Vitamins & Nutrition
1. What this chapter covers, and how NEET PG actually tests it
Vitamin questions look like pure recall, and are usually revised that way, with poor retention.
The syndromes become far easier when read as consequences of a biochemical role.
Vitamin K carboxylates clotting factors, so its deficiency causes bleeding. Vitamin B6 serves transamination and decarboxylation, including that of glutamate to GABA, so its deficiency causes seizures. Niacin becomes NAD, so its deficiency affects the tissues with the highest turnover — skin, gut and brain.
Once the role is known, the syndrome is derivable and the treatment is obvious.
Two further organising ideas carry substantial weight. Fat-soluble vitamins are stored and can therefore reach toxic levels, while water-soluble vitamins are generally excreted. And the folate-B12 pair is separated by one metabolite, which is the single most reliably tested discrimination in the chapter.
This chapter covers the fat-soluble vitamins, the B group as cofactors, vitamin C with the folate and B12 pair, and trace elements with protein-energy malnutrition.
| In scope here | Deliberately out of scope |
|---|---|
| Fat-soluble vitamin deficiency and toxicity | Detailed rickets radiology (see Orthopedics) |
| B vitamin cofactor roles and deficiency syndromes | Anaemia classification and blood film detail (see Pathology) |
| Folate and B12 discrimination, vitamin C | Nutritional programme delivery and surveys (see PSM) |
| Zinc, copper, selenium, iodine; kwashiorkor and marasmus | Enteral and parenteral feeding protocols (see Medicine) |
2. The fat-soluble vitamins
2.1 Why storage matters
Vitamins A, D, E and K are absorbed with dietary fat and stored in the liver and adipose tissue.
Two consequences follow, and both are examinable.
Any condition causing fat malabsorption causes deficiency of all four together — cystic fibrosis, cholestasis, coeliac disease and terminal ileal resection being the standard causes.
And because they are stored rather than excreted, toxicity is possible, which is rarely true of the water-soluble group.
2.2 The four vitamins
Vitamin A exists as retinal in rhodopsin and as retinoic acid controlling epithelial differentiation.
Deficiency therefore affects vision and epithelium in a predictable sequence: night blindness first, since rod function fails earliest, then conjunctival dryness, Bitot's spots, and finally corneal ulceration and keratomalacia.
Follicular hyperkeratosis reflects the epithelial role, and susceptibility to severe measles reflects the immune one — which is why vitamin A is given routinely in measles in deficient populations.
Toxicity is important because it is iatrogenic. Excess causes raised intracranial pressure, hepatotoxicity and bone pain, and retinoids are potently teratogenic, which is why isotretinoin demands strict contraception.
Vitamin D deficiency causes rickets in children — bowed legs, rachitic rosary, craniotabes and widened wrists — and osteomalacia in adults, where bone pain and proximal myopathy predominate.
Vitamin E is a membrane antioxidant, so deficiency causes haemolytic anaemia, particularly in preterm neonates, together with a spinocerebellar syndrome and peripheral neuropathy.
That neurological picture closely resembles Friedreich ataxia, and the resemblance is a favourite exam pairing.
Vitamin K is required by a carboxylase that adds a second carboxyl group to glutamate residues on factors II, VII, IX and X, and proteins C and S.
Without carboxylation those factors cannot bind calcium and are functionally inert, even though they are present in normal amounts.
Newborns are deficient because placental transfer is poor and the gut is not yet colonised, which is why intramuscular vitamin K is given at birth to prevent haemorrhagic disease of the newborn.
Warfarin acts on this pathway, inhibiting vitamin K epoxide reductase and thereby preventing regeneration of the active cofactor.
3. The B vitamins as cofactors
3.1 Thiamine
Thiamine pyrophosphate serves four dehydrogenase or transferase reactions: pyruvate dehydrogenase, alpha-ketoglutarate dehydrogenase, transketolase and branched-chain alpha-ketoacid dehydrogenase.
Since two of those are central to aerobic glucose metabolism, deficiency impairs energy production in the tissues most dependent on it.
The syndromes divide by which tissue fails first.
Dry beriberi is peripheral neuropathy. Wet beriberi is high-output cardiac failure with oedema. Wernicke encephalopathy is the triad of confusion, ophthalmoplegia and ataxia.
Untreated Wernicke progresses to Korsakoff syndrome, with irreversible anterograde amnesia and confabulation.
Thiamine must be given before glucose in a suspected case, because a glucose load consumes the remaining thiamine in glycolysis and can precipitate acute Wernicke encephalopathy.
