Vitamin D Deficiency: Signs, Testing, and Treatment

Why is vitamin D deficiency so common?

Vitamin D is produced in the skin when it is exposed to UV-B light from the sun. Most people in temperate climates, those who spend most of the day indoors, and those with darker skin tones — which requires more sun exposure to produce the same amount of vitamin D — are at higher risk of getting too little. Diet contributes a smaller portion: fatty fish, egg yolks, and fortified dairy or plant milks are the main food sources, but it is difficult to maintain adequate vitamin D from diet alone.

Analysis of NHANES data from 2001 to 2018 found that approximately 22% of U.S. adults had moderate vitamin D deficiency (serum 25-hydroxyvitamin D below 50 nmol/L), and an additional 41% were insufficient, with non-Hispanic Black Americans bearing a disproportionate burden 1Ref 1Cui A, Xiao P, Ma Y, Fan Z, Zhou F, Zheng J, Zhang L (2022).Prevalence, trend, and predictor analyses of vitamin D deficiency in the US population, 2001–2018.NHANES analysis (71,685 subjects, 2001–2018): 22% moderate deficiency, 41% insufficiency; elevated burden in non-Hispanic Black Americans and women.. A separate analysis of 2011–2014 data found 5% of Americans at risk of deficiency and 18% at risk of inadequacy, underscoring how consistently low vitamin D status appears across population samples 2Ref 2Herrick KA, Storandt RJ, Afful J, Pfeiffer CM, Schleicher RL, Gahche JJ, Potischman N (2019).Vitamin D status in the United States, 2011–2014.NHANES 2011–2014 (n=16,180): 5.0% of Americans at risk of vitamin D deficiency, 18.3% at risk of inadequacy; highest deficiency risk in non-Hispanic Black individuals (17.5%)..

What are the symptoms of low vitamin D?

Vitamin D deficiency can be completely asymptomatic, especially when levels are only mildly low. When symptoms do appear, they tend to be non-specific:

• Fatigue and low energy — one of the most commonly reported symptoms. Research has found associations between low vitamin D and fatigue, and correcting deficiency may help in people with documented low levels 5Ref 5Di Molfetta IV, Bordoni L, Gabbianelli R, Sagratini G, Alessandroni L (2024).Vitamin D and Its Role on the Fatigue Mitigation: A Narrative Review.Evidence for vitamin D’s role in mitochondrial function and fatigue pathways; association between low vitamin D and fatigue symptoms; correcting deficiency associated with fatigue improvement.
• Bone aches — vitamin D is essential for calcium absorption and bone mineralization; severe deficiency causes softening of the bones (osteomalacia in adults)
• Muscle weakness or cramps
• Low mood — associations between low vitamin D and depression have been studied, though the causal relationship is not fully established
• Frequent infections — vitamin D plays a role in immune regulation

These symptoms are shared by many conditions including iron deficiency anemia, thyroid problems, and poor sleep. A blood test is the only reliable way to distinguish vitamin D deficiency from other causes.

What blood test is used, and what do the levels mean?

The test is called 25-hydroxyvitamin D (often written as 25(OH)D). It measures the storage form of vitamin D in the bloodstream.

The Endocrine Society's clinical practice guideline defines serum thresholds as follows 3Ref 3Holick MF, Binkley NC, Bischoff-Ferrari HA, Gordon CM, Hanley DA, Heaney RP, Murad MH, Weaver CM (2011).Evaluation, Treatment, and Prevention of Vitamin D Deficiency: an Endocrine Society Clinical Practice Guideline.Defines serum 25(OH)D thresholds: deficiency <20 ng/mL, insufficiency 21–29 ng/mL, sufficiency 30–100 ng/mL; provides treatment dosage recommendations.:

| Level | Typical interpretation | |---|---| | Below 20 ng/mL | Deficiency — most guidelines recommend treatment | | 21–29 ng/mL | Insufficiency — whether to treat is more nuanced | | 30–100 ng/mL | Sufficient for most adults | | Above 100–150 ng/mL | Potentially too high (risk of toxicity with supplementation) |

The Endocrine Society and other organizations differ slightly on exact cut-offs. Most clinicians aim to keep levels in the 30–60 ng/mL range for general health, though bone health guidelines may use higher targets for specific populations.

