Is it possible to live on animal foods alone?
TL:DR - Yes - cultures have survived off of only animal foods for millenia and modern people have been doing carnivorous diets for decades with no ill health.
Humans require 10 essential vitamins, essential fats, bioavailable proteins, minerals, and water. Humans do not require any carbohydrate (sugar, starch, or fiber) and do not need micronutrients that come in plants.
Clinical Effects of Inadequate Intake
The lower limit of dietary carbohydrate compatible with life apparently is zero, provided that adequate amounts of protein and fat are consumed. However, the amount of dietary carbohydrate that provides for optimal health in humans is unknown. There are traditional populations that ingested a high fat, high protein diet containing only a minimal amount of carbohydrate for extended periods of time (Masai), and in some cases for a lifetime after infancy (Alaska and Greenland Natives, Inuits, and Pampas indigenous people) (Du Bois, 1928; Heinbecker, 1928). There was no apparent effect on health or longevity. Caucasians eating an essentially carbohydrate-free diet, resembling that of Greenland natives, for a year tolerated the diet quite well (Du Bois, 1928). However, a detailed modern comparison with populations ingesting the majority of food energy as carbohydrate has never been done.
It has been shown that rats and chickens grow and mature successfully on a carbohydrate-free diet (Brito et al., 1992; Renner and Elcombe, 1964), but only if adequate protein and glycerol from triacylglycerols are provided in the diet as substrates for gluconeogenesis. It has also been shown that rats grow and thrive on a 70 percent protein, carbohydrate-free diet (Gannon et al., 1985). Azar and Bloom (1963) also reported that nitrogen balance in adults ingesting a carbohydrate-free diet required the ingestion of 100 to 150 g of protein daily. This, plus the glycerol obtained from triacylglycerol in the diet, presumably supplied adequate substrate for gluconeogenesis and thus provided at least a minimal amount of completely oxidizable glucose.
The ability of humans to starve for weeks after endogenous glycogen supplies are essentially exhausted is also indicative of the ability of humans to survive without an exogenous supply of glucose or monosaccharides convertible to glucose in the liver (fructose and galactose). However, adaptation to a fat and protein fuel requires considerable metabolic adjustments.
The only cells that have an absolute requirement for glucose as an oxidizable fuel are those in the central nervous system (i.e., brain) and those cells that depend upon anaerobic glycolysis (i.e., the partial oxidation of glucose to produce lactate and alanine as a source of energy), such as red blood cells, white blood cells, and medulla of the kidney. The central nervous system can adapt to a dietary fat-derived fuel, at least in part (Cahill, 1970; Sokoloff, 1973). Also, the glycolyzing cells can obtain their complete energy needs from the indirect oxidation of fatty acids through the lactate and alanine-glucose cycles. In the absence of dietary carbohydrate, de novo synthesis of glucose requires amino acids derived from the hydrolysis of endogenous or dietary protein or glycerol derived from fat. Therefore, the marginal amount of carbohydrate required in the diet in an energy-balanced state is conditional and dependent upon the remaining composition of the diet. Nevertheless, there may be subtle and unrecognized, untoward effects of a very low carbohydrate diet that may only be apparent when populations not genetically or traditionally adapted to this diet adopt it. This remains to be determined but is a reasonable expectation.
1. He spent altogether eleven and one-half years within the arctic circle.
2. He lived for a number of days, totaling nine years, on an exclusive meat diet.
3. He lived for nine successive months on an exclusive meat diet.
4. He reached his maximum weight while subsisting on meat (fish).
5. His sense of physical and mental well being was at its best during that period of his life.
6. He found that the exclusive meat diet worked as well when he was inactive as when active, and as well in hot weather as in cold.
7. Constipation was never present. One month's entire absence from exercise produced neither constipation nor muscular weakness.
8. His hair thickened, and his scalp became healthier.
9. Teeth decay was apparently much less rapid. Stefansson avers that not a single case of constipation was observed in 600 exclusively meat-eating Eskimos for a period of three years
Neither Stefansson nor any of his men, so far as we could determine, suffered any ill effects from long continued meat diet.
Two normal men volunteered to live solely on meat for one year, which gave us an unusual opportunity of studying the effects of this diet. The term “meat,” as used by us, included both the lean and the fat portions of animals. The subjects derived most of their calories from fat and the diet was quite different from what one, who uses the term “meat” as including chiefly lean muscle, would expect. Rubner called attention to the fact that a man cannot live on meat alone because of the physical limitation of the apparatus of mastication. He was evidently considering only lean meat as fat offers little difficulty. It is well known that the Eskimos have lived on an almost exclusive meat diet for generations. Certain explorers in the North also have subsisted for long periods on meat. Dr. Vilhjalmur Stefansson in particular has demonstrated that it is feasible for travelers in the arctic region to “live off the country,” which means living on meat alone. The experiences of Stefansson and his companions have been given in his book “The Friendly Arctic”. He spent over 11 years in arctic exploration, during 9 years of which he lived almost exclusively on meat. Stimulated by this experience, Stefansson and Andersen, the latter a member of one of the expeditions, voluntarily agreed to eat nothing but meat for 1 year while they continued their usual activities in the temperate climate of New York.