"A stipulative definition specifies how you're going to use a term. Since your usage may be a new one, it's unfair to criticize a stipulative definition for clashing with conventional usage. Stipulative definitions should be judged, not as correct or incorrect, but rather as useful or useless." - Harry J Gensler
The Carnivore Diet as a protocol has a stipulative definition. There are arguments over what exactly that definition is. I prefer the simple description established in the "Zero Carb" (ZC) community of eating "only from the animal kingdom". I do not think it is useful to stratify the diet into "levels", for example — a low carb diet including lots of meat doesn't need a special name; it doesn't appear to have special properties except in comparison to a diet lacking in animal foods.
Nonetheless, many hearing the term Carnivore Diet immediately take issue with the name because of conventional usage. Inevitably they want to argue about whether humans are biologically classified as carnivores. This chapter introduces that question, not because it matters to the definition of a Carnivore Diet, but because consideration of what it means to be carnivorous will help us evaluate the appropriateness of plant exclusion in humans.
Before we can ask if humans are carnivores or if carnivorous diets are appropriate it's important to get straight what we mean by "carnivore". Otherwise, spurious objections will distract us from the discussion. Often when people use the word carnivore, what they are really talking about is the observed behaviour of a species, or a group. That is, they are referring to what the animal eats in the sense of what they have been seen eating, or what the range of foods is that members of the species eat. This may seem intuitively obvious, but there is a major problem with this definition. I will illustrate with a couple of examples.
It is a surprising, but known phenomenon that deer are sometimes found munching on bones from carcasses in the wild , or grazing on baby birds . It is thought to be a source of minerals for them . Like deer, it's also not uncommon to see cows and sheep eating hatchlings . Nonetheless, few would argue that this means these animals are better classified as omnivores than as herbivores. To the contrary, these animals represent one of the most stereotypical classes of herbivores.
On the other end of the spectrum, consider the domestic cat. There are probably domestic cats alive now from a long line of many generations of cats eating a diet of exclusively commercial cat kibble containing corn starch. By a behavioural definition we ought to classify domestic cats as omnivores. After all, starch is a mainstay of their diet! But this is ridiculous. Felines are the quintessential carnivore. They are, indeed, obligate carnivores, meaning they have reduced ability to even digest plants, and there are several nutrients they cannot get from plant precursors, such that they cannot survive for significant periods without eating meat .
These two examples show that what looks like omnivory can be driven on the one hand by a nutrient-poor environment that forces an animal to seek food outside its normal niche, or by economic or convenience-based motivations.
"Animals are classified as carnivorous, herbivorous or omnivorous principally on the basis of structural and anatomical characteristics; whilst such characteristics are well suited to erecting taxonomies, they do not always correspond exactly to the behaviour of every group or individual comprising the taxon, for the simple reason that animals tend to adopt the course of least resistance and do not always do that for which they are best adapted." — P.F. Wilkinson 
If our heuristic for determining which of the several classes of "-vores" an animal belongs to is based on what they've sometimes put in their mouths, or even what they've subsisted on in recent generations, then we're all "omnivores", and there's little point even having a classification. A definition that applies to everything is meaningless.
For this reason, I prefer a physiological definition. In particular, the questions I think are relevant when asking if a species is carnivorous are the following:
Does the animal, in its natural, evolved environment , have a requirement to get at least some nutrition from meat?
To put it slightly differently: would it be extremely difficult or implausible for that animal to get sufficient nutrition in its evolved environment without including some meat?
Does the animal have the necessary adaptations to get all or most of its nutritional needs met by animal-sourced food?
That is, are there adaptations in the animal that make a primarily carnivorous diet viable, for the species to thrive even in the absence of significant plant contribution.
If the answers to these questions are yes, then the animal is carnivorous for the purposes of this book. This definition covers obligate carnivores like felines, that can't get all their required nutrition from plants, and it also covers facultative carnivores like dogs, that do have the ability to derive enough nutrition from plants to survive the short term . Whether it is too liberal a definition, covering also animals we really do want to call omnivores we'll reconsider below.
What fuels an herbivore?
