of Vitamin D2 and Vitamin D3 in Improving Vitamin D Status: Systematic Review and Meta-Analysis. Nutrients. 2021 Sep 23;13(10). DOI: 10.3390/nu13103328. PMID: 34684328. PMCID: PMC8538717.
  37. Bleizgys A. Vitamin D dosing: basic principles and a brief algorithm (2021 update). Nutrients. 2021 Dec 10;13(12). DOI: 10.3390/nu13124415. PMID: 34959969. PMCID: PMC8709011.
  38. Cashman KD, Ritz C, Kiely M, Odin Collaborators. Improved Dietary Guidelines for Vitamin D: Application of Individual Participant Data (IPD)-Level Meta-Regression Analyses. Nutrients. 2017 May 8;9(5). DOI: 10.3390/nu9050469. PMID: 28481259. PMCID: PMC5452199.
  39. Todd JJ, McSorley EM, Pourshahidi LK, Madigan SM, Laird E, Healy M, et al. Vitamin D3 supplementation in healthy adults: a comparison between capsule and oral spray solution as a method of delivery in a wintertime, randomised, open-label, cross-over study. Br J Nutr. 2016 Oct 11;116(8):1402–8. DOI: 10.1017/S0007114516003470. PMID: 27724992.
  40. Helde Frankling M, Norlin A-C, Hansen S, Wahren Borgström E, Bergman P, Björkhem-Bergman L. Are Vitamin D3 Tablets and Oil Drops Equally Effective in Raising S-25-Hydroxyvitamin D Concentrations? A Post-Hoc Analysis of an Observational Study on Immunodeficient Patients. Nutrients. 2020 Apr 26;12(5). DOI: 10.3390/nu12051230. PMID: 32357579. PMCID: PMC7282031.
  41. Grossmann RE, Tangpricha V. Evaluation of vehicle substances on vitamin D bioavailability: a systematic review. Mol Nutr Food Res. 2010 Aug;54(8):1055–61. DOI: 10.1002/mnfr.200900578. PMID: 20425758. PMCID: PMC3033429.
  42. Cavalier E, Jandrain B, Coffiner M, Da Silva S, De Niet S, Vanderbist F, et al. A Randomised, Cross-Over Study to Estimate the Influence of Food on the 25-Hydroxyvitamin D₃ Serum Level after Vitamin D₃ Supplementation. Nutrients. 2016 May 20;8(5). DOI: 10.3390/nu8050309. PMID: 27213447. PMCID: PMC4882721.
  43. Hermes WA, Alvarez JA, Lee MJ, Chesdachai S, Lodin D, Horst R, et al. Prospective, Randomized, Double-Blind, Parallel-Group, Comparative Effectiveness Clinical Trial Comparing a Powder Vehicle Compound of Vitamin D With an Oil Vehicle Compound in Adults With Cystic Fibrosis. JPEN J Parenter Enteral Nutr. 2017 Aug;41(6):952–8. DOI: 10.1177/0148607116629673. PMID: 26903303. PMCID: PMC4993695.
  44. Conteville LC, Vicente ACP. Skin exposure to sunlight: a factor modulating the human gut microbiome composition. Gut Microbes. 2020 Sep 2;11(5):1135–8. DOI: 10.1080/19490976.2020.1745044. PMID: 32339065. PMCID: PMC7524261.
  45. De Pessemier B, Grine L, Debaere M, Maes A, Paetzold B, Callewaert C. Gut-Skin Axis: Current Knowledge of the Interrelationship between Microbial Dysbiosis and Skin Conditions. Microorganisms. 2021 Feb 11;9(2). DOI: 10.3390/microorganisms9020353. PMID: 33670115. PMCID: PMC7916842.
  46. Alfredsson L, Armstrong BK, Butterfield DA, Chowdhury R, de Gruijl FR, Feelisch M, et al. Insufficient sun exposure has become a real public health problem. Int J Environ Res Public Health. 2020 Jul 13;17(14). DOI: 10.3390/ijerph17145014. PMID: 32668607. PMCID: PMC7400257.
  47. Ashok T, Palyam V, Azam AT, Odeyinka O, Alhashimi R, Thoota S, et al. Relationship between vitamin D and thyroid: an enigma. Cureus. 2022 Jan 10;14(1):e21069. DOI: 10.7759/cureus.21069. PMID: 35165540. PMCID: PMC8826546.
  48. Lagunova Z, Porojnicu AC, Aksnes L, Holick MF, Iani V, Bruland OS, et al. Effect of vitamin D supplementation and ultraviolet B exposure on serum 25-hydroxyvitamin D concentrations in healthy volunteers: a randomized, crossover clinical trial. Br J Dermatol. 2013 Aug;169(2):434–40. DOI: 10.1111/bjd.12349. PMID: 23551243.
  49. Lee YM, Kim SA, Lee DH. Can Current Recommendations on Sun Exposure Sufficiently Increase Serum Vitamin D Level?: One-Month Randomized Clinical Trial. J Korean Med Sci. 2020 Mar 2;35(8):e50. DOI: 10.3346/jkms.2020.35.e50. PMID: 32103645. PMCID: PMC7049620.
  50. Joh H-K, Hwang S-S, Cho B, Lim CS, Jung S-E. Effect of sun exposure versus oral vitamin D supplementation on serum 25-hydroxyvitamin D concentrations in young adults: A randomized clinical trial. Clin Nutr. 2020 Mar;39(3):727–36. DOI: 10.1016/j.clnu.2019.03.021. PMID: 30987813.
  51. Salvadori G, Lista D, Burattini C, Gugliermetti L, Leccese F, Bisegna F. Sun exposure of body districts: development and validation of an algorithm to predict the erythemal ultra violet dose. Int J Environ Res Public Health. 2019 Sep 27;16(19). DOI: 10.3390/ijerph16193632. PMID: 31569692. PMCID: PMC6801664.
  52. Table 4, Ultraviolet (UV) Index Levels – The Surgeon General’s Call to Action to Prevent Skin Cancer – NCBI Bookshelf [Internet]. [cited 2023 Jul 18]. Available from:
  53. UV Index Scale | US EPA [Internet]. [cited 2023 Jul 18]. Available from:
  54. Home | Dietary Supplement Label Database (DSLD) | NIH Office of Dietary Supplements [Internet]. [cited 2022 Sep 8]. Available from:

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