Overview
Vitamins are organic compounds required by the body in small amounts that cannot be synthesized internally (with a few exceptions) and must be obtained from the diet. They function mainly as coenzymes and catalysts for essential metabolic reactions, and their deficiency or excess produces well-characterised clinical syndromes.
This lesson classifies vitamins by solubility into water-soluble (B-complex and C) and fat-soluble (A, D, E, K) groups, covering their food sources, recommended daily allowances, deficiency diseases, and toxicity risks — essential knowledge for interpreting nutritional and metabolic laboratory findings.
Learning Objectives
After this lesson you will be able to…- Classify vitamins as fat-soluble or water-soluble
- Describe the functions, sources, and deficiency states of the water-soluble B-complex vitamins and vitamin C
- Describe the functions, sources, and deficiency/toxicity states of the fat-soluble vitamins A, D, E, and K
- Explain why fat-soluble vitamins carry a greater risk of toxicity than water-soluble vitamins
- Relate specific vitamin deficiency syndromes to their classic clinical presentations
Clinical Story
Why This MattersA 6-year-old child from a low-income household presents with bowed legs and a flattened back of the skull. The physician suspects rickets and orders serum calcium, phosphate, and vitamin D levels. Understanding that vitamin D is essential for intestinal calcium absorption and bone mineralisation explains why its deficiency produces this classic skeletal deformity — and highlights the laboratory's role in confirming nutritional deficiency diagnoses.
Core Concepts
Vitamins are classified by solubility. Fat-soluble vitamins (A, D, E, K) are stored in the body for long periods and pose a greater toxicity risk with megadose supplementation. Water-soluble vitamins (the eight B-complex vitamins and vitamin C) are not stored significantly and are excreted in urine when in excess, making deficiency more common but toxicity rare.
| Vitamin | Key Function | Deficiency |
|---|---|---|
| B1 — Thiamin | Energy release, nerve function | Beriberi |
| B2 — Riboflavin | Energy release, vision, skin | Cracked lips, sore tongue |
| B3 — Niacin | Energy production, skin/nerve health | Pellagra |
| B6 — Pyridoxine | Protein metabolism, RBC formation | Dermatitis, anemia |
| Folate (B9) | Cell growth, RBC formation | Neural tube defects, anemia |
| B12 — Cobalamin | DNA synthesis, nerve maintenance | Pernicious anemia, neuropathy |
| Biotin | Carbohydrate/fat/protein metabolism | Rare; fatigue, dermatitis |
| Pantothenic Acid | Energy production, hormone synthesis | Rare |
Vitamin C is essential for collagen synthesis, wound healing, immune function, and as an antioxidant working alongside vitamin E. It cannot be synthesized or stored by the body, so daily intake (90 mg/day males, 75 mg/day females) is essential. Severe deficiency causes scurvy, characterised by loss of collagen strength, bleeding gums, and poor wound healing.
Vitamin A supports vision, bone growth, reproduction, gene expression, and immune regulation. Sources include animal products (retinol) and beta-carotene from orange/dark green vegetables. Deficiency causes xerophthalmia and night blindness; toxicity (>3,000 mcg RAE) can cause severe birth defects and increased hip fracture risk.
Vitamin D increases intestinal calcium absorption and supports bone mineralisation; it is synthesized in skin upon UV exposure and obtained from fortified dairy and oily fish. Deficiency causes rickets in children (bowed legs, skull flattening) and osteomalacia/osteoporosis in adults. Toxicity causes hypercalcemia and can be dangerous, especially in infants.
Vitamin E (tocopherol) is a key antioxidant protecting cell membranes, red blood cells, and vitamins A and C from oxidative damage; deficiency is rare, mainly in premature infants or fat malabsorption. Vitamin K, partly produced by intestinal bacteria, is essential for normal blood clotting; deficiency causes hemorrhage and is treated prophylactically in newborns who lack intestinal flora at birth.
Laboratory Principle
Most vitamins function as coenzymes or coenzyme precursors, binding to specific apoenzymes to enable essential metabolic reactions — for example, thiamin pyrophosphate (from B1) is required for pyruvate dehydrogenase, and pyridoxal phosphate (from B6) is essential for transaminase reactions. Laboratory vitamin assays typically use immunoassay or chromatographic (HPLC) methods to directly quantify circulating vitamin levels, or measure functional biomarkers (e.g. serum calcium/ALP for vitamin D status, INR for vitamin K status).
Equipment Required
Reagents & Materials
| Reagent / Material | Concentration / Grade | Purpose | Storage |
|---|---|---|---|
| Vitamin D (25-OH) immunoassay kit | Working strength | Quantifies serum 25-hydroxyvitamin D, the best marker of vitamin D status | 2–8°C |
| Vitamin B12/Folate immunoassay kit | Working strength | Measures serum B12 and folate for anemia workup | 2–8°C |
| Prothrombin time (PT) reagent | Thromboplastin-based | Assesses vitamin K-dependent clotting factor activity | 2–8°C |
| Calcium & ALP reagents | Working strength | Supportive markers for vitamin D-related bone metabolism | 2–8°C |
Step-by-Step Procedure
Confirm the clinical suspicion (e.g. anemia, bone pain, bleeding tendency, poor wound healing) that warrants a specific vitamin panel.
Collect a fasting venous blood sample into the appropriate tube; protect light-sensitive analytes (e.g. vitamin A, riboflavin) from light exposure.