That ordering is a genuine clinical rule and a favourite question, and it follows directly from thiamine's role in pyruvate dehydrogenase.
3.2 Riboflavin, niacin and pyridoxine
| Vitamin | Cofactor role | Deficiency |
|---|---|---|
| B2 riboflavin | FAD and FMN | Angular cheilitis, glossitis, corneal vascularisation |
| B3 niacin | NAD and NADP | Pellagra: dermatitis, diarrhoea, dementia |
| B6 pyridoxine | Transamination, decarboxylation, ALA synthase | Seizures, sideroblastic anaemia, peripheral neuropathy |
Pellagra's three Ds affect the three highest-turnover tissues, since every one depends heavily on NAD-linked metabolism. The dermatitis is characteristically photosensitive, giving the Casal's necklace distribution.
Niacin can be synthesised from tryptophan, which creates three secondary causes of pellagra worth knowing.
Hartnup disease impairs tryptophan absorption. Carcinoid syndrome diverts tryptophan into serotonin synthesis. Isoniazid depletes pyridoxine, which is required for the conversion.
Niacin in pharmacological doses causes flushing, which is prostaglandin-mediated and reduced by aspirin — a mechanism, not merely an association.
Pyridoxine deficiency causes seizures because glutamate decarboxylase requires it to make GABA, so inhibitory neurotransmission fails.
It also serves ALA synthase, the rate-limiting enzyme of haem synthesis, which is why deficiency produces a sideroblastic anaemia.
Isoniazid is the classic cause, which is why pyridoxine is co-prescribed with it routinely.
Note that pyridoxine is one of the few water-soluble vitamins with a genuine toxicity — a sensory peripheral neuropathy in excess.
3.3 Biotin and pantothenate
Biotin serves the carboxylases — pyruvate carboxylase, acetyl-CoA carboxylase and propionyl-CoA carboxylase.
Deficiency is uncommon but has two memorable causes: prolonged raw egg white consumption, since avidin binds biotin, and prolonged antibiotic use suppressing gut flora.
Pantothenate becomes coenzyme A, and its deficiency is rare enough to be tested mainly as a matching item.
3.4 Reading a B vitamin question backwards
Because each B vitamin serves a defined set of reactions, a described deficiency can be worked back to the vitamin without recalling syndrome names.
Seizures point to pyridoxine, since GABA synthesis by glutamate decarboxylase depends on it.
A sideroblastic anaemia points to pyridoxine as well, since ALA synthase requires it — and the two features together, in a patient on antituberculous therapy, make the answer nearly certain.
Lactic acidosis with a neurological presentation points to thiamine, because pyruvate dehydrogenase is blocked and pyruvate is diverted to lactate.
A photosensitive rash with diarrhoea and confusion points to niacin, since NAD-dependent metabolism fails first in the highest-turnover tissues.
Angular cheilitis with corneal vascularisation points to riboflavin, and dermatitis with alopecia in a patient on long-term antibiotics or eating raw eggs points to biotin.
This backwards reading is faster and more robust than forward recall, because a stem gives features and asks for the vitamin, not the reverse.
It also protects against the commonest error in this area, which is recognising a syndrome name without being able to attach it to a deficiency.
4. Vitamin C, folate and B12
4.1 Vitamin C
Ascorbate is the cofactor for prolyl and lysyl hydroxylase in collagen synthesis, reduces dietary iron from ferric to ferrous to aid absorption, and acts as an antioxidant.
Scurvy is therefore a collagen disease, giving perifollicular haemorrhage, swollen bleeding gums, poor wound healing and, in children, subperiosteal haemorrhage.
The coagulation screen is normal, because the bleeding is from vascular fragility rather than a clotting defect.
Excess intake predisposes to oxalate renal stones.
Its role in reducing dietary iron explains a practical point: ascorbate taken with a meal substantially improves absorption of non-haem iron, which matters in populations relying on plant-based diets.
4.2 The folate and B12 discrimination
Both deficiencies cause a megaloblastic anaemia, because both are required for thymidine synthesis and therefore for DNA replication.
Only B12 deficiency causes neurological disease, specifically subacute combined degeneration of the dorsal columns and corticospinal tracts.
The reason lies in their differing enzymatic roles.
B12 serves two reactions: methylmalonyl-CoA mutase, and homocysteine methyltransferase which regenerates methionine and simultaneously releases folate from its methyl-trapped form.
Folate serves only the second of these.
So the discriminating metabolite is methylmalonic acid.
| Finding | Folate deficiency | B12 deficiency |
|---|---|---|
| Megaloblastic anaemia | Yes | Yes |
| Homocysteine | Raised | Raised |
| Methylmalonic acid | Normal | Raised |
| Neurological signs | No | Yes |
One further point is clinically critical rather than merely academic.
