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What is Vitamin D all about?

Vitamin D is more than just a vitamin. Because our bodies can make Vitamin D in our skin when it is exposed to good sunlight, Vitamin D is more specifically considered a hormone. Vitamin D has many important jobs in your body. It keeps your bones strong by helping your body absorb and use nutrients. Vitamin D allows your muscles to move efficiently, and nerves need it to carry messages throughout your body. Recently, increasing attention has been given to the role of Vitamin D in boosting the immune system.

Vitamin D for Good Bone and Muscle Health

Vitamin D is necessary for strong bones and muscles. Without Vitamin D, our bodies cannot effectively absorb calcium, a mineral that is essential to bone health. Calcium is not very well absorbed by the human intestines. When calcium and Vitamin D interact in your bloodstream, they activate cells in your bones to help make your bones stronger. Vitamin D also aids in keeping muscles strong. A study published in Plos One showed a positive relationship between muscle strength and vitamin D intake. If you don’t get enough vitamin D as you age, you are more likely to suffer falls and broken bones.

Vitamin D and COVID-19

Vitamin D is crucial for a healthy functioning immune system. Immune System cells including B-cells and T-cells have receptors for vitamin D and rely on the vitamin to function properly. Vitamin D keeps your immune system balanced.  Prior research has found a link between vitamin D and a decrease in viral respiratory infections including the ‘common cold’. A growing body of circumstantial evidence now also specifically links outcomes of COVID-19 and Vitamin D status.

In some studies, low Vitamin D seems to correlate with poor COVID-19 outcomes.

Does Vitamin D aid our defense to COVID-19? Risk of Vitamin D deficiency correlates with many other known COVID-19 risk factors including age, obesity, diabetes, and chronic illness. In addition, certain populations including African-American, Hispanic, and elderly --- have BOTH disproportionate morbidity and mortality from COVID-19 and high rates of vitamin D deficiency.  Vitamin D deficiency could be one of many factors involved in determining COVID-19 outcomes, and it's a problem that can be corrected safely and cheaply. There seems to be very little downside to addressing and correcting Vitamin D levels - especially in high-risk populations.

A recent University of Chicago study published in JAMA Network Open found that patients with "likely deficient" Vitamin D status ha doubled the risk of testing positive for COVID-19 versus those with "likely sufficient" vitamin D. There is a lot of research to still be done, but Vitamin D supplementation could be especially important for older individuals since they are at high risk of both poor outcome from COVID-19 and vitamin D deficiency. The "possible" link to COVID-19 morbidity and mortality has become an additional reason to take Vitamin D intake seriously.

How can you get vitamin D?

It’s best to get vitamin D from sunlight and food, but you can also take it in a supplement. Only a small proportion of vitamin D comes from the food we eat, but it is still important to include vitamin D rich foods in your diet. Foods that contain Vitamin D naturally include: fatty fish like salmon, tuna, and mackerel; beef liver, cheese, and egg yolks; and mushrooms have a small amount. Foods fortified with vitamin D in the US include milk, breakfast cereal, and some orange juice, yogurt, and soy drinks.

There are few foods that naturally contain high levels of Vitamin D.

Sunlight is the best natural source of Vitamin D. Your body produces Vitamin D when the sun shines directly on your skin. Just 10 to 15 minutes of sunlight without sunscreen a couple of times a week is usually sufficient to maintain normal vitamin D levels. Always take care not to burn, especially during the strong sunshine in the middle of the day. Even on cloudy days, your body can still produce Vitamin D from sunlight, but it can take a little longer. Speak with your physician about the specific amount of sun exposure recommended for you each week.

If you are 65+ years, not exposed to much sun or a pregnant or breast-feeding woman, or have been diagnosed with Vitamin D deficiency, your physician may advise or prescribe a daily vitamin D supplement.

Why the increased focus on Vitamin D today?

