Creating a scientifically semi-valid super-soldier, addendum: Metabolism

Question

A while back Demigan had a running series of posts about various aspects biological enhancement for the purpose of designing super-soldiers with an element of (pseudo) science behind them.

You can find his previous questions here:

Creating a scientifically semi-valid super-soldier, part 1: Skeleton

Creating a scientifically semi-valid super-soldier, part 2: nervous system

Creating a scientifically semi-valid super-soldier, part 3: Physical shock resistance

Creating a scientifically semi-valid super-soldier, part 4: respiratory system

Creating a scientifically semi-valid super-soldier, part 5: Heart and circulatory system

Creating a scientifically semi-valid super-soldier, part 6: Radiation protection

Creating a scientifically semi-valid super-soldier, part 7: Hearing

Creating a scientifically semi-valid super-soldier, part 8: Communication

Creating a scientifically semi-valid super-soldier, part 9: Temperature control

One thing I didn't really see mentioned, which I was wondering about, is metabolism. Obviously, a super-solider with powered-up muscles, nervous system, etc. is going to be burning a LOT of energy, even at rest. The post about temperature control is basically asking how to deal with all the excess heat that is created as a result.

Metabolism is the process by which the body converts food/fuel into the energy it needs to function. The higher the energy needs of the organism, the greater the amount of fuel required, or the more efficient the conversion process must be.

In nature, we do see that some creatures have more efficient metabolisms than others. For instance, warm-blooded beasties have more efficiently designed metabolisms than cold-blooded beasties because they need to use additional energy warming their bodies and therefore need to use the energy that they have in a more efficient way.

Increasing metabolic efficiency would also probably help with the temperature problem a bit as well. This article notes:

no energy transfer can be perfectly efficient – that's a basic law of physics. Instead, each time energy changes forms, some amount of it is converted into a non-usable form. In the reactions of an animal's metabolism, much of the energy stored in fuel molecules is released as heat.

Logically, the more efficient we can make this conversion, the less heat will be produced as waste.

Given a super-soldier's high rate of energy usage, how do we stop them from needing to eat constantly, 24/7? The less we have to feed them, and the longer they can survive cut off from supplies, the better. Any ideas for improving these guys' metabolic efficiency or generally finding ways to feed them a more "normal" amount while still powering their high energy needs?

To stay in keeping with Demigan's posts, a good answer needs to be limited to a biological solution, where a body can build, repair and maintain it.

Answer 1

As high as possible, with the ability to regulate it downwards.

When well supplied and in a combat situation their metabolisms would spike. Soldiers would be able to move and think quicker, and recover from injuries faster. If well supplied while marching this would also give them a mobility advantage compared to other footsloggers over rough terrain (any other terrain and they should be in transports).

When not fighting, they can regulate their metabolism downwards to a more manageable level. Arguably, humans are probably a little too highly metabolised for the large majority of warfare. Sitting in trenches waiting for an attack doesn't require that much energy. They can save on supplies by reducing their metabolism. Platypodes and Echidnas are able to maintain a fairly active lifestyle with body temperatures around 32 degrees celsius. Something like that would allow for a moderately alert fighting force that doesn't require quite so much energy to maintain.

When not on active duty, they could enter a sort of torpor or hibernation to reduce their metabolism as close to zero as possible. For storage during peacetime, or transport at some distance from the front lines this would be ideal. In protracted conflicts, you could always have the majority of your fighting force in torpor with only a select few alert sentries (provided you're confident you can wake your sleeping soldiers in time to respond to an attack). There's also a benefit of having an intermediate torpor state where they are still capable of some activity in case they are cut off from supplies for a protracted amount of time, but still at risk from the enemy.

Answer 2

It requires preserving blood around vital organs, allowing peripheral tissues to starve, just as whales and seals sort the body's oxygen supply by being cut off from the air. This proved so effective that over time it became the norm even among active super soldiers; The only drawback: they will have a dead-pale color. This skin color is a strategy for increasing their fuel consumption. When lactate levels on the surface of tissues become too high - or when they feed - blood is redistributed to the skin, and their body turns red. Also, during sleep, their metabolic activity will fall by about half of normal values.

Answer 3

Several improvements

1. The ability to digest what we call dietary fiber, specifically cellulose, and use it for energy. All that is needed here is an enzyme that has the ability to break down cellulose and use it for energy. Making any plant or tree into a source of pure sugar.
2. Low basal metabolic rate, similar to Ynneadwraith's answer, this is the amount of energy that the soldier's body needs to maintain itself while at rest. There is no reason why this has to be high, as it is only during exertion when this needs to change. The same thing happens in all humans already, so little modification is needed.
3. Large storage capacity. Normally this is called fat in humans. The super soldier would need to be able to make and burn fat at an accelerated rate in order to charge and discharge their energy reserves (fat).