#16. The Principles of Energy Metabolism

Hi, I am Dr. Robert Bilkovski. Welcome to this podcast series on Indirect Calorimetry. In this first episode, we will go over the principles of energy metabolism.

The goals of this podcast are to:

• Provide you with an understanding of the difference in calories between fat, carbohydrates and proteins,
• Discuss the components of energy expenditure, which includes Resting Energy Expenditure, the Diet-induced Thermogenesis and Activity Energy Expenditure, and
• How measurement of energy expenditure can inform feeding status, whether that being over-fed or under fed. This will be guided by the RQ or respiratory quotient.

Let´s begin. First, to put nutrition and calorie mix into perspective, we must understand how the body acts at rest. During periods of rest, like we are now listening to this podcast, the body preferentially metabolizes fat rather than carbohydrates.

In contrast, during periods of activity, the body switches to preferential utilization of carbohydrates. For those athletes amongst us, who have heard of the importance of muscle glycogen and the need to consume carbohydrates during periods of endurance activities?

The RQ, or Respiratory Quotient is utilized to inform not only calorie mix but adequacy of feeding. First the goal for calorie mix is an RQ between 0.8 and 1.0 which implies a balanced mix of calories¹.

While a respiratory quotient that is less than 0.7 suggests predominantly fat utilization and that the patient is at risk of being overfed.

In contrast, a respiratory quotient that is greater than 1.3 suggests predominant carbohydrate consumption and the risk of over-feeding may be present.

In summary, an RQ or respiratory quotient close to 0.8 is ideal and most closely reflects the metabolism of the body at rest.

Measurement tools used to determine energy expenditure includes indirect calorimetry, while other equations and algorithms can be used for this measure; and these estimates will be discussed in more detail in future podcasts.

Indirect calorimetry is a measurement of respiratory gas exchange at the alveolar level in order to inform cellular metabolic activity. What indirect calorimetry can measure includes total energy expenditure and respiratory quotient (as we discussed earlier).

RQ is derived from the oxygen consumption, which is also noted as VO2, and the production or elimination of carbon dioxide which is noted as VCO2.

• VO2 is a reflection of the balance between oxygen delivered to the tissue and the metabolic needs of that tissue, these metabolic needs can change between sleeping, exercise, or the extreme cases of burns and sepsis.
• VCO2, in contrast, is a measurement that reflects oxidation of nutrients at the cellular level, where VCO2 measurements are dependent on the adequacy of ventilation at the alveoli.
• In summary, the RQ is derived from the following equation, of VCO2 divided by VO2.

More importantly, the respiratory quotient at steady state is equal to the resting energy metabolism.

As stated earlier, RQ can be helpful to inform nutrient mix as well as caloric needs. The goal is to provide adequate calories through fats, proteins and carbohydrates while at the same time avoiding over and under-feeding to strike that balance, one must understand factors that may increase or decrease energy expenditure of a patient.

A few notable examples that increase energy expenditure includes fever and sepsis, pain and large open wounds such as burns.

In contrast, a reduction in energy expenditure can result from advanced age, obesity and the use of narcotic analgesics and neuromuscular blocking agents².

Both over and under-feeding have the potential for unwanted patient complications, namely, prolongation of mechanical ventilation. Over-feeding also increases the risk of infection, hypercapnia as well as liver and bile duct flow issues. In contrast, under-feeding can decrease lean muscle mass and result in diminished muscle function^3,4.

In closing, the measurement of energy expenditure provides the clinician with a tool that allows them to tailor nutritional support, in order to prevent over-feeding and under-feeding.

The next podcast will go into more details on ways energy expenditure can be determined at the bedside.

Thank you for listening to this podcast on the Principles of Energy Metabolism. In the next podcast we will compare indirect calorimetry and predictive equations.

References

1. McClave, S. A., et al. (2003). "Clinical use of the respiratory quotient obtained from indirect calorimetry." JPEN J Parenter Enteral Nutr 27(1): 21-26
2. Weissman, C., et al. (1986). "The energy expenditure of the mechanically ventilated critically ill patient. An analysis." Chest 89(2): 254-259
3. Singer, P., et al. (2011). "The tight calorie control study (TICACOS): a prospective, randomized, controlled pilot study of nutritional support in critically ill patients." Intensive Care Med 37(4): 601-609.Petros, S., et al. (2016). "Hypocaloric vs Normocaloric Nutrition in Critically Ill Patients: A Prospective Randomized Pilot Trial." JPEN J Parenter Enteral Nutr 40(2): 242-249.
4. Casaer, M. P., et al. (2011). "Early versus late parenteral nutrition in critically ill adults." N Engl J Med 365(6): 506-517