Key Facts

Nutrition provides the substrates for energy metabolism, tissue repair, and physiological regulation. Understanding nutritional biochemistry is essential for recognising deficiency states, prescribing supplementation, and managing nutritional support.

Epidemiology

Malnutrition (undernutrition) affects approximately 3 million people in the UK, costing the NHS an estimated £19.6 billion annually. Obesity (BMI ≥30) affects ~28% of UK adults. Micronutrient deficiencies (iron, vitamin D, B12, folate) are common in both developing and developed countries.

Aetiology

Macronutrients:

• Carbohydrates (4 kcal/g): Primary energy source; glucose is the obligate fuel for brain and red blood cells
• Proteins (4 kcal/g): Structural, enzymatic, immune functions; essential amino acids must be obtained from diet
• Fats (9 kcal/g): Energy storage, cell membrane structure, hormone synthesis; essential fatty acids (omega-3, omega-6)

Micronutrients:

• Water-soluble vitamins (B complex, C): Not stored significantly; regular intake needed
• Fat-soluble vitamins (A, D, E, K): Stored in liver and adipose tissue; malabsorption causes deficiency
• Trace elements: Iron, zinc, copper, selenium, iodine — enzyme cofactors

Pathophysiology

Refeeding syndrome pathophysiology:

• Starvation → catabolic state, depletion of intracellular electrolytes, glycogen exhaustion
• Refeeding → insulin secretion → glucose uptake and anabolic processes → massive intracellular shift of phosphate, potassium, magnesium
• Consequences: Hypophosphataemia (most dangerous), hypokalaemia, hypomagnesaemia → cardiac arrhythmias, respiratory failure, cardiac failure
• NICE CG32 criteria for high risk: BMI <16, unintentional weight loss >15% in 3-6 months, little/no nutritional intake >10 days

Vitamin Deficiencies

• Thiamine (B1): Wernicke's encephalopathy (triad: confusion, ophthalmoplegia, ataxia); beriberi (wet = cardiac failure; dry = neuropathy)
• Niacin (B3): Pellagra (dermatitis, diarrhoea, dementia — the 3 Ds)
• B6 (pyridoxine): Peripheral neuropathy, sideroblastic anaemia, seizures (isoniazid-induced)
• Folate: Megaloblastic anaemia, neural tube defects (prevention: 400mcg preconception)
• B12: Megaloblastic anaemia, SACD, glossitis, neuropsychiatric symptoms
• Vitamin C: Scurvy — bleeding gums, petechiae, poor wound healing
• Vitamin A: Night blindness, Bitot's spots, xerophthalmia, keratomalacia
• Vitamin D: Rickets/osteomalacia, proximal myopathy, bone pain
• Vitamin K: Bleeding, prolonged PT/INR

Red Flags

• Acute Wernicke's encephalopathy — medical emergency, give Pabrinex IV immediately
• Refeeding risk in malnourished patients — check phosphate, K⁺, Mg²⁺ before and during feeding
• B12 deficiency with neurological symptoms — treat urgently (neurological damage may be irreversible)

Bedside

• BMI calculation: <18.5 underweight, 25-29.9 overweight, ≥30 obese
• MUST (Malnutrition Universal Screening Tool): 5-step screening for malnutrition risk
• Mid-arm muscle circumference: Nutritional status assessment

Bloods

• FBC and film: Microcytic (iron), macrocytic (B12/folate) anaemia
• Iron studies: Ferritin, serum iron, TIBC, transferrin saturation
• B12, folate: Serum levels
• 25-OH vitamin D: <25 nmol/L deficient, 25-50 insufficient
• Bone profile: Calcium, phosphate, ALP
• Albumin, pre-albumin: Nutritional markers (though albumin is also an acute phase reactant)
• Phosphate, Mg²⁺, K⁺: Essential before refeeding
• HbA1c, lipid profile, TFTs: Metabolic assessment

Imaging

• DEXA scan: Bone density if vitamin D deficiency with fracture risk
• X-ray: Rickets (cupping/fraying of metaphyses), osteomalacia (Looser zones)

Special Tests

• Methylmalonic acid (MMA): Elevated in B12 deficiency (distinguishes from folate deficiency)
• Homocysteine: Elevated in both B12 and folate deficiency
• Intrinsic factor and parietal cell antibodies: Pernicious anaemia
• Schilling test: Historical test for B12 malabsorption (rarely performed now)

Non-pharmacological

• Dietary counselling: Dietitian referral for malnutrition, obesity, or specific deficiencies
• MUST screening: All hospital inpatients on admission
• Oral nutritional supplements (ONS): Fortisip, Ensure — if oral intake inadequate
• Nasogastric/nasojejunal feeding: If unable to meet requirements orally
• TPN (total parenteral nutrition): If GI tract non-functional — specialist nutrition team oversight

Pharmacological

• Thiamine: Pabrinex IV (B/C vitamins) 2 pairs TDS for 3-5 days for Wernicke's; oral thiamine 100mg TDS for prophylaxis
• B12: Hydroxocobalamin 1mg IM alternate days × 2 weeks (if neurological symptoms: 3 weeks), then every 3 months
• Folate: Folic acid 5mg OD for 4 months; 400mcg OD preconception (5mg if risk factors)
• Iron: Ferrous sulphate 200mg BD-TDS; IV iron (ferric carboxymaltose) if oral intolerant/malabsorption
• Vitamin D: Colecalciferol loading 50,000 IU weekly × 6 weeks, then 800-2000 IU daily maintenance
• Vitamin K: Phytomenadione 10mg IV for bleeding/high INR
• Refeeding protocol: Start at 10 kcal/kg/day (5 kcal/kg if very high risk), gradually increase; supplement phosphate, K⁺, Mg²⁺, thiamine

Referral Criteria

• Complex nutritional support — nutrition team/dietitian
• Suspected eating disorder — psychiatry
• Refractory deficiency despite replacement — gastroenterology (malabsorption workup)

• Wernicke's encephalopathy: If untreated, ~20% mortality, ~80% progress to Korsakoff's syndrome (irreversible amnesia)
• B12 deficiency: Haematological features fully reversible; neurological features may be permanent if treatment delayed >6 months
• Iron deficiency: Excellent prognosis with replacement; must investigate cause in men and post-menopausal women
• Refeeding syndrome: Mortality up to 20% if unrecognised; preventable with careful refeeding and electrolyte monitoring
• Vitamin D deficiency: Responds well to supplementation; skeletal deformities from rickets may persist

Vitamin Deficiency Summary

Trace Element Deficiencies