Metabolic fuel is a well-known parameter which measures the body’s fuel preference in energy production. 

Your body primarily relies on three sources of fuel for energy: carbs, fats, and proteins. Proteins aren’t a major source of energy as the body uses them primarily as building blocks. Fats and carbs, our body’s main energy sources, are partnered up to fuel us in varying situations. However, in every given situation, the ratio of fat to carb usage shifts.

In order to live our bodies must regulate the fluxes of multiple fuel sources to support changing metabolic rates that result from variations in physiological circumstances. The aim of fuel selection strategies is to exploit the advantages of individual substrates while minimizing the impact of disadvantages from years of torpor to seconds of sprinting. The regulation of energy metabolism is a complex challenge because the fuels available vary greatly in stored quantity, energy density, speed of conversion to ATP and water solubility. 

RQ (respiratory quotient) is the gold standard measurement for directly determining metabolic fuel usage. However, it is an invasive, challenging and unavailable technique for most. RER (respiratory exchange ratio) is currently the preferred method for determining metabolic fuel. RER estimates the relative contribution of carbohydrate and lipids to overall energy expenditure, via CO2 production and O2 uptake, in an indirect manner. 

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