What is the main source of fuel in sepsis?

Main Source of Fuel in Sepsis: Fatty Acids

The main source of fuel in sepsis is fatty acids (option B). During sepsis, there is a significant shift in metabolic pathways with lipolysis becoming a dominant process, making fatty acids the primary energy source 1.

Metabolic Changes in Sepsis

Fatty Acid Metabolism

• Sepsis triggers inflammatory cytokines (IL-1β, TNF) and stress hormones (cortisol, adrenaline, glucagon) that dramatically increase lipolysis of adipose tissue 1
• This results in elevated plasma free fatty acids (FFAs) and triglycerides, which can increase up to four-fold in septic patients 1
• Fatty acids become the primary fuel source as lipid metabolism and turnover are significantly increased during critical illness 1

Glucose Metabolism

• While hyperglycemia is common in early sepsis due to insulin resistance, glucose is not the main fuel source 1
• Glucose utilization is primarily redirected to immune cells to promote aerobic glycolysis and immune function 1
• In later stages of sepsis, hypoglycemia often develops due to peripheral glucose usage and anorexia 1

Other Metabolic Substrates

• Ketone bodies, which are produced from fatty acids, are actually suppressed in sepsis 1
• Amino acids may be mobilized but are not the predominant energy source 1

Pathophysiological Evidence

The shift to fatty acid utilization in sepsis is supported by several key observations:

1. Catabolic State: Sepsis induces a highly catabolic state where larger molecules are broken down to meet increased energy demands 1

2. Lipolysis Upregulation: Inflammatory mediators and stress hormones significantly increase hormone-sensitive lipase activity, leading to enhanced breakdown of lipid stores 1

3. Respiratory Quotient Changes: Studies show that hypermetabolic patients (including those with sepsis) have a respiratory quotient of approximately 0.76-0.90, indicating predominant fat utilization even when glucose is administered 2

4. Persistent Fat Oxidation: Unlike in depleted but non-septic patients, septic patients continue to utilize endogenous fat as an energy source even when given large glucose loads 2

Clinical Implications

Understanding that fatty acids are the main fuel source in sepsis has important clinical implications:

• Metabolic Support: Parenteral nutrition strategies should account for the body's preference for fatty acid utilization during sepsis 1

• Potential Complications: The buildup of fatty acids can lead to toxicity and organ damage if utilization pathways become impaired 1

• Glycemic Control: While glucose control remains important, targeting overly tight glycemic control (80-100 mg/dL) may be harmful; a target of 140-180 mg/dL is now recommended 3

• Nutritional Considerations: Excessive carbohydrate administration may not be beneficial as septic patients preferentially utilize fat stores regardless 2

In conclusion, while multiple fuel sources are utilized during sepsis, fatty acids represent the predominant energy substrate due to enhanced lipolysis and the body's metabolic shift toward fat utilization even in the presence of adequate glucose.