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What is the minimum caloric intake for an adult? I'm not talking about a healthy diet. There's plenty of advice on that. I'm wondering what the absolute limits of H. sapiens are. When there's a multi-year famine, and the body is willing to shut down as much of its capacity as possible without actually failing to function for years at a time, how much energy is it consuming?

I know the Basal Metabolic Rate is a key factor to this, but the numbers I have seen for it are all "healthy" BMRs. I'm interested in the unhealthy numbers. For example, I'd consider maintenance of temperature optional for these circumstances (assume we have a temperate enough climate), and I presume heart rate and breathing will go down, decreasing those loads. Obviously there's limits as to how many environmental constraints can be addressed (if I knew what environments this was studied in, I'd likely know which studies to look for!). And obviously not everyone is the same, but that's an issue in every human study ever completed.

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  • $\begingroup$ Threr's no single answer to this question, just generalizations. It depends on the person, their state of health when starvation starts, and what you mean by "truly starving". To me, truly starving means no food whatsoever. If you want an answer, there's only one place to get it, and it's not here. Hint: these experiments were done in the 40's. $\endgroup$ – anongoodnurse Oct 12 '17 at 20:30
  • $\begingroup$ jn.nutrition.org/content/135/6/1347.full $\endgroup$ – Count Iblis Oct 12 '17 at 21:24
  • $\begingroup$ How much water does it take for a plant to keep some leaves? how long is a piece of string? $\endgroup$ – com.prehensible Oct 12 '17 at 21:26
  • $\begingroup$ @anongoodnurse The existence of variation does not make the question less amenable to scientific analysis. The confounding factors you mention, along with many other relevant factors, I'm sure, as well as the problem of defining the outcome variable can all be in principle addressed empirically. Although doing experiments on the subject is unethical, naturally occurring events of famine or voluntary starvation can be also studied. Surely we all agree that qualified generalisations and statistical analysis are the bedrock of biological research. $\endgroup$ – vkehayas Oct 12 '17 at 21:41
  • $\begingroup$ @anongoodnurse The challenge of defining starving is why I started with my first paragraph defining the term as I use it. I specifically wanted to step away from the anorexia/bulimia related content because the focus there is how to make those people healthy, not just continue functioning. $\endgroup$ – Cort Ammon Oct 12 '17 at 21:56
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A better example is the stories of tibetan monks who wrote accounts of their experience to survive for months or years on faith alone, and watch their hair and nails fall out in the search for wisdom. they could tell you how fast a coma arrives from eating just 300 calories per day.

There isn't a minimum amount though, because it's all subjective for time until, temperature, survival chores, time until coma, time until difficulty moving... you can probably keep someone in an emaciated coma with intravenous injections, or someone could stay awake with high exposure risk to illness, hair loss, nail loss. The question is vague, and there are 100 scientific studies about calorie restriction, so you are hoping someone will read 10-20 pages until they find some information for you.

It also depends how many watts the human needs to use in that situation to stay alive.

In 30'C you probably need 30 watts to stay warm, and 20 degrees about 60 watts, so the value can double based on temperature, you must change your question a bit.

60 watts basal metabolic rate body temperature would use 70 calories per hour, about a slice of bread, so 1400 calories per day without bedcovers. It depends a lot on ambient temperature and exercise, for example 1 mile of walking uses 100 calories. At less than 1000 calories per day, exercise becomes difficult and the person gets thin. Jogging uses about 250 watts. Extreme athletes and triathletes peak at about 650 watts for sprinting.

There are 100ds of citations on caloric defficiency and restriction, with values around 350 - 700 per day, which change metabolism considerably:

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3014770/

https://www.ncbi.nlm.nih.gov/pubmed/2718912

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  • $\begingroup$ I don't think it answers the question. $\endgroup$ – anongoodnurse Oct 12 '17 at 20:32
  • $\begingroup$ I'm not reading through 20 pages of rambling science articles to find out that there isn't an answer! There isn't an absolute minimum, same as there isn't a minimum amount of water for a plant to survive, and people don't starve for years. $\endgroup$ – com.prehensible Oct 12 '17 at 21:25

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