Vitamin D Deficiency: A Comprehensive Guide

Updated: April 9, 2024

woman getting vitamin d outside

Your body has over 30 trillion cells, and each one of them has a vitamin D receptor.  That tells us that vitamin D is the only vitamin that every cell in your body requires!  The vitamin D receptor is a switch that triggers the cell to express vital functions such as immune strength, bone health, mood balance, and more!  

Without a steady supply of vitamin D, these functions will slow down, waver, and eventually cease to operate effectively. That’s why not just your bone health, but virtually every aspect of your health hinges on getting enough vitamin D [1]. 

But are we getting enough?

According to the National Institutes of Health (NIH), “vitamin D deficiency is more prevalent than ever, and high-risk populations should be screened.” The specific numbers vary depending on the risk group, but included in the NIH’s list of high-risk groups are nursing home residents, elderly patients, African American and Hispanic individuals—and women with osteoporosis.

In fact, the Linus Pauling Institute at Oregon State University reports that dietary intake surveys indicate 81% of children and adolescents (age 2-18 years) and 95% of adults are not meeting the Estimated Average Requirement (EAR). 

That’s a problem because vitamin D has a hand in some of your most vital physiological processes, including bone health, immunity, cardiovascular health, and more. Simply put, if your vitamin D levels are low, your health will suffer.

And for many of us, our vitamin D levels are low. 

So, how do we get vitamin D? That’s the easy question because we get vitamin D from just three sources: food, the sun, and supplements. The real question we need to answer is, “Are we getting enough, and if not, why?”

So let’s dig in and uncover why so many people struggle with low vitamin D levels, the best way to test your vitamin D, and what you can do to get your levels back up to an optimal range. 

Risk Factors for Vitamin D Deficiency

woman inside not getting her vitamin d

Why are we all so vitamin D deficient? It’s because we get vitamin D from three sources: food, sunlight, and supplementation. Whether or not we are getting a sufficient amount depends on how much we are getting from each one, or from a combination of any or all of the three [2]. 

Here are some of the most common risk factors for vitamin D deficiency:

Lack of Sunlight

Vitamin D is often called the “sunshine vitamin” because when our skin is exposed to the sun, our bodies can manufacture its own vitamin D. Yale Medicine dermatologist David J. Leffell,  says, “We each have vitamin D receptor cells that, through a chain of reactions starting with conversion of cholesterol in the skin, produce vitamin D3 when they’re exposed to ultraviolet B (UVB) from the sun.” 

But, according to Harvard’s T.H. Chan School of Public Health, the following are some of the reasons exposure to UVB light is decreased, subsequently decreasing vitamin D absorption:

  • “Correctly applied”, the use of sunscreen can reduce vitamin D absorption by more than 90%.
  • Wearing full clothing that covers the skin.
  • Spending limited time outdoors.
  • Darker skin tones due to having higher amounts of the pigment melanin, which acts as a type of natural sunscreen. 
  • Older ages when there is a decrease in 7-dehydrocholesterol levels and changes in skin, and a population that is likely to spend more time indoors.
  • Certain seasons and living in northern latitudes above the equator where UVB light is weaker. For example, in the northern hemisphere, people who live in parts of the U.S., Canada, and Norway can’t make enough vitamin D from the sun for up to six months of the year.  Similarly, in the southern hemisphere, people living in Buenos Aires, and Cape Town, South Africa make far less vitamin D from the sun during their winter months (June through August) than they can during their spring and summer months. Though the body can store vitamin D from summer sun exposure, it needs to last for many months, but by late winter, many people in these higher-latitude locations can find themselves vitamin D deficient [3]. 


Obesity is an epidemic all its own in the US, and unfortunately, along with all the other side effects that come with excess weight, it appears that vitamin D levels also take a hit. 

Since vitamin D is a fat-soluble vitamin, once it enters your body (if it isn’t used immediately), it gets stored in your fat cells. This means that for people that have excess body fat, the vitamin D they produce or take in gets sequestered away in their fat tissue and doesn’t make it to circulation. In essence, the increased number of fat cells dilutes the available vitamin D [4].

