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In reference to the classic Indestructible Edible Trope, how long can an adult subsist on largely junk food? I realize that "largely" is vague, so feel free to clarify as desired to give a helpful answer (one way or the other). To motivate things:

Imagine a world shortly after the apocalypse. Something very quickly resulted in the "depopulation" of most of the USA (or similar country). Best bet is a virulent plague with a 95% mortality rate. Because of how quickly it spread and killed people, those who survived the plague have basically found themselves in a "fairly" undisturbed world. For the purposes of this question that mainly means that grocery stores, gas stations, etc, are mostly still stocked with goods.

Of course power goes out quickly, so anything that needs to be refrigerated goes bad very soon. This leaves hardier staples, but from what I can tell the longest lasting foods are largely pure carbs with very little nutrition other than calories. Some examples:

  1. Dry pasta (1-2 years)
  2. White rice (4-5 years, but only 6-8 months for brown rice)
  3. Unpopped popcorn (indefinitely)
  4. Cereal (6-8 months)

My own impressions lead me to believe that these are also some of the more common items in a grocery store, and so they would be the easiest to find in large quantities for a post-apocalyptic scavenger. However, these foods (especially white rice and white pasta) have very little nutritional value other than calories and some minerals. Certainly very little protein, and no fat - fat especially would make them go bad faster, I suspect, so high-fat foods are most likely out after a few months (although I think canned nuts might last 6 months or a bit more).

Still, our survivors need to figure out how to become independent and produce their own food without the help of modern infrastructure. For the purposes of this question our survivors apparently have access to sufficient quantities of clean water as well as shelter. We're ignoring the risk posed by large quantities of dead bodies rotting everywhere. The survivor(s) are small in numbers and we'll presume there is enough food left in stores: the only thing that will hamper their long-term scavenging is the expiration date of the food (the actual one, not the one printed on the back).

How long do our survivors have to figure out how to become self-sufficient with food sources before malnutrition becomes a serious problem? Or can they survive more or less indefinitely off of such low-quality food? I'm also open to frame challenges, aka let me know if there are actually more than enough nutritious and long-lasting food sources around and there is nothing to worry about.

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  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – L.Dutch Dec 30 '18 at 7:53
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    $\begingroup$ The question title should be edited to remove the discrepency between its false premise and the legitimate question in the body. $\endgroup$ – R.. Jan 1 at 19:04
  • $\begingroup$ None of the foods you listed are "pure" carbs. Example common foods with nearly 100% of their calories as carbohydrates are refined sugars (white sugar), syrups (corn syrup), and refined starches (corn starch). You could maybe say "carb-heavy" for what you've listed, or are you actually talking about pure sugar? $\endgroup$ – Nick T Jan 2 at 5:40

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Given that there is enough food in convenience stores / supermarkets (ie. little enough people have survived that the quantity of food is not a problem) quite a few years, by which time they'd hopefully find local library (as I'm not too optimistic about their skillsets) and learn to plant, grow and eat the natural food outside the city to survive indefinitely.

First, there is "best by" date, and there is "will probably cause digestive problems" date - and there is quite some "sub-prime" time in between them.

There are plenty of stuff which will hold for few years or more, and provide you with more then enough nutrients needed for comfortable survival for few years:

