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.
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. This marker is part of the Healthspan Assessment panel available to my 1-on-1 patients.
Why Vitamin D matters
- Calcium absorption and bone strength
- Immune balance
- Muscle strength
- 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, and in my practice they are available only to patients working 1-on-1 with me.
What low Vitamin D can mean
- Limited sun exposure
- Darker skin
- Older age and obesity
- Malabsorption, such as celiac disease
- Some medications
What high Vitamin D can mean
- Excess supplementation, which can raise calcium and cause kidney problems
What affects Vitamin D
- Season and latitude
- Skin tone and age
- Body weight
- Gut absorption
Ways to support healthy levels
- Sensible sun exposure
- Foods such as fatty fish, egg yolks and fortified foods
- 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; 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
- Fatigue, bone pain or muscle weakness
- Frequent infections or low mood in winter
- Osteoporosis risk
- 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.
Want help making sense of your labs?
A free 15-minute consultation with Dr. Kneessi. No cost, no obligation. In-office in Odenton, MD or by telehealth.
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/
- 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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