A vitamin D level of 21 ng/mL is considered low. Low levels of vitamin D can occur from prolonged, inadequate intake through diet, limited exposure to sunlight, poor kidney function resulting in impaired conversion of 25(OH)D, or inadequate vitamin D absorption.
Low vitamin D can negatively affect your health and put you at increased risk for, and severity of COVID-19, diabetes, and cancer. Having low levels of vitamin D may also increase your risk for certain autoimmune and neurodegenerative diseases like multiple sclerosis and Alzheimer’s.
Diet low in vitamin D
Older age (partly due to a decline in the skin’s ability to synthesize vitamin D with age)
Living in cold or northern climates (people who live above 37°N latitude cannot synthesize vitamin D year-round)
Limited sun exposure (such as those who spend the majority of their time indoors or are covered when outside)
Pregnancy and breastfeeding
Cystic fibrosis, Crohn's, celiac, kidney, and liver disease, or other conditions that interfere with vitamin D absorption or synthesis
Obesity (greater amounts of subcutaneous fat sequester more of the vitamin)
Gastric bypass surgery
To improve vitamin D levels:
Increase the amount of vitamin D-rich foods you eat. Good sources include salmon, herring, canned tuna, eggs (with the yolk), and fortified foods like milk, yogurt, and breakfast cereals
Get 10–30 minutes of unprotected midday sunlight most days
Take a vitamin D supplement. How much you should take depends on your level of deficiency. For a level of 21, you will likely need to take 5,000 IU daily for several months to significantly improve your vitamin D status.
If levels do not improve after 3 months, review your supplements with an expert or talk to your doctor.
If you are vitamin D deficient, a daily supplement is typically needed to get levels into the optimal range (40-80 ng/mL) over time.
For levels of 21 ng/mL, talk to your healthcare provider. Daily supplementation of vitamin D3 (cholecalciferol) should be considered to achieve adequate body stores of vitamin D.
National Institutes of Health. (2021, March 26). Office of Dietary Supplements - Vitamin D. National Institutes of Health – Office of Dietary Supplements. https://ods.od.nih.gov/factsheets/VitaminD-HealthProfessional/
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