VITAMINS & NUTRIENTS

Vitamin D and B12 are the two nutrient markers most commonly tested on blood work panels — and two of the most commonly deficient. Here is what your results mean and when supplementation makes sense.

Medically reviewed by Missy Zammichieli, DNP, APRN, FNP-BC · Updated March 2, 2026

THE BOTTOM LINE

Vitamin D and B12 are the two nutrient markers most commonly tested on blood work panels — and two of the most commonly deficient. Vitamin D deficiency affects roughly 42% of U.S. adults. B12 deficiency is especially common in vegetarians, vegans, older adults, and people on certain medications.

Both are fixable once identified, and blood work takes the guesswork out of whether you actually need to supplement.

INTRODUCTION

Vitamins and minerals are foundational to nearly every process in your body — from bone strength and immune defense to brain function and red blood cell production. Unlike cholesterol or blood sugar, which your body manufactures in large quantities, several essential vitamins and minerals must come from food, supplements, or sun exposure. When levels fall short, the effects can be subtle for months or years before symptoms appear.

Blood work offers a direct measurement of circulating nutrient levels, removing the guesswork from supplementation and dietary planning. This article focuses on the two nutrient markers most commonly tested: Vitamin D and B12. It covers what each marker measures, what published guidelines consider adequate, what drives deficiency, and how to interpret your results in context. For a deeper look at why vitamin D and micronutrients are critical bottlenecks in optimization — including dosing strategies and the science behind each nutrient — see our companion article.

CORE BIOMARKERS

Vitamin D (25-Hydroxyvitamin D)

What it measures: The 25-hydroxyvitamin D blood test measures the primary circulating form of vitamin D — the form your liver produces after vitamin D enters the body through sun exposure, food, or supplementation. It is considered the most reliable indicator of overall vitamin D status.

Why it matters: Vitamin D functions more like a hormone than a traditional vitamin. It plays well-documented roles in:

• Bone health: Vitamin D is required for calcium absorption. Prolonged deficiency leads to reduced bone density and, in severe cases, softening of the bones.
• Immune function: Vitamin D receptors are present on most immune cells. Research indicates that deficiency is associated with increased susceptibility to infection.
• Muscle function: Low vitamin D levels are associated with reduced muscle strength, impaired physical performance, and increased fall risk — particularly in older adults.
• Mood: Observational studies have identified associations between low vitamin D and depressive symptoms, though causation has not been firmly established.

How common is deficiency? A national survey found that about 42% of U.S. adults are vitamin D deficient (below 20 ng/mL). Prevalence is significantly higher among Black adults (82%) and Hispanic adults (69%) compared to white adults (30%).

Published guidelines on levels:

Classification	25(OH)D Level
Deficiency	Below 20 ng/mL
Insufficiency	21–29 ng/mL
Sufficiency	30 ng/mL and above

The Endocrine Society recommends at least 30 ng/mL. The National Academy of Medicine takes a more conservative position, concluding that 20 ng/mL meets the needs of most people for bone health, with a recommended intake of 600 IU/day for adults under 70 and 800 IU/day for adults over 70. The Endocrine Society has suggested that individuals at risk for deficiency may need 1,500–2,000 IU/day to maintain levels above 30 ng/mL.

The gap between these guidelines reflects ongoing scientific debate about optimal levels for outcomes beyond bone health.

What causes low vitamin D:

• Sun exposure: Vitamin D is made in the skin from UVB exposure. Latitude (anywhere north of roughly Atlanta), time of year, sunscreen use, and time spent indoors all reduce production.
• Skin pigmentation: Darker skin reduces UVB-driven vitamin D production, which is a primary driver of higher deficiency rates in Black and Hispanic populations.
• Obesity: Vitamin D is fat-soluble and gets stored in body fat, reducing circulating levels. People with BMI above 30 tend to have significantly lower vitamin D levels.
• Malabsorption: Conditions affecting fat absorption — including celiac disease, Crohn’s disease, and gastric bypass surgery — impair vitamin D uptake.
• Age: Aging reduces the skin’s capacity to make vitamin D.

Vitamin B12 (Cobalamin)

What it measures: A serum B12 test measures circulating cobalamin, a water-soluble vitamin essential for DNA synthesis, red blood cell formation, and neurological function.

Why it matters:

• Neurological function: B12 is required for myelin synthesis — the insulating sheath around nerve fibers. Deficiency can cause numbness and tingling in hands and feet, cognitive changes, and in severe prolonged cases, irreversible nerve damage.
• Red blood cell production: B12 deficiency produces abnormally large red blood cells, leading to a type of anemia characterized by fatigue, weakness, and shortness of breath.
• Cardiovascular relevance: B12 helps convert homocysteine to methionine. Deficiency leads to elevated homocysteine, which has been associated with cardiovascular risk.

Published reference ranges: The WHO defines serum B12 below 203 pg/mL as deficient. Most labs use a range of approximately 200–900 pg/mL, though deficiency symptoms can occur even within the low-normal range. Additional testing (methylmalonic acid, homocysteine) may be warranted when levels fall between 200–400 pg/mL.

What causes low B12:

• Dietary insufficiency: B12 is found almost exclusively in animal products (meat, fish, dairy, eggs). Vegan diets contain virtually no natural B12. Research indicates that up to 86% of vegans have low B12 levels.
• Pernicious anemia: An autoimmune condition where the body attacks intrinsic factor, the protein required for B12 absorption. This is the most common cause of severe B12 deficiency in older adults.
• Medication use: Metformin (commonly prescribed for type 2 diabetes) reduces B12 absorption — studies show a 2.4-fold increased risk of deficiency. Proton pump inhibitors (PPIs) and H2 blockers reduce the stomach acid needed to release B12 from food.
• Age-related malabsorption: An estimated 10–30% of adults over 50 have reduced ability to absorb food-bound B12 due to lower stomach acid production.

