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So I read recently this article https://dothemath.ucsd.edu/2011/11/mpg-of-a-human/ as I was trying to design a sci fi alien race for my story and I found out that a human only has an efficiency of converting food to energy of about 25%, what would happen if somehow through a fluke of biology this alien race being very similar to humans had a metabolism with an efficiency of 75%.

What would the effects be? Would it allow them to go for weeks without air or food?

Would it allow them to go for weeks without air or food?

Well. A human can survive several weeks without food, so another similar organism which could make better use of its reserves might reasonably be able to last longer. Three times as efficient, over nine weeks of starvation. Seems reasonable.

Air though, that's a different matter.

Being fuel efficient isn't quite the same as being oxidiser efficient. You might be able to burn up three times as much of the food you ingest as a human does, but that burning process is still gonna require oxygen. I'm not sure how much of the oxygen a human takes into their system is actually "wasted" so I can't answer this bit definitively, but I'd expect that for the same metabolic rate your peeps might have exactly the same oxygen requirements as a human.

Whether or not that is the case, there are other reasons for breathing, such as ridding yourself of gas-phase metabolic byproducts. You can actually last a surprisingly long time on a lungful of pure oxygen (this is often done in emergency procedures like endotracheal intubation which involve you not breathing for a bit whilst people ram things down your windpipe) but the level of $CO_2$ in your lungs will slowly increase. As it does, the ability of your blood to rid itself of excess $CO_2$ is impaired and eventually halted entirely (it requires a concentration gradient to diffuse into your lungs), resulting in respiratory acidosis and eventually death.

Breath holding may be improved by looking at how other animals do it, such as whales or wulruses or crocodiles... a superefficient metabolism is unnecessary, and probably orthogonal.

Fuel efficiency means you need less fuel to accomplish the same work.

If you have a super fuel efficient ugly little car that your friends are ashamed to ride in, it requires less gas than my 1970 Mustang to go the same distance. But you can ride with me if you want. We will split gas costs.

So too your efficient people. For the same activity they require less fuel than a human with normal metabolism.

It is interesting to consider whether existing human "racial" types might have intrinsically different fuel efficiencies.

I am struggling to make this into an engaging scifi concept, though. "Dude who does not need to eat much" does not grab me and shake me in the same way as "Dude who must eat immense amounts" or "dude who eats weird stuff".

What would the effects be? Would it allow them to go for weeks without air or food?

There are animals which are just like that on Earth. They're called reptiles.

A lot of the energy we mammals eat goes towards keeping a constant temperature. Reptiles don't have the same thermal regulation mechanism, so while they are sluggish at night and during winter, they need much less food. And they use much more of the energy content of the food they eat for other things like moving around and keeping a heartbeat.

For the record, some quick googling brings us figures such as crocodiles being able to survive for a whole year without eating (by hibernating in times of drought), and it's usual for them to live on around 50 "full" meals per year (less than one per week).

That's what comes to my mind when you mention super-efficient metabolisms.

You need to define what you mean by "metabolism."

The article you link seems to be discussing how well your body can extract energy from food, not how "efficiently" it uses that energy, so there is a bit of ambiguity.

Would it allow them to go for weeks without air or food?

The short answer is yes and no, if your aliens can extract energy from food more efficiently, then they require less of that food. However, it's not that they're burning fewer calories; they're just getting more from the same amount of food. So then for air? The answer is no. There is a difference in extracting calories from food and using those calories. If we're saying digestive efficiency is higher here, being able to extract calories from food more efficiently doesn't mean your alien's metabolism rate is lower; your alien still needs the same amount of oxygen to properly burn the calories in the food it takes in.

Personally, I find this answer to be a bit boring, so let's delve deeper into other ways your alien can have a "more efficient metabolism" and what they mean.

True Metabolic Efficiency

A concept of "true" metabolic efficiency would mean that your alien is able to use less energy to power its biology. And when it comes to powering biology, obviously we're going to be talking about mitochondria. In all eukaryotic metabolism, glucose is used to power the creation of ATP. The ATP is then used to power the structures of the cell. This happens through glycolysis and the citric acid cycle. Oxygen is used to essentially "burn" the glucose and produce NADH and pyruvate, which are used by the mitochondria to power molecular pumps. These pumps move protons between the membranes of the mitochondria to create a charge differential. This charge differential them moves electrons in the electron transport chain, which essentially uses electricity to power cellular machinery that assembles ATP. Since ATP acts as energy storage, you can think of all this grand machinery as a simple way of charging tiny biological batteries. It's a bit more complex than this, but for our purposes this explanation is sufficient.

Now, if you increase the efficiency of the electron transport chain, you're able to assemble more ATP with less input. Unfortunately, this chain is already pretty efficient since it's had 4 billion years to perfect, but for world building purposes let's go all Jeff Goldblum here and assume "life has found a way."

What would it mean if this electron transport chain, or indeed the mitochondria as a whole, is more efficient than that of a human? Well, we already have some biological examples: birds.

Birds already have more efficient mitochondria than humans, and it leads to a lot of interesting effects.

Mitochondrial processes are the prime source of oxidative stress, which causes damage to the cell, and is a main contributor to aging. Oxygen is a very nasty chemical; it's quite reactive, and its greek name means "acid maker." Our cells are using it to literally burn glucose, but it cuts both ways: the oxygen can burn the cell itself and cause damage. This is because waste products and intermediary products called free radicals leak out of the mitochondria as part of its normal metabolic processes. The cell has to spend energy neutralizing the free radicals and repairing damage from them, which all effects efficiency. Mitochondria in birds are more efficient, and produce far less of the waste products and free radicals that cause this damage in the first place. The result? Birds have extraordinarily long life-spans for creatures their size. For example, the african grey parrot and macaw both have lifespans on the order of 60 years, despite being warm blooded with high metabolic rates. You'd be hard pressed to find a mammal of similar size that lives as long.

Birds also process oxygen much more efficiently than mammals. Although this is due mostly to the way their lungs are constructed, their highly efficient mitochondria no doubt also play a role.

So, to answer your question, if you want your aliens to have a truly more efficient metabolism, say their mitochondria (or whatever equivalent organelle) are highly efficient.

In this particular case, you might even be able to say they can go a long time without air. Probably not weeks, but definitely longer than a creature of the same size and metabolic rate.

You could say they process glucose more efficiently, and their molecular pumps and the electron transport chain are much more efficient. In this case, yes, they could go a long time without food. You could even throw in better digestive efficiency to make them extract more calories from their food as well for an even bigger punch.

In addition, because their mitochondria are super efficient, they would likely live much longer. A human with bird mitochondria might actually live for 200 years or more, so you could say your aliens live for a very long time.