You Are Probably Vitamin D-Ficient, and Here’s Why It Matters – a Lot

Often called “the sunshine vitamin,” vitamin D is unique in that it is a vitamin AND a hormone your body can make with help from the sun.

But despite the ability to get vitamin D from food, supplements, and the sun, an estimated one billion people worldwide are deficient.

Why is this concerning? Well, a growing body of evidence suggests that vitamin D plays a much broader role in disease prevention and optimal health than previously believed.

For example, a new study published in The BMJ on February 15 found that vitamin D protects against acute respiratory infections including colds and flu.

Lead researcher Professor Adrian Martineau from QMUL said of the findings:

This major collaborative research effort has yielded the first definitive evidence that vitamin D really does protect against respiratory infections. Our analysis of pooled raw data from each of the 10,933 trial participants allowed us to address the thorny question of why vitamin D ‘worked’ in some trials, but not in others.

The bottom line is that the protective effects of vitamin D supplementation are strongest in those who have the lowest vitamin D levels, and when supplementation is given daily or weekly rather than in more widely spaced doses.

Vitamin D fortification of foods provides a steady, low-level intake of vitamin D that has virtually eliminated profound vitamin D deficiency in several countries. By demonstrating this new benefit of vitamin D, our study strengthens the case for introducing food fortification to improve vitamin D levels in countries such as the UK where profound vitamin D deficiency is common.

How does vitamin D protect against respiratory infections? Researchers believe that it boosts levels of antimicrobial peptides – natural antibiotic-like substances – in the lungs. Results of the study fit with the observation that colds and flu are the most common during winter and spring, when levels of vitamin D are at their lowest. This may also explain why vitamin D protects against asthma attacks, which are commonly triggered by respiratory viruses.

Here’s a quick rundown of other recent findings on vitamin D…

Lower levels of vitamin D in the blood have been associated with active disease in people with Crohn’s disease and ulcerative colitis (UC), inflammatory bowel diseases that cause long-lasting inflammation and ulcers in the colon. Low levels of vitamin D also appear to be linked with an increase in the risk of clinical relapse in people with UC.

Vitamin D deficiency may increase the risk of chronic headache.

Vitamin D may reduce severe asthma attacks, and low blood levels have been linked to increased risk of attacks in children and adults with asthma.

A five-year research project found that a daily dose of vitamin D3 improves heart function in people with chronic heart failure. Other studies have found that low vitamin D levels were associated with higher risk of heart failure, sudden cardiac death, stroke, overall cardiovascular disease, and cardiovascular death.

Researchers at Kyoto University recently identified a new way vitamin D helps control the balance of lipids in the body. This finding could advance development of new treatments for metabolic disorders and certain cancers.

A study published in 2016 found that extra vitamin D can restore good bacteria in the gut and may help improve metabolic syndrome (thus reducing the risk of diabetes and heart disease), “Based on this study, we believe that keeping vitamin D levels high, either through sun exposure, diet or supplementation, is beneficial for prevention and treatment of metabolic syndrome,” says Professor Stephen Pandol, at Cedars-Sinai Medical Center, USA, who collaborated with Yuan-Ping Han’s research group at Sichuan University, China in the study.

Researchers at the University of California, San Diego School of Medicine found that higher levels of vitamin D – specifically serum 25-hydroxyvitamin D – are associated with a correspondingly reduced risk of cancer.

Dozens of studies suggest an association between low vitamin D levels and increased risks of colon and other cancers. The evidence is strongest for colorectal cancer, with most (but not all) observational studies finding that the lower the vitamin D levels, the higher the risk of these diseases.

In late 2016, a systematic review of seven studies found evidence that vitamin D deficiency is associated with an increased risk of developing bladder cancer. More clinical studies are needed to confirm the findings, researchers said.

A Northwestern University study found a major link between low levels of vitamin D and aggressive prostate cancer. “Vitamin D deficiency may predict aggressive prostate cancer as a biomarker,” said lead investigator Dr. Adam Murphy, an assistant professor of urology at Northwestern University Feinberg School of Medicine and a Northwestern Medicine urologist. “Men with dark skin, low vitamin D intake or low sun exposure should be tested for vitamin D deficiency when they are diagnosed with an elevated PSA or prostate cancer. Then a deficiency should be corrected with supplements.” This study was not the first to find a link between low vitamin D and prostate cancer: A 2015 study found that taking vitamin D supplements could slow or even reverse the progression of less aggressive, or low-grade, prostate tumors without the need for surgery or radiation, and a 2014 study found that vitamin D deficiency was an indicator of aggressive prostate cancer and spread of the disease.

Vitamin D3 supplementation was found to help improve symptoms of autism in a 2016 trial.

Research suggests that low levels of vitamin D are associated with mood disorders and depression.

Low vitamin D levels have been linked with a risk of cognitive decline, mental impairment, and Alzheimer’s disease.

