10.4 Vitamins, Nutritional Assessment & Deficiency States
Key Takeaways
- Vitamins and Nutrition is one of the four sub-areas of Special Chemistry (15% to 20% of the C examination), alongside endocrinology, therapeutic drug monitoring, and toxicology.
- 25-hydroxyvitamin D, not 1,25-dihydroxyvitamin D, is the analyte for assessing vitamin D nutritional status because its half-life is 2 to 3 weeks; LC-MS/MS resolves D2, D3, and the interfering 3-epi-25-hydroxyvitamin D3.
- Methylmalonic acid is elevated only in B12 deficiency while homocysteine is elevated in both B12 and folate deficiency, which is the decisive laboratory discrimination because folate alone corrects the anemia while B12 neuropathy progresses.
- Serum vitamin E must be interpreted as a ratio to total serum lipids because tocopherol is transported on lipoproteins, and vitamin K deficiency prolongs the PT/INR first because factor VII has the shortest half-life.
- Prealbumin (half-life about 2 days) tracks short-term nutritional repletion far better than albumin (about 3 weeks), but all nutritional proteins are negative acute-phase reactants and must be interpreted with CRP.
10.4 Vitamins, Nutritional Assessment & Deficiency States
[!NOTE] Blueprint placement: Special Chemistry (15% to 20% of the examination) has exactly four sub-areas — Endocrinology, Vitamins and Nutrition, Therapeutic Drug Monitoring, and Toxicology. Vitamins are therefore roughly a quarter of that domain, and "hormones/vitamins" is also one of the 15 procedure areas the BOC accepts as documented clinical chemistry experience. Expect biochemical role, deficiency syndrome, specimen handling, and assay interference — in that order of emphasis.
Fat-Soluble Vitamins (A, D, E, K)
Fat-soluble vitamins are absorbed with dietary lipid and require bile salts and pancreatic lipase. Any cause of fat malabsorption — cystic fibrosis, cholestasis, celiac disease, pancreatic insufficiency, bariatric surgery, abetalipoproteinemia — produces a combined A/D/E/K deficiency. They are stored in liver and adipose tissue, which is why they are the vitamins that cause toxicity.
Vitamin A (Retinol)
- Transport: Retinol circulates bound to retinol-binding protein (RBP), which travels as a complex with transthyretin (prealbumin). That coupling is why RBP and prealbumin fall together in protein-energy malnutrition, and why RBP is retained in renal failure (falsely raising apparent vitamin A status).
- Deficiency: Night blindness is the earliest sign, followed by conjunctival xerosis, Bitot spots, and keratomalacia; also follicular hyperkeratosis and impaired immunity.
- Toxicity: Acute — nausea, vomiting, and pseudotumor cerebri (raised intracranial pressure). Chronic — hepatotoxicity, alopecia, bone pain. Retinoids are teratogenic.
- Measurement: HPLC or LC-MS/MS on serum, protected from light and promptly frozen. Typical adult serum retinol is about 30 to 80 ug/dL.
Vitamin D
Section 9.1 covers the endocrine control of calcium; here the point is analytical. 25-hydroxyvitamin D is the assessment analyte because its half-life is 2 to 3 weeks and it reflects total supply from diet, supplements, and cutaneous synthesis. 1,25-dihydroxyvitamin D has a half-life of hours, is tightly regulated by PTH, and is ordered only for specific problems (renal disease, granulomatous hypercalcemia, hereditary rickets) — ordering it as a nutritional screen is a classic error.
- LC-MS/MS is the reference method because it separately quantifies vitamin D2 (ergocalciferol) and D3 (cholecalciferol) and resolves the 3-epi-25-hydroxyvitamin D3 isomer that inflates immunoassay results, especially in infants.
- Widely used interpretive thresholds are deficiency below 20 ng/mL and insufficiency 20 to 30 ng/mL; thresholds differ between the Endocrine Society and the Institute of Medicine, so report against your laboratory's stated criteria.
Vitamin E (Alpha-Tocopherol)
- Role: The principal lipid-phase antioxidant, protecting polyunsaturated fatty acids in membranes from peroxidation.
- Deficiency: Hemolytic anemia in premature infants; in older children and adults, a spinocerebellar syndrome with ataxia, areflexia, and peripheral neuropathy, seen in abetalipoproteinemia and chronic cholestasis.
