Vitamin D: Test, Don't Guess
Low vitamin D is genuinely linked to worse bone, muscle and immune outcomes, and about 1 in 4 people in the US have levels too low for bone health. But the large randomized trials tell a more sobering story: correcting a low level with a supplement has not been shown to prevent cancer, heart disease or fractures in people who are not already deficient. The honest move is to test your actual blood level, not guess or blanket-dose, and act on the number with your doctor.
Get a 25(OH)D blood test A standard 25-hydroxyvitamin D blood draw, ask for it at your next physical or order one directly if it is not routine at your clinic
Routine population screening for vitamin D is not currently recommended for people without symptoms or risk factors (USPSTF), but if you have a real reason to check, limited sun exposure, darker skin at a northern latitude, malabsorption, bone or muscle symptoms, testing is the only way to know your actual number instead of guessing.
Compare your result to both reference standards Under 12 ng/mL: deficient. 12 to under 20: inadequate for bone health (NIH). The Endocrine Society's clinical guideline sets a higher bar for at-risk patients: under 20 ng/mL deficient, 30 ng/mL minimum for sufficiency, 40 to 60 ng/mL to comfortably clear that bar. Above 50 ng/mL, watch for potential adverse effects.
Two respected US bodies use different cutoffs, which is why the same number can be called normal by one doctor and low by another; know which standard your result is being read against.
Get 10 to 30 minutes of midday sun on bare skin, several times a week Shorter for lighter skin, longer for darker skin or higher latitudes; skip this or use sun protection instead if you burn easily or have a personal or family history of skin cancer
Skin makes vitamin D from UVB exposure; it is free and, for people with reasonable sun access, often enough on its own to close a mild gap.
Supplement D3, not D2, and consider pairing it with K2 Reference points only, not a personal dose: the RDA is 600 IU/day for ages 14 to 70 and 800 IU/day over 70; a randomized trial (BEST-D) found 4,000 IU/day was needed to reliably reach the 40 ng/mL+ range some clinicians target; the upper limit for adults is 4,000 IU/day without medical supervision. Confirm your own dose with your doctor.
D3 (cholecalciferol) is the form used in essentially all the trial evidence cited here; going above the upper limit without medical supervision is not something this page recommends.
Take your D3 dose with the largest meal of the day, ideally one with some fat in it No extra cost or effort, just timing
One trial found taking vitamin D with the largest meal raised serum levels by about 50% versus fasting; a later, smaller trial did not replicate the effect, so treat this as a free, no-downside habit rather than a guaranteed fix.
Recheck your 25(OH)D level about 8 to 12 weeks after starting or changing a dose One repeat blood test
Vitamin D moves slowly; retesting too early will not reflect your new dose, and retesting is the only way to know if what you are doing is actually working.
Know what correcting a deficiency will, and will not, do Fixing a genuine deficiency supports bone and muscle health. In large randomized trials of generally healthy adults, vitamin D supplementation has not been shown to lower rates of cancer, heart disease or fractures, and a US task force says there is not enough evidence to recommend routinely screening for it in people without symptoms.
This is the central, honest tension in the vitamin D evidence: the association between low levels and poor health is real and repeatedly observed, but large randomized trials correcting that low level in already-healthy people have mostly failed to move hard outcomes.
This protocol works from your actual levels. Testing partner coming soon.
- 25(OH)D blood test
We may earn a referral fee if you book testing through a future partner link; it will never affect which tests are listed here. This is not medical advice or a diagnosis.
The case for testing is straightforward: about 1 in 4 people in the US carry levels too low for bone and overall health, and correcting a genuine deficiency supports bone density and muscle function, both well established.
View sources
FoundMyFitness #083: How Vitamin D, Omega-3s & Exercise May Increase Longevity (Dr. Rhonda Patrick)
What's the Optimal Level of Vitamin D? (Dr. Peter Attia & Dr. Rhonda Patrick)
FoundMyFitness: a new study on vitamin D deficiency
Vitamin D: Articles, Videos & Studies (FoundMyFitness topic hub)
Vitamin D: Fact Sheet for Health Professionals (NIH Office of Dietary Supplements)
Vitamin D Supplements and Prevention of Cancer and Cardiovascular Disease (Manson et al., the VITAL trial, NEJM 2019)
Supplemental Vitamin D and Incident Fractures in Midlife and Older Adults (LeBoff et al., VITAL ancillary study, NEJM 2022)
Screening for Vitamin D Deficiency in Adults: US Preventive Services Task Force Recommendation Statement (JAMA 2021)
Evaluation, Treatment, and Prevention of Vitamin D Deficiency: an Endocrine Society Clinical Practice Guideline (Holick et al., JCEM 2011)
Vitamin D status and ill health: a systematic review (Autier et al., Lancet Diabetes Endocrinol, 2014)
Not medical advice. This page is for education only and is not a substitute for professional medical care. Consult a qualified clinician before changing your health routine.
Independent curation. YourProtocol is an independent platform. This protocol is based on the publicly available work of Rhonda Patrick and is not created, reviewed, endorsed by, or affiliated with Rhonda Patrick or PhD, FoundMyFitness.
Is this for you
- Anyone told their vitamin D is 'a little low' and not sure if it matters
- People with limited sun exposure (indoor jobs, northern winters, consistent coverage)
- Darker skin tones at northern latitudes, who need more sun time for the same synthesis
- Anyone about to buy a vitamin D supplement 'just in case' who has never actually tested
Cautions
- This page is intentionally cautious: correcting a real deficiency is well supported for bone and muscle health, but taking vitamin D broadly to prevent cancer, heart disease or fractures is not supported by the largest randomized trials to date; do not expect it to do more than the evidence shows
- Vitamin D toxicity is rare but real at very high, sustained doses, well above the 4,000 IU/day adult upper limit; doses above that require medical supervision, especially alongside calcium supplements or kidney disease
- If you are pregnant, breastfeeding, have kidney disease, sarcoidosis, or another condition that affects calcium or vitamin D metabolism, talk to your doctor before supplementing
- If you take thiazide diuretics, digoxin, or calcium supplements, tell your doctor before starting a vitamin D supplement, since these can interact.
- Educational only, not medical advice
Common questions
Some links earn us a commission. It never affects our picks or grades. See our affiliate disclosure.