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Vitamin D Test: Fasting or Not? What the Science Says

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At a glance

  • Test / Serum 25-hydroxyvitamin D, 25(OH)D [2]
  • Units / ng/mL or nmol/L; 1 ng/mL = 2.5 nmol/L [2]
  • Meal-timing trial / 62 healthy older adults; monthly 50,000 IU vitamin D3 taken with a high-fat meal, a low-fat meal, or no meal over 90 days [1]
  • 25(OH)D at 30 and 90 days / No significant difference between the three meal groups [1]
  • 12-hour D3 rise / 241 nmol/L with the low-fat meal vs 207 high-fat and 201 no meal [1]
  • Population requirement / 20 ng/mL (50 nmol/L) covers at least 97.5% of people (IOM 2011) [2]
  • 2024 Endocrine Society / No longer endorses 30 ng/mL or specific sufficiency cutoffs [3]

What Does the 25-OH Vitamin D Test Measure?

The test measures serum 25-hydroxyvitamin D, also called 25(OH)D or calcidiol. Results are reported in ng/mL or nmol/L, and 1 ng/mL equals 2.5 nmol/L; 20 ng/mL is the same as 50 nmol/L [2].

Does Fasting Change the Result?

The closest test of the question is a 90-day randomized trial of 62 healthy older adults. Each person received monthly 50,000 IU vitamin D3 doses taken with a high-fat meal, a low-fat meal, or no meal [1]. That design tested food eaten with the vitamin D dose, not food eaten before the blood draw, so it does not settle whether a meal before the draw shifts the number.

What the Meal-Timing Trial Found

Meal fat changed short-term absorption. The mean 12-hour rise in vitamin D3 was 200.9 nmol/L with no meal, 207.4 nmol/L with a high-fat meal, and 241.1 nmol/L with a low-fat meal; the low-fat group was significantly higher than the other two groups (P=0.038) [1]. That is a measure of absorbed D3, not the 25(OH)D value used to judge vitamin D status.

By 30 and 90 days, the three groups did not differ significantly in their 25(OH)D increases [1].

Does Taking Vitamin D With Food Matter?

Dietary fat is not required to raise 25(OH)D [1]. In the same trial, a vitamin D3 dose taken with no meal produced 25(OH)D increases at 30 and 90 days that were not significantly different from doses taken with a high-fat or low-fat meal [1].

The trial used monthly 50,000 IU vitamin D3 doses in healthy older adults over 90 days, so it speaks to that dose, form and schedule rather than every vitamin D product [1].

How to Read the Number

The 2024 Endocrine Society guideline says the society no longer endorses its 2011 target of 30 ng/mL (75 nmol/L) and no longer endorses specific 25(OH)D levels to define sufficiency, insufficiency and deficiency [3][5]. It also says there is no clear evidence defining an optimal level for disease prevention [3].

The Institute of Medicine 2011 report used 20 ng/mL (50 nmol/L) as the level covering at least 97.5% of the population [2]. It said levels above 30 ng/mL were not consistently associated with more benefit and that levels above 50 ng/mL should raise concern about potential adverse effects [6]. The CDC NHANES analysis uses below 30 nmol/L (about 12 ng/mL) as at risk of deficiency and 30 to 49 nmol/L as at risk of inadequacy [7]. The USPSTF 2021 statement notes that no single serum cutpoint defines deficiency [8].

Does Everyone Need This Test?

The 2024 Endocrine Society guideline suggests against routine 25(OH)D testing in all the populations it considered, including people with obesity or dark skin, when there is no established indication for testing [3]. The USPSTF 2021 found the evidence insufficient to assess screening asymptomatic, nonpregnant adults for vitamin D deficiency (I statement) [8].

