Overview
Nutrition is the requirement of the body from external sources to sustain life — providing energy (carbohydrates, fats, proteins), building blocks (proteins, amino acids) and functional molecules (micronutrients, vitamins). A reduced, unbalanced or excessive nutrient intake causes deficiency disorders, protein-energy malnutrition, or diseases of over-nutrition such as obesity, diabetes and cardiovascular disease.
This lesson covers basal metabolic rate, thermogenesis, the nutritional aspects of carbohydrates, lipids and proteins, the components of a balanced diet, and the two classical protein-energy malnutrition syndromes — kwashiorkor and marasmus — which remain important clinical presentations in laboratory and community health settings.
Learning Objectives
After this lesson you will be able to…- Define nutrition and basal metabolic rate (BMR), and explain factors affecting BMR.
- Describe the nutritional aspects of carbohydrates, lipids and proteins.
- Enumerate the essential amino acids and essential fatty acids.
- Describe the composition of food and what constitutes a balanced diet.
- Describe the causes, symptoms and treatment of kwashiorkor and marasmus.
Clinical Story
Why This MattersAn 18-month-old child recently weaned off breast milk onto a starchy, protein-poor diet is brought to the clinic with a distended abdomen, swollen feet, and patchy, discoloured hair. The physician suspects kwashiorkor and orders serum albumin and total protein. The lab technologist must understand how protein deficiency leads to hypoalbuminemia and oedema to correctly flag this critical finding.
Core Concepts
BMR is the energy required by the body at complete rest (Kcal/hour/m² body surface area), calculated from oxygen consumption (1 L O₂ ≈ 4.825 Kcal) and body surface area (Du Bois formula). Average adult male BMR is ~40 Kcal/hr/m²; adult female ~36 Kcal/hr/m². BMR rises in pregnancy, fever, growth, and with higher muscle mass, and is influenced by age, sex, activity level and pathological state.
Thermogenesis is heat production to maintain body temperature, occurring through muscle movement/shivering (electron transport chain leakage) and brown adipose tissue, where an uncoupling protein dissipates the proton gradient as heat instead of ATP.
Carbohydrates yield 4 Kcal/g and are the body's chief, cheapest and easiest-to-digest energy source. The brain uses glucose almost exclusively, consuming ~20–25% of total energy intake. Main dietary forms are starch (grains, pulses, tubers), sugars (mono/disaccharides in fruits, vegetables, milk), and cellulose (indigestible fibre providing bulk/roughage). Carbohydrates exert a protein-sparing effect, allowing dietary amino acids to be used for protein synthesis rather than energy. No essential carbohydrates exist since the body can synthesise them from other sources.
Lipids store energy (9 Kcal/g), form structural components of cells, and act as hormones and vitamin carriers. Essential fatty acids — alpha-linolenic acid, linoleic acid and arachidonic acid — cannot be synthesised in the body and must comprise ≥3% of energy in adults (5–6% in children). Diets should favour polyunsaturated over saturated fats to reduce atherosclerosis risk. Cholesterol is structurally essential but excess intake (RDA 300 mg/day) is linked to cardiovascular disease.
Proteins (4 Kcal/g) are the primary structural and functional building blocks of the body. Of 20 amino acids, 9 are essential (must be obtained from diet): histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine.
Protein quality depends on digestibility coefficient (DC) and biological value (BV); Net Protein Utilization (NPU) = (DC × BV)/100. Animal (first-class) proteins have high biological value and complete amino acid profiles; vegetable (second-class) proteins are often deficient in specific amino acids but can be complemented through mutual supplementation (e.g. rice + pulses). Protein requirement is ~2.5 g/kg in infancy, ~1 g/kg in adulthood, and higher during pregnancy/lactation (~2–2.5 g/kg).
A balanced diet supplies all nutrients in correct proportion for an individual's biological needs, without deficiency or excess. It should include cereals, pulses, vegetables and fruits, milk and milk products, oils and fats, sugars, and (for non-vegetarians) animal proteins/eggs. A typical energy split is ~55% carbohydrate, 30% fat, 15% protein. Energy requirements vary by body weight, sex, and activity — e.g. a sedentary adult man needs ~2320 Kcal/day, while heavy work raises this to ~3490 Kcal/day.
Kwashiorkor occurs in children weaned onto a diet deficient in protein but adequate in calories: muscle wasting, retarded growth, patchy hyper/hypo-pigmented skin, brittle hypo-pigmented hair with a "flag sign", and a classically distended, oedematous belly due to loss of plasma oncotic pressure (low albumin) and fatty liver. Prevented by adding protein-rich foods (milk, eggs, pulses, groundnuts) when weaning.
Marasmus is a disorder of both protein and calorie deficiency, causing gross muscle wasting, loss of subcutaneous fat, and growth retardation, often worsened by infections and diarrhoea. Correction requires increasing protein and energy intake, supplementing vitamins/minerals, correcting dehydration and treating infections.
Laboratory Principle
Serum albumin and total protein are measured colorimetrically — albumin using the bromocresol green (BCG) dye-binding method, where albumin binds the dye to produce a colour change proportional to concentration, and total protein using the biuret method, where peptide bonds react with copper sulphate in alkaline solution to form a violet complex read photometrically. These simple, robust assays make protein status assessment widely available even in resource-limited laboratories.