Before there were humans as we know them, there were hominids, and if we go back far enough, these hominids are believed to have been predominantly herbivorous, meaning they were adapted to eating plants as their primary food. This herbivorous heritage is the context in which humans differentiated themselves evolutionarily, and it is those differences that made us what we are. Some argue that we should eat like our closest relatives, the chimpanzees, and other great apes. However, our evolutionary path has led us far from the paths of those others. The closest extant relative of the carnivorous cetaceans (whales and dolphins) is thought to be the herbivorous hippopotamus , and yet it would be foolhardy to suggest the orca's family history entails a need for a plant-based diet. Closest does not imply close.
What is that diet that our non-human relatives have like? The diet of an herbivore looks superficially like it might consist of something akin to our modern fruits and vegetables. It is often presumed, then, that this would be a high-carbohydrate, low fat diet, and that therefore the animal gets most of its energy from carbohydrates and little from fat. In fact, in a sense, the opposite is true.
To see why herbivores can be thought of as having fat-based metabolisms, we have to understand some anatomy and physiology. The energy strategy of herbivores is to eat copious amounts of low-quality forage, subjecting it to a long digestive process, often multiple times. When I say "low quality", I'm not making an aesthetic judgment, it's a term from nutritional ecology.
Fibre and Dietary Quality
"Dietary quality" is a measure of nutritional value proposed by primatologists who were trying to understand foraging behaviour in primates . The researchers divided the dietary components of the primates they were studying into three classes: The highest quality foods for primates were animal-sourced foods. It is a little recognised fact that most herbivores do get some prey in their diets, primarily insects, and some primates such as chimpanzees hunt small game. Animal-sourced foods have the most bioavailable nutrients, protein, and energy. The reproductive parts of plants, such as flowers, bulbs, fruits, and seeds come in at moderate quality. They also have relatively greater nutritive value compared to other plant parts. They can't be depended on as a mainstay, though, because they are scarcer, more seasonal, and often defended with physical barriers like shells or thorns, or chemical defenses. Plants that don't defend against the eating of their reproductive parts tend to extinction quickly for obvious reasons. Nonetheless, if you can get them, they are of value, though considerably less than animal food. The lowest quality foods are the structural components of plants, such as roots, stems, and leaves.
The reason these latter parts are so poor nutritionally, is that they are mostly made of fibre. As it happens, vertebrates don't have the ability to digest fibre. We cannot break down the bonds that hold the cells together where the energy is locked away. Most of the carbohydrate on the planet is stored in plants in this form, and it would be utterly inaccessible to mammals except for a very clever hack.
Many microbes, it turns out, can digest fibre, and what herbivores have done is specialised their anatomy to house "fermenting" microbes that take fibre as input and then output fat. By dint of this symbiotic relationship, herbivores actually get some 60-80% of their energy from fatty acids that are the result of this fermentation process . Herbivores can be differentiated by where in the gastrointestinal tract they house their microbial "fat factories".
Location, location, location
The major parts of the gastrointestinal tract, in order from entry to exit are the stomach, the small intestine, the caecum, and the colon (large intestine). The relative proportions of these components reflect digestive specialisation. Of these, the stomach, caecum, and colon can be specialised for fermentation The small intestines specialise in assimilation.
Foregut fermenters typically have multichambered stomachs for fermentation. Examples of foregut fermenters are ruminants such as sheep, cattle, deer, and kangaroos; and pseudoruminants such as hippopotamuses and camels. There are even some ruminant primates, the colobus monkeys. The stomach chambers house vats of microbes that ferment fibre into fat. The animals regurgitate food and chew it again to expose more cellulose surface area and add more saliva, which appears to be important for pH levels .
Subjecting the same food to many digestive passes creates efficient energy extraction from low quality food. The remains after this process is complete then pass through the rest of the gastrointestinal tract including the small intestine. This gives opportunity for the bodies of the microbes to be digested, too, contributing protein nutrition.
Hindgut fermenters, including horses, elephants, rabbits, and many great apes, (as well as herbivorous reptiles and birds) relegate the fermentation process to the caecum and colon. This means fermentation comes after the small intestine. One disadvantage to this solution is that the leftovers from the fermentation process that the colon doesn't absorb have nowhere to go but out, having missed the small intestines. That includes protein from the bodies of the microbes themselves.
Moreover, a single pass, even through these specialised organs, is less efficient than the multiple pass strategies of the foregut fermenter. For these reasons, hindgut fermenters often engage in coprophagy, the eating again of food after it has passed through the digestive tract. Our heritage is from hindgut fermentation.
