# ATP energy "quantitative" output?

• jerromyjon
So in summary, the average human consumes about 2000 kcal per day, which converts to about 100 watts. Most humans consume about 2000 kcal per day, which converts to about 100 watts, meaning that to power an average American household, you would need to burn one adult body per week.f

#### jerromyjon

Adenosine triphosphate cycle is what releases stored energy for the body to use throughout to do work, right? So I'm just curious if is easy to guess what the total output in an average human converted to electrical potential in a "sustained demand" versus total conversion of entire consumable mass? When I say entire consumable mass I mean like percentage of components your body can break down for energy if you had another body to maintain consumption... it's just a silly curiosity, my ballpark guess would be high kilowatts/second just "doing hard work with your muscles and the circulatory work" but then maybe gigawatts total "biologically available" energy?

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Most humans consume about 2000 kilocalories per day, which converts to about 100 watts. Note that watt is a unit of power (energy/time), not energy, and I have no idea what quantity would have units of kilowatts/second.

Most humans consume about 2000 kilocalories per day, which converts to about 100 watts
That doesn't make sense to me at all, I mean on a physical output ability, say pedalling a generator bike, I could (very poorly efficiency-wise) produce energy through work.
"In an effort to raise awareness about energy use and global warming, the BBC even made a TV-programme in which an entire household was powered via these generators, with 80 cyclists generating up to 14 kW."
What I am trying to get a ballpark idea about is bypassing all physical work, taking the energy right out of the blood sugar, so to speak, there would have to be more energy than what your physical work produces?

I'm not sure how the heat produced factors into this.

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Here is how heat generation factors into this:
Endothermic organisms generate heat energy to maintain body temperature -- and regulate it within some temperature range. Mammals are endothermic, humans are mammals.

The price of being endothermic is that energy is consumed just to make heat. Wikipedia on BMR:
Basal metabolic rate (BMR) is the minimal rate of energy expenditure per unit time by endothermic animals at rest.
The advantage of being endothermic is that the organism can behave normally at a broad range of ambient temperatures, unlike ectothermic (cold blooded) reptiles that have to warm up after a cold night before being fully functional.

The BMR for humans is not fixed; it is a function of the mass of fat free tissue per unit of body weight. For this reason BMR declines in adult humans at about 1%-2% per year, largely because of the loss of fat-free tissue, mostly muscle. Or, more personally, this is why middle-aged people tend to gain weight.

Also there are other metabolic pathways than ATP (and friends) that transfer energy - NADH for example. Alternatively neurons store energy and later released as an electric potential. And ATP may not be available everywhere all the time in every cell.

So, in fact if you 'take out' heat you kill the person as a future source of ATP.

Okay, so go back to square one with that extra bit and see what you want to to ask.

Ok, so the average human has a mass of ~70 kg. For simplicity, let's model this as 70 kg of steak. According to Google, a 251 g steak has about 679 kilocalories of energy, which gives about 2.7 kcal/g or about 11 MJ/kg. Therefore, if you were to stick a human body into a biomass incinerator, I would expect to be able to extract around 770 MJ or 210 kilowatt-hours of energy (though of course not all of that energy could be captured with 100% efficiency). Therefore, to power an average American household, which consumes about 911 kWh per month, you would need to burn one adult body per week (or maybe two to account for efficiency losses).

So, in fact if you 'take out' heat you kill the person as a future source of ATP.
I meant as you do work your heat production increases which sweat releases through evaporation, which is a huge waste of energy on the human powered generator bikes, all that heat is free to dissipate rather than be utilised for more work.
you would need to burn one adult body per week (or maybe two to account for efficiency losses).
Yikes! So if you could keep breaking down mass to keep working you'd be down to nothing in a week, or less! For some reason I thought there would be more energy density but then it is just mostly water.

Let's account for metabolic requirements and such:

You seem to want a formula: http://www.uz.zgora.pl/~jleluk/animacje/show_thumbnails.pl.htm

What you're asking for is not simple. Why? Because blood glucose does not directly equate to energy.

Example of one reason: There are too many places for glucose to go in the lipid section in that horrendous chart. Healthy humans cannot use all of the carbohydrate from one meal at a single go. Not to mention gluconeogenesis - creating glucose from proteins - may contribute to the glucose load. You eat carbs that equate to 100g of pure glucose. It takes a while for your body to digest and extract all the glucose. But it is not gobbled up by cells all at one time. It gets converted to molecules like glycogen, kind of like a fuel dump. All of that converting back and forth consumes energy. And the second law of thermodynamics takes its toll as well. Glucosee blood levels (diabetes anyone?) and their controls are complex.

It is clear you want some kind of 'number' or formula.

Since I don't like to make up stuff - these guys made an effort to answer your question - what is the efficiency of the use of 2000 kCal/day humans use? They give a number: we "fritter" away about 90kCal of heat per hour. So if you like, consider 90kCal/hour as the theoretical possible production of useful energy. Also consider the energy donor as past tense as @Ygggdrasil mentioned.

http://mb-soft.com/public2/humaneff.html