Centrifuge and separate serum/plasma promptly; store at recommended temperature until analysis, especially for fat-soluble vitamins requiring stability precautions.
Run the appropriate immunoassay, HPLC, or coagulation test as indicated by the requested vitamin panel.
Compare results to age- and sex-specific reference ranges, correlate with clinical findings (e.g. MCV for B12/folate anemia), and report to the requesting physician.
Flow Diagram
Quality Control
Run manufacturer-provided control material at two levels for each vitamin immunoassay run; verify calibration curves regularly, particularly for light-sensitive and unstable analytes such as vitamin A and folate.
Participate in relevant External Quality Assessment programmes for vitamin B12, folate, and vitamin D testing to confirm inter-laboratory comparability and detect assay-specific bias.
Reference Values
Normal Ranges⚠️ Reference ranges may vary between laboratories. Always apply your laboratory's established reference intervals.
Clinical Interpretation
| Finding | Possible Significance | Action / Follow-up |
|---|---|---|
| Vitamin D < 20 ng/mL | Vitamin D deficiency — risk of rickets (children) or osteomalacia (adults) | Correlate with calcium, phosphate, ALP; recommend supplementation |
| Vitamin B12 low with macrocytic anemia (high MCV) | Megaloblastic anemia due to B12 deficiency (possible pernicious anemia) | Check intrinsic factor antibodies; initiate B12 replacement |
| Prolonged PT/INR | Possible vitamin K deficiency or malabsorption | Assess dietary/malabsorption causes; vitamin K supplementation |
| Vitamin A markedly elevated | Vitamin A toxicity — risk of birth defects, hip fracture | Discontinue high-dose supplementation; monitor liver function |
Common Errors & How to Avoid Them
Cause: Both are light-sensitive and degrade rapidly, causing falsely low results
Prevention: Protect samples from light using amber tubes or foil wrapping.
Cause: Recent food intake can transiently alter some vitamin levels and interfere with lipid-based assays
Prevention: Confirm fasting status before collection where required.
Cause: Prolonged storage without proper handling can degrade folate levels
Prevention: Separate and analyze promptly, or freeze per assay-specific stability guidelines.
Laboratory Tips from the Bench
Always interpret vitamin B12 results alongside folate and a complete blood count — both deficiencies cause macrocytic anemia, and B12 deficiency alone can mask a coexisting folate deficiency.
When faced with unexplained bleeding and a prolonged PT, always consider vitamin K deficiency or malabsorption (e.g. biliary obstruction, chronic antibiotic use) before assuming a primary coagulation factor defect.
Remember fat-soluble vitamins with 'A DEK of cards' — vitamins A, D, E, and K are the fat-soluble ones, stored in the body and carrying greater toxicity risk than water-soluble vitamins.
Important Notes
Newborns lack the intestinal bacteria needed to produce vitamin K and have low placental transfer, putting them at risk of haemorrhagic disease of the newborn — hence the routine vitamin K injection given at birth.
High-dose folate supplementation can correct the anemia of B12 deficiency while allowing irreversible neurological damage to progress silently — always test B12 before initiating folate therapy in suspected megaloblastic anemia.
Interactive Quiz
Test Your KnowledgeFlashcards
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Clinical Case Study
Apply Your KnowledgeBrought by his mother with progressive bowing of both legs and difficulty walking over the past year. He has had minimal sun exposure and a diet low in dairy products.
Low vitamin D with low phosphate and markedly elevated alkaline phosphatase reflects impaired bone mineralisation due to inadequate intestinal calcium absorption, consistent with nutritional rickets.
- →Vitamin D deficiency impairs intestinal calcium absorption, leading to compensatory bone changes seen as elevated ALP.
- →Classic skeletal findings in children include bowed legs (genu varum) and skull flattening.
- →Treatment involves vitamin D and calcium supplementation along with increased safe sun exposure.
Frequently Asked Questions
Water-soluble vitamins are not significantly stored in body tissues; any excess is readily filtered by the kidneys and excreted in urine. Fat-soluble vitamins, by contrast, are stored in adipose tissue and the liver, allowing them to accumulate to toxic levels with excessive supplementation.
Folate is essential for DNA synthesis and rapid cell division during early foetal development. Adequate folate intake before conception and in early pregnancy significantly reduces the risk of neural tube defects such as spina bifida.
Yes — deficiencies often occur together in malnutrition, malabsorption, or alcoholism (e.g. combined thiamin, folate, and B12 deficiency). Laboratories should consider a broader nutritional panel when one deficiency is confirmed, especially in high-risk patients.
Quick Revision
10-Minute ReviewKey Takeaways
- Vitamins are essential organic micronutrients that mainly function as coenzymes in metabolism.
- Solubility (fat vs water) determines storage, deficiency risk, and toxicity potential.
- Each vitamin has a characteristic deficiency syndrome — beriberi, pellagra, scurvy, rickets, and more.
- Fat-soluble vitamin toxicity is a real clinical risk with megadose supplementation, unlike most water-soluble vitamins.
- Laboratory testing of vitamin levels and their functional markers (calcium, ALP, PT/INR, MCV) supports accurate nutritional diagnosis.
Competency Checklist
Track Your MasteryReferences
- National Institute of Open Schooling. Biochemistry — Module: Vitamins (Lesson 10).
- Murray RK, et al. Harper's Illustrated Biochemistry. 30th ed. McGraw-Hill.
- Institute of Medicine (US). Dietary Reference Intakes for Vitamins.