Equipment Required
Reagents & Materials
| Reagent / Material | Concentration / Grade | Purpose | Storage |
|---|---|---|---|
| Bromocresol Green (BCG) reagent | Working reagent | Serum albumin estimation | 2–8 °C, protect from light |
| Biuret reagent | Alkaline copper sulphate | Total protein estimation | Room temperature, tightly capped |
| Protein calibrator/standard | Traceable, certified | Instrument calibration | 2–8 °C |
Step-by-Step Procedure
Collect venous blood in a plain tube; a fasting sample is preferred for a complete nutritional/metabolic panel.
Centrifuge at 3000–4000 rpm for 10 minutes to obtain clear serum.
Add BCG reagent for albumin or biuret reagent for total protein in the ratio specified by the kit insert.
Incubate for the specified time, then read absorbance at the appropriate wavelength against a reagent blank and calibrator.
Calculate albumin/total protein concentration and, if needed, the albumin-to-globulin ratio; report against reference ranges.
Flow Diagram
Quality Control
Run two-level protein controls with each batch and monitor with Levey-Jennings charts; recalibrate when reagent lots change.
Participate in an EQAS programme for clinical chemistry protein assays and review peer-group comparison reports each cycle.
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 |
|---|---|---|
| Low serum albumin with oedema | Kwashiorkor (protein malnutrition) | Assess dietary history; initiate protein-rich refeeding |
| Low total protein, low body weight-for-height | Marasmus (protein-calorie malnutrition) | Assess for infection/dehydration; graded refeeding |
| Low BMR | Hypothyroidism, malnutrition, sedentary state | Correlate with thyroid function tests |
| High cholesterol/LDL | Excess saturated fat/cholesterol intake | Dietary counselling; assess cardiovascular risk |
Common Errors & How to Avoid Them
Cause: Recent food intake can transiently elevate triglycerides and alter protein fractions.
Prevention: Confirm fasting status (typically 9–12 hours) before sample collection.
Cause: Causes haemoconcentration, falsely raising total protein and albumin.
Prevention: Release the tourniquet within one minute.
Cause: Oedema in kwashiorkor can mask underlying muscle wasting on weight assessment alone.
Prevention: Use MUAC and clinical examination alongside weight to assess true nutritional status.
Laboratory Tips from the Bench
In suspected malnutrition, always assess both albumin (chronic protein status) and a rapid-turnover protein like prealbumin/transthyretin (acute nutritional changes) when available.
Mid-upper arm circumference (MUAC) is a quick, reliable field screening tool for protein-energy malnutrition in children, independent of oedema-related weight distortion.
Remember "Kwashiorkor = pot belly + oedema (protein-poor, calorie-okay)" vs "Marasmus = wasted and wiry (both protein and calories deficient)."
Important Notes
Severely malnourished children require gradual, carefully monitored refeeding — aggressive nutritional replenishment can precipitate dangerous electrolyte shifts (refeeding syndrome), particularly hypophosphatemia.
Combining two incomplete vegetable protein sources deficient in different amino acids (e.g. cereals + pulses) yields a nutritionally complete protein profile, an important concept for vegetarian diet planning.
Interactive Quiz
Test Your KnowledgeFlashcards
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Clinical Case Study
Apply Your KnowledgeRecently weaned off breast milk onto a starchy cereal-based diet, now with a distended abdomen, bilateral pedal oedema, and patchy, discoloured, brittle hair.
Severely low albumin causing oedema, with near-normal body weight (masked by fluid retention), hair changes and anaemia from reduced globin synthesis, is characteristic of kwashiorkor rather than marasmus.
- →Oedema can mask true muscle wasting on weight-based assessment alone.
- →Low albumin reflects impaired hepatic protein synthesis from prolonged protein deficiency.
- →Refeeding must be gradual to avoid refeeding syndrome.
Frequently Asked Questions
Kwashiorkor results from protein deficiency with adequate calorie intake, causing oedema and a distended belly. Marasmus results from combined protein and calorie deficiency, causing gross wasting without oedema.
They typically lack one or more essential amino acids in adequate amounts and are less efficiently digested/absorbed than animal proteins, though combining different plant sources (mutual supplementation) can overcome this limitation.
Roughage (cellulose) adds bulk to food, aids peristaltic movement through the digestive tract, and traps water in the large intestine, supporting healthy bowel function even though it provides no direct energy.
Quick Revision
10-Minute ReviewKey Takeaways
- BMR reflects the body's resting energy requirement and is influenced by age, sex, muscle mass and pathological state.
- Carbohydrates are the primary and most economical energy source; the brain depends on glucose.
- Lipids provide the highest calorific value and supply essential fatty acids not made by the body.
- Nine amino acids are essential and must be supplied by dietary protein.
- A balanced diet combines cereals, pulses, vegetables/fruits, dairy, fats and (optionally) animal proteins in the correct proportion.
- Kwashiorkor and marasmus are distinct protein-energy malnutrition syndromes with different causes and management.
Competency Checklist
Track Your MasteryReferences
- Vasudevan DM, Sreekumari S, Vaidyanathan K. Textbook of Biochemistry for Medical Students. 8th ed.
- Park K. Park's Textbook of Preventive and Social Medicine. Nutrition chapter.
- NIOS Medical Laboratory Technology curriculum — Biochemistry Module, Lesson 15: Nutrition.