Because folate can bypass the block in DNA synthesis, giving folate alone to a B12-deficient patient corrects the anaemia while the neurological damage continues to progress — and it removes the anaemia that would have prompted investigation.
So B12 must be excluded before folate is given, and this reasoning appears regularly in examinations.
The two also differ in reserve. Folate stores last only a few months, while B12 stores last years, which is why dietary folate deficiency appears quickly and dietary B12 deficiency takes a long time to manifest.
Causes of B12 deficiency worth knowing are pernicious anaemia, terminal ileal disease or resection, Diphyllobothrium latum, metformin and nitrous oxide, which irreversibly oxidises the cobalt atom.
5. Trace elements and protein-energy malnutrition
5.1 Trace elements
| Element | Deficiency features |
|---|---|
| Zinc | Acrodermatitis enteropathica, poor wound healing, hypogonadism, dysgeusia, alopecia |
| Copper | Anaemia with normal iron studies, neutropenia, osteoporosis |
| Selenium | Keshan cardiomyopathy |
| Iodine | Goitre, and cretinism in the neonate |
Copper deficiency deserves a moment of reasoning. Copper is required by lysyl oxidase for collagen cross-linking and by enzymes in iron mobilisation, which is why the anaemia occurs despite adequate iron stores.
Zinc deficiency is the one that produces a recognisable rash, symmetrical and periorificial with acral involvement, and it responds rapidly to replacement.
Zinc is also required by a large number of enzymes and transcription factors, which is why its deficiency simultaneously impairs wound healing, taste, immunity and growth — a breadth that makes it easy to overlook as a unifying cause.
Iodine deficiency remains the commonest preventable cause of intellectual disability worldwide, since maternal deficiency deprives the developing fetal brain of thyroid hormone at a critical period. Universal salt iodisation exists specifically to address this.
Note also the interaction between excess zinc and copper: high-dose zinc induces intestinal metallothionein, which binds copper and prevents its absorption, so prolonged zinc supplementation can itself produce copper deficiency.
5.2 Kwashiorkor and marasmus
These are the two poles of protein-energy malnutrition and are distinguished by one finding.
Marasmus is total energy deficiency, producing severe wasting of both fat and muscle, an aged appearance, and no oedema. Appetite is often preserved.
Kwashiorkor is protein deficiency with relatively preserved calorie intake, and its defining feature is oedema.
The oedema arises because hypoalbuminaemia lowers plasma oncotic pressure, and the same protein deficiency causes hepatic fatty change, since apolipoproteins for lipid export cannot be made.
Kwashiorkor also produces skin desquamation, the flag sign of alternating pale and dark hair bands recording periods of poor and better nutrition, and marked apathy or irritability.
The presence of oedema is therefore the discriminator, and the deceptively well-covered appearance it produces is why kwashiorkor can be underestimated clinically.
Refeeding syndrome is the shared danger. Reintroducing carbohydrate drives insulin release, which pushes phosphate, potassium and magnesium into cells, and the resulting hypophosphataemia can cause cardiac and respiratory failure.
Feeding must therefore be cautious and electrolytes monitored, with thiamine given first for the reason described earlier.
The paradox is worth stating explicitly: the more depleted the patient, the more dangerous their first adequate meal, so nutritional urgency and refeeding caution pull in opposite directions.
5.3 Assessing nutritional status
Several measures are used, and each answers a different question.
Body mass index is weight divided by height squared, and it assesses chronic status in adults but says nothing about body composition or recent change.
Mid-upper arm circumference is the field measure of choice in children, because it requires no equipment beyond a tape and changes little with age between six months and five years.
Weight for height identifies acute wasting, while height for age identifies chronic stunting.
That distinction matters for interpretation. A stunted child has suffered prolonged deprivation, whereas a wasted child is acutely malnourished now — and the two demand different responses.
Serum albumin is a poor marker of nutrition in acute illness, because it is a negative acute phase reactant and falls with inflammation regardless of intake.
Prealbumin has a shorter half-life and tracks change more responsively, but shares the same limitation.
Worked clinical vignettes
Q1. A patient with alcohol dependence is brought in confused. Which intervention must precede intravenous glucose, and why?
Pick an option to check your answer.
Show explanation
Solution. Thiamine pyrophosphate is required by pyruvate dehydrogenase, so metabolising a glucose load consumes whatever thiamine remains.
In an already depleted patient this can precipitate acute Wernicke encephalopathy, which may then progress to irreversible Korsakoff syndrome.