- Interpretation trap: Tocopherol is carried on lipoproteins, so serum concentration tracks serum lipids. In hyperlipidemia a "normal" tocopherol may represent functional deficiency and in hypolipidemia a low value may be spurious. Report the tocopherol-to-total-lipid (or to cholesterol plus triglyceride) ratio when lipids are abnormal.
Vitamin K
- Role: Cofactor for the gamma-glutamyl carboxylase that carboxylates glutamate residues on factors II, VII, IX, and X and proteins C and S, creating the calcium-binding Gla domains required for coagulation.
- Deficiency: Prolonged PT/INR first (factor VII has the shortest half-life), then prolonged aPTT. Causes include newborn status (hemorrhagic disease of the newborn — hence the prophylactic injection), prolonged broad-spectrum antibiotics, fat malabsorption, and warfarin, which inhibits vitamin K epoxide reductase.
- Marker: PIVKA-II (protein induced by vitamin K absence, or des-gamma-carboxy prothrombin) is a sensitive marker of functional vitamin K deficiency and is also used as a tumor marker for hepatocellular carcinoma.
Water-Soluble Vitamins
Water-soluble vitamins are minimally stored (B12 is the great exception, with hepatic stores lasting years), are excreted in urine, and rarely cause toxicity — with the important exceptions of pyridoxine neuropathy and biotin's analytical interference.
| Vitamin | Active Form / Role | Deficiency Syndrome | Laboratory Notes |
|---|---|---|---|
| B1 (thiamine) | Thiamine pyrophosphate: pyruvate dehydrogenase, alpha-ketoglutarate dehydrogenase, transketolase | Wet and dry beriberi; Wernicke-Korsakoff; high-anion-gap lactic acidosis | Measure whole-blood thiamine diphosphate by HPLC, not serum; erythrocyte transketolase activation is the older functional assay |
| B2 (riboflavin) | FAD/FMN | Angular cheilitis, glossitis, seborrheic dermatitis | Erythrocyte glutathione reductase activity coefficient |
| B3 (niacin) | NAD/NADP | Pellagra: dermatitis, diarrhea, dementia | Synthesized from tryptophan; carcinoid tumors divert tryptophan and can precipitate pellagra |
| B6 (pyridoxine) | Pyridoxal-5'-phosphate (PLP): transaminases, ALA synthase | Sideroblastic anemia, seizures, peripheral neuropathy (also from excess pyridoxine) | ALT is more PLP-dependent than AST; IFCC methods add PLP to reagent so deficiency does not falsely lower measured activity. Isoniazid antagonizes B6 |
| B7 (biotin) | Carboxylase cofactor | Rare: alopecia, dermatitis, acidosis | High-dose supplements cause severe streptavidin-biotin immunoassay interference (Section 10.1) |
| B9 (folate) | Tetrahydrofolate: one-carbon transfer, thymidylate synthesis | Megaloblastic anemia; neural tube defects | Serum folate reflects recent intake; erythrocyte folate reflects tissue stores over the prior 3 months |
| B12 (cobalamin) | Methylcobalamin, adenosylcobalamin | Megaloblastic anemia plus subacute combined degeneration of the cord | Requires intrinsic factor; hepatic stores last years |
| C (ascorbate) | Prolyl/lysyl hydroxylase (collagen); reducing agent | Scurvy: perifollicular hemorrhage, bleeding gums, poor wound healing | Extremely labile: preserve with metaphosphoric acid, protect from light, freeze promptly |
Separating B12 Deficiency from Folate Deficiency
Both produce a macrocytic, megaloblastic anemia with hypersegmented neutrophils, and serum concentrations of both can sit in an indeterminate zone. Two metabolites resolve it:
| Metabolite | B12 Deficiency | Folate Deficiency |
|---|---|---|
| Methylmalonic acid (MMA) | Increased | Normal |
| Homocysteine | Increased | Increased |
MMA accumulates only when adenosylcobalamin-dependent methylmalonyl-CoA mutase is impaired, making it the B12-specific marker; homocysteine rises in both because both vitamins feed methionine synthase. The distinction is not academic — treating a B12 deficiency with folate corrects the anemia while the neurological damage progresses.
Additional B12 pitfalls: intrinsic factor blocking antibodies can interfere with competitive-binding B12 immunoassays and produce falsely normal results in frank pernicious anemia; nitrous oxide irreversibly oxidizes cobalamin and can precipitate acute deficiency; and pregnancy, oral contraceptives, and multiple myeloma alter transcobalamin/haptocorrin binding proteins and shift measured totals without changing tissue status.