  • pasta, flour, oil, muesli etc. - stuff that might have a year or so specified by manufacturer, but will hold for much longer if unopened.
  • sugar and honey which will hold for quite a few years (or decades) without any problem.
  • lot of canned fruit and vegetables - from corn, beans, beetroot, pickles, olives, fruit compote, jams, etc. which will hold for 2-5 years by their (pessimistic) "best before" dates, and will probably be edible a few years after that
  • canned meat and spam and fish and spam and other meals with a lots a preservatives which will hold for 5 years at least (I'm just looked at some fish is spicy vegetable sauce whose best-by date is 3 years in the future, and I'm pretty sure I haven't bought it this year) and will provide you with essential fats/amino acids.
  • dried fruit, nuts will be good for a year at least (or more if taken care of)
  • dried and salted meat like beef jerky might hold for years
  • peanut butter, ketchup, dehydrated soups etc will last for few years.
  • fruit juices with preservatives, beers, wines, tea etc. will also hold from one to quite a few years
  • food supplements like "dissolving tablets" will also provide minerals and vitamins (although not in ideal healthy form, more than enough for survival).
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    $\begingroup$ I'm with you on most of this, but beer gets bad rather quickly, unless we're talking about pasteurised stuff. ;-) And ordinary black tea is completely stale after a year. I'd rather drink pure water. $\endgroup$ – Karl Dec 29 '18 at 0:22
  • $\begingroup$ The only thing useful on here is "canned meat". (Which is a pretty poor meat product.) The rest is just carb-filler. "Pure carbs" - see the question title - are completely useless to humans; you'll be dead in a few weeks. $\endgroup$ – Fattie Dec 29 '18 at 0:57
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    $\begingroup$ @Fattie and spam and peanut butter and fish are also not carbs. Heck, even regular corn are also only about 6-7 times less carbs then proteins. Beetrot about 5 times less. Beans have only double the carbs than proteins+fats. Peanuts have 4 times as much proteins and fats as they have carbs! etc... Your "100% carbs" is nonsense. Also, much of mentioned food is rich in other nutrients; there is much more that you need in your diet than simplistic carbs/proteins/fats division! $\endgroup$ – Matija Nalis Dec 29 '18 at 1:12
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    $\begingroup$ @Fattie sunflower nuts actually have the same amount of protein as carbs, pistachio's have 3/4 protein/carbs, even pasta is about 1/5 protein carbs. And peanuts have a whopping 50% more protein than carbs, i mentioned sunflower first because of its superior nutrient balance $\endgroup$ – Richie Frame Dec 29 '18 at 5:30
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    $\begingroup$ @MatijaNalis According to some estimates, wheat is still by far the biggest protein source in human diet. Does it have lots of carbs? Sure. But it has huge amount of protein as well (in fact, more than soybeans for the same cultivated area! Though of course you would want to rotate your wheat with lentils to replenish nitrates in the soil). $\endgroup$ – Luaan Dec 31 '18 at 14:20
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A few months at best

It's the lack of vitamins that gets you.

Vitamin C deficiency is known colloquially as scurvy and was one of the biggest killers of sailors in the age of exploration.

Simply, you need your fruit and veg or you're going to die.

You can get around this if you have supplies of concentrated fruit juice or vitamin supplement pills, but you've got to have those vitamins.

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    $\begingroup$ Yep, scurvy was my first thought too. $\endgroup$ – Tim B Dec 28 '18 at 19:15
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    $\begingroup$ scurvy can be cured by extracting vitamin C from some evergreen trees, which are supposedly still healthy, by boiling the bark source. It's an old trick that european explorers learned from native american tribes when they first got there. Fruits come in season but trees always have bark, supposing our survivors know this they should be ok scurvy-wise $\endgroup$ – Alexandre Aubrey Dec 28 '18 at 20:49
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    $\begingroup$ @Spencer And you don't have to eat parboiled rice day-in day-out. ;-) If there's a really serious problem, the solution often is grow potatoes. en.wikipedia.org/wiki/The_Martian_(film) $\endgroup$ – Karl Dec 29 '18 at 0:10
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    $\begingroup$ @Fattie that's just not true. While "pure carb" is certainly not very healthy, it will certainly won't kill you in a week. Heck, I've just been drinking water with some sugars, vitamins and minerals for week (fizzy tablet supplements) some months ago and aside for hunger at the start and end of the period did not even feel any discomfort, much less felt like being on the brink of dying. $\endgroup$ – Matija Nalis Dec 29 '18 at 1:17
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    $\begingroup$ Would the prevalence of multivitamins and supplements at supermarkets (in the US at least) be able to offset this? I feel they have a fair shelf life. $\endgroup$ – ndm13 Dec 29 '18 at 1:56
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The medical term is kwashiorkor.

It was first identified in Africa in 1935. Mothers would stop breast-feeding their child when they became pregnant again. The older children would get plenty of calories from a high-carbohydrate diet, but were no longer getting protein (from the breast milk). The name "kwashiorkor" is a word in Ga which means "the sickness the baby gets when the new baby comes".