Not your mother's fruits and vegetables
Although other great apes are often described as frugivores, fruit is only one part of their diets. They also eat pith, fibrous stems and stalks, and leaves. Compared with domesticated fruits from our thousands of years of selective breeding, wild fruits are much higher in fibre (and lower in sugar) . As an example of the diet of our herbivorous primate relatives, one study found the western lowland gorilla gets about "2.5% energy as fat, 24.3% protein, 15.8% available carbohydrate, with potentially 57.3% of metabolizable energy from short-chain fatty acids (SCFA) derived from colonic fermentation of fiber." 
As the name reveals, short-chain fatty acids are fats. However, SCFAs are metabolised differently from medium- or long-chained fatty acids, so the downstream effects of a diet that's high in SCFAs is not exactly the same as one high in the long-chain fatty acids that humans typically eat, though there are similarities.
The three primary SCFAs produced by gut microbes are acetate, propionate, and butyrate. Depending on which microbes dominate, which in turn depends partly on diet, the ratio of these can vary . Usually it's mostly acetate with smaller amounts of the other two. Typically, studies estimate ratios of around 3:1:1 in humans [ibid]. Even though this means that herbivores are using fats for energy, it doesn't mean that they are normally in a ketogenic state, as we will see later. In fact, propionate is a gluconeogenic substrate, contributing to glucose synthesis in the body [ibid].
If it is meaningless to call cats and deer omnivores, even though they both ingest plants, is there any sense in having the omnivore category at all? Omnivory is a good classification for animals that have the anatomical and physiological ability to make good use of both strategies. It entails great flexibility, and it comes in degrees. Pigs and boars, bears, and dogs are points on a spectrum of decreasing aptitude for thriving on plants. Again we can see this by examining their digestive tracts. Boars have comparatively extensive colons, much like hindgut fermenters, and do, as it happens, eat plenty of plant material. Because their digestive systems allow them to digest plants well, they have freedom to be opportunistic, which is advantageous. Bears have less colon than boars, and some draw the line between omnivores and carnivores with bears, racoons, and other members of the Carnivoran order. That is, many consider bears carnivores, if only hypocarnivores.
The terms hypo-, meso-, and hypercarnivore are meant to distinguish levels of carnivory by the proportion of meat in their diets. It's worth noting that a mere 70% meat qualifies as hypercarnivore by these definitions ! Yet, there is no definitive ratio of plant eating to meat eating that demarcates omnivores from carnivores. One of the animals least adapted to eat plants without usually being classed an obligate carnivore, is the dog. Yet whether the dog should be considered an omnivore or a facultative carnivore is hotly debated in some circles.
I think the reason for the controversy is that those who believe that dogs should definitely eat some plants for optimal health must argue that dogs are omnivores, because not only is a facultative carnivore able to survive on plants, but not thrive long term, as discussed previously, but more importantly, a facultative carnivore is thought not to need dietary plant intake for optimal health. In this view, any time a dog eats some portion of its diet as plants, it is, by definition, eating something of inferior quality needlessly, and thus it seems a questionable practice.
This argument about dogs may not be resolvable, as there seems to be no way to falsify either position. However, the Internet is full of anecdotes on both sides of the question, with dogs faring best either with some plants or with none. In a case like this, it seems prudent for an individual dog owner to experiment with both kinds of diet and with an open mind.
Even for an animal best classified as an omnivore, and thus having the ability to get adequate nutrition from a wide variety of sources, this does not mean that all options are equally nutritious or that it will eat everything with equal propensity and no discrimination whatsoever. If you were at an all-you-can-eat buffet, you would survey the offerings and make your selections based on your preferences (and, if you're human, based on your beliefs!), Just as the Hadza will eat tubers, but prefer other foods when they are available .
Different foods have different quality of nutrition, and animals are wired to detect and prefer food high in protein, energy, and nutrients in short supply. The label omnivore can thus be insidious, and used to justify feeding or eating low quality food "because we can" to the detriment of the eater.
In other words, claiming that people ought to eat "in moderation" from all potential food sources, is failing to recognise that some foods are simply more nutritious than others. There is no virtue in eating something simply because you can, even if it is a source of nutrients. It must always come back to a comparison with the best alternative.
Facultative carnivores are of particular interest. For a carnivore, having the ability to get nutrition from plants is a survival advantage. It can carry an animal through periods when the "preferred" food is scarce. To call an animal with this ability an omnivore is misleading, though. It makes it sound as though anything goes, that the animal is just as happy and healthy on any of a variety of foods, or even that they must partake in a variety. That is simply not the case.
