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Vitamin D: Test, Don't Guess

The honest, test-first approach to vitamin D: find out your actual level, correct a real deficiency with sunlight and D3, and know clearly what the evidence does and does not support before you spend money chasing a number.

Not endorsed · based on the published work of Rhonda Patrick
At a glance
Time
Quarterly
Steps
7
Difficulty
Beginner
Limited 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.
The protocol

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
Why ↓

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.

USPSTF, JAMA 2021
In your stack: 25(OH)D blood test

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.
Why ↓

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.

NIH Office of Dietary Supplements; Holick et al., Endocrine Society Guideline, JCEM 2011

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
Why ↓

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.

NIH Office of Dietary Supplements (causes of deficiency)

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.
Why ↓

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.

NIH Office of Dietary Supplements; BEST-D trial, PMC5306173
In your stack: Vitamin D3 supplement (consider D3 plus K2)

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
Why ↓

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.

Mulligan & Licata, J Bone Miner Res 2010; contrast Rackham et al., PMC4882721

Recheck your 25(OH)D level about 8 to 12 weeks after starting or changing a dose

One repeat blood test
Why ↓

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.

Endocrine Society Guideline, JCEM 2011

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.
Why ↓

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.

Manson et al. (VITAL), NEJM 2019; LeBoff et al. (VITAL), NEJM 2022; Autier et al., Lancet Diabetes Endocrinol 2014
The evidence 10
Limited

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.

FoundMyFitness #083: How Vitamin D, Omega-3s & Exercise May Increase Longevity (Dr. Rhonda Patrick) Listen open.spotify.com Plays here
What's the Optimal Level of Vitamin D? (Dr. Peter Attia & Dr. Rhonda Patrick) Watch youtube.com
FoundMyFitness: a new study on vitamin D deficiency Watch instagram.com
Vitamin D: Articles, Videos & Studies (FoundMyFitness topic hub) Read foundmyfitness.com
Vitamin D: Fact Sheet for Health Professionals (NIH Office of Dietary Supplements) Read ods.od.nih.gov
Vitamin D Supplements and Prevention of Cancer and Cardiovascular Disease (Manson et al., the VITAL trial, NEJM 2019) Read nejm.org
Supplemental Vitamin D and Incident Fractures in Midlife and Older Adults (LeBoff et al., VITAL ancillary study, NEJM 2022) Read pubmed.ncbi.nlm.nih.gov
Screening for Vitamin D Deficiency in Adults: US Preventive Services Task Force Recommendation Statement (JAMA 2021) Read jamanetwork.com
Evaluation, Treatment, and Prevention of Vitamin D Deficiency: an Endocrine Society Clinical Practice Guideline (Holick et al., JCEM 2011) Read academic.oup.com
Vitamin D status and ill health: a systematic review (Autier et al., Lancet Diabetes Endocrinol, 2014) Read pubmed.ncbi.nlm.nih.gov

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
Does taking vitamin D actually prevent disease?
Not according to the largest randomized trials. The VITAL trial (25,871 healthy US adults, 2,000 IU/day for 5 years) found no reduction in cancer or cardiovascular disease incidence, and a related VITAL analysis found no significant drop in fractures. Vitamin D still matters for bone and muscle health if you are actually deficient, but it is not a proven disease-prevention pill for people with already-adequate levels.
What vitamin D level counts as deficient?
By NIH's reference range, under 12 ng/mL is deficient and 12 to under 20 is inadequate for bone health. The Endocrine Society uses a higher clinical bar for at-risk patients: under 20 ng/mL is deficient, and 30 to 60 ng/mL is the target range for sufficiency. Ask which standard your result is being read against.
Should I get tested for vitamin D even if I feel fine?
A US task force (USPSTF) says there is not enough evidence to recommend routine vitamin D screening in adults without symptoms or risk factors. If you have limited sun exposure, darker skin at a northern latitude, a condition that affects absorption, or you are about to spend money on a supplement, testing first is the more informed move.
How much vitamin D should I take?
This page gives reference points, not a personal dose: the RDA is 600 to 800 IU per day depending on age, and one randomized trial found 4,000 IU per day was needed to reliably reach a higher target range. The right dose for you depends on your tested level and should be confirmed with your doctor, especially near or above the 4,000 IU per day upper limit.
Does it matter if I take vitamin D on an empty stomach?
Possibly. One trial found taking it with your largest, fattiest meal raised blood levels by about 50% compared with fasting, but a smaller later trial did not find the same effect. Taking it with food costs nothing extra, so it is a reasonable habit even though the evidence is mixed.
Is vitamin D3 or D2 better?
D3 (cholecalciferol) is the form used in nearly all of the trial evidence cited on this page and is generally considered more effective at raising and sustaining blood levels than D2 (ergocalciferol).
25(OH)D blood test
Look for: A single blood draw, at a clinic or a direct-to-consumer lab
Vitamin D3 supplement (consider D3 plus K2)
Look for: Reference dose only, confirm with your doctor and your own blood level
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