Nutrition and malnutrition

Energy from food

• Fat yields more than twice as much energy per gram because its carbons are more reduced (less oxygen) than those of carbohydrate. Complete oxidation of one palmitate (C16) gives about 106 ATP (129 by older counting), against about 30–32 ATP from one glucose.
• Total energy expenditure = basal metabolic rate (60–70%) + physical activity (most variable) + thermic effect of food (about 10%; highest for protein).
• Typical adult needs: man about 2,500 kcal/day, woman about 2,000; pregnancy adds about 300 kcal/day in later trimesters and lactation about 500.

Basal metabolic rate

• BMR is the energy used at complete rest, awake, fasting and at comfortable temperature, mainly by liver, brain, heart, kidney and muscle. It is set chiefly by thyroid hormone, which increases Na⁺/K⁺-ATPase activity and mitochondrial oxygen use.

Remember Thyroid hormone is the main long-term setter of BMR; fever and catecholamines raise it acutely.

A balanced diet

• Salt under 5 g/day; at least five portions (400 g) of fruit and vegetables a day.
• Essential amino acids (cannot be made): phenylalanine, valine, threonine, tryptophan, isoleucine, methionine, histidine, leucine, lysine; arginine is semi-essential in children. Tyrosine becomes essential in PKU.
• Protein quality: egg protein is the reference (biological value about 100). Cereals lack lysine and pulses lack methionine, so eating them together (complementation) gives complete protein.
• Glycaemic index compares the blood glucose rise from a food with that from pure glucose.

Essential fatty acids

• Humans cannot insert double bonds beyond carbon 9, so linoleic acid (omega-6) and α-linolenic acid (omega-3) must come from the diet. Arachidonic acid becomes essential only if linoleic acid is lacking.
• Linoleic acid leads to arachidonic acid (prostaglandins, leukotrienes); α-linolenic acid leads to EPA and DHA (brain and retina).
• Rich sources: vegetable seed oils (sunflower, corn, soybean) for omega-6; fish oils and flaxseed for omega-3. Coconut oil is mostly saturated and contains the least essential fatty acid.
• Deficiency (long fat-free TPN): scaly dermatitis, hair loss, poor wound healing, growth failure.

Dietary fibre

• Non-starch polysaccharides that human enzymes cannot digest; aim for about 25–30 g/day.

• Colonic bacteria ferment fibre to short-chain fatty acids (butyrate feeds colonocytes). Excess bran adds phytate, which reduces mineral absorption.

Protein-energy malnutrition

• Marasmic kwashiorkor combines both.
• WHO severe acute malnutrition (6–59 months): weight-for-height below −3 Z, MUAC under 115 mm, or bilateral pitting oedema.
• Stunting (low height-for-age) means chronic malnutrition; wasting (low weight-for-height) means acute; underweight (low weight-for-age) mixes both.
• Nutritional assessment uses anthropometry, growth charts, clinical signs, dietary recall and proteins: albumin (half-life about 20 days) reflects long-term status, prealbumin (transthyretin) about 2 days reflects acute change.

Exam trap Oedema separates kwashiorkor from marasmus; weight alone can mislead because fluid hides the wasting.

Managing severe acute malnutrition

• The child's physiology is reductive: low metabolic rate, weak heart, sodium-loaded cells, depleted potassium, and infection without fever. Overloading with fluid, sodium, protein or iron is dangerous.
• Stabilisation phase (days 1–7): treat or prevent hypoglycaemia, hypothermia and dehydration (oral ReSoMal, low in sodium, high in potassium; IV only for shock), correct electrolytes (potassium, magnesium), give broad-spectrum antibiotics to all, correct micronutrients but withhold iron, and start cautious feeding with F-75 (75 kcal/100 mL, low protein) in small frequent feeds, every 2–3 hours day and night.
• Rehabilitation phase: switch to F-100 or ready-to-use therapeutic food for catch-up growth; start iron now; sensory stimulation and follow-up.
• Refeeding syndrome: when feeding restarts, insulin drives phosphate, potassium and magnesium into cells, causing hypophosphataemia, arrhythmia, heart failure and Wernicke encephalopathy. Start slowly and give thiamine first.

Obesity

• BMI = weight (kg) ÷ height² (m²). WHO: underweight under 18.5; normal 18.5–24.9; overweight 25–29.9; obese 30 or more; class III 40 or more. Asian cut-offs are lower because cardiometabolic risk rises at lower BMI: the Asia-Pacific classification used in South Asian guidelines calls 23–24.9 overweight and 25 or more obese (WHO public-health action points are 23 and 27.5).
• Worked example: 81 kg and 1.80 m gives 81 ÷ 3.24 = BMI 25: overweight by the international scale but obese by Asian cut-offs.
• Central obesity predicts risk better than BMI: waist over about 90 cm (men) or 80 cm (women) in South Asians; waist-to-hip ratio.
• Appetite control: the lateral hypothalamus is the feeding centre and the ventromedial nucleus the satiety centre; a ventromedial lesion causes overeating and obesity. Leptin from fat cells reduces appetite (obese people are leptin resistant); ghrelin from the stomach increases it.
• Complications: type 2 diabetes, hypertension, dyslipidaemia, coronary disease, stroke, obstructive sleep apnoea, osteoarthritis, fatty liver, gallstones, PCOS, varicose veins, cancers (endometrial, breast, colon, kidney, oesophageal adenocarcinoma), depression.
• Secondary causes: hypothyroidism, Cushing syndrome, drugs (antipsychotics, insulin, sulphonylureas, steroids, valproate), hypothalamic lesions, Prader–Willi syndrome.
• Cushing syndrome versus simple obesity: features favouring Cushing are wide purple striae, proximal muscle weakness, easy bruising, thin skin and osteoporosis; central fat and plain striae occur in both. Screen with the overnight dexamethasone suppression test or 24-hour urinary free cortisol.
• Management: diet and activity; drugs (orlistat, a lipase inhibitor; GLP-1 receptor agonists such as semaglutide; tirzepatide); bariatric surgery for BMI 40 or more, or 35 or more with comorbidity (lower thresholds for Asians).

How FCPS asks about this

• Energy per gram: fat 9 kcal, alcohol 7 kcal, carbohydrate and protein 4 kcal; palmitate yields about 106 ATP.
• Factor that increases BMR: hyperthyroidism (thyroid hormone is the main regulator); fever and pregnancy also raise it.
• Essential fatty acids: linoleic and α-linolenic acid; coconut oil has the least.
• Benefit of soluble fibre in oats and pulses: lowers LDL and slows glucose absorption.
• Feature separating kwashiorkor from marasmus: oedema (also fatty liver, flaky-paint skin).
• Feed in the stabilisation phase of complicated SAM: F-75 in small frequent feeds; iron withheld until rehabilitation.
• Feature suggesting Cushing syndrome rather than simple obesity: purple striae, proximal myopathy, easy bruising.