In this book I will argue that humans, like dogs, are facultative carnivores, even though our primate heritage shaped our carnivory in a different way, making us look different from other carnivores and even making our needs different. Although we can make use of plants as a nutrient source, we don't have to, and in some cases we are better off minimising plant intake.
Footnotes & References
PAMELA J. PIETZ and DIANE A. GRANFORS "White-tailed Deer (Odocoileus virginianus) Predation on Grassland Songbird Nestlings," The American Midland Naturalist 144(2), 419-422, (1 October 2000). https://doi.org/10.1674/0003-0031(2000)144[0419:WTDOVP]2.0.CO;2
"Although probably opportunistic, deer predations clearly were deliberate and likely are more common than generally believed."
| In fact, it was once believed that botulism was a phosphorous deficiency disease, because cattle will sometimes chew on carrion when deficient in phosphorous, and this was associated with the symptoms of botulism. Of course we now know the botulism came from bacteria on the carcasses, not the phosphorous deficiency that prompted the cattle to eat from them. See https://en.wikipedia.org/wiki/Osteophagy|
As a news story this never seems to get old.
There are also journal reports.
Furness, R. W. “Predation on Ground-Nesting Seabirds by Island Populations of Red Deer Cervus Elaphus and Sheep Ovis.” Journal of Zoology 216, no. 3 (November 1988): 565–73. https://doi.org/10.1111/j.1469-7998.1988.tb02451.x.
"If the habit of eating bone from seabird chicks was novel and aberrant one might expect each deer or sheep to tackle chicks differently and clumsily, but both species showed remarkably uniform and precise techniques of extracting the parts they wanted from chicks. The parts ingested were those with high bone content but little flesh, skin or feathers. Attacks on shearwater chicks by deer seemed always to begin with decapitation and only sometimes were leg and wing bones extracted. Sheep more often bit off legs, and decapitation was less common. However, sheep neatly severed legs and wings and swallowed these, whereas deer chewed the legs and carpal regions of wings and so extracted fragmented bone without ingesting feet, skin or feathers. Leg or wing amputations by sheep were often not fatal and wounds healed over, whereas no live shearwater chick showing evidence of attacks by deer was found among large numbers handled for ringing. The proportion of chicks mutilated by sheep or deer was small, but at its peak on Foula, about five tern chicks were mutilated for every sheep grazing in the area, and on Rhum about three shearwater chicks were attacked for each deer present."
Nack, Jamie L., and Christine A. Ribic. “APPARENT PREDATION BY CATTLE AT GRASSLAND BIRD NESTS.” The Wilson Bulletin 117, no. 1 (March 2005): 56–62. https://doi.org/10.1676/04-056.
"Videotaped evidence of cattle re-moving nestlings and eggs from ground nestssuggests that the impact of cattle on grasslandbird nests has been underestimated in the past."
Plantinga, Esther A., Guido Bosch, and Wouter H. Hendriks. “Estimation of the Dietary Nutrient Profile of Free-Roaming Feral Cats: Possible Implications for Nutrition of Domestic Cats.” British Journal of Nutrition 106, no. S1 (October 12, 2011): S35–48. https://doi.org/10.1017/S0007114511002285.
"The domestic cat’s wild ancestors are known to be obligatory carnivores, consuming predominantly prey. The consumption of a diet composed of animal tissues throughout evolution has led to unique digestive and metabolic adaptations (often referred to as idiosyncrasies) (11 – 14). Reduction of redundant enzymes and modification of enzyme activities will have had specific advantages in terms of energy expenditure (11). Examples of these adaptations include:
| Wilkinson, P. F. "Ecosystem models and demographic hypotheses: predation and prehistory in North America." Models in archaeology, edited by DL Clarke. London: Methuen (1972): 543-576.|
| While I don't wish to get into an embroiled argument about the definition of natural — something I've already discussed in Chapter 2 — what I wish to exclude is recent technologically-based formulations to replace meat that were obviously not present in the evolutionary environment. I make this distinction because the ability to synthesise nutrients in a lab, gather them from remote regions, or isolate them and concentrate them from plants doesn't have any bearing on why an animal's physiology is the way it is. One might argue, for example, that people can survive and thrive as vegans, because they can manufacture or outsource the missing nutrients. That may be a valid argument for being able to avoid eating meat here and now, but it is not an argument against evolved human need for animal nutrition.|
Ullrey, D. E. (2004). "Mammals: Carnivores". In Pond, Wilson (ed.). Encyclopedia of Animal Science. CRC Press. p. 591. ISBN 978-0-8247-5496-9.