Thiamine before glucose is a genuine clinical rule derived directly from the cofactor's role. Answer: (b).
Q2. A patient has a megaloblastic anaemia with raised homocysteine but a normal methylmalonic acid. Which deficiency is present?
Pick an option to check your answer.
Show explanation
Solution. B12 serves two reactions, methylmalonyl-CoA mutase and homocysteine methyltransferase. Folate serves only the latter.
So both deficiencies raise homocysteine, but only B12 deficiency raises methylmalonic acid.
A normal methylmalonic acid therefore identifies folate deficiency, which is consistent with the absence of neurological signs. Answer: (b).
Q3. A child with severe malnutrition has pitting oedema, an enlarged fatty liver, desquamating skin and alternating pale bands in the hair. Which diagnosis is this, and what causes the oedema?
Pick an option to check your answer.
Show explanation
Solution. Oedema is the defining feature separating kwashiorkor from marasmus.
Protein deficiency with relatively preserved calories causes hypoalbuminaemia, so plasma oncotic pressure falls and fluid moves into the interstitium.
The same protein deficiency prevents apolipoprotein synthesis, so lipid accumulates in the liver, and the flag sign records alternating periods of poorer and better nutrition. Answer: (b).
7. Common exam traps
- Forgetting that fat malabsorption causes all four fat-soluble deficiencies together. Cystic fibrosis, cholestasis and ileal disease affect A, D, E and K simultaneously.
- Overlooking vitamin toxicity. Fat-soluble vitamins accumulate, and retinoids are potently teratogenic.
- Giving glucose before thiamine. The glucose load consumes remaining thiamine and can precipitate Wernicke encephalopathy.
- Forgetting the secondary causes of pellagra. Hartnup disease, carcinoid syndrome and isoniazid all reduce niacin availability from tryptophan.
- Missing that isoniazid causes pyridoxine deficiency. Seizures and peripheral neuropathy follow, which is why pyridoxine is co-prescribed.
- Treating folate and B12 as interchangeable. Only B12 deficiency raises methylmalonic acid and causes neurological disease.
- Giving folate before excluding B12 deficiency. The anaemia corrects while neurological damage progresses unmonitored.
- Overlooking refeeding syndrome. Hypophosphataemia from insulin-driven intracellular shift can be fatal.
Summary
- Fat-soluble vitamins are stored, so fat malabsorption causes all four deficiencies together and toxicity is possible, unlike the water-soluble group.
- Vitamin A deficiency progresses from night blindness through Bitot's spots to keratomalacia, and its excess is teratogenic and raises intracranial pressure.
- Vitamin D deficiency causes rickets in children and osteomalacia with proximal myopathy in adults.
- Vitamin E deficiency causes haemolysis and a spinocerebellar syndrome resembling Friedreich ataxia.
- Vitamin K carboxylates factors II, VII, IX and X with proteins C and S, and warfarin acts by blocking regeneration of the active cofactor.
- Thiamine serves pyruvate dehydrogenase and three related enzymes, so deficiency causes beriberi and Wernicke encephalopathy, and thiamine must precede glucose.
- Niacin deficiency causes pellagra's dermatitis, diarrhoea and dementia, with secondary causes in Hartnup disease, carcinoid syndrome and isoniazid therapy.
- Pyridoxine deficiency causes seizures through failed GABA synthesis and sideroblastic anaemia through impaired ALA synthase, classically from isoniazid.
- Biotin serves the carboxylases and is bound by avidin in raw egg white.
- Vitamin C is required for collagen hydroxylation and iron absorption, so scurvy causes vascular fragility with a normal coagulation screen.
- Folate and B12 deficiency both cause megaloblastic anaemia and both raise homocysteine, but only B12 deficiency raises methylmalonic acid.
- Only B12 deficiency causes subacute combined degeneration, and folate given alone corrects the anaemia while neurological damage progresses.
- Folate stores last months while B12 stores last years, which is why dietary deficiency appears at very different rates.
- Zinc deficiency causes a periorificial rash with poor healing, and copper deficiency causes anaemia with normal iron studies.
- Marasmus is wasting without oedema, while kwashiorkor has oedema from hypoalbuminaemia together with fatty liver and the flag sign.
- Refeeding syndrome causes hypophosphataemia through insulin-driven intracellular shift and can be fatal, so replacement must be cautious.
- B vitamin questions are best read backwards, from the described feature to the reaction it serves, since stems supply features and ask for the vitamin.
- Iodine deficiency is the commonest preventable cause of intellectual disability worldwide, and prolonged high-dose zinc can itself induce copper deficiency.