Older Age

As you age, your ability to synthesize vitamin D through your skin (via sunlight) diminishes. This is a natural side effect of the aging process, and it’s one of the primary reasons we see so much vitamin D deficiency in older populations [5].


Certain medications can block or interfere with vitamin D synthesis, while others increase its degradation. Some examples include laxatives, steroids, cholesterol-lowering drugs, and weight-loss drugs [6]. 

Low Dietary Intake or Malabsorption of Vitamin D

Although sunlight is the primary source of vitamin D for most people, you can still get a fair percentage of your vitamin D (10-50%) through foods like salmon, cod, sardines, eggs, and mushrooms. 

Unfortunately, many people today suffer from malabsorption syndromes like IBS, IBD, Crohn’s disease, and Celiac disease, all of which will make it more difficult to absorb nutrients like vitamin D [7][8].

Certain Health Conditions

Health conditions like kidney disease and liver disease can impair vitamin D synthesis and activation due to these conditions’ impact on enzymes involved in vitamin D activity [9][10].  

Signs and Symptoms of Vitamin D Deficiency

Vitamin D deficiency symptoms often come on slowly, so it’s helpful to know what to look for. Some of the most common signs that your vitamin D levels are low include:

Signs of Vitamin D Deficiency

  • Muscle weakness or cramps
  • Bone or joint pain
  • Chronic pain
  • Fatigue
  • Hair loss
  • Depression and/or anxiety
  • Restless sleep
  • Poor concentration
  • Weakened immunity 
  • Headaches

Long-Term Concerns Of Vitamin D Deficiency

If your vitamin D levels remain chronically low, it can lead to more serious issues, including:

  • Osteoporosis 
  • Osteomalacia 
  • Rickets (in children)
  • Secondary hyperparathyroidism

Other conditions linked to low vitamin D levels include:

  • Diabetes [11]
  • Dementia [12] 
  • Heart disease [13] 
  • Fertility [14]
  • PCOS [15] 
  • Autoimmunity [16]
  • Metabolic syndrome [17]
lady inside her house

How To Test For Vitamin D Deficiency

A simple blood test will tell you how much vitamin D is in your circulation and indicate whether or not you need to increase your vitamin D levels. It used to be that you had to have your blood drawn at a lab, but now you can test your vitamin D levels with a simple at home test. All it takes is a quick prick of your finger, then mail the sample off and you’ll have your results back in around five business days.

How To Increase Your Vitamin D Levels

If you find that your vitamin D levels are low or just want to get ahead of potential deficiency, there are a handful of ways to increase your levels

First, try to incorporate more vitamin D rich foods into your diet. There are not a lot of foods high in vitamin D, but vitamin D does naturally occur in certain foods like salmon, cod, sardines, tuna, mackerel, beef liver, and eggs. When choosing food, it’s important to look for food that’s been properly raised. Studies suggest that wild-caught salmon is higher in vitamin D (and other nutrients) than farmed salmon. Similarly, eggs from free-range pastured chickens that were exposed to sunlight exhibited three to four times higher levels of vitamin D than chickens raised indoors.

vitamin d rich foods

The only vegetarian source of vitamin D is found in mushrooms exposed to UV light. 

The lack of vitamin D in many foods is why foods like milk, cereal, and some orange juices are vitamin D2- and D3-fortified. In fact, vitamin D enriched foods helped reduce rickets in children, but we haven’t seen the same result when it comes to osteoporosis in adults.

Next, try to get more natural sunlight throughout the day. Synthesis of vitamin D through your skin via sunlight is one of the most effective ways to naturally increase your levels, with some research showing that just twenty minutes per day of sun exposure can prevent vitamin D deficiency [18]. That’s why vitamin D is called the “sunshine vitamin.” 

That said, this comes with two disclaimers. First, studies show that due to the UV rays and location of the sun during certain times of the year (November to February), sun exposure may result in little to no vitamin D synthesis. This is especially relevant in certain areas in the Northern US [19][20].