Recent research has stressed the importance of Vitamin D — not just for good bone health, but also for possibly preventing chronic disease when we are older. The pandemic and lockdown have kept many of us indoors. Walking, outdoor sports and social activities have been put on hold, but good health needs a bit of outdoors too. Modern fast-food diets, limited physical activity and decreased milk intake all make it harder to maintain healthy Vitamin D levels.

Risk factors for having low vitamin D levels include:

  • Age over 50
  • Very limited sun exposure (i.e. housebound, mobility limitations)
  • Have kidney disease or conditions that affect how their bodies absorb minerals
  • Have darker skin
  • Are lactose intolerant, meaning they can’t digest the sugar in dairy foods
  • Strict vegan diet
  • Infants who are breastfeeding
  • Live in Northern states (with less sun exposure)
  • Taking certain prescription medications
RELATED ARTICLE: COVID-19 Reopening and Immunity

How much Vitamin D do we need?

Vitamin D is measured in units called "International Units," or IUs. According to the Institute of Medicine's Food and Nutrition Board and the NIH Office of Dietary Supplements, the Recommended Daily Allowances (RDA) for Vitamin D is:

  • 400 IU/day for children aged 0-12 months
  • 600 IU/day for children aged 1-18 years
  • 400-800 IU/day for adult < 50 years
  • 800-1000 IU/day for adult > 50 years
Speak with your physician about your need to take a Vitamin D supplement.

These are the dosages that seem to prevent bone disease but may not be the amount that will result in your healthiest bones and muscles. It is important to speak with your physician about the Vitamin D dosage recommended for you.

How do doctors test Vitamin D levels?

The best way to know your Vitamin D status is to have a blood test which checks the level in the blood. It is important that the correct blood test is done. Measuring the level of 25-hydroxy Vitamin D reflects your body’s level from both diet and sunlight. When your doctor knows the exact level of Vitamin D in your bloodstream, he or she can make the most accurate recommendation of how much Vitamin D you should take.

A simple blood test is the most reliable way to know your Vitamin D status.

A blood level less than 20 ng/mL can increase risk of bone disease, and more than 150 ng/mL can be harmful. Most experts recommend maintaining blood levels between 40 and 70 ng/mL to assure good absorption of calcium for healthy bones.

What are the risks of taking vitamin D?

Our skin cannot make too much Vitamin D — it stops when there is enough in the blood, but it is possible to overdose on Vitamin D supplements. However, it is possible to overdose on Vitamin D supplements. Too much vitamin D can cause loss of appetite, excessive urinating, nausea, and weight loss. High doses of vitamin D can also lead to disorientation, bone pain and kidney stones. Because too much Vitamin D can be dangerous, doses greater than 2000IU/day should be taken cautiously, as advised by your doctor and based upon the results of blood tests.


Vitamin D is important for good muscle health. People with very low Vitamin D blood levels may be more likely to experience muscle cramps, bone or joint pain, and may be at higher risk of complications from COVID-19. Although more research needs to be done, we know that Vitamin D is vital for healthy bones, muscles and our immune system. Studies even confirm that older people who get adequate Vitamin D seem to fall less, have fewer bone fractures and better muscle function. Speak with your physician about your risk of low Vitamin D levels and consider being tested.

Source: Vitamin-D and COVID-19: do deficient risk a poorer outcome?, The Lancet
Vitamin D and your health Infographic.
What strategy are you using during the pandemic to increase time outdoors and optimize your Vitamin D levels? Please SHARE any information that has been useful. Subscribe to Blog below for regular updates or CONTACT US with questions, comments and ideas for future posts.
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Cory Calendine, MD is an Orthopaedic Surgeon and founding partner of the Bone and Joint Institute of Tennessee at Williamson County Hospital in Franklin, TN. Dr. Calendine is an expert in Joint Replacement, specializing in Hip and Knee Surgery. From diagnosis through treatment, the Orthopaedic Surgical experts at the Bone and Joint Institute use the latest techniques and technology to improve care for people with musculoskeletal problems. For more information, please contact our office or schedule your appointment today.  

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