Signs and symptoms include:

  • lower immune response, thinning hair, and tooth loss. These all rely on proteins.
  • muscle breakdown. This includes both skeletal muscle and the respiratory diaphragm. The body reclaims proteins, which are broken down into amino acids that are released into the blood.
  • increase in blood urea nitrogen. Amino acids are converted into glucose in the liver, by removing the nitrogen.
  • fat deposits in the liver. The high carbohydrate diet actually encourages the liver to store the excess carbohydrate as fat.
  • liver failure. With less protein to work with, the liver cells shrink and get replaced by the fat deposits.
  • reduced blood proteins. Albumin and clotting factors are proteins that are normally made in the liver.
  • swelling (edema), particularly in the ankles and feet. Reduced albumin draws water into cells by osmosis, and tissues swell.
  • low blood volume. The fluid has shifted to other spaces.
  • distended abdomen. The enlarged fatty liver and edema in the abdomen (ascites) pushes out the abdomen, which the weakened diaphragm cannot hold in.
  • rashes.
  • irritability.

kwashiorkor

The amount of time depends on the person's protein reserve. Children are more commonly affected than adults, because they have fewer protein reserves. Children can die in a matter of weeks. Adults can often continue for several months.

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  • $\begingroup$ Twinkies and popcorn have enough protein and last for decades $\endgroup$ – Underminer Dec 31 '18 at 21:33
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Even if you have vitamin supplements stored, lack of fats will kill you before lack of protein. There's a phenomenon called "rabbit starvation" -- someone surviving in the wild whose only meat is wild rabbits will die in a few months because there isn't enough fat in the meat -- some vitamins are fat soluble and can't be absorbed, even with adequate vegetation in the diet, without enough fat intake.

Protein catabolism is a genuine issue: it's why people in WWII concentration camp photos look like living skeletons; their bodies have metabolized their own muscle to keep their brains and organs running. But if you have zero fat in a calorie-sufficient diet, you'll still waste and die even if there's enough protein to avoid digesting your own muscles.

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  • $\begingroup$ Any idea how long that might take in an otherwise healthy (at first) adult? $\endgroup$ – conman Dec 28 '18 at 19:38
  • $\begingroup$ Given it was mentioned (by Les Stroud) as relevant to wilderness survival of someone who becomes lost in the deep woods, I'd guess the time period is two or three months -- certainly more than a week or two, likely less than half a year (anyone ought to be able to walk out in that kind of time, if they aren't injured or too weak from starvation). $\endgroup$ – Zeiss Ikon Dec 28 '18 at 19:42
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    $\begingroup$ Your body can make fats from carbohydrates it cant make protein from carbs. "rabbit starvation" aka protein poisoning. requires a lack of carbs as well. lack of protein may kill you first, PEM has a plethora of symptoms. $\endgroup$ – John Dec 28 '18 at 20:42
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    $\begingroup$ I have seen a timeframe of a week for brain issues to develop when you have carbohydrates but no fatty acids. Note that this is a lot shorter than the survival time without food--if you have no food the body uses stored fat and in doing so provides the essential fatty acids. This was found with IV nutrition. $\endgroup$ – Loren Pechtel Dec 30 '18 at 0:20
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    $\begingroup$ @LorenPechtel Interesting. So you're saying that if you knew ahead of time that (for some strange reason) you would only have access to pure carbs and water for a week, you'd be better off not eating anything? $\endgroup$ – Dan Dec 30 '18 at 7:09
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I question the premise of the question.

In most places where you can find dry grains and pasta, you can also find dry (and canned) beans, lentils, peas, soya, chickpeas, nuts, etc. Combining those with grains will give you a complete protein containing all the essential amino acids needed to sustain the human body. These pulses can, similarly to rice, last for many years if stored away from moisture in airtight conditions away from sunlight. Hermetic food can also be kept for a very, very long time.

Indeed, the vitamins are a bigger worry. Your heroes need either hermetic juices, hermetic fruits/vegetables (and even milk or eggs, for B12) or dietary supplements.