Second, because UVB light can burn the skin and cause skin cancer, you want to be sure to avoid burning or over exposure to the sun. The sun is most intense at midday which means it will take less time to get sufficient sunlight, but if you’re concerned about overexposure, simply choose to spend time in the sun in the mornings or afternoon. 

Sunlight offers other health benefits as well. It supports better sleep and helps set your circadian rhythms by regulating the levels of serotonin and melatonin. It’s also been found to protect against type 1 diabetes, multiple sclerosis, and several forms of cancer..

Ideally, you can sustain healthy vitamin D levels through diet and exposure to sunlight. But not everyone can or does.  This is why so many people are turning to vitamin D supplements. 

Adding a high-quality vitamin D supplement to your regimen is an excellent way to ensure you keep your vitamin D at an optimal level. Fortunately, your body doesn’t process vitamin D differently, regardless of whether it comes from food, the sun, or supplementation. When searching for a vitamin D supplement, make sure that it’s in the D3 form,as opposed to D2, because vitamin D3 is more active and, therefore, more efficient at raising your vitamin D levels. 

You’ll also want to ensure that your supplement contains an adequate dose of vitamin D.

d3 complete bottle

Optimal Level and Recommended Intake of Vitamin D

There is some debate about the optimal level of vitamin D in the blood.

Though there is wide agreement that 20 ng/mL or below constitutes vitamin D deficiency, the range of what constitutes an optimal level of vitamin D can vary. That said, the National Institute of health places an optimal level between 30-50 ng/mL. That aligns with other experts who find the optimal level to be “concentrations around 48 ng/mL.” The evidence suggests that levels in the low 40s is a reasonable and healthy target to shoot for.

Recommended Intake of Vitamin D

The simple answer is, it depends on a variety of factors such as age, weight, skin color, and even geographical location affect the amount of Vitamin D a person needs. At the lower level, recommendations from The National Institutes Of Health and the Endocrine Society range from 400-2000 IU. Whereas a recent large-scale study from Germany recommends a much higher range. That’s why it’s always best to discuss your Vitamin D needs with your healthcare provider, who has the full overview of your personal needs and medical history.

AlgaeCal’s Vitamin D3 Complete contains 1000 IUs, plus vitamins A, E, and K because, when combined, these four vitamins complement one another to support an impressive range of health benefits.

Ultimately, your optimal levels will depend on your current vitamin D levels, health status, and health goals.


Vitamin D is crucial to so many aspects of your health, that it’s probably a good idea to make sure you’re not deficient. If you have yet to check your vitamin D levels, a simple home test will let you know where you stand.

In the meantime, taking a high-quality vitamin D3 supplement can help you stay ahead of potential deficiencies and give you peace of mind that your body is receiving the nutrients it needs – even when you’re stuck inside all day. 

For more on the latest research related to health, nutrition, and aging gracefully, sign up for the AlgaeCal Newsletter.


How much vitamin D3 should I take daily?

It depends on a variety of factors such as age, weight, skin color, and even geographical location affect the amount of Vitamin D a person needs. At the lower level, recommendations from The National Institutes Of Health and the Endocrine Society range from 400-2000 IU. Whereas a recent large-scale study from Germany recommends a much higher range. That’s why it’s always best to discuss your Vitamin D needs with your healthcare provider, who has the full overview of your personal needs and medical history.

What are the main benefits of vitamin D3?

Vitamin D3 is the most active form of vitamin D, making it the most efficacious in raising your vitamin D levels. This nutrient plays a crucial role in bone health and immunity and has a hand in many other physiological processes.

What are the symptoms of vitamin D3 deficiency?

Some of the most common symptoms of vitamin D deficiency include muscle aches or weakness, fatigue, mood changes, sleep disturbance, hair loss, weakened immunity, and poor concentration.

What are the symptoms of vitamin D3 deficiency?

Some of the most common symptoms of vitamin D deficiency include muscle aches or weakness, fatigue, mood changes, sleep disturbance, hair loss, weakened immunity, and poor concentration.

