Quick answer: The 25-hydroxy vitamin D test is the best measure of your vitamin D status. Vitamin D supports bones, immunity, muscles and mood. Low levels are common, especially in winter, with darker skin, obesity, older age and limited sun. Sunlight, food and supplements can raise levels, guided by testing, since too much vitamin D can be harmful.
Key takeaways
The 25-hydroxy vitamin D test is the most useful measure of your vitamin D status.
Low levels are common in winter and with darker skin, higher body weight, older age or limited sun.
Sun, food and supplements can raise levels, and retesting matters because too much vitamin D can also cause harm.
What is Vitamin D?
Vitamin D is a fat-soluble vitamin and hormone made in the skin from sunlight and obtained from food and supplements. The 25-OH form in the blood reflects your stores.
Why Vitamin D matters
Vitamin D matters for calcium absorption and bone strength, immune balance, muscle strength, and mood and brain function.
Understanding your result
Reference ranges vary by lab, age and sex. Your provider interprets your result against the lab’s range and your overall picture, rather than chasing a single number. Functional labs are optional. They can be helpful in more complex cases.
What low Vitamin D can mean
Low vitamin D can come with limited sun exposure, darker skin, older age and obesity, malabsorption such as celiac disease, and some medications.
What high Vitamin D can mean
High vitamin D usually comes from excess supplementation, which can raise calcium and cause kidney problems.
What affects Vitamin D
Vitamin D is affected by season and latitude, skin tone and age, body weight, and gut absorption.
Ways to support healthy levels
Sensible sun exposure and foods such as fatty fish, egg yolks and fortified foods help, along with supplements guided by testing (see who should consider vitamin D3). According to the NIH Office of Dietary Supplements, the upper limit for adults is 4,000 IU a day unless your provider recommends otherwise, so discuss your dose with your provider.
Vitamin D and healthy aging
Vitamin D influences gene expression in immune and other cells and supports communication between cells. Low levels are linked with more inflammation, weaker bones and muscles, and poorer immune function as we age.
Related tests and articles: parathyroid hormone, osteoporosis prevention.
When to see a naturopathic doctor
It may be time to see a naturopathic doctor if you have fatigue, bone pain or muscle weakness, or frequent infections or low mood in winter. The same is true if you have osteoporosis risk, or if you take vitamin D and want to check your level.
See a medical provider promptly for severe muscle weakness, confusion, excessive thirst or kidney stones, which can signal very high vitamin D or calcium.
What the research shows
How common is low vitamin D? Using national NHANES data from 2005 to 2006, Forrest and Stuhldreher measured 25-hydroxyvitamin D in 4,495 US adults. Using a cutoff of 20 ng/mL or less, 41.6% were deficient, including 82.1% of Black adults and 69.2% of Hispanic adults. Deficiency was also more common with obesity, lower education and no daily milk. As a one-time survey, this study shows who tends to run low but cannot show cause and effect.
Does raising vitamin D matter? A 2017 BMJ meta-analysis by Martineau and colleagues combined individual data from 25 randomized trials with 11,321 people. Vitamin D lowered the odds of at least one acute respiratory infection by about 12% overall (odds ratio 0.88), and more in people who started below 25 nmol/L (about 10 ng/mL), with an odds ratio of 0.30. Daily or weekly doses helped, while large occasional doses did not. The trials were very different from each other, so the average effect is modest.
Form matters too. A 2012 meta-analysis by Tripkovic and colleagues pooled randomized trials that directly compared vitamin D3 with vitamin D2 in adults. D3 raised blood 25-hydroxyvitamin D levels significantly more than D2, mostly when given in large single doses. With daily dosing, the difference was no longer significant. The included trials were small and varied, and the authors noted that effects by age, sex and ethnicity still need study.
Vitamin D at a glance
| Topic | What the article explains |
|---|---|
| Why it matters | Calcium absorption and bone strength; immune balance; muscle strength; and mood and brain function |
| What can affect it | Season and latitude; skin tone and age; body weight; and gut absorption |
| Supportive habits discussed | Sensible sun exposure; foods such as fatty fish, egg yolks and fortified foods; supplements guided by testing; and according to the NIH Office of Dietary Supplements, the upper limit for adults is 4,000 IU a day unless your provider recommends otherwise; discuss your dose with your provider |
Questions to bring to your clinician
These questions can help you get the most out of your next visit.
When should I retest my vitamin D after making a change, so we know I am in a healthy range?
How should I read my vitamin D alongside the rest of my labs and my symptoms?
How often would it make sense to recheck my vitamin D in my situation?
Which daily habits or medications could be influencing my vitamin D before we retest?
Frequently asked questions
What is the best test for vitamin D?
The 25-hydroxy vitamin D blood test.
Can I get enough vitamin D from the sun?
Sometimes in summer, but many people need food or supplements, especially in winter.
Can you take too much vitamin D?
Yes. Excess can raise calcium and cause problems. Test and follow guidance.
Should I take vitamin K2 with D?
Some practitioners pair them for bone and calcium balance. Ask your provider, especially if you take warfarin.
Sources
- Forrest KY, Stuhldreher WL. Prevalence and correlates of vitamin D deficiency in US adults. Nutr Res 2011. PMID: 21310306. https://pubmed.ncbi.nlm.nih.gov/21310306/
- Martineau AR, Jolliffe DA, Hooper RL, et al. Vitamin D supplementation to prevent acute respiratory tract infections: systematic review and meta-analysis of individual participant data. BMJ 2017. PMID: 28202713. https://pubmed.ncbi.nlm.nih.gov/28202713/
- Tripkovic L, Lambert H, Hart K, et al. Comparison of vitamin D2 and vitamin D3 supplementation in raising serum 25-hydroxyvitamin D status: a systematic review and meta-analysis. Am J Clin Nutr 2012. PMID: 22552031. https://pubmed.ncbi.nlm.nih.gov/22552031/
- Marcinowska-Suchowierska E, et al. Vitamin D toxicity: a clinical perspective. Front Endocrinol (Lausanne) 2018. PMID: 30294301. https://pmc.ncbi.nlm.nih.gov/articles/PMC6158375/
- Christakos S, et al. Vitamin D and intestinal calcium absorption. Mol Cell Endocrinol 2011. PMID: 21664413. https://pmc.ncbi.nlm.nih.gov/articles/PMC3405161/
- MedlinePlus: Vitamin D Test
- NIH Office of Dietary Supplements: Vitamin D (Consumer Fact Sheet)
This article is for educational purposes only and is not medical advice. It does not replace an individual evaluation. Talk with your healthcare provider before starting supplements or changing medications, especially if you are pregnant, nursing, or managing a health condition.
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