Vitamin D could play a role in the prevention and treatment of a number of different conditions, including type 1 and type 2 diabetes, hypertension, glucose intolerance, multiple sclerosis, and macular degeneration, according to research.

Laboratory studies show that vitamin D can reduce cancer cell growth and plays a critical role in controlling infections. Many of the body’s organs and tissues have receptors for vitamin D, and scientists are studying its other possible functions.

As you can see, vitamin D plays an incredibly vital role in overall health, and deficiency has been linked with many serious health conditions.

Vitamin D deficiency can also cause:

  • impaired immune system functioning, which puts you at a higher risk for infection
  • rickets, a condition that most commonly occurs in children that causes bone softening (bone softening in adults is called osteomalacia)
  • insulin resistance, which affects your ability to use insulin to process blood sugar
  • thin or brittle bones, which increases your risk for osteoporosis

Vitamin D serves several important functions in the body, including:

  • promoting calcium absorption
  • maintaining normal calcium and phosphate levels
  • promoting bone and cell growth
  • reduced risk of bone fractures
  • may help increase muscle strength
  • reducing inflammation

Causes of Deficiency
As noted at the beginning of this article, it is estimated that a billion people worldwide are deficient in vitamin D. There are several possible causes of deficiency:

  • Strict vegetarian or vegan diets. Most natural sources of vitamin D are animal-based, including fish and fish oils, egg yolks, fortified milk, and beef liver.
  • Inadequate exposure to sunlight, or wearing sunscreen while outdoors (sunscreen blocks the sun’s ability to stimulate vitamin D production).
  • Having darker skin – the pigment melanin reduces the skin’s ability to make vitamin D in response to sunlight exposure.
  • Kidneys failing to convert vitamin D to its active form. This can be a function of aging.
  • Certain health conditions, including Crohn’s disease, cystic fibrosis, and celiac disease – these conditions can affect the intestine’s ability to absorb vitamin D from food.
  • Obesity – vitamin D is extracted from the blood by fat cells, altering its release into the circulation.

Symptoms of Deficiency

For many people, symptoms of vitamin D deficiency are subtle – if they have symptoms at all. Often, symptoms don’t appear until levels get very low or have been low for quite some time.

Possible symptoms include:

  • Bone pain and muscle weakness
  • Frequent bone fractures
  • Soft bones that may result in deformities
  • Unexplained fatigue
  • Constant respiratory problems
  • Depression
  • Seasonal Affective Disorder (SAD), a type of depression believed to be related to a lack of sunlight
  • Chronic infections

Because it is possible to take too much vitamin D, it is important to ask your healthcare practitioner to give you a blood test to see if you are deficient.

Vitamin D Supplementation

Vitamin D supplementation is not a simple matter. Dr. Mercola says there are many things to consider, including selecting the correct kind of vitamin D, dosing correctly, and monitoring blood levels:

What your body requires is vitamin D3 and not vitamin D2, the synthetic form commonly prescribed by physicians. One microgram of vitamin D3 or 25-hydroxyvitamin D3 is about five times more potent in raising serum 25(OH)D than an equivalent amount of vitamin D2. Aside from being less effective, vitamin D2 can also pose potential harm to your body.

It is very, very important to have your blood levels of vitamin D tested before and during supplementation. There are two vitamin D tests currently being offered: 1,25(OH)D and 25(OH)D. The correct test to order is 25(OH)D – also known as 25-hydroxyvitamin D – because it is the better marker of overall D status.

Dosage Guidelines

Dr. Mercola recommends optimizing vitamin D levels via a combination of sun exposure, supplements, and food “to maintain a healthy blood level of 40-60 ng/ml year round.” He says that based on research, 40 ng/ml appears to be the “magic number” for a wide range of health benefits, and to reach that level, adults need about 8,000 IUs per day. This is a general guideline, Dr. Mercola cautions. It’s important to have your healthcare provider monitor your blood levels.

While the Institute of Medicine (IOM) recommends 20 nanograms per milliliter (ng/ml) of serum concentration of 25-hydroxy vitamin D as an adequate level, or 600 IUs a day up to age 70 and 800 IUs if you’re over 70, many vitamin D researchers believe that’s not even enough to prevent osteomalacia, let alone take advantage of vitamin D’s additional health benefits, Dr. Mercola says.

Dietary sources of vitamin D

As mentioned previously, it is nearly impossible to obtain enough vitamin D through food, but the following are the best sources:

  • grass-fed butter
  • egg yolks
  • cheese
  • wild Alaskan salmon
  • sardines
  • beef liver

Vitamin K, Calcium, Magnesium, and Vitamin A

There’s an intricate relationship between vitamin D, vitamin K, calcium, magnesium, and vitamin A. Here’s what you need to know about each, and how they interact.

Vitamin K

The “K” in vitamin K comes from the German koagulation. Coagulation refers to the process of blood clot formation.

Vitamin K allows the body to use calcium to perform its clotting function. If you are low in vitamin K, your body may not be able to use calcium in this way. Vitamin K also helps form and maintain our bones and teeth via activating a protein called osteocalcin that helps the body use calcium and deposit it where it is needed.

