Why is vitamin D important?

Vitamin D is a fat-soluble vitamin that performs an important role in calcium homeostasis and bone metabolism. Although vitamin D is called a “vitamin” (a required nutrient obtained from the diet), it is not truly a vitamin. It acts more like a hormone since the body can synthesize it from cholesterol after the skin is exposed to UVB rays from the sun.^1,2

Vitamin D is estimated to be involved in the regulation of up to 2000 genes, giving it significant input into the critical processes happening in cells throughout the body!^3,4 Consequently, this vital nutrient has been linked to virtually every health condition under the sun, from bone and muscle health, to brain health, pregnancy, immune activity, cardiovascular functions and more.^1,2

In contrast, vitamin D deficiency can lead to significant health consequences, and it’s estimated that 42% of American adults are deficient.^1,5 Below are some of the ways vitamin D has been shown to impact health.

The benefits of vitamin d3 and consequences of deficiency

Vitamin D is known for helping to balance minerals like calcium, phosphorus, sodium, and magnesium for healthy bone formation and mineralization.⁶ Although it plays a significant role in skeletal health, vitamin d benefits go well beyond bone health to all parts of the body. For example, vitamin D can act as an antioxidant, regulate immune activity, support cardiovascular health, modulate blood sugar balance, regulate neurotransmitter synthesis and more. ⁷

Vitamin D from food, supplementation, or sun exposure, is originally in an inactive form. It must shuttle first through the liver, to become 25(OH) vitamin D, and then to the kidney, to become biologically active 1,25(OH)₂ vitamin D. Once biologically active, vitamin D travels to organs throughout the body (intestines, pancreas, kidney, lungs, thyroid, etc.) affecting cellular health and function.

Below are some of the organ systems that benefit from vitamin D, and the negative consequences of vitamin D deficiency in select health conditions.⁷

Vitamin D and musculoskeletal health

Vitamin D supports the normal structure and function of bones and muscles. Historically, the consequences of chronic vitamin D deficiency have been on full display in cases of rickets, a skeletal disorder that results in weakening and distortion of bone growth in children without food sources of vitamin D and low sun exposure.³ Vitamin D deficiency also affects adults, as evidenced by an increased risk in certain types of fractures and forms of muscles weakness.^3,8

Vitamin D and brain and nervous system

 Vitamin D substantially affects the brain and nervous systems. Studies have shown that supplemental vitamin D may enhance mood during the winter season when vitamin D synthesis from the sun is extremely low.^9,10 In addition, research suggests that low vitamin D levels may be associated with increased sleepiness and sleep difficulties.^11,12

Vitamin D and pregnancy

Growing evidence also supports the benefits of vitamin D during pregnancy. Recent studies have found that during pregnancy, vitamin D may reduce the risk of many pregnancy complications.^13–18 For more information about the role of vitamin D during pregnancy check out “The Importance of Vitamin D During Pregnancy”.

Vitamin D and immunity and respiration

Since vitamin D plays a role in regulating the immune system, it may also have a positive influence on immune-related conditions, and may help to diminish their symptoms.³ For example, adequate amounts of vitamin D during childhood and adulthood have been shown to decrease the exacerbation of chronic respiratory issues and their symptoms.^18,19 

Studies have also shown that pregnant women taking 4,400 IU of vitamin D per day may reduce the risk of respiratory issues in their newborn to three-year-old children, compared to pregnant women who took just 400 IU per day.²⁰ Self-reported incidences of seasonal respiratory complications have shown to be significantly lower in women taking 800 IU per day compared to placebo.^21,22 For more information check out “Don’t Forget Vitamin D this Cold and Flu Season”.

