# What is the minimum energy needed to stay alive?

This is a bit of an odd question, but here goes.

Your body needs energy to function. Each organ (brain, heart, etc.) need different amounts of energy to function properly. So it stands to reason that your heart's minimum required energy is higher than that of your spleen (just an example). So here's my question:

"Which organ requires the most minimum energy, and how much energy is the minimum?"

I'm asking this question because I'm trying to figure out "if energy were removed from a body, such that all parts of the body had the exact same amount of energy, what would be the minimum energy needed to make sure no organs failed" (this has to do with World Building, to an extent).

Furthermore, it'd be very helpful to know how much energy would be required for said organ not only for survival, but also to maintain a relatively normal, or even active, functionality. Would this equal the energy normally found in a person, or could it be lower?

If I need to clarify further, just let me know. Thank you in advance!

• In general, organs will start to fail if blood pressure drops below 60 for sustained periods of time. Even for short periods of time, being below 60 can cause permanent damage. While some may fail sooner than others (re: certain organs need more energy) surviving while even one organ fails (more than likely the kidneys) without medical attention is nearly impossible. May 4 '17 at 22:33

## Resting energy

The number you are looking for is basically the energy the body needs in a resting state.
When resting, we assume that no energy is used for any actions but vital ones, as the body is not exactly doing anything.

## Measuring that energy

Measuring that energy is slightly more complicated. Here is my solution to it:
With any machine, we are talking about efficiency. This is basically the percentage of energy that is actually used and not turned into heat.

The efficiency of the human respiratory system (the ATP cycle) is 39%. This means that 61% of the energy is transferred as body heat and 39% are actually being used.
(The maximum practical efficiency of combustion engines is 35-37%, just for comparison.)

## Doing the maths

The energy of our body heat is approximately 80 watt. Because 61% of the total energy vital for our body is the body heat, the 80 Watt are 61% of the total energy necessary.

Therefore, the total energy needed for the body is 130 watt or 130 Joule / second.

## The result in perspective

This energy is the same as 468kJ per hour or 11MJ per day. Converting this into kcal results in 2629kcal.

This is roughly 200kcal more than WebMD lists as requirements for a sedentary average male. Why could this be the case? It is probably because the body heat energy is difficult to measure and prone for errors. If it was 60 watt instead, we would end up with 2000kcal / day, which seems a bit more realistic to me.

Edit: According to Jason's answer, my end result should be around 1200kcal / day. This leaves me wondering what has messed up my calculations...

## Note on Wikipedia

I couldn't find a link to an online version of the source that Wikipedia cites. There seems no other reliable reference out there that has a number. This is why I cited wikipedia.

• According to the wiki article, anyways, the Mifflin St. Jeor equation is one of the most accurate predictors (of an average) to date. The paper (pay barrier) is here and from the abstract it seems it was derived empirically, but the paper may have other details. My gut tells me it's too complex a subject to be concisely summarized, and so digging in to some of those papers might be your best bet for an explanation (the Harris ones, too). I suspect your case is due to an oversimplification regarding respiratory efficiency and its relation to BMR. Apr 27 '17 at 20:18
• @JasonC Yeah, this will probably be the case. My gut tells me that I would be more likely to underestimate the number though than overestimating. Magnitude 2 is striking though. Thanks for the link, I'll have a read Apr 27 '17 at 20:20
• Btw you can get higher than 1200 with the Mifflin St. Jeor equation. If you plug in the proper age, weight, height, for whatever WebMD defined as the "average [fe]male" you'd probably get closer. Wikipedia's cited example number is for a 55-year old, 130 lb, 5'6" woman, which is much closer to WebMD's sedentary female number for that age. Also BMR has a strict set of criteria, so there's that, too. And WebMD uses a different equation. This seems to be one of those "fuzzy" things. Apr 27 '17 at 20:29

It sounds like what you are looking for is the basal metabolic rate, which is the rate that you expend energy at rest, which roughly translates to the minimum rate that you need to consume energy in order to survive long term.

It varies significantly from person to person based on a large number of factors. A number of formulas to estimate it for a given person are present in that article, as well as details on factors that contribute to variance. Too much to summarize here, but BMR is the term you'll want to search for for more information.

As for the metabolic rates of various tissues, there is also a lot of research on that as well, here is one example (archive), and general search terms are things like "energy expenditure per organ".