The Hierarchy of Fuel Selection - Which Fuel Do You Preferentially Burn?

The oxidative hierarchy follows the body’s relative storage capacity for the different substrates and their role in ensuring survival. In this context hierarchy=priority (of fuel selection)

1. Thus, the hierarchy is dominated by alcohol because the body has practically no storage capacity for it and must be eliminated by oxidative disposal through rapid hepatic oxidation (1,2,3).

2. Carbs come next in fuel selection. Due to the relatively small storage capacity for carbohydrates as glycogen (4,5,6,7) and the need to maintain glucose homeostasis within tight limits, ingestion of excess carbohydrates causes an acute autoregulatory increase in their oxidation (5,8,9,10,11). Lipogenesis is quantitatively unimportant in humans (12,13).

3. Protein comes next. For quite some time there was some dispute as to which was dominant, carbs or protein (14), but there’s clear evidence in favor of carbohydrate, at least under certain circumstances (11). Protein oxidation is related to protein ingestion (15).

4. Fat appears at the base of the hierarchy of fuel selection because there is virtually infinite capacity for fat storage. Thus, the rate of fat oxidation is regulated by the presence or absence of the other macronutrients. Futhermore, there is no autoregulatory mechanism between fat intake and fat oxidation, in other words, fat doesn’t regulate its own oxidation after intake (6,16,17,18), and is further reduced under conditions of caloric excess (6,7).

Fat oxidation is not increased after a single meal (16,17) and over an entire day excess fat intake does not increase fat oxidation (18). Increases in fat oxidation occur secondary to increases in the body fat mass (19). Not even with 50% excess calories from fat does fat promote its oxidation, oxidation rates remain practically equal to baseline before the overfeeding (20).

­ The studies cited are all tightly controlled using calorimetry, isotopic tracers, etc. Context is post-prandial, overfeeding, underfeeding, men and women obese and lean, short term and long term. Given what is available to the body the body will channel those substrates in a priory fashion following that hierarchy=priority.

We can also think of insulin, what does insulin do? Suppresses lipolysis when glucose is present.

Here is the twist: in practice you are usually burning a mix of fuels (21). Even when carbohydrates are the main fuel source fat oxidation is not 100% suppressed.

For example, in one study 24-hr total substrate oxidation (whole-room calorimetry) was measured in Sedentary Lean (n=10), Sedentary Obese (n=9), Sedentary Reduced-Obese (n=7) and in Physically Active Reduced-Obese (n=12) men and women with or without an acute exercise bout (21).  

All meals were prepared by a metabolic kitchen and had the same macronutrient composition: 15% protein, 30% fat, 55% carbohydrate.

Twenty-three hour data on respiratory gas exchange were extrapolated to 24-hr values, here is what happens:

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21.Audrey Bergouignan, Elizabeth H. Kealey, Stacy L. Schmidt, Matthew R. Jackman, and Daniel H. Bessesen. Twenty-Four Hour Total and Dietary Fat Oxidation in Lean, Obese and Reduced-Obese Adults with and without a Bout of Exercise. PLoS One. 2014; 9(4): e94181.