By Diane L. Schneider, MD, Medially reviewed by Dr. C.H. Weaver M.D. 10/2020

Vitamin D requires sunshine for activation and as winter approaches many individuals will be deficient in this important vitamin. Vitamin D is a fat-soluble vitamin that comes from dietary supplements, foods such as fortified milk and cereal, certain kinds of fish, and exposure to sunlight. Vitamin D is important for bone health, and some research suggests that it may also reduce the risk of certain types of cancer and reduce the severity of or prevent viral respiratory infections. (7) In recent years some clinical studies have shown that higher plasma levels of vitamin D are associated with improved survival in colorectal cancer patients. (1,2)

Why Is Vitamin D Important?

The main function of vitamin D is to regulate calcium balance in your body. Vitamin D regulates how well calcium is absorbed from the intestine. With adequate vitamin D, you absorb about 35% of the calcium that you take in from foods, drinks, and supplements. If your vitamin D level is low, the efficiency of calcium absorption drops to 10 to 15 percent. When not enough calcium is consumed, bone releases calcium to keep everything running and this can result in the gradual development of osteoporosis.

Benefits beyond Bone Health

In addition to its role in maintain bone health recent research has suggested that vitamin D has other benefits, including reduced the risk of heart disease, cancer, diabetes, autoimmune diseases, and infectious respiratory diseases. Most of these findings were based on observational studies, however one recent clinical trial suggest higher doses of vitamin D can delay the recurrence of colon cancer and other trials are under way to further investigate the effects of vitamin D on cancer and heart disease.(2)

Vitamin D and COVID-19

Some but not all studies suggest that patients with lower vitamin D levels who are diagnosed with COVID-19 tend to have worse outcomes and researchers recently reported that people hospitalized for COVID-19 who were deficient in vitamin D were more likely to have severe symptoms or die from COVID-19. Dr. David Meltzer and colleagues at The University of Chicago Medicine just published that patients who have lower vitamin D levels are more likely to test positive for COVID-19. (4-6)

Vitamin D is clearly important for a healthy immune system and research suggests it can play a role in preventing viral respiratory lung infections. (7) According to Dr. Anthony Fauci “If you are deficient in vitamin D, that does have an impact on your susceptibility to infection” While ongoing research will ultimately determine the relationship between vitamin D deficiency and COVID-19 infections it makes sense to make sure individuals are at least no deficient in this important vitamin.

How Much Vitamin D Do You Need?

How much vitamin D you need is widely debated with different groups providing different recommendations. In light of growing evidence of widespread vitamin D deficiency, the International Osteoporosis Foundation (IOF) has released a new position statement with increased vitamin D recommendations for older men and women. The statement was published in Osteoporosis International. (1)

Based on a growing body of evidence, the IOF has made the following recommendations:

  • The estimated average vitamin D requirement for older adults to reach appropriate blood levels of the nutrient is 800 to 1,000 international units (IU) per day.
  • Individuals who are obese, have osteoporosis, have limited sun exposure, or suffer from malabsorption may need to increase their intake to 2,000 IU/day.
  • High-risk individuals are encouraged to undergo testing to measure blood levels of vitamin D and then supplement accordingly.

Because there is such a widespread deficiency of vitamin D, the IOF hopes that the new recommendations will help to prevent falls and fractures in the older population.

People who have questions about the level of vitamin D that’s right for them are advised to talk with their physician.

Food Sources of Vitamin D

While you might assume that food would be the ideal source of vitamin D, the reality is that few foods naturally contain vitamin D. The main natural food sources of vitamin D are oily fish, like salmon, mackerel, and sardines. Egg yolks contain only small amounts of vitamin D. In the United States, all milk is fortified with vitamin D. An 8-ounce glass of milk contains 100 IU. Other fortified foods include cereals and some brands of orange juice and yogurt.

Can You Take Too Much Vitamin D?

Vitamin D is a fat-soluble vitamin that is stored in fat. Vitamin D taken at recommended doses is generally not of a concern. Classic toxicity with high blood levels of calcium and kidney and liver damage occurs with blood levels in the 200 to 400 ng/mL range as a result of high vitamin D intake. In general, daily intake up to 10,000 IU is thought to be safe. You should consult your healthcare provider to individualize the right dose for you.

Should You Be Screened for Vitamin D?

Most organizations do not recommend universal screening for assessing vitamin D blood levels. The US Preventive Services Task Force concluded this year that the benefits as well as any potential harm from vitamin D screening and early interventions cannot be determined. Individuals at high risk of low vitamin D, however, such as those with obesity, osteoporosis, celiac disease, or Crohn’s disease, are advised to have vitamin D levels checked. Winter and early spring, when sun exposure is lowest, is the best time of year to check. The best indicator of vitamin D levels is the serum 25-hydroxyvitamin D (25OHD), which is determined by sun exposure and vitamin D intake.

People who have questions about the level of vitamin D that’s right for them are advised to talk with their physician.

Diane L. Schneider, MD*, is a geriatrician, epidemiologist, author, and co-founder of An experienced writer and public speaker, Dr. Schneider is frequently featured as an osteoporosis expert on numerous television and radio shows and in Internet articles, magazines, newspapers, and medical publications. Dr. Schneider is the author of The Complete Book of Bone Health (Pro­metheus Books, 2011; $21).*


  1. Dawson_hughes B, Mithal A, Bonjour JP, et al. IOF position statement: Vitamin D recommendations for older adults. Osteoporosis International. DOI 10:1007/s00198-010-1285-3.
  2. Higher Vitamin D Levels Linked with Reduced Risk of Colon Cancer
  3. Vitamin D sufficiency, a serum 25-hydroxyvitamin D at least 30 ng/mL reduced risk for adverse clinical outcomes in patients with COVID-19 infection
  4. Vitamin D concentrations and COVID-19 infection in UK Biobank
  5. Association of Vitamin D Status and Other Clinical Characteristics With COVID-19 Test Results
  6. Vitamin D supplementation to prevent acute respiratory tract infections: systematic review and meta-analysis of individual participant data
  7. Jennifer Garner & Dr. Fauci’s Candid COVID-19 Chat