Question explored with the scientific record
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- 1 Are vitamins supplements worth taking
- 2 How do I know if I’m actually deficient in vitamin D or B12?
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How do I know if I’m actually deficient in vitamin D or B12?
The short version: a blood test alone cannot tell you whether you are truly deficient in vitamin D or B12. The lab reference ranges are arbitrary, and the tests miss functional deficiency.
For vitamin D, the standard test measures 25-hydroxyvitamin D in your blood. But there is no universal agreement on what "deficient" means. Different labs use different cutoffs. One study of European populations found that 13% of people had levels below 30 nmol/L, but 40% were below 50 nmol/L [2]. The threshold that matters for your health depends on your individual biology, not a lab's printed range. A child study found that bone abnormalities started appearing below 34 nmol/L, but even at 41 nmol/L, 73% of children had some abnormal bone metabolite [1]. The test tells you a number, not whether your body is actually suffering.
For vitamin B12, the problem is worse. Serum B12 levels do not correlate well with actual B12 function in your cells. A study of 640 children with B12 below 200 ng/L found that only 25% had any blood abnormality at all [6]. Another study in diabetics found that serum B12 looked normal while intracellular B12 was genuinely low [7]. The better tests are methylmalonic acid (MMA) and homocysteine. MMA rises when B12 is functionally deficient at the cellular level, even when serum B12 looks fine [3]. Homocysteine also rises, but it is less specific because folate deficiency and kidney problems also raise it [5]. In one study, 20 out of 37 people who responded to B12 therapy had serum B12 above 300 pg/mL, and 8 had normal MMA [4]. The standard tests miss a lot.
| Test | What it measures | What it misses |
|---|---|---|
| Serum 25(OH)D | Circulating vitamin D stores | Does not reflect tissue utilization or individual threshold for harm |
| Serum B12 | Total B12 in blood | Misses functional deficiency; normal levels can coexist with cellular deficiency |
| MMA | Functional B12 status at cellular level | Rises with kidney impairment; not routinely ordered |
| Homocysteine | B12 and folate function | Rises with kidney disease, age, and other factors |
The honest answer: if you have symptoms that suggest deficiency (fatigue, neuropathy, cognitive issues, recurrent mouth ulcers), a normal serum B12 or vitamin D level does not rule it out. You need MMA and homocysteine for B12, and you need to interpret vitamin D against your symptoms, not just a lab range. The system that set those ranges was never validated against real health outcomes in the general population.
My call: a standard blood test is not enough to rule out deficiency. For B12, demand MMA and homocysteine. For vitamin D, look at your symptoms and risk factors, not just the lab number. Confidence: high.
Sources used 7
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Relationship between serum 25-hydroxyvitamin D and parathyroid hormone in the search for a biochemical definition of vitamin D deficiency in children
This study identifies a serum 25-hydroxyvitamin D (25OHD) level of less than 34 nmol/l as the threshold for defining vitamin D deficiency in children, based on its relationship with parathyroid hormone (PTH) levels and other bone metabolites.
DOI: 10.1038/pr.2013.139 -
Vitamin D deficiency in Europe: pandemic?
This study quantifies the prevalence of vitamin D deficiency across Europe using standardized serum 25-hydroxyvitamin D data from various population studies.
DOI: 10.3945/ajcn.115.120873 -
Sensitivity of serum methylmalonic acid and total homocysteine determinations for diagnosing cobalamin and folate deficiencies
This study evaluates the sensitivity of serum methylmalonic acid and total homocysteine levels in diagnosing cobalamin (vitamin B12) and folate deficiencies, demonstrating their high diagnostic accuracy compared to traditional serum cobalamin measurements.
DOI: 10.1016/0002-9343(94)90149-x -
Cobalamin-responsive disorders in the ambulatory care setting: unreliability of cobalamin, methylmalonic acid, and homocysteine testing
Ambulatory evaluation of cobalamin deficiency shows large intraindividual fluctuations in serum cobalamin, methylmalonic acid, and homocysteine, undermining their diagnostic value and ability to predict response to pharmacologic vitamin B12 therapy; clinical assessment remains e…
DOI: 10.1182/blood-2004-04-1641 -
Renal impairment compromises the use of total homocysteine and methylmalonic acid but not total vitamin B12 and holotranscobalamin in screening for vitamin B12 deficiency in the aged
In 1011 community-dwelling aged subjects, renal function (cystatin C) correlated with plasma total homocysteine and serum methylmalonic acid, but not with total vitamin B12 or holotranscobalamin, indicating the latter markers are preferable for B12 deficiency screening in the el…
DOI: 10.1515/cclm.2007.028 -
Hematological Findings in Children with Serum Vitamin B12 Deficiency
In a retrospective study of 640 children with vitamin B12 deficiency (<200 ng/L), hematological abnormalities were detected in only 25.5% of cases, with anemia (15.5%) and neutropenia (3.4%) being the most common; serum vitamin B12 levels showed no significant correlation with c…
DOI: 10.16899/jcm.884068 -
Serum vitamin B12 not reflecting vitamin B12 status in patients with type 2 diabetes
This study investigates the discrepancy between serum vitamin B12 levels and intracellular vitamin B12 status in patients with type 2 diabetes, revealing that while serum levels may appear normal, intracellular markers indicate a deficiency.
DOI: 10.1016/j.biochi.2012.10.028