What happens when your vitamin D is really low?

Chronically low vitamin D can impair calcium absorption and lead to issues with bone health, including osteomalacia and osteoporosis. Furthermore, low vitamin D levels may increase your risk for several other health conditions, including diabetes, heart disease, dementia, autoimmune disease, and more.

What is the difference between vitamin D and vitamin D3?

Vitamin D3 is a form of vitamin D. The primary difference between the two is that vitamin D3 is more active and, therefore, more effective than other forms of vitamin D.

Article Sources

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  3. Naeem, Z. (2010). Vitamin d deficiency-an ignored epidemic. International journal of health sciences, 4(1), V.
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  5. Boucher, B. J. (2012). The problems of vitamin d insufficiency in older people. Aging and disease, 3(4), 313.
  6. Gröber, U., & Kisters, K. (2012). Influence of drugs on vitamin D and calcium metabolism. Dermato-endocrinology, 4(2), 158-166.
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  8. Ham, M., Longhi, M. S., Lahiff, C., Cheifetz, A., Robson, S., & Moss, A. C. (2014). Vitamin D levels in adults with Crohn's disease are responsive to disease activity and treatment. Inflammatory bowel diseases, 20(5), 856-860.
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Article Comments

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  1. Teresa Robinson

    February 22, 2024 , 12:18 am

    Vitamin D is also necessary for those who have had cancer.

  2. Yoori AlgaeCal

    February 22, 2024 , 5:00 pm

    Thank you for your comment, Teresa! If you are interested, a study HERE indicates that vitamin D blood levels from 60-80 ng/ml may be needed to reduce cancer risk!

    – Yoori

  3. Leslie Sciolaro

    February 22, 2024 , 4:05 pm

    Why are D4 and K1 and K2 not mentioned here?

  4. Samantha AlgaeCal

    February 23, 2024 , 11:06 am

    Leslie, this article discusses two forms of vitamin D – D2 and D3 – which are commonly found in supplements. It also explains that D3 is a superior form because it is more active and efficient in raising levels. Although vitamin D and vitamin K work together, the article focuses on vitamin D specifically. If you are interested in learning more about the two forms of Vitamin K, we have another article available HERE. I hope this helps! 🙂
    – Sam

  5. Judy Miller

    February 22, 2024 , 9:48 pm

    My son, who is a roofer and in the sun a lot was diagnose with low Vitamin D. What reason is there for this? He was also diagnosed with Sampter’s Triad at about the same time? Is there a connection? His ethnicity is primarily Finnish with the other half, Scandinavian/German/English. We all live in northern Minnesota.

  6. Samantha AlgaeCal

    February 23, 2024 , 10:28 am

    Judy, there are quite a few things that can affect vitamin D production from the sun, like the amount of pigment in your skin. We suggest having a chat with your healthcare provider for their best judgment and recommendations on your son’s situation as we are not medical professionals!
    – Sam

  7. Judy

    February 23, 2024 , 8:11 am

    Is it true too much Vitamin D can cause urinary incontinence?

  8. Samantha AlgaeCal

    February 23, 2024 , 11:50 am

    Judy, studies show that urinary incontinence may be associated with vitamin D deficiency but not with excessive intake or levels. That said, it’s always a good idea to consult with a doctor or pharmacist regarding this information!
    – Sam

  9. Carla Frisbee

    February 23, 2024 , 3:46 pm

    I’ve been taking Vit D3 5000 IU daily for years. No D3 level check done previously. Plus I’ve taken MK72 (Menaquione-180mcg) daily for many years also.
    I’m concerned w/ cost increasing w/ D3 complete due recommending 1-6 tablets daily. & each bottle has 60 capsules. There’s no way to know how to know how many to take! I’m an RN & have studied foods, nutrition & alternative health for 45 years. Now I’m on a fixed income & increased costs of living. I do know 7 months after taking the AlgeaCal Complete & Strontium my DEXA scan had improved! I just can’t keep paying so much for you products. Any suggestions are appreciated? Thanks

  10. Yoori AlgaeCal

    February 26, 2024 , 5:23 pm

    Thank you for reaching out and sharing your concern, Carla. AlgaeCal is certainly an investment towards your bone-health, but we are glad to hear that you’ve had an increase in bone density while taking AlgaeCal!

