A *very* special dual-fuel car analogy for the human body that I just invented.

The human body is like a very special dual-fuel car.

From http://www.aa1car.com/library/alternative_fuels.htm

In this very special dual-fuel car:-

Glucose is represented by Ethanol, 'cos Ethanol is a carbohydrate, according to Robert Lustig ;-)
Glucose is C₆H₁₂O₆. Ethanol is C₂H₆O. 3(C₂H₆O) = C₆H₁₈O₃. It's not very close, but it'll do!

Caprylic acid is represented by Octane, 'cos fatty acids are hydrocarbons, don'tcha know? ;-)
Caprylic acid is CH₃(CH₂)₆COOH and Octane is CH₃(CH₂)₆CH₃, which is actually pretty close.

Storage depots:

 

Carbohydrates:

For Ethanol, there's a large storage tank (≡ muscle glycogen) and a small storage tank (≡ liver glycogen). The contents of the large storage tank cannot be used to top-up the small storage tank, but the contents of the small storage tank can be used to top-up the large storage tank. The contents of the small storage tank are used to fuel a generator (≡ Hepatic Glucose Production) to keep the ECU (≡ brain) working at all times. The contents of the large storage tank are used to fuel the engine.

Fats:

For Octane, there's a large storage tank (≡ subcutaneous adipose tissue) and a small storage tank (≡ visceral adipose tissue). The contents of the small storage tank are used to produce hormones etc. The contents of the large storage tank are used to fuel the engine.

Substrate Utilisation:

When the car is driven at low speed, the engine burns mostly Octane (≡ RQ=0.7).
When the car is rapidly accelerating or driven at high speed, the engine burns mostly Ethanol (≡ RQ=1).
When the car is being driven intermediately, the engine burns a mixture of Octane & Ethanol.

Overeating/Undereating:

 

Carbohydrates:

If the large Ethanol storage tank becomes full, excess Ethanol overspills to the small storage tank.
If the small storage tank becomes full, a gizmo kicks-in and converts excess Ethanol into Octane (≡ De-Novo Lipogenesis).
It also shifts fuel usage of the engine towards Ethanol, to deplete Ethanol as quickly as possible.
If the small storage tank becomes full, the car malfunctions (≡ fatty liver).

Conversely, if the small storage tank becomes nearly empty, it shifts fuel usage of the engine towards Octane, to conserve Ethanol.

Fats:

If the large Octane storage tank becomes full, excess Octane overspills to the small storage tank.
If the small storage tank becomes full, the car malfunctions (≡ insulin resistance/metabolic syndrome/type 2 diabetes).