The subject is a healthy male in his twenty, weight is 60kg in the nude and 1.75m tall, he has been fasting for a week and I gave him 1.5 litre of distilled water and fed him 500g of potatoes before locking him away in isolation. He had already agreed to volunteer for my clinical trial of special dieting methods and would be paid 1 millions Euro should he manage to survive for a year with only his droppings and piss.

Can my test subject claim the 1 million dollar cheque or should I flee the country for scamming people?

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    $\begingroup$ Basic math tells you NO: Do not forget you process some of the food. You also sweat out water into air. So, even if he could live off his feces, he is most probably going to die out of thirst $\endgroup$ Sep 5, 2016 at 6:54
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    $\begingroup$ @Pavel we process most of the food, not just some. Evolution optimized us really well. $\endgroup$
    – Mołot
    Sep 5, 2016 at 7:15
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    $\begingroup$ The stuff your body excretes is poison: it's why your body gets rid of it--- it can't be processed further. Why would putting it back in improve your chances of survival? $\endgroup$
    – nzaman
    Sep 5, 2016 at 10:58
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    $\begingroup$ If he consumed his own excrement and urine, he would die sooner than if he didn't do it. $\endgroup$
    – vsz
    Sep 5, 2016 at 13:25
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    $\begingroup$ @Madlozoz raw potatoes are perfectly edible, at least those which grow here in my country google.ru/maps/place/Russia $\endgroup$
    – MolbOrg
    Sep 5, 2016 at 22:09

4 Answers 4


Estimates on water loss per day just from breathing seem to be around 300-500ml (based on a web search: obviously do your own research for the specific conditions you're interested in. It will depend on activity and air humidity and other things).

This adds up to well over 100kg a year, which is greater than your test subject's entire body mass plus the 1.5l of water you gave him. This is without counting sweat and urine, so we don't even have to consider what happens to those. There is absolutely no chance whatever of him surviving if has no means to replenish the water he exhales. For a few days that might be OK, there's some "spare" in the body plus he's got his 1.5l. For a year, forget it.

If you provide him with some kind of complete environmental water recycling system, or materials to fashion one for himself (condense water out of the air by night, that sort of thing), then we could go on to talk about food. But as things stand it's pretty irrelevant since he'll die of thirst before food becomes a major issue.

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    $\begingroup$ Along these lines, I recommend Dune's stillsuits. They explored how far you could get away with surviving in a hostile science fiction world. $\endgroup$
    – Cort Ammon
    Sep 5, 2016 at 16:01
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    $\begingroup$ Yea, with 1.5 L water to last for a year? I really don't get why anything else in anyone else's answers are necessary. TL;DR: you die without water $\endgroup$ Sep 5, 2016 at 19:30

He would probably die

Especially because he is too thin.
But not as fast as some people may think

Potato diet

Recently, some guy made the stunt of living 2 month with only potatoes. But it was 20 potatoes a day which weigh around 2 kg.

Also, 2 months is not that much. In extreme starvation, normal people survive up to 2 months with only water.

Who needs calories anyway

500 g raw potatoes is about 1600 kJ of nutritional value. Can a man survive 1 year with 25% of basal energy intake?

As mentionned earlier, most people die after 2 month of fasting. Little food can buy you some time, but probably not that much.

BUT... in a case of extreme obesity, a man fasted for 382 day under medical supervision.
He was given some salt, vitamins, yeast and unlimited "non-caloric liquid".

You could imagine that potatoes could give the salt and vitamins intake (not as good, but your scenario allows your test subject to loose sight, teeth and sanity).

However, digesting your own fat needs A LOT of water. 1.5 liters cannot be enough.

Drink your own pee

The role of urine is to get rid of the waste in the blood.
As such, it is not really toxic. Not much bacteria and whatever toxin are in the drunk urine will be filtered (again) by the kidney.
But if you re-inject the toxins, your pee will always get more yellow and at some point, your kidneys will surrender.
Especially if you are fasting as (whatever snake oil seller may claim) you would produce much more toxins.

I'd say that urine would get toxic within 1 week only. If you have a micro filter, you're good.

eat your own poop

What the hell would you expect from that??

Some argue on the Internet that the bacteria would not kill you. But if you eat a significant quantity, the nitrate surely will.

Anyway, poop is expelled because your digestive tract doesn't want it. Keep in mind that digestion needs plenty of calories and water so don't try to digest useless stuff.

In the end

Lock a 200 kg man with 3 l water and 500 g potatoes a day and I'd say he has a good 15% chance of beeing a happy, toothless, blind, crazy winner.
Tell him not to eat his own poop.

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    $\begingroup$ What I recall (and just re-googled) from wilderness survival information is that drinking your pee is useful the first time around, but after that it's too concentrated (sodium, potassium, etc.), and it does more harm than good (like drinking seawater). The 2nd time around is possibly ok, or maybe break-even. If you have muscle injuries, the extra strain of drinking urine is probably even worse. The US Army Field Manual recommends against it.. TL:DR urine drinking -> extra day or two, maybe. $\endgroup$ Sep 5, 2016 at 8:49
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    $\begingroup$ 15% chance to survive a year witout water. - I would claim that this is extremely optimistic. $\endgroup$
    – Taemyr
    Sep 5, 2016 at 13:17
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    $\begingroup$ Well... that's why I recommend 3 liter per days in my last sentence. An hypocaloric diet would also dehydrate you severally, hence the large water amount $\endgroup$
    – Madlozoz
    Sep 5, 2016 at 14:00
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    $\begingroup$ @PeterCordes That's because the body gets much more strict with water processing as you start dehydrating - but it can't absorb the water already filtered. The second time, your body is most likely already in a conservation mode, and only expels as little liquid as possible - the concentration of salts etc. is much higher. As for seawater, the last study I remember suggests that drinking seawater is better than drinking nothing - human kidneys are good enough at filtering to make it a net water gain, even though it does strain them a lot. Still, you want to drink very little of it. $\endgroup$
    – Luaan
    Sep 5, 2016 at 15:53
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    $\begingroup$ The question does not describe 500 g potatoes per day. It is per year. $\endgroup$ Sep 5, 2016 at 19:15

Your volunteer should have checked the Internet before declining. Faeces and urine will have little influence because reprocessing is of little benefit and may in the case of urine even shut down your body; they are waste products after all. Some actual figures:

  • Water AND Food - 18 days is the world record. In a holding cell.
  • Food – 1-2 months on average before dying
  • Water – 3-7 days on average before dying



Your system has no caloric source. If diet is fatty, then quite some calories can stay not digested and go to excrement. Some of your dead cells can get there, too. Doesn't really matter. Best you can get is 100% of what was in your food plus a little something that was already in your body anyway.

Carbon dioxide is a way to say goodbye to calories. It ends up in air, not in poop, and it is no longer usable anyway.

Eating poop or not, doesn't matter in any substantial way. A year on 500g of potatoes is not possible.

For piss, it could take him quite a while. If he's not sweating too much, and not losing much with breathing, it could take a while. This is something controlled externally, by the one who holds controls on temperature and humidity dials.


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