Q5 LONG ANSWER 10M ★ High-yield

Define nitrogen balance. Describe protein turnover and protein quality. Compare kwashiorkor with marasmus.

Biochemistry · 1st Professional — Pre-Clinical

Protein Turnover

Protein turnover = the continuous synthesis and degradation of body protein.

About 300–400 g of protein is turned over daily, against a dietary intake of only 60–100 g
Most amino acids released are reused; only about 20% are oxidised
Half-lives vary widely — ornithine decarboxylase 11 minutes, albumin 20 days, collagen years

Pathways of degradation

System	Location	Handles
Ubiquitin–proteasome	Cytosol; ATP-dependent	Short-lived and abnormal proteins; the selective route
Lysosomal	Lysosome; ATP-independent	Long-lived and extracellular proteins; non-selective

Proteins destined for the proteasome are tagged with ubiquitin; the N-terminal residue (the N-end rule) and pest sequences determine the rate

Nitrogen Balance

Nitrogen balance = the difference between nitrogen intake and nitrogen excretion.Protein is about 16% nitrogen, so 1 g nitrogen = 6.25 g protein.

State	Meaning	Occurs in
Nitrogen equilibrium	Intake = output	Normal healthy adult
Positive balance	Intake > output — tissue being built	Growth, pregnancy, lactation, convalescence, athletic training
Negative balance	Output > intake — tissue being lost	Starvation, trauma, burns, sepsis, fever, immobilisation, uncontrolled diabetes, deficiency of any single essential amino acid

CLINICAL PEARL

A negative balance can be caused by the lack of one essential amino acid, even when total protein intake is generous. Protein synthesis is all-or-none: the limiting amino acid halts the whole process, and the remainder are deaminated.

Protein Quality

Biological value — the proportion of absorbed nitrogen retained. Egg 100, milk 85, meat 75, rice 65, wheat 50
Limiting amino acid — lysine in cereals, methionine in pulses
Mutual supplementation — cereals with pulses (dal–chawal) gives a combined value approaching that of animal protein. The traditional Indian diet is well designed in this respect
Daily requirement 0.8–1 g/kg in adults; up to 2 g/kg in infancy

Protein Energy Malnutrition

Feature	Kwashiorkor	Marasmus
Deficiency	Protein, with adequate calories	Total calories including protein
Age	1–3 years (after weaning)	Under 1 year
Oedema	Present — the defining feature	Absent
Weight for age	60–80% (masked by oedema)	Below 60%
Muscle wasting	Present but hidden	Severe and obvious
Subcutaneous fat	Preserved	Lost — "old man" face
Serum albumin	Markedly low	Normal or slightly low
Fatty liver	Present	Absent
Skin and hair	Flaky paint dermatosis; flag sign; sparse pale hair	Relatively normal
Appetite	Poor	Good (voracious)
Mood	Apathetic, irritable	Alert but miserable

Why kwashiorkor causes oedema and fatty liver

Oedema — ↓ albumin → ↓ plasma oncotic pressure → fluid moves into the interstitium
Fatty liver — no apoprotein can be made, so VLDL cannot be exported and triglyceride accumulates
Free radical damage and aflatoxin exposure are also implicated

Management of Severe Malnutrition

Treat life-threatening problems first — hypoglycaemia, hypothermia, dehydration, electrolyte imbalance, infection
Feed cautiously at first
Refeeding syndrome — rapid feeding drives phosphate, potassium and magnesium into cells → arrhythmia, cardiac failure and death. The commonest cause of death in the first days of treatment
Then catch-up growth with high-energy feeds, micronutrients and sensory stimulation

Applied Aspects

Marasmic kwashiorkor — features of both; the commonest severe form seen in Indian hospitals
Nutritional oedema — bilateral, pitting and starting in the feet; the cardiac, renal and hepatic causes have their own signs
Hypoglycaemia and hypothermia are the two commonest causes of death in the first 48 hours and must be looked for actively
Vitamin and mineral deficiencies coexist — zinc, potassium, magnesium, vitamin A and iron must all be corrected
Iron is withheld in the first week of treatment — free iron promotes bacterial growth and oxidative damage
Recovery is judged by weight gain of 10–15 g/kg/day during the catch-up phase
Prevention — exclusive breast feeding for 6 months, timely complementary feeding, immunisation and growth monitoring
Serum albumin is a poor marker of acute nutrition — its half-life is 20 days. Prealbumin (2 days) and retinol-binding protein (12 hours) respond faster
Negative nitrogen balance after major surgery or burns is driven by cortisol and cytokines and cannot be fully reversed by feeding alone
India carries a very large burden of childhood undernutrition; the commonest picture is marasmic kwashiorkor, with features of both
Anthropometry — weight for height detects acute wasting, height for age detects chronic stunting; mid-upper arm circumference is the simplest field measure

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