Vitamin D, also known as the “sunshine” vitamin, is essential for our health. Vitamin D is not a vitamin but a prohormone-substance that converts to a hormone. The kidneys produce vitamin D, controlling blood calcium concentration and impacting the immune system. It is also known as calcitriol, ergocalciferol, calcitriol, and cholecalciferol.
The body produces vitamin D in a chemical reaction when the skin becomes exposed to sunlight. This reaction produces cholecalciferol, and the liver converts it to calcitriol. The kidneys then convert it to calcitriol, the active form in the body. Vitamin D helps absorb calcium for blood calcium levels to reach their optimal level. This function enables bone mineralization required for strong, healthy bones. Food sources such as oily fish, egg yolks, mushrooms, and fortified foods contain vitamin D2 and D3. However, sunlight is the best source. Vitamin D supplementation is necessary in climates without adequate UVB rays and scarce food sources.
Approximately 40% of Americans present with vitamin D deficiency. People of color are at the most significant risk for deficiency. Studies during the COVID-19 pandemic show that vitamin deficiency may be associated with a higher risk of testing positive for the virus. In one review, over 80% of patients hospitalized with COVID-19 were vitamin D deficient.
The role of vitamin D
Vitamin D deficiency is primarily associated with rickets, a disease in which the bone tissue does not adequately mineralize, resulting in soft bones and deformities. Research is revealing a multitude of health conditions related to a vitamin D deficiency. The symptoms and health risks of vitamin deficiency include:
- Bone pain and muscle weakness
- Increased risk of death from cardiovascular disease
- Cognitive impairment in older adults
- Severe asthma in children
- Infections and immune system disorders
- Multiple Sclerosis
- Falls in older people
- High blood pressure
- Low bone density/Osteoporosis
- Rheumatoid Arthritis
- Thyroid Disease
Vitamin D and sunlight
The amount of vitamin D that our skin produces varies on factors such as:
- The season: This factor depends on where we live. Warmer climates will have more extended periods of sun exposure, whereas colder climates reduce the amount of sun exposure.
- The amount of cloud cover and air pollution will determine sun exposure: Carbon particles in the air from burning fossil fuels, wood, and other materials scatter and absorb UVB rays, reducing vitamin D production.
- Where we live: Cities near the equator emit higher Ultra-Violet (UV) light levels. The UV-B light in sunlight causes our skin to produce vitamin D.
- The melanin content of our skin: Melanin is the pigment that causes our skin to tan. The darker our skin, the more sun exposure is needed to get sufficient vitamin D from the sun. Fair-skinned individuals absorb more vitamin D.
- Age: Individuals over the age of 50 produce less vitamin D than their younger counterparts. Spending less time outdoors, existing kidney problems, and fewer skin receptors account for vitamin deficiency among this population. An estimated 40% of older people living in sunny South Florida are vitamin D deficient.
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Causes of vitamin D deficiency
Cystic fibrosis, Crohn’s disease, and Celiac disease do not allow the intestines to absorb vitamin D through supplements.
Weight loss surgeries reducing the stomach’s size or bypassing the small intestine increase the difficulty of consuming adequate quantities of specific nutrients, minerals, and vitamins.
Vitamin D deficiency is more common in obesity with a body mass index greater than 30. Fat cells keep the vitamin D isolated so that it is not released. Obese individuals need to consume larger doses of vitamin D to maintain normal vitamin D levels.
Kidney and liver diseases reduce the amount of an enzyme required to convert vitamin D to a form used in the body. Lack of this enzyme results in producing insufficient levels of vitamin D in the body.
Since vitamin D is fat-soluble, people with problems digesting fats will not absorb vitamin D.
Other factors contributing to vitamin D deficiency include:
- Mobility: Homebound or inactive individuals cannot use sun exposure as a source of vitamin D.
- Breast Milk: A woman’s breast milk only contains a small amount of vitamin D. Formula is also low in vitamin D. Therefore, infants who are exclusively breastfed are at risk of vitamin D deficiency.
- Self-imposed or medically restricted diets: Eliminating various nutrients, including vitamin D, increases the risk of deficiencies. Diets low in vitamin D are more common in individuals with a milk allergy or lactose intolerance or those who consume a vegan or ovo-vegetarian diet.
- Medications: Laxatives, steroids, cholesterol-lowering drugs, seizure-control drugs (phenobarbital, phenytoin), tuberculosis (rifampin), weight-loss (orlistat) can lower vitamin D levels.
- Occupations: Employees who work indoors, shift workers, and medical residents are often deficient in vitamin D due to limited sun exposure. Outdoor workers fare better; however, 75% are vitamin D insufficient while 48% are deficient.
