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Vitamin D: A Reason to Bask in the Sun

Updated: May 30, 2022


While the title may imply we're suggesting that you grab your favorite beach towel and head to the beach as fast as you can, we're not advising a two to three hour sun bathe to get as tan as you can. We're simply suggesting a 15 to 30 minute walk or relaxation in the sun, with its beautiful rays beaming down on you, as a part of your daily health habits. This practice can stimulate vitamin D synthesis in your body, which supports bone, heart, immune, and mental health.


Are you getting enough?

Vitamin D is well known for supporting bone health. However, research is showing it plays a role in so much more from supporting heart health to immune function to fertility to mood. Vitamin D deficiency is defined as vitamin D levels less than 30 nmol/L while vitamin D insufficiency is defined as vitamin D levels between 30 to 50 nmol/L. Vitamin D deficiency affects about one billion people worldwide, while 50 percent of the population has vitamin D insufficiency. Factors that affect vitamin D status include diet, exposure to sunshine, geographical location, skin pigmentation, malabsorption, and kidney or liver disease. Ensuring you're getting enough through sun exposure, diet, or supplementation is important for optimal health. How do you know? Let's dive in...


But First—What is Vitamin D?

Vitamin D is a fat-soluble vitamin with many important physiological functions in the body including promoting bone health, muscle function, immune function, cardiovascular health, and has also been found to be an important modulator of the reproductive system and fertility. Vitamin D has two primary forms, vitamin D2 and D3, but many other types of vitamin D form through metabolism. Vitamin D can be obtained through some foods, exposure to sunlight, or supplementation. Vitamin D2 (ergocalciferol) is derived from fortified and plant sourced foods. In the past, vitamin D3 was only derived from animal sourced foods and exposure to sunlight. However, vegan vitamin D3 supplements, made from lichen (a type of algae) are now available. Vitamin D3 (cholecalciferol) is the more readily absorbed form of vitamin D.


Science Alert!

When skin is exposed to UVB radiation, 7-dehydrocholesterol (a cholesterol precursor) in the skin is converted to vitamin D3. Both vitamin D3 and vitamin D2 are metabolized to 25-hydroxyvitamin D (abbreviated as 25(OH)D), the major circulating metabolite, or form of vitamin D, in the liver. If there is a need for vitamin D in the body, 25(OH)D is then converted to 1,25-dihydroxyvitamin D (abbreviated as 1,25(OH)2D) in the kidneys, which is the active form that exerts physiological functions and is characteristically a steroid hormone. Both the kidneys and liver play critical roles in maintaining optimal levels of vitamin D, which is why individuals with kidney or liver disease may have suboptimal vitamin D levels.


Who's At Risk?

The serum level of 25(OH)D is measured to determine the adequacy of vitamin D status. The Institute of Medicine (IOM) has defined four categories of vitamin D status including risk of deficiency, risk of inadequacy, sufficiency, and above normal, which may be reason for concern. A 25(OH)D serum level of less than 30 nmol/L is associated with vitamin D deficiency; a serum level of 30–50 nmol/L is considered insufficient; a level greater than 50 nmol/L is considered adequate; and a serum level greater than 125 nmol/L is linked to adverse effects. Data from 2011–2014 found that 18 percent of individuals in the United States were at risk of insufficiency, while 5 percent were at risk of deficiency. Race, vitamin D intake, sun exposure, obesity, age, and physical activity all impact individual vitamin D status.

Darker skin tones contain more melanin, which limits the amount of vitamin D able to be produced from exposure to sunlight. This is because the melanin in the skin competes for UVB rays making those with darker complexions more susceptible for vitamin D deficiency. Additionally, where you live in relation to the equator is another factor that places you at risk for vitamin D deficiency. The further from the equator you are, the shorter the days and the less UVB rays. Dark skinned individuals or those living far from the equator may require longer exposure to sunlight or supplementation to maintain adequate vitamin D levels.

Age is also a contributing factor to vitamin D status. Aging impacts vitamin D and calcium metabolism by reduction in 1,25(OH)2D production and decreased calcium absorption. For this reason, the recommended daily allowance of vitamin D for the elderly increases to 800 IU/day from 600 IU/day.

Obesity is also a risk factor for vitamin D deficiency. Individuals with greater amounts of subcutaneous fat and a body mass index (BMI) of 30 or greater are more likely to have a lower serum 25(OH)D level than non-obese individuals. Therefore, individuals with a BMI of 30 or greater may require higher daily intakes of vitamin D to maintain adequate serum 25(OH)D levels.

The Many Roles of Vitamin D


Vitamin D and Bone Health


Vitamin D is an important nutrient needed to maintain blood calcium balance and, therefore, bone health. Once vitamin D2 and vitamin D3 are metabolized to 1,25(OH)2D, it is released into the small intestine where it increases the amount of calcium and phosphorus absorbed into the bloodstream. This form of vitamin D also mobilizes calcium from bone and increases uptake of calcium in kidneys, both in order to maintain blood calcium balance. The parathyroid hormone, released in response to low blood calcium levels, regulates the synthesis of 1,25(OH)2D, and, therefore, calcium absorption.