Vitamin D and cardiovascular health

In addition to the respiratory system, research shows that vitamin D helps to reduce the risk of various complications of the heart and blood vessels.^23,24 For example, researchers reported that daily vitamin D supplementation using a wide range of doses (i.e., 600 IU, 2,000 IU, 4,000 IU) decreased stiffness of specific blood vessels after only eight weeks, compared to placebo. Even more compelling, the influence of vitamin D was observed in a dose-response relationship, meaning that as more vitamin D was taken, more improvement in blood vessel structure was observed.²⁵

Vitamin D and metabolism

Vitamin D is involved in numerous metabolic processes throughout the body, with research showing that vitamin D can positively influence blood sugar imbalances and metabolic complications.^26–29 Obesity is a significant risk factor for many metabolic and cardiovascular imbalances. Research suggests that the prevalence of vitamin D deficiency may be higher among people with obesity (and to a lesser degree in people who are overweight) and those with blood-sugar imbalances.³⁰ Importantly, the association between vitamin D and metabolic complications may be dose-dependent—the better one’s vitamin D status, the lower the risk of many of these imbalances.

Sources and forms of vitamin D

Vitamin D from sunlight

Sunlight can be a potent natural driver of vitamin D synthesis. A 20-minute total body exposure to sunlight produces approximately 10,000–20,000 IU of vitamin D.^2,3 Anything that reduces or interferes with UVB radiation from sunlight penetrating the skin affects the skin’s ability to make vitamin D. Many factors influence vitamin D production by the body, from geographical location to environmental conditions.

• Geographical location, time of year, time of day: Differences in sun exposure greatly depend on the angle at which sunlight reaches the earth, which changes based on where one lives, the time of year, and the time of day. Those living at higher latitudes, (above 30^oN or 30^oS) receive less UVB radiation than those living closer to the equator, and this limits how much vitamin D can be made by the skin.³¹ Vitamin D synthesis is lower during the winter season, and in early morning and late afternoon, when skin exposure to sunlight is minimal.⁵
• Skin pigmentation: Melanin is a pigment in the skin, hair, and eyes that acts as a natural sunscreen by absorbing UVB radiation.³² The more melanin in the skin, the less vitamin D will be produced. Dark skin is estimated to have a natural SPF of 15, effectively reducing vitamin D synthesis by up to 99%.³³ It can take someone with darker skin 3-6 times longer to make the same amount of vitamin D as someone with a lighter skin tone.² 
• Aging: The body’s ability to make vitamin D diminishes with age. For the body to make vitamin D, the presence of a compound called 7-dehydrocholesterol is needed when UVB rays from sunlight penetrate the skin. However, with increasing age, smaller amounts of this compound are produced. Consequently, even with the same amount of sun exposure, less vitamin D is produced. It’s estimated that the elderly can have up to a 75% reduction in their ability to make vitamin D due to lower levels of 7-dehydrocholesterol.^5,34
• Sunscreen: Acting in similar fashion as melanin, sunscreen blocks the absorption of UVB radiation. As a result, when sunscreen is properly applied it can reduce how much vitamin D is made by the skin by up to 99%.³² 
• Other factors: Time indoors, cloudiness, fog, smog, and clothing all decrease the amount of UVB rays that reach the skin, and as a result, the amount of vitamin D that can be made.³¹

Dietary sources of vitamin D

Natural food sources of vitamin D3 (or cholecalciferol, a primarily animal-based form of vitamin D) include fatty fish, egg yolks, and cod liver oils. For vitamin D2 (or ergocalciferol, the plant-based source of vitamin D), shiitake mushrooms are an excellent source. The table below provides estimates for the amounts of vitamin D3 (naturally occurring and fortified) and D2 that may be found in certain food sources.  

*Table is modified from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3571641. **Minimal erythemal dose (MED) in a bathing suit. Exposure of arms and legs to 0.5 MED is equivalent to ingesting ~3000 IU vitamin D3.

Supplements: Which form of vitamin D is best?

Two forms of vitamin D are available for supplementation: D2 and D3. These two forms primarily differ in structure and sources. Although both forms are converted by the liver into calcitriol [25(OH) vitamin D], the same form of vitamin D measured in the blood to determine a person’s vitamin D status, strong evidence indicates that vitamin D3 is more effective at sustaining vitamin D blood levels over time than vitamin D2.^35–38

Vegans and others who prefer a plant-based source of vitamin D typically choose to supplement with vitamin D2, compared to animal-based vitamin D3 sources. Fortunately, there is a plant-based vitamin D3 available in the form of lichen, a fungi and algae plant-like organism. 