Screening and diagnosis for vitamin D deficiency/toxicity
A blood test will measure vitamin D levels. Measures of vitamin D deficiency range from 12-30 ng/mL. A blood level of less than 12 mg/mL signals a vitamin D deficiency. Vitamin D3 increases blood levels significantly more than vitamin D2. However, taking high doses of vitamin D3 for prolonged periods can result in toxicity when blood levels rise above 150 ng/mL. The vitamin stored in body fat is released slowly into the bloodstream. Therefore, the effects of toxicity may extend for several months after taking supplements. Although rare, vitamin D toxicity can be dangerous. Vitamin D toxicity or hypercalcemia symptoms include nausea, abdominal pain, vomiting, dizziness, weight loss, hardening of the arteries, irregular heart beat, fatigue, confusion, excessive thirst, and frequent urination. Excessive vitamin D can also cause bone pain and calcium stones. It is not possible to reach dangerously high levels through diet and sun exposure alone.
The daily current recommended amount of vitamin D is 600 International Units (IU) for adults under 70 and 800 IU for older adults. Infants (0-12 months) require 400 IU. The safe upper limit for adults is up to 4,000 IU. Intake in the range of 40,000-100,000 IU/day is associated with toxicity in humans.
One study from Rutgers’s School of Public Health assessed the effect of vitamin D on cognitive function on three groups of obese and overweight women between the ages of 50 and 70 in a randomized control trial. One group took 600 IU daily of vitamin D for a year. Another group took 2,000 IU daily, and the third took 4,000 IU daily. The results indicated that memory and learning improved for the group that took 2,000 IU daily but did not take the higher dosage. Further findings show that reaction time slowed at the 2,000 IU daily dose and was significantly slower at the higher dosage, potentially increasing the risk for falls.
How much time in the sun is safe and efficient for producing vitamin D?
Campaigns to reduce outdoor time due to increasing cancer risks result in limited sun exposure and vitamin D insufficiency and deficiency. Current public health advice on sun exposure needs reconsidering to communicate a better balance between the benefits and the risks of sunlight, especially at higher latitudes where ambient levels of UVR are lower even in summer.
Research shows that midday sun exposure is optimal for producing vitamin D. At noon, the sun is at its highest peak when its UVB rays are most intense. Furthermore, exposure later in the day can increase cancer risk. In the UK, 13 minutes of midday sunlight three times per week during the summer is adequate for Caucasians. Another study recommends 30 minutes of midday summer sun exposure in Oslo, Norway, equivalent to consuming 10,000-20,000 IU of vitamin D.
Living further from the equator reduces the ability to produce vitamin D from sun exposure. The ozone layer absorbs the sun’s rays; therefore, people in these areas need to spend more time in the sun. Additionally, people who live farther from the equator may not produce vitamin D from the sun for six months during the winter months.
Some scientists recommend exposing one-third of the area of our skin to the sun. According to this recommendation, a tank top and shorts for 10-30 minutes three times per week during the summer is sufficient exposure for light-skinned individuals. Sunscreen use prevents sunburn by blocking the UVB rays; sunscreen use lowers vitamin D production. Nevertheless, few people apply enough sunscreen to block the UVB rays or use sunscreen irregularly, so the effects are insignificant.
Although sun exposure is vital for producing vitamin D, excessive exposure can result in sunburn, cancer, aging skin, heatstroke, dehydration, eye damage, skin changes, and lowered immune system. Certain medications can cause photosensitivity and phototoxicity, such as:
- Cordarone (a heart medication)
- Tricyclic anti-depressants,
- Acne medications (Accutane and Retin A)
- Thiazide diuretics
- Older antihistamines
- Birth control pills
The “sunshine” vitamin is essential for our health. Vitamin D insufficiency and deficiency are common globally due to changing lifestyles minimizing outdoor occupations, and Western diets low in this vitamin. Campaigns warning against the damage of sun exposure in measured doses results in more people avoiding sun exposure and covering their bodies. Supplementation at recommended doses is essential to reach and maintain optimal levels when sun exposure and diet are inadequate in producing vitamin D.
- Harvard Medical School (2020). 6 things you should know about vitamin D. https://www.health.harvard.edu/staying-healthy/6-things-you-should-know-about-vitamin-d
- Healthline (2021). The Benefits of Vitamin D. https://www.healthline.com/health/food-nutrition/benefits-vitamin-d
- Kidwell, L. (2021). Vitamin D: An Important Factor for Overall Health. Rush University Medical Center. https://www.rush.edu/news/vitamin-d-important-factor-overall-health
- Mayo Clinic (2020). What is vitamin D toxicity? Should I be worried about taking supplements? https://www.mayoclinic.org/healthy-lifestyle/nutrition-and-healthy-eating/expert-answers/vitamin-d-toxicity/faq-20058108
- Neuroscience News (2019). More vitamin D may improve memory but too much may slow reaction time. Rutgers University. https://neurosciencenews.com/vitamin-d-memory-reaction-time-10896/
- Raman, R. (2018). How to Safely Get Vitamin D from Sunlight. https://www.healthline.com/nutrition/vitamin-d-from-sun
- Science News (2019). Does sunscreen inhibit vitamin D levels? Maybe not. https://www.sciencedaily.com/releases/2019/05/190509115443.htm
- Shye, R. (2021). These Medications Can Make You More Sensitive to the Sun. https://www.goodrx.com/blog/avoid-the-sun-if-you-take-these-drugs/
- WebMD (2021). Vitamin D Deficiency. https://www.webmd.com/diet/guide/vitamin-d-deficiency#1