"Animals that eat only animal prey are sometimes called strict or obligate carnivore to distinguish them from facultative carnivores that eat mostly prey but also consume nonanimal foods. Felids are strict carnivores and, in the wild, obtain most of their food by predation on the tissues of mammals, birds, or fish. Their domestic representative, the cat (Felis catus), differs in several respects in its metabolism and nutrient requirements from the domestic dog (Canis familiaris), a canid that is a facultative carnivore. It is presumed that these differences are an evolutionary consequence of their respective ancestral diets."
| Nikaido, M., A. P. Rooney, and N. Okada. “Phylogenetic Relationships among Cetartiodactyls Based on Insertions of Short and Long Interpersed Elements: Hippopotamuses Are the Closest Extant Relatives of Whales.” Proceedings of the National Academy of Sciences 96, no. 18 (August 31, 1999): 10261–66. https://doi.org/10.1073/pnas.96.18.10261.|
Sailer, Lee Douglas, Steven J. C. Gaulin, James S. Boster, and Jeffrey A. Kurland. “Measuring the Relationship between Dietary Quality and Body Size in Primates.” Primates 26, no. 1 (January 1985): 14–27. https://doi.org/10.1007/BF02389044.
"Primate diets can be conveniently described in terms of three principal food. types: (1) "structural" parts of plants, including leaf material, stems, bark and other plant material that contains a high proportion of structural carbohydrates such as cellulose; (2) "reproducive" parts of plants, such as flowers and flower buds, ripe and unripe fruits, nectar and other resins, all of which contain reduced proportions of structural carbohydrates and higher proportions of digestible sugars than the foods in the first category; and (3) "animal matter", including vertebrate and invertebrate prey (GAULIN & SAILER, 1984a). This is a useful classification of food types for many reasons, but of central importance here is the fact that it groups foods simultaneously by abundance and quality. Structural parts are abundant but are characterized by low nutrient density. Animal matter is nutrient-rich but sparsely distributed. Reproductive parts are intermediate in both nutrient density and supply (GAULIN, 1979)."
Bergman, E. N. “Energy Contributions of Volatile Fatty Acids from the Gastrointestinal Tract in Various Species.” Physiological Reviews 70, no. 2 (April 1, 1990): 567–90. https://doi.org/10.1152/physrev.19184.108.40.2067.
"After subtracting such energy losses as methane in the eructated gases and nitrogenous compounds in the urine, the three VFA then would account for 65-75% of the total metabolizable energy. When it is realized that the higher or five-to seven-carbon VFA can add up to about an additional 5% of the energy and that some VFA are produced and absorbed from the cecum and large intestine, it is obvious that the total VFA produced make up a very large part of the useful energy made available to the animal. The total available VFA easily could account for 80% of the animal’s daily energy requirements."
| Kay, R. N. “The Influence of Saliva on Digestion in Ruminants.” World Review of Nutrition and Dietetics 6 (1966): 292–325. https://doi.org/10.1159/000391428.|
Schwitzer, Chr, S Y Polowinsky, and C Solman. “Fruits as Foods – Common Misconceptions about Frugivory,” n.d., 39.