Multivitamins typically include either 400, 500, or 1000 IU vitamin D in the form of D2 or D3 or a combination of both. Vitamin D stand-alone supplements can be found in vitamin D3 form with values of 400–50,000 IU.³

Vitamin D dosage: How much is enough? 

Dosage recommendations for vitamin D

General vitamin D supplementation guidelines have been proposed by various federal and research-based institutions. The table below provides recommendations from three agencies and institutions, including the Endocrine Society whose guidelines most reflect recent research.

Keep in mind, the FDA recently changed its regulations for reporting vitamin D values in nutrition supplement facts. As of January 2020, all supplement companies are required to report amounts of vitamin D in micrograms (mcg) instead of the current International Units (IU) (1 mcg = 40 IU). 

Below are some general guidelines:

*4000–6000 IU/day is mother’s required intake if infant is not receiving 400 IU/day.

IOM = Institute of Medicine; RDA = Recommended Dietary Allowance; RDI = Recommended Dietary Intake; IU = International Units.

[Table is modified from Evaluation, Treatment, and Prevention of Vitamin D
Deficiency: an Endocrine Society Clinical Practice
Guideline https://www.ncbi.nlm.nih.gov/pubmed/21646368].

Most vitamin D researchers agree that vitamin D levels are sufficient at levels >30 ng/mL (or 75 nmol/L). Levels of 20 ng/mL or below (< 50 nmol/L) are considered indicative of vitamin D deficiency, while blood levels 21–29 ng/mL (or 51-74 nmol/L) may indicate insufficiency.³⁹

Why testing for vitamin D is important 

Numerous factors determine how vitamin D supplementation will affect an individual’s vitamin D blood levels, including baseline vitamin D levels, age, body weight, digestive health, diet, current medications, and more. Because the same dose of vitamin D in one person can result in different blood levels in another, measuring blood levels is the most efficient way to determine someone’s vitamin D status.

Blood levels of vitamin D, measured as calcitriol [25(OH) vitamin D], can be easily and accurately detected with a blood test requested by your doctor. 

• The lower a person’s vitamin D blood levels prior to supplementation, the more vitamin D is required to achieve sufficient levels. Researchers reported that a 50% higher dose of vitamin D was needed to reach sufficient levels (>75 nmol/L or 30 ng/mL) in patients with lower baseline vitamin D levels, compared to patients with higher baseline levels.⁴⁰
• With a higher body mass index (BMI), more vitamin D is needed to reach target blood levels. According to one study, after supplementing with 2000 IU of vitamin D per day, those with a higher BMI had significantly lower vitamin D levels than those with a lower BMI.⁴¹
• Other studies show that age also has an important influence on vitamin D levels after supplementation.^42–44 Research suggests that as age increases, it may take more amounts of vitamin D to increase blood levels.⁴²

How much vitamin D is too much?

While the proper care and precautions should be exercised when taking dietary supplements, research suggests that the risk of taking too much vitamin D3 is extremely rare. In fact, a growing body of research indicates that most people need to increase their daily vitamin D intake.¹ Clinical Practice Guidelines indicate the following:³

• Infants (up to 1 year): up to 2,000 IU is safe
• Children and adolescents (1–18 years): up to 4,000 IU is safe
• Adults (> 18 years): up to 10,000 IU is safe

These levels are based on studies demonstrating that the use of these amounts within their respective age groups was safe, and no signs of vitamin D toxicity were reported.³⁹ It’s also important to note that research shows the same safety profile for supplemental vitamin D3 doses beyond 40,000 IU/day!²

In closing

Despite its significance for overall health, research suggests that most people are not getting enough through sun exposure and diet. Testing, supplementation, appropriate sun exposure, and consuming dietary sources of vitamin D provide a balanced approach to achieving vitamin D sufficiency.