Requirements and Starvation

Describe the normal nutritional requirements

Fasting

Fasting is the metabolic state achieved after complete digestion and absorption of a meal.

Fasting can be divided into:

• Early fasting
  Less than 24 hours.
  • Plasma glucose falls due to consumption
    Leads to hormonal changes:
    • Insulin release decreases, causing:
      • Liver
        • Decreased glycogenesis
        • Increased gluconeogenesis
      • Muscle
        • Decreased glucose utilisation
        • Decreased glycogenesis
        • Decreased protein synthesis
      • Fat
        • Decreased lipogenesis
          Due to:
          • Decreased glucose uptake
          • Decreased TG uptake
        • Increased lipolysis
    • Adrenaline release increases, causing:
      • Decreased insulin release
      • Increased lipolysis
      • Increased muscle FFA use
      • Increased hepatic glycogenolysis and gluconeogenesis
    • Glucagon release increases
  • Cellular metabolism alters:
    • Decreased glucose uptake by non-obligate glucose consumers
      e.g. Muscle.
    • Increased FFA and ketone body use
      β-oxidation of FFAs to meet ATP requirements, leading to formation of ketone body.

• Sustained fasting
  Greater than 24 hours. See starvation below.

Starvation

Starvation is the failure to absorb sufficient calories to sustain normal body function, requiring the body to survive on endogenous stores.

• Days:
  • Energy is conserved through reduction in movement
  • Hormonal changes
    • Increased gluconeogenesis, using glycerol, lactate, and amino acids
    • Insulin concentrations fall further
    • Cortisol levels increase
    • Glucagon levels peak at 4 days
  • Metabolic changes
    • Glucose use continues to fall, and FFA use increases
    • Further fall in muscle protein synthesis

• Weeks:
  • Tissues adapt to metabolise ketones (with plasma levels rising up to 7 mmol.L^-1), and gluconeogenesis falls
    • The brain still requires 100g of glucose per day
  • BMR falls
  • All but life-saving movement ceases
  • Death typically occurs after 30-60 days, when muscle catabolism weakens the respiratory muscles such that secretions can no longer be cleared, and pneumonia occurs

Refeeding Syndrome

Refeeding syndrome is a deranged metabolic state that occurs with feeding after a period of prolonged fasting, typically >5 days.

There are three pathogenic mechanisms:

• A large spike in insulin causes increased cellular uptake (and low plasma levels) of:
  • Glucose
  • Magnesium
  • Phosphate
  • Potassium
• Sodium and water retention occurs, which may precipitate cardiac failure
• Increased carbon dioxide production increases minute ventilation and work of exhausted respiratory muscles

Management is by slow institution of feeding and aggressive electrolyte management.

References

1. Chambers D, Huang C, Matthews G. Basic Physiology for Anaesthetists. Cambridge University Press. 2015.
2. ANZCA August/September 2001