Also, I would like to point out that grains are not "pure carbs". Even white rice contains 2.6% protein. Pasta (from wheat, unenriched), has 5.8%. Oats contain 16.9%. Moreover, these proteins consist of amino acid residues. Your heroes will need both them, and the protein from legumes. If they only were to eat beans, for example, they would also get deficiency. Both grains and legumes are needed to get a complete protein. Some notable exceptions to this are the soya bean, amaranth, chia and quinoa, which are complete proteins in themselves.

Hermetic eggs, meat or dairy are also options for getting protein, however, I suspect such products may be in a shorter supply in an apocalypse situation.

I think as long as your heroes know a little bit about the basics of nutrition, they should be able to dodge protein deficiency.

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  • $\begingroup$ Proteins are not essential amino acids. Proteins may or may not contain essential amino acids. $\endgroup$ – Peter Mortensen Dec 30 '18 at 17:39
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    $\begingroup$ @PeterMortensen Proteins are large biomolecules, or macromolecules, consisting of one or more long chains of amino acid residues. A linear chain of amino acid residues is called a polypeptide. A protein contains at least one long polypeptide. The amino acid residues essential to humans are Tryptophan, Threonine, Isoleucine, Leucine, Lysine, Methionine, Cystine, Phenylalanine, Tyrosine, Valine, Arginine, Histidine, Alanine, Aspartic acid, Glutamic acid, Glycine, Proline and Serine. In common speech this is called protein. $\endgroup$ – Revetahw Dec 30 '18 at 17:46
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White rice, flour, pasta, and popcorn are not pure carbs. Rice has about 8% of its calories from protein. Flour is about 11%, pasta about 14% and popcorn about 12%.

It's recommended you get at least 10% of your calories from protein (about 0.8g/kg body weight). This is not the bare minimum required for survival; it's a recommendation. These grains should be sufficient in this regard.

These grains alone do not have all the essential amino acids (wheat, for instance, is deficient in lysine). However, when combined with lentils or other pulses, legumes or seeds, you can get all the essential amino acids. Dried lentils and pulses have long shelf lives.

The remaining necessities are all things you need in small amounts: vitamins, minerals, omega-3 fatty acids, salt, etc. Salt, of course, doesn't expire. For the others, I would raid a pharmacy or supermarket for supplements.

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If we assume nothing but carbs (e.g. white rice), they'll have salt/electrolyte problems before lack of fat or vitamins fells them. But if we must add only from the snacks, the right candy may make all the difference!

In the reducto-ad-carbohydrate case of white rice and nothing else, the lack of salt and related electrolytes will cause significant health problems within a week -- well before lack of fat or vitamins A - D become issues. So salt is a priority; ditto Potassium, Magnesium in short order. (Salt + Salt substitute.)

But what about the junk food; can it significantly help this poor diet? If they get to choose just one junk-food thing to add to this 'diet,' what's a good choice?

Candy, especially chocolate/peanut-based candies: Reese's PB cups, peanut M&Ms, Mr. Goodbar. Fat from both chocolate and the peanuts, a little salt in most candy, some protein from the peanuts, maybe a trace of fat-soluble vitamins A and D. A trail mix heavy in salted peanuts and raisins would also work pretty well, assuming that's sufficiently junky. Most chips/crisps will provide fat and salt, but not much protein.

Do I want to eat this diet? No. But If I had to supplement a ton of white rice with a single junk-food choice, I'd go for the peanut M&Ms. Thence multi-vitamin pills.

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    $\begingroup$ I‘m pretty sure you won’t get electrolyte deficits within a single week, unless you are sweating excessively or suffering from diarrhea. Otherwise surviving a week without food would be impossible. $\endgroup$ – Michael Dec 29 '18 at 11:35
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    $\begingroup$ @Michael I second that. Some people can go a month without any food. Short of something causing the materials to leave the body faster (excessive salt intake) there's no reason to believe they cannot survive (albeit weakly) for a month. $\endgroup$ – The Great Duck Dec 29 '18 at 18:55
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    $\begingroup$ White rice still has 3% - 10% protein (so it is more than carbohydrates) $\endgroup$ – Peter Mortensen Dec 30 '18 at 17:37
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    $\begingroup$ @Michael After a week without salt, you'll get dizzy because your body simply won't contain enough water anymore. You need the salts to keep the water in your body. Without salt, you can drink as much as you want, you will loose a significant amount of body weight due to water loss. And you don't need to sweat much to trigger this effect within a week. $\endgroup$ – cmaster Dec 31 '18 at 1:28
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    $\begingroup$ That said, salt is available in typical stores in quantities that'll more than outlast the other stocks, and it's basically got an infinite shelf life. So, no problem for the apocalyptic survivors. $\endgroup$ – cmaster Dec 31 '18 at 1:30
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Not a problem.

