What is the primary source of fuel in critical illness or acute injury?

Primary Source of Fuel in Critical Illness or Acute Injury

The primary source of fuel in critical illness or acute injury is skeletal muscle, which is broken down to provide amino acids for gluconeogenesis and to support the body's increased metabolic demands. 1

Metabolic Response in Critical Illness

Critical illness and acute injury trigger a complex metabolic response characterized by distinct phases:

1. Acute Early Phase (ICU day 1-2):

   • Marked by insulin resistance and accelerated glucose production
   • Often called "stress diabetes" or "diabetes of injury"
   • Skeletal muscle breakdown begins rapidly

2. Acute Late Phase (ICU day 3-7):

   • Continued catabolic state with significant tissue breakdown
   • Altered pathways of energy production
   • Sustained protein catabolism from skeletal muscle

3. Recovery Phase (after ICU day 7):

   • Anabolic recovery phase
   • Resynthesis of lost tissue begins
   • Body becomes more metabolically able to process delivered nutrients 1

Why Skeletal Muscle is the Primary Fuel Source

During critical illness or acute injury:

• Skeletal muscle provides amino acids that are essential for:

  • Gluconeogenesis (production of new glucose)
  • Supporting increased metabolic demands
  • Synthesis of acute phase proteins
  • Immune function 1

• This process leads to significant muscle wasting, which explains why:

  • Higher protein requirements (1.2-2 g/kg/day) are recommended to minimize muscle loss
  • Patients experience profound weakness after critical illness 1

Role of Other Potential Fuel Sources

While skeletal muscle is the primary fuel source, other metabolic sources play supporting roles:

• Liver:

  • Plays a crucial role in glucose metabolism
  • Increases glucose production through upregulation of both gluconeogenesis and glycogenolysis
  • However, the liver requires amino acids from skeletal muscle breakdown to produce glucose 1, 2

• Fat:

  • Lipid metabolism is increased as fatty acids are used as a fuel source
  • This is secondary to skeletal muscle breakdown 1

• Amino Acids:

  • While amino acids are crucial, they come primarily from skeletal muscle breakdown
  • They are not a primary storage form of energy but rather the product of muscle catabolism 1

Clinical Implications

Understanding that skeletal muscle is the primary fuel source has important clinical implications:

• Hyperglycemia Management:

  • Normalization of glucose is primarily achieved by reducing endogenous glucose production rather than increasing glucose disposal 1, 3
  • In traumatized patients, hyperglycemia is due to increased endogenous glucose production 3

• Nutritional Support:

  • Energy provision should be equal or lower than measured energy expenditure during the acute phase
  • Less than 100% of energy expenditure should be targeted in the early phase due to endogenous glucose production
  • Increases in nutritional support are appropriate in the stable/recovery phase 1
  • Overfeeding in the early phase may be harmful as endogenous production is already enhanced 1

Common Pitfalls in Management

• Overfeeding: Providing excessive calories in early critical illness can worsen outcomes by interfering with the body's endogenous metabolic response
• Inadequate protein: Failing to provide sufficient protein can accelerate muscle loss
• Ignoring the phase of illness: Nutritional needs change throughout the course of critical illness, requiring adjustments to feeding strategies
• Focusing solely on glucose control: While important, this addresses only one aspect of the complex metabolic derangements

The evidence clearly demonstrates that skeletal muscle serves as the primary fuel source during critical illness or acute injury, with the liver, fat metabolism, and amino acid utilization playing supporting roles in the overall metabolic response.