How much vitamin D do most adults need?

Vitamin D3 (cholecalciferol) is the preferred supplemental form. A 2021 systematic review and meta-analysis of 24 studies found that D3 raised serum 25(OH)D levels approximately 15.7 nmol/L more than D2 (ergocalciferol), with consistent advantages across dosage levels and demographics 4Ref 4Balachandar R, Pullakhandam R, Kulkarni B, Sachdev HS (2021).Relative Efficacy of Vitamin D2 and Vitamin D3 in Improving Vitamin D Status: Systematic Review and Meta-Analysis.Meta-analysis of 24 studies (n=1,277): D3 (cholecalciferol) raised serum 25(OH)D by approximately 15.7 nmol/L more than D2 (ergocalciferol) and more effectively reduced PTH levels..

General guidance from the NIH Office of Dietary Supplements: - The U.S. RDA for adults up to age 70 is 600 IU per day; for adults over 70, it is 800 IU per day - The Tolerable Upper Limit for most adults is 4,000 IU per day from supplementation (not food) - When treating confirmed deficiency, clinicians often prescribe higher doses temporarily — sometimes 2,000–4,000 IU daily for adults, or a weekly high-dose regimen — to rebuild stores before dropping to a maintenance dose

Dose should be guided by your blood level, not by a standard number. Someone with severe deficiency needs more than someone whose level is borderline low. Over-supplementing can cause toxicity — a rare but real risk — characterized by elevated blood calcium, nausea, confusion, and kidney problems.

Vitamin D is fat-soluble, so taking it with a meal that contains some fat improves absorption.

Can vitamin D help with fatigue specifically?

A 2024 narrative review found evidence that vitamin D plays a role in mitochondrial function, oxidative stress, and the pathways involved in fatigue 5Ref 5Di Molfetta IV, Bordoni L, Gabbianelli R, Sagratini G, Alessandroni L (2024).Vitamin D and Its Role on the Fatigue Mitigation: A Narrative Review.Evidence for vitamin D’s role in mitochondrial function and fatigue pathways; association between low vitamin D and fatigue symptoms; correcting deficiency associated with fatigue improvement.. Correcting deficiency in people with documented low levels has been associated with improvements in reported fatigue. However, giving vitamin D to people with already-adequate levels does not appear to reduce fatigue.

This distinction matters: supplementing when your level is fine is unlikely to boost energy. Confirming your level first prevents both unnecessary supplementation and missed deficiency.

Who should be routinely tested?

Vitamin D testing is not currently recommended as a universal screening test for all adults. The U.S. Preventive Services Task Force issued an “I” (insufficient evidence) grade in 2021, concluding that evidence is lacking to determine whether screening asymptomatic adults leads to better health outcomes 6Ref 6US Preventive Services Task Force (2021).Vitamin D Deficiency in Adults: Screening — Final Recommendation Statement.Grade I (insufficient evidence): USPSTF concluded evidence is lacking to determine whether screening asymptomatic adults for vitamin D deficiency improves health outcomes (April 2021).. However, testing is reasonable for people who:

• Have symptoms suggesting deficiency (fatigue, bone pain, muscle weakness)
• Have limited sun exposure or live at high latitudes
• Have a condition affecting fat absorption (Crohn’s disease, celiac disease, gastric bypass)
• Have osteoporosis or are at high risk for bone fractures
• Are taking medications that interfere with vitamin D metabolism (some anticonvulsants, glucocorticoids)
• Are older adults, especially those in care settings

A Gale primary care clinician can decide whether testing makes sense for you based on your specific situation.