ADDITIONAL NUTRITIONAL MARKERS

Some panels include additional nutrient markers:

Folate (Vitamin B9)

Works closely with B12 in DNA synthesis and red blood cell formation. Deficiency can independently cause anemia and is a well-established risk factor for neural tube defects during pregnancy.

Iron, Ferritin, and TIBC

If your panel includes iron markers, these show whether your body has enough iron stored and available. Iron deficiency is common in active populations. One important note: ferritin also rises with inflammation, so your provider may look at it alongside hs-CRP to get an accurate picture.

Magnesium

Involved in over 300 enzymatic reactions — muscle contraction, nerve function, blood sugar regulation, and bone health. Survey data suggest that roughly half the U.S. population consumes less than the recommended daily amount.

WHAT AFFECTS YOUR RESULTS

Several factors can shift nutrient marker results:

• Season: Vitamin D levels are typically highest in late summer and lowest in late winter. A result drawn in February may look different from one drawn in August.
• Diet and supplementation: B12 levels respond to intake and supplementation. Vitamin D supplementation typically takes 6–8 weeks to reach steady-state levels in blood.
• Medications: Metformin, PPIs, and certain anticonvulsants can reduce absorption of B12 or vitamin D. Providing a medication list to your provider supports accurate interpretation.
• Absorption conditions: Celiac disease, inflammatory bowel disease, and gastric surgery can impair absorption of multiple nutrients at once.
• Body composition: Higher body fat is associated with lower circulating vitamin D. Weight changes can shift levels independent of intake.
• Supplement timing: Taking a B12 supplement shortly before a blood draw can spike levels, potentially masking a tissue-level deficiency. Some providers recommend testing before morning supplementation.

COMMON QUESTIONS

“Is my vitamin D level good enough?”

That depends on which guideline your provider follows. The National Academy of Medicine considers 20 ng/mL sufficient for the general population based on bone health data. The Endocrine Society recommends at least 30 ng/mL, particularly for individuals at risk of deficiency. Both agree that levels below 20 ng/mL constitute deficiency. If your level falls in the 20–30 ng/mL range, your provider can help interpret it based on your individual risk factors.

“Can I take too much vitamin D?”

Yes. Because vitamin D is fat-soluble, it can accumulate. Toxicity — while rare from normal supplementation — can occur with sustained high doses, leading to elevated blood calcium with symptoms including nausea, kidney stones, and kidney damage. The tolerable upper limit is set at 4,000 IU/day for adults, though some individuals under medical supervision may safely take more. Blood testing is the most reliable way to guide dosing.

“Why is B12 important for vegetarians?”

B12 is naturally present almost exclusively in animal-derived foods. Individuals following vegetarian and especially vegan diets are at substantially elevated risk of deficiency without supplementation or fortified food intake. Because prolonged B12 deficiency can cause irreversible nerve damage, guidelines from multiple organizations recommend that vegans supplement B12 or consume B12-fortified foods regularly.

References

1. Holick MF, Binkley NC, Bischoff-Ferrari HA, et al. Evaluation, treatment, and prevention of vitamin D deficiency: an Endocrine Society clinical practice guideline. Journal of Clinical Endocrinology & Metabolism. 2011;96(7):1911-1930.
2. Aranow C. Vitamin D and the immune system. Journal of Investigative Medicine. 2011;59(6):881-886.
3. Bischoff-Ferrari HA, Dawson-Hughes B, Staehelin HB, et al. Fall prevention with supplemental and active forms of vitamin D: a meta-analysis of randomised controlled trials. BMJ. 2009;339:b3692.
4. Anglin RE, Samaan Z, Walter SD, McDonald SD. Vitamin D deficiency and depression in adults: systematic review and meta-analysis. British Journal of Psychiatry. 2013;202:100-107.
5. Forrest KY, Stuhldreher WL. Prevalence and correlates of vitamin D deficiency in US adults. Nutrition Research. 2011;31(1):48-54.
6. Institute of Medicine. Dietary Reference Intakes for Calcium and Vitamin D. Washington, DC: The National Academies Press; 2011.
7. Stabler SP. Vitamin B12 deficiency. New England Journal of Medicine. 2013;368(2):149-160.
8. Green R, Allen LH, Bjorke-Monsen AL, et al. Vitamin B12 deficiency. Nature Reviews Disease Primers. 2017;3:17040.
9. World Health Organization. Serum and red blood cell folate concentrations for assessing folate status in populations. Vitamin and Mineral Nutrition Information System. Geneva: WHO; 2015.
10. Pawlak R, Parrott SJ, Raj S, Cullum-Dugan D, Lucus D. How prevalent is vitamin B12 deficiency among vegetarians? Nutrition Reviews. 2013;71(2):110-117.
11. Aroda VR, Edelstein SL, Goldberg RB, et al. Long-term metformin use and vitamin B12 deficiency in the Diabetes Prevention Program Outcomes Study. Journal of Clinical Endocrinology & Metabolism. 2016;101(4):1754-1761.
12. Camaschella C. Iron-deficiency anemia. New England Journal of Medicine. 2015;372(19):1832-1843.
13. Rosanoff A, Weaver CM, Rude RK. Suboptimal magnesium status in the United States: are the health consequences underestimated? Nutrition Reviews. 2012;70(3):153-164.

This article is for informational purposes only and does not constitute medical advice. Blood work results should be interpreted by a qualified healthcare provider in the context of your complete medical history, medications, and clinical presentation.

UNDERSTAND YOUR NUMBERS

Stop guessing about supplementation. A blood panel shows exactly where your nutrient levels stand — so you can make informed decisions about what your body actually needs.

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