    Carla, the first step we suggest is to check your vitamin D blood level. You see, the amount of vitamin D you need depends on your blood level and health goals. You may or may not need additional supplementation on top of the 1600 IU of vitamin D AlgaeCal Plus provides you with per daily 4 capsules! We suggest checking your vitamin D blood level, and discussing with your doctor on the optimal amount of vitamin D supplementation that would be best for YOU, and adjust your D3 complete dosage accordingly :). If you only need to take 1 softgel of D3 complete, for example, each bottle should be a 2 month supply!

    – Yoori

  11. Nancy Snyder

    February 24, 2024 , 6:33 am

    What symptoms result from a too high level of vitamin D?

  12. Yoori AlgaeCal

    February 26, 2024 , 5:31 pm

    Great question, Nancy! Excessive levels of vitamin D, known as vitamin D toxicity, can lead to various symptoms and health problems such as nausea, fatigue, kidney stones, hypercalcemia, etc. Please note that vitamin D toxicity is rare and usually occurs only with extremely high doses of vitamin D supplements. Most cases of excessive vitamin D intake are due to over-supplementation rather than from diet or sunlight exposure alone. Please refer to the section “Vitamin D Toxicity” in this blog article for more information, and let us know if you have any follow-up questions! 🙂

    – Yoori

  13. Helen Foote

    April 12, 2024 , 8:50 am

    I had successful parathyroid surgery at Normal Parathyroid Center in June 2023. I take AlgaeCal Plus (not Strontium Boost). My vit D reading was 52 ng/mL in July 2023, November 2023 vit D reading was 58 ng/mL, April 2024 vit D reading is 61 ng/mL. Your article here says: “National Institute of health places an optimal level between 30-50 ng/mL”. How do I bring my current vit D reading down without lowering my AlgaeCal Plus dosage? I have severe osteoporosis so I really don’t want to lower my AlgaeCal dosage of 4 pills per day if I don’t need to. Could the parathyroid surgery in June 2023 be making my vit D intake more efficient than before surgery? Other than vit D that is in the AlgaeCal Plus I do not supplement vit D. Please help.

  14. Yoori AlgaeCal

    April 15, 2024 , 2:47 pm

    Thank you for reaching out, Helen! Please note that there is some debate about the optimal level of vitamin D in the blood. For example, the Endocrine Society HERE suggests maintaining levels above 30 ng/mL, some researchers HERE suggest maintaining level preferably at 40–60 ng/mL to achieve the optimal overall health benefits of vitamin D and another study HERE indicates that levels from 60-80 ng/ml may be needed to reduce cancer risk. Ultimately, your optimal levels will depend on your current health status and health goals.

    As we are not medical professionals, we suggest discussing with your doctor if the daily 1600 IU of vitamin D in AlgaeCal Plus is the right fit for you, and follow their best advice. HERE is our Doctor Information Sheet for your convenience :). I hope this helps!

    – Yoori

This article features advice from our industry experts to give you the best possible info through cutting-edge research.

Lara Pizzorno
MDiv, MA, LMT - Best-selling author of Healthy Bones Healthy You! and Your Bones; Editor of Longevity Medicine Review, and Senior Medical Editor for Integrative Medicine Advisors.,
Dr. Liz Lipski
PhD, CNS, FACN, IFMP, BCHN, LDN - Professor and Director of Academic Development, Nutrition programs in Clinical Nutrition at Maryland University of Integrative Health.,
Dr. Loren Fishman
MD, B.Phil.,(oxon.) - Medical Director of Manhattan Physical Medicine & Rehabilitation and Founder of the Yoga Injury Prevention Website.,
Prof. Didier Hans
PHD, MBA - Head of Research & Development Center of Bone Diseases, Lausanne University Hospital CHUV, Switzerland,