The best form of vitamin K is K2, according to Dr. Mercola. K2 helps regulate where calcium ends up in the body, and it may help prevent calcium from being deposited in the arteries (which is a huge risk factor for heart disease). It also has been found to play an essential role in bone metabolism, and studies suggest that it can help prevent osteoporosis and fractures.

Vitamin K2 is mainly found in certain animal foods, including high-fat dairy products from grass-fed cows, liver, kefir, and egg yolks. Certain cheeses such as Brie and Gouda (these two are particularly high in K2, containing about 75 mcg per ounce), and fermented foods such as natto and vegetables fermented using a starter culture of vitamin K2-producing bacteria are decent sources. Most fermented vegetables are not really high in vitamin K2 and come in at about 50 mcg per serving, Dr. Mercola says, but if specific starter cultures are used they can have ten times as much, or 500 mcg per serving.


Because there is a strong calcium-vitamin K relationship, being deficient in vitamin K can cause calcium levels to build up and deposit in our soft tissues.

Vitamin D increases calcium levels in the body, and vitamin K helps the body use calcium. Taking large doses of vitamin D when there’s a vitamin K deficiency present can lead to problems like atherosclerosis (calcification of the arteries).


Magnesium is an important mineral that aids in vitamin D production and use. It appears to regulate the sensitivity of our tissues to vitamin D.

Because magnesium is used in vitamin D metabolism, some researchers believe that supplementing with high levels of vitamin D might cause an even greater magnesium deficiency in people who are already deficient.

Research has shown that magnesium supplementation, taken along with vitamin D supplementation, was more effective at correcting a vitamin D deficiency than vitamin D supplementation alone.

Vitamin A

Vitamin A can prevent vitamin D are both fat soluble (fat soluble vitamins are absorbed in the intestines along with fat and are stored in the liver) and can reach toxic levels in the body, but some studies shown that they can balance each other out. If you are deficient in one, high doses of the other may cause problems. The lower your vitamin A level is, the more dangerous excess vitamin D becomes.

Some studies suggest that increasing vitamin A can also help reduce any calcium buildup that may occur with excess vitamin D.

Vitamin D Toxicity

Vitamin D toxicity, also called hypervitaminosis D, is a rare but potentially serious condition that can occur when you have excessive amounts of vitamin D in your body. It is usually caused by mega-dosing vitamin D supplements – your body will regulate how much vitamin D it produces via sun exposure, and it is very difficult to take too much via food sources.

From the Vitamin D Council:

Vitamin D toxicity, where vitamin D can be harmful, usually happens if you take 40,000 IU per day for a couple of months or longer, or take a very large one-time dose.

Vitamin D is fat-soluble, which means your body has a hard time getting rid of it if you take too much. When you take large amounts of vitamin D, your liver produces too much of a chemical called 25(OH)D.

When your 25(OH)D levels are too high, this can cause high levels of calcium to develop in your blood. High blood calcium is a condition called hypercalcemia.

Symptoms of vitamin D toxicity include feeling sick, poor appetite, extreme thirst, frequent urination, constipation or diarrhea, abdominal pain, muscle weakness or pain, bone pain, feeling confused, and feeling tired.

If you suspect that you are experiencing vitamin D toxicity, the Vitamin D Council says to have a blood test to check your 25(OH)D levels.

Levels above 150 ng/ml are considered potentially toxic and potentially harmful.

Very high levels of 25(OH)D can develop if you:

  • take more than 10,000 IU/day (but not equal to) every day for 3 months or more. However, vitamin D toxicity is more likely to develop if you take 40,000 IU/day every day for 3 months or more.
  • take more than 300,000 IU in a 24 hour period.

If you have taken this much vitamin D, seek medical attention. Your health providers will get your calcium and 25(OH)D levels tested.

Sun Exposure Guidelines

Some vitamin D researchers have found that somewhere between 5–30 minutes of sun exposure between 10 AM and 3 PM at least twice a week to the face, arms, legs, or back without sunscreen usually leads to sufficient vitamin D synthesis. For guidelines on how to use natural sun exposure to optimize your vitamin D benefits, please see Dr. Mercola’s article Little Sunshine Mistakes that Can Give You Cancer Instead of Vitamin D.

Indoor light therapy can help, too.


Considering that an estimated one billion people worldwide are believed to be low in vitamin D, odds are your level is low too (unless you are already supplementing). Having your healthcare provider check sounds like a good idea, considering all of the health risks associated with deficiency and all of the benefits associated with proper levels, doesn’t it?

Additional Reading

Vitamin K2 And The Calcium Paradox: How a Little-Known Vitamin Could Save Your Life by Kate Rheaume-Bleue

The Vitamin D Solution: A 3-Step Strategy to Cure Our Most Common Health Problems by Michael F. Holick Ph.D. M.D.

The Magnesium Miracle by Carolyn Dean Md Nd

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