"We looked at the nutrient and energy content of 201 individual wild fruits and 13 samples of multiple different fruits from primate habitat countries around the world, taken from 20 different literature sources (Goodall 1977; Hladik 1977; Nagy and Milton 1979; Milton and Jenness 1987; Ganzhorn1988; Leighton 1993; Conklin and Wrangham 1994; Dasilva 1994; Hamiltonand Galdikas 1994; Wrangham et al. 1998; Atsalis 1999; Matsumoto-Odaand Hayashi 1999; Serio-Silva et al. 2002; Milton 2003; Powzyk and Mowry2003; Curtis 2004; Ganzhorn et al. 2004; Worman and Chapman 2005; Danishet al. 2006; Rothman et al. 2007b). All fruits were identified by the respective authors as being a food source for frugivorous primates through either direct observation of feeding or analysis of faeces. Additionally, we collected fruit samples from 68 plant species that were utilized as food resources by blue-eyed black lemurs (Eulemur flavifrons) in northwest Madagascar. Samples were subjected to Weender analysis and detergent analysis. We also looked at the nutrient content of 101 commercially cultivated fruits and 96 commercially cultivated vegetables, taken from research using European and US retailed fruits being fed to zoo animals (Milton 2003; Schmidt et al. 2001; Schmidt et al. 2005, S.Y. Polowinsky and C. Schwitzer, unpubl.), and data held by Zootrition®V2.5 (Saint Louis Zoo, USA) and the United States Department of Agriculture (Agriculture 2007). We defined vegetables in a culinary sense as herbaceous plants or plant parts which are cultivated, commercially available and regularly eaten as unsweetened or salted food by humans in Europe and/or North America. Thus, our vegetable sample also contained some fruits in the botanical sense (i. e. fleshy reproductive organs of plants), such as e. g. tomatoes, cucumbers and squash. We did not subdivide vegetables into different plant parts such as fruits, leaves, roots and tubers etc, since this would have resulted in sample sizes too small for statistical analysis.
"The nutritional values included in our study were: ash, crude fat, crude protein, crude fibre, neutral detergent fibre (NDF), acid detergent fibre (ADF),acid detergent lignin (ADL), total sugars, metabolizable energy (ME) (calculated using the standard Atwater energy equivalents for humans of 4, 4 and 9 kcal/g of protein, carbohydrate and fat, respectively), calcium, phosphorus, sodium,potassium, magnesium and vitamin C. The choice of micronutrients was determined to some extent by the availability of data rather than by concept. All values were converted to percentage of dry matter if not already recorded as such, except for energy which was recorded as kcal/100 g of dry matter.The wild fruits included in our sample were from a variety of countries, and we attempted to get representative samples from the 4 primate habitat regions: Madagascar, Africa, Asia and the Neotropics (Figure 1). Research detailing the nutritional values of wild fruits consumed by primates in Asia was somewhat lacking, as was data from all regions regarding individual sugars and micronutrients. Data were tested for normal distribution using the Kolmogorov-Smirnovone-sample test. Independent samples T-tests (2-tailed) were carried out for normally distributed variables (i. e. crude ash, crude fibre, NDF, ADF,ADL), and Mann-Whitney U tests for those variables found not to be normally distributed (i. e. crude fat, crude protein, total sugars, metabolizable energy), using SPSS®12.0 (SPSS Inc., USA), to compare wild and cultivated fruits and vegetables; p was set at <.05 and <.01 to accept results as significant and highly significant, respectively.
"As expected, differences were found in the nutritional qualities of cultivated and wild fruits. The wild fruits had a significantly larger mean percentage of crude fibre (26.6±14.4% DM>12.8±5.6% DM; p<0.01), NDF (47.2±18.1%DM>12.4±5.4% DM; p <0.01), ADF (42.7±14.8% DM>8.5±4.8% DM; p<0.01), ADL (21.9±9.0% DM>3.1±2.0% DM; p<0.01), crude protein (7.7±5.0% DM>5.7±2.8% DM; p<0.01) and ash (5.2±2.1% DM>3.6±1.6% DM; p<0.01). The cultivated fruits were found to have a significantly larger mean percentage of sugar (48.6±20.1% DM>17.4±16.3% DM; p <0.01), more than double that found in wild fruits, and also had significantly higher mean ME (359.4±50.0 kcal/100g DM>209.1±122.1 kcal/100g DM; p<0.01).I nformation regarding the individual sugars and micronutrient content of wild fruits was too sparse for reliable statistical analysis, but the available data suggest that cultivated fruits are 10 to over 100 times richer in individual sugars than wild fruits and are comparably poorer sources of several essential minerals."
| The Western Lowland Gorilla Diet Has Implications for the Health of Humans and Other Hominoids, David G. Popovich J. Nutr. October 1, 1997 vol. 127 no. 10|
Besten, Gijs den, Karen van Eunen, Albert K. Groen, Koen Venema, Dirk-Jan Reijngoud, and Barbara M. Bakker. “The Role of Short-Chain Fatty Acids in the Interplay between Diet, Gut Microbiota, and Host Energy Metabolism.” Journal of Lipid Research 54, no. 9 (September 2013): 2325–40. https://doi.org/10.1194/jlr.R036012.