Ascorbate as an Analytical Interferent
Vitamin C is a potent reducing agent, which makes it both a physiological antioxidant and a laboratory nuisance. It consumes the hydrogen peroxide generated in peroxidase-coupled Trinder reactions, producing falsely low results for glucose (glucose oxidase), cholesterol, triglyceride, and uric acid, and it causes false-negative urine dipstick glucose and blood pads and false-negative guaiac fecal occult blood tests. Section 2.3 works through the glucose case in detail.
Protein Markers of Nutritional Status
Serum proteins are used to grade protein-energy malnutrition and to monitor nutritional support. Their usefulness is governed entirely by half-life — and undermined entirely by inflammation.
| Marker | Approximate Half-Life | Representative Interval | Comment |
|---|---|---|---|
| Retinol-binding protein | ~12 hours | 3 - 6 mg/dL | Most rapid responder; accumulates in renal failure |
| Prealbumin (transthyretin) | ~2 days | 18 - 45 mg/dL | Standard marker for monitoring short-term nutritional repletion |
| Transferrin | ~8 - 10 days | 200 - 360 mg/dL | Confounded by iron status: iron deficiency raises it (Section 9.3) |
| Albumin | ~18 - 21 days | 3.5 - 5.0 g/dL | Too slow for acute monitoring; also lost in nephrotic syndrome and enteropathy |
[!WARNING] All four are negative acute-phase reactants. In a septic or post-surgical patient they fall because of the inflammatory response, not because of nutrition, so a low prealbumin in that setting does not prove malnutrition. Always interpret them alongside CRP: while CRP is rising or high, nutritional interpretation is suspended; once CRP falls, a rising prealbumin genuinely tracks nutritional repletion.
Malnutrition Phenotypes
- Marasmus: Global energy deficiency. Severe wasting of fat and muscle with relatively preserved visceral proteins; albumin is often near-normal, and edema is absent.
- Kwashiorkor: Protein deficiency with adequate carbohydrate intake. Hypoalbuminemia with edema, fatty liver, and depressed visceral proteins despite a deceptively preserved body weight.
- Refeeding syndrome: Re-introducing carbohydrate to a chronically starved patient triggers an insulin surge that drives phosphate, potassium, and magnesium intracellularly, producing life-threatening hypophosphatemia (Section 9.2). Monitor phosphorus, potassium, and magnesium daily during the first days of repletion, and give thiamine before the glucose load.
Specimen Handling Summary
| Analyte | Requirement |
|---|---|
| Vitamins A, E, B6, folate, B12 | Light-protected (amber) tubes; separate promptly; freeze if not analyzed immediately |
| Vitamin C | Stabilize with metaphosphoric acid, protect from light, freeze; deteriorates within hours otherwise |
| Thiamine | Whole blood (EDTA), light-protected, frozen — serum is inadequate |
| 25-hydroxyvitamin D | Serum, stable; LC-MS/MS preferred to resolve D2/D3 and the 3-epi metabolite |
| Fat-soluble vitamin panels | Fasting specimen preferred; recent supplement doses distort results |
The methods themselves follow a simple division: LC-MS/MS or HPLC for the fat-soluble vitamins, 25-hydroxyvitamin D, thiamine, and B6; competitive-binding chemiluminescent immunoassay for B12 and folate; and enzymatic or functional activation assays for the older riboflavin and transketolase measurements.
A 74-year-old vegan patient presents with a macrocytic anemia (MCV 118 fL), hypersegmented neutrophils, and progressive paresthesias with impaired vibratory sense. Serum vitamin B12 is 232 pg/mL and serum folate is 6.1 ng/mL, both reported as low-normal. Which pair of results would establish clinically significant B12 deficiency rather than folate deficiency?
A clinical chemistry laboratory is asked to monitor nutritional repletion in a patient on day 4 of total parenteral nutrition following emergency bowel resection. Prealbumin is 9 mg/dL (reference 18 - 45 mg/dL), albumin is 2.6 g/dL, and C-reactive protein is 148 mg/L. How should the technologist interpret the prealbumin result?
A patient with primary biliary cholangitis and marked hypercholesterolemia (total cholesterol 620 mg/dL) is evaluated for fat-soluble vitamin status. Serum alpha-tocopherol is reported at the lower end of the reference interval, yet the patient has ataxia, areflexia, and a peripheral neuropathy. What is the most appropriate analytical interpretation?