Frequently asked questions

Do I need to fast before a 25-OH vitamin D test?
Fasting has not been shown to change the result. In a 90-day trial of 62 healthy older adults taking monthly 50,000 IU vitamin D3 with a high-fat meal, a low-fat meal or no meal, the groups had no significant difference in 25(OH)D increases at 30 or 90 days [1].
Did a high-fat meal work better than a low-fat meal with vitamin D3?
No. Short-term D3 absorption over 12 hours was highest with the low-fat meal, but after 30 and 90 days the groups had no significant difference in 25(OH)D increases [1].
Does vitamin D need to be taken with food?
Dietary fat is not required to raise 25(OH)D. In the trial, 25(OH)D increases at 30 and 90 days did not differ significantly between the no-meal, high-fat-meal and low-fat-meal groups [1].
Is 30 ng/mL the target for vitamin D?
The 2024 Endocrine Society guideline no longer endorses the 2011 target of 30 ng/mL or specific cutoffs for sufficiency, insufficiency and deficiency [3][5]. The IOM 2011 report used 20 ng/mL (50 nmol/L) as the level covering at least 97.5% of the population [2].

References

  1. Dawson-Hughes B, Harris SS, Palermo NJ, Ceglia L, Rasmussen H. Meal conditions affect the absorption of supplemental vitamin D3 but not the plasma 25-hydroxyvitamin D response to supplementation. J Bone Miner Res. 2013;28(8):1778-83. Meal conditions affect the absorption of supplemental vitamin D3 but not the plasma 25-hydroxyvitamin D response to supplementation
  2. Ross AC, Manson JE, Abrams SA, Aloia JF, Brannon PM, Clinton SK, et al. The 2011 report on dietary reference intakes for calcium and vitamin D from the Institute of Medicine: what clinicians need to know. J Clin Endocrinol Metab. 2011;96(1):53-8. The 2011 report on dietary reference intakes for calcium and vitamin D from the Institute of Medicine: what clinicians need to know
  3. Demay MB, Pittas AG, Bikle DD, Diab DL, Kiely ME, Lazaretti-Castro M, et al. Vitamin D for the Prevention of Disease: An Endocrine Society Clinical Practice Guideline. J Clin Endocrinol Metab. 2024;109(8):1907-1947. Vitamin D for the Prevention of Disease: An Endocrine Society Clinical Practice Guideline
  4. Tripkovic L, Lambert H, Hart K, Smith CP, Bucca G, Penson S, 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;95(6):1357-64. Comparison of vitamin D2 and vitamin D3 supplementation in raising serum 25-hydroxyvitamin D status: a systematic review and meta-analysis
  5. Official web page. Official web page
  6. The 2011 Report on Dietary Reference Intakes for Calcium and Vitamin D from the Institute of Medicine: What Clinicians Need to Know - PMC. The 2011 Report on Dietary Reference Intakes for Calcium and Vitamin D from the Institute of Medicine: What Clinicians Need to Know - PMC
  7. Herrick KA, Storandt RJ, Afful J, Pfeiffer CM, Schleicher RL, Gahche JJ, et al. Vitamin D status in the United States, 2011-2014. Am J Clin Nutr. 2019;110(1):150-157. Vitamin D status in the United States, 2011-2014
  8. US Preventive Services Task Force, Krist AH, Davidson KW, Mangione CM, Cabana M, Caughey AB, et al. Screening for Vitamin D Deficiency in Adults: US Preventive Services Task Force Recommendation Statement. JAMA. 2021;325(14):1436-1442. Screening for Vitamin D Deficiency in Adults: US Preventive Services Task Force Recommendation Statement
  9. Marcinowska-Suchowierska E, Kupisz-Urbańska M, Łukaszkiewicz J, Płudowski P, Jones G. Vitamin D Toxicity-A Clinical Perspective. Front Endocrinol (Lausanne). 2018;9:550. Vitamin D Toxicity-A Clinical Perspective
  10. Araki T, Holick MF, Alfonso BD, Charlap E, Romero CM, Rizk D, et al. Vitamin D intoxication with severe hypercalcemia due to manufacturing and labeling errors of two dietary supplements made in the United States. J Clin Endocrinol Metab. 2011;96(12):3603-8. Vitamin D intoxication with severe hypercalcemia due to manufacturing and labeling errors of two dietary supplements made in the United States
  11. U.S. National Library of Medicine, DailyMed. Ergocalciferol capsules (Rx) prescribing information. Ergocalciferol capsules (Rx) label
Evidence overview for Vitamin D Test: Fasting or Not? What the Science Says