"Changes in dietary fibers drive changes in the composition of gut microbiota. Although diet is a major determinant of the colonic microbiome, the host genetic background and the colonic milieu also exert a strong influence on the microbial composition in the large intestine (51–54). The microbial activity in turn also affects the colonic milieu. Together, this causes a strong variation of the microbial population between individuals. In this section we will discuss this variation, the mechanisms of microbial SCFA production, and the interaction between microbial composition, microbial SCFA production, and the colonic milieu."
"Acetate, propionate, and butyrate are present in an approximate molar ratio of 60:20:20 in the colon and stool (9–11). Depending on the diet, the total concentration of SCFAs decreases from 70 to 140 mM in the proximal colon to 20 to 70 mM in the distal colon (12). A unique series of measurements in sudden-death victims (n = 6) showed that the acetate:propionate:butyrate ratio in humans was similar in the proximal and distal regions of the large intestine (11). In the cecum and large intestine, 95% of the produced SCFAs are rapidly absorbed by the colonocytes while the remaining 5% are secreted in the feces (12–15)."
"To prevent high SCFA concentrations in blood, the liver clears the major part of propionate and butyrate from the portal circulation (105). Propionate acts as a precursor for gluconeogenesis in the liver (6). In ruminants, with isotope dilution techniques, the contribution of propionate to glucose synthesis was calculated to vary between 45 and 60% (106). It is unclear if this is similar in nonruminants, because ruminants depend on SCFAs for 80% of their maintenance energy (31). After conversion of propionate into propionyl-CoA by propanoate:CoA ligase (AMP-forming), propionyl-CoA is converted to succinyl-CoA in three consecutive steps catalyzed by propionyl-CoA carboxylase, methylmalonyl-CoA epimerase, and methylmalonyl-CoA mutase. Succinyl-CoA enters the tricarboxylic acid (TCA) cycle and is converted to oxaloacetate, the precursor of gluconeogenesis (107). In humans the extent to which propionate contributes to energy metabolism is unknown due to the lack of data on true production rates of propionate. Concentrations of propionate in portal blood and hepatic venous blood suggest that around 30% of propionate is taken up by the liver (11, 105). Peripheral tissues take up the remainder of propionate because peripheral venous blood levels were 23% lower compared with hepatic venous blood levels. In another study it was estimated that humans use 50% of the propionate as a substrate for hepatic gluconeogenesis (108). The general view is that the liver clears a large fraction of propionate from the portal circulation, but absolute values are still unknown."
This categorization of levels of carnivory originally came from Valkenburgh 1988, but the names hypo- meso-, and hypercarnivore referring to those classes started with Holliday and Steppan:
"Of the recognized carnivoran ecomorphs,the niche of the meat specialist, or hypercarnivore, is associated with a diet comprising more than 70% meat, in contrast to the generalist (Van Valkenburgh 1988, 1989), which may eat 50–60% meat with vegetable matter and invertebrates making up the remainder of the diet."
Holliday, Jill A, and Scott J Steppan. “Evolution of Hypercarnivory: The Effect of Specialization on Morphological and Taxonomic Diversity,” n.d., 21.
Valkenburgh, Blaire Van. “Carnivore Dental Adaptations and Diet: A Study of Trophic Diversity within Guilds,” n.d., 27.
Marlowe, Frank W., and Julia C. Berbesque. “Tubers as Fallback Foods and Their Impact on Hadza Hunter-Gatherers.” American Journal of Physical Anthropology 140, no. 4 (December 2009): 751–58. https://doi.org/10.1002/ajpa.21040.
"The Hadza are hunter-gatherers in Tanzania. Their diet can be conveniently categorized into five main categories: tubers, berries, meat, baobab, and honey. We showed the Hadza photos of these foods and asked them to rank them in order of preference. Honey was ranked the highest. Tubers, as expected from their low caloric value, were ranked lowest. Given that tubers are least preferred, we used kilograms of tubers arriving in camp across the year as a minimum estimate of their availability. Tubers fit the definition of fallback foods because they are the most continuously available but least preferred foods. Tubers are more often taken when berries are least available. We examined the impact of all foods by assessing variation in adult body mass index (BMI) and percent body fat (%BF) in relation to amount of foods arriving in camp. We found, controlling for region and season, women of reproductive age had a higher %BF in camps where more meat was acquired and a lower %BF where more tubers were taken. We discuss the implications of these results for the Hadza. We also discuss the importance of tubers in human evolution."