There are literally food sources all around us. The only way your survivors would have to survive on "junk food" is if they were somehow trapped in an industrial city or in the middle of a desert, and even then they would learn to find alternate sources of food as part of surviving. If it's a situation where they have adequate clean water, they'll be able to find edibles.

Once they manage to get into the country, they should very quickly find other survivors who actually know how to grow food and/or have a rough understanding of which wild foods are good to eat. Heck, they'll find good food literally growing on trees or out of the ground, if they come upon an orchard or a cabbage farm.

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  • $\begingroup$ This is completely wrong. It is very, very difficult to "forage!" for real food. $\endgroup$ – Fattie Dec 29 '18 at 0:55
  • $\begingroup$ @Fattie except that plants of farms would continue to grow on their own and just be more wild meaning that if located during the proper time there will already be an abundant food source. Considering that most of rural land is devoted to farming it wouldn't be difficult to stumble into one by accident. $\endgroup$ – The Great Duck Dec 29 '18 at 18:52
  • $\begingroup$ Can you give some specifics? I certainly wouldn't know how to find food in any of the forests around me (and there are plenty of them). $\endgroup$ – conman Dec 29 '18 at 19:04
  • $\begingroup$ Duck, that season's crop (if it was the right time of year) might be available, assuming someone knew how to harvest it in some way. After a month though, fields just turn to rubbish. Farming is totally artificial. $\endgroup$ – Fattie Dec 29 '18 at 19:21
  • $\begingroup$ @Fattie except that the land would not completely die out. I see no reason to assume that a field a square kilometer in size would completely die out and have absolutely no instance of the crop growing naturally the next year. Yes, it would turn wild but it would certainly maintain the plants that were growing there previously if only as a natural side effect of those plants having the advantage. Weeds are not going to overtake an area that large that fast. $\endgroup$ – The Great Duck Dec 31 '18 at 10:56
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To summarize some of the other answers: the survivors will need to combine dried grains, dried legumes, jarred/canned fats, and canned fruits & veggies to get a complete daily meal, reserving the more limited supplies of canned meat for holidays. The many bottles of vitamins & minerals on drugstore shelves should shore up deficiencies for some time to come, while the survivors learn to turn city lots into truck gardens. (I hope someone knows about crop rotation!)

There's two big problems with the "oh, it's easy" solution outlined above. One is adequate storage conditions. I was born before common A/C in the US's Upper Midwest, so I know that in most locations food would have to be moved to long-term storage in a basement, or it will go rancid quite quickly. How much gets moved puts an upper limit on the food available a year after the Disaster.

This leads to the second big problem: hoarding. People don't have to hoard the entire store, just one key item in the complex set of ingredients above, to create a famine in a land of plenty. (As famines in our world often occur.) Most likely, the clever hoarders who stock up on fats (especially, yes, chocolate) will be the ones who find themselves both wealthy and hated. Government will be essential!

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Canned Food

Ignore the expiration dates. Canned food lasts for decades.

My grandfather was an American soldier during the Korean War. Due to his special dietary requirements, he survived on food left over from WWII. Crackers stored in plastic in sealed boxes lasted (due to their low water contented.) Canned fish also lasted that long.

This article, quoting the US FSIS says that canned foods can be safe indefinitely.

Vitamins

Modern stores are loaded with nutritional supplements. Again ignoring expiration dates, dry vitamin capsules can last at least a decade. Even if it degrades somewhat, there should be plenty to keep the survivors from deficiency.

Between the two, there will be adequate nutrition for your survivors for at least a decade or until they run out of supplies.

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