How about if you consumed nothing but…blood?
I mean, it’s got everything you need in it, otherwise you’d already be dead.
Hopefully, you’d get yours from a relatively clean and uninfected source.
What say y’all…could you live on blood?
If I had to live on just one thing, it’d be peanut butter. Protein, carbs, even a little fiber. It keeps for yonks, and all you need to eat it is a finger (or a spoon if you wanna get fancy).
Potatoes were the main source of calories for the Irish, but they’re not so great for vitamins. For that, there’s cabbage (another staple food for the Irish, and indeed for lower class folks around the world).
Lembas
[sub]Well, it’s not sci fi! 
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Leaving aside the Soylent Green jokes, I think by the OP definition, ground up people might work. Run a healthy human through a meat grinder and pelletize the result.
Shouldn’t that food have all the nutrients a human requires? I leave the practical aspects of obtaining a source, etc., to the entrepreneurs amongst us.
“People Chow…Bodies Do A Body Good!”
Some humans live well over a year on breastmilk alone. 
Potato skins do contain some betacarotene, so they are completely lacking in vitamins. But yes, the Irish must have supplemented their potato and milk diet with some vegetables.
As far as liquid diets and fiber, there was a recent report that coffee is high in fiber. However, I’m fairly sure that blood, which someone suggested above, has little, if any, fiber. We consume fiber, but it just stays in the intestines.
Why is fiber required in the human diet? Is it just the stimulation of the digestive system, or is there something else? If the former, then blood should do the trick by itself. At least if it leaks out of a hole in one’s duodenum, it does an outstanding job of stimulating the intestines (as I can attest from personal experience).
My best bet would be soybeans. It’s a legume high in protein, Vitamin C, fiber and is green, so I assume further nutritional elements would be present.
Although I gather there are negatives associated with soy consumption as well.
Buckwheat is the best known single source diet containing everything necessary to support life. Buckwheat by the way is not a grain or cereal but the seed of a plant whose closest relative is rhubarb. Known as Kasha is Russia and Soba in Korea and Japan, it has sustained hundreds of generations. If you had the educational experience of being a prisoner in the Soviet Gulags you would have been fed buckwheat three times a day in the form of Kasha porridge or soup, go dig out your old copy of ’ One Day in the Life of Ivan Denisovitch’ and read about meal time. Buckwheat is coming on very strong in the last year do to it’s low cost and high nutrition, even to the point of being a main ingrediant in TOSTITOS new multi- grain chip, so now you can get your buckwheat at 7-11. Buckwheat also has the huge advantage of a very short growing season, 45 days, and stores for years.
The other food which might qualify are pumpkins, but the caveat here is you must eat the entire plant- leaves, blossoms, flesh and seeds. Though naitve to the Americas, as are potatoes, Central Africa has successfully created a diet based on the entire Pumpkin grown year round supplemented with peanuts which is fairly healthy. Most Central Africans have very litttle meat and dairy in thier diet, not by choice, but for economic and refridgeration reasons. Who knows, coming soon buckwheat-pumpkin TWINKIES.
Maybe, if you ate it raw, but a lot of nutrients are broken down by cooking. I don’t think that you can get enough of all of the necessary vitamins from any cooked meat.
Doug actually does have a point here. As I understand it, peanuts contain the majority of the necessary nutrients for survival, though of course not in the appropriate proportions. It’s not the perfect food, but one that is probably the closest (outside of blood, Soylent Green, and other reprocessed-people comestibles) to ideal for a single-source human food. I am not sure precisely what essential nutrients are missing, but as I understand it, a couple of amino acids and a few each of vitamins (B-12 is the only one I’m certain of on this list) and minerals would be the sole missing nutrients. Obviously, fiber and fluid intake would also be needed, but we’re looking at sustenance.
I don’t know about forever, but I can attest to the fact one can go 27.5 years on it.
There’s alway the question of what single food source on which you could survive, to which the answer is nothing, of course, but I’ve always wanted to know what the fewest number of foods you could eat and have a healthy, balanced diet. Something like beans, spinach and buckwheat, or whatever it might be.
If you can alter the seasoning I could live on chicken and vegetable soup 
I don’t have a cite for it, but Marilyn Vos Savant postulated that if a supply of water was always available, one could live on dry dog food.
I heard that you can survive on **bread, bananas and orange juice ** (but have no cite).
I heard flour, lard, and carrots. I druther die.