Bone remodeling (the breakdown and rebuilding of bone) occurs continually throughout life in adult bones. Vitamin D’s function in enhancing calcium and phosphorus absorption in the intestine aids in the maintenance of bone health so that calcium is not mobilized from bone (bone breakdown) in order to maintain normal blood calcium levels. Vitamin D deficiency leads to decreased calcium absorption and the release of calcium from the bones. This is why bone loss can occur when there are long periods of inadequate vitamin D intakes.

Vitamin D insufficiency is associated with osteoporosis, while vitamin D deficiency leads to rickets (growth abnormalities in children) and osteomalacia (softening of the bones in adults). Adequate consumption of vitamin D can help prevent bone disease. In addition to maintaining bone health, vitamin D also supports muscle function.

Vitamin D and Immunity

Adequate vitamin D levels support immunity by helping your body be less susceptible to infection. (Fun fact: vitamin D was used to treat infections such as tuberculosis before antibiotics were used!) The lack of sunlight during the winter months, resulting in less vitamin D production, may be one reason why you may be more likely to catch the common cold and flu in the winter months.


Those with autoimmune disorders such as rheumatoid arthritis, multiple sclerosis, diabetes mellitus, or inflammatory bowel disease may also benefit from vitamin D. Low levels of vitamin D have been correlated with increased severity of autoimmune disorders such as multiple sclerosis and rheumatoid arthritis.

Vitamin D may also have an anti-inflammatory role, and, as such, has been studied in relation to cancer incidence and mortality. Observational studies have shown associations between low serum levels of 25(OH)D and increased risk of cancer as well as death from cancer. Some clinical trial results support this association.

Recently, there has been a lot of conversation around the potential protective effect that vitamin D demonstrates with COVID-19. While the evidence is inconclusive at this time, vitamin D might decrease your chance of acute respiratory infection which is common with the contraction of COVID-19 and can lead to more severe symptoms. Overall, it is important to ensure your vitamin D levels are normal during any time, but especially during COVID times.

Vitamin D and Mood

Ever notice how, in the winter months, your mood shifts in comparison to in the summer? This is most commonly known as seasonal depression or seasonal affective disorder (SAD) and vitamin D may play a role in this. As the winter months get colder and days shorter, people spend less time outside in the sunshine, which may lead to vitamin D insufficiency or deficiency. A systematic review and meta-analysis of 14 observational studies found an association between deficient or low levels of 25(OH)D and depression. However, clinical trials conducted to date do not support this claim. Although you shouldn’t rely on vitamin D to treat depression, maintaining adequate serum levels in conjunction with necessary medication may help to support your overall mood.


Vitamin D and Cardiovascular Health



While heart health may not be the first thing you think of when you hear vitamin D, vitamin D has been shown to be associated with cardiovascular health. Low serum 25(OH)D levels have been linked to increased risk of stroke, heart disease, heart attack, and early death. This is just another reason to make sure you’re getting enough vitamin D from your diet, the sun, or a supplement, if needed.

Vitamin D and Fertility

Observational studies have found links between vitamin D status and fertility in both men and women. In women, vitamin D status has been found to be associated with metabolic and endocrine disturbances, with vitamin D deficiency potentially contributing to the development of polycystic ovary syndrome (PCOS). Additionally, serum 25(OH)D levels may correlate with in vitro fertilization (IVF) outcomes in women. Further, vitamin D may play a role in regulating endometriosis. Endometriosis is a chronic inflammatory disease and involves the poor regulation of the immune response. Because vitamin D is a known regulator of the immune system it may be able to help with managing inflammatory diseases like endometriosis. In men, vitamin D status is positively associated with sperm function and testosterone levels.

Sources of Vitamin D


Exposure to sunlight is the primary way in which individuals meet vitamin D needs, however, the diet can also provide a source of vitamin D. Those following a plant-based diet should look for foods fortified with vitamin D such as fortified orange juice, cereals and plant-based milk. Additionally, mushrooms can be a good source of vitamin D2, the less absorbable form of vitamin D. To ensure adequate vitamin D intake when following a plant-based diet, especially in the winter months when there is less time outside, a supplement may be recommended. Available supplements include vitamins D3 and D2. Vitamin D3 may come from animal sources (sheep's wool) or lichen (plant-based) while D2 is always plant-derived. There is some evidence showing that vitamin D3 can increase 25(OH)D serum levels more effectively than D2. If your doctor or dietitian has recommended a vitamin D supplement, check the ingredient label to make sure the source is vegan or made with lichen if you are following a vegan lifestyle.

While rare, vitamin D toxicity can occur, therefore it is important to check with your health care provider before starting a supplement. Your health care provider can assess your vitamin D status and necessity for supplementation with a simple blood draw.


Get some sunshine!

As illustrated by the numerous functions that vitamin D plays in the body, it is incredibly important for maintaining overall health. Vitamin D insufficiency is common in the general population, plant-based or not. Look for vitamin D fortified foods, enjoy a little natural sunlight daily, and take a vitamin D supplement, if needed. Next time you need an excuse to take a moment to soak up those mood-boosting afternoon sun rays for 15–30 minutes, remember all of the essential roles vitamin D plays in your body!

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Hi there! My name is Kendall Dennis and I am a graduate student in Georgia State University's Coordinated Program for Dietetics. I'm passionate about plant-based eating and the power our dietary choices have in impacting animals, the environment, and ourselves. I'm so excited to be pursuing food and nutrition as a career path. When I'm not studying, you can find me hiking, playing with my dog, baking, or binge-watching mystery shows.

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