Humans are physiologically adapted to animal source foods

Animals thrive best on diets resembling the ones to which they are physiologically adapted; i
t would be unlikely that Homo sapiens constitutes an exception to this principle [Leroy & Cofnas 2020]. Any discussion on the healthiness of animal source foods (ASFs) should at least address whether or not ASFs are part of our species-appropriate diets (while acknowledging that there may not be a single optimal human diet). From an evolutionary angle [see elsewhere], Homo sapiens emerged with the anatomical and physiological equipment of a habitual rather than facultative meat eater [Henneberg et al. 1998]. Those who argue that the human diet is naturally herbivorous based on a phylogenetic relationship with apes [Barnard & Leroy 2020], overlook the divergences that occurred during evolution [Leroy & Barnard 2020]. Undeniably, our hominin ancestors are adapted to procure nutrients from both plants and ASFs, whereas veganism is without evolutionary precedent [O'Keefe et al. 2022].
 
Anatomical adaptations

Although one has to be careful when interpreting the incomplete human fossil record [Wood & Smith 2022], the probable conclusion is that a substantial ASF intake during most of human evolution has contributed to cranial-dental and intestinal morphological change, human erect posture, reproductive characteristics, longer lifespan, and brain expansion [Isler & Van Shaik, 2014; Baltic & Boskovic 2015; Mann 2007, 2018]. In addition, the gut microbiome is thought to have changed substantially [Moeller et al. 2014; Amato et al. 2015; Beasley et al. 2015], likely towards an adaptation to higher meat and fat intake [Moeller et al. 2014; Domínguez-Rodrigo et al. 2021]. Anatomical and genetic changes in the human lineage [Okerblom et al. 2018] are suggestive of ecological adaptations that involve the pursuit and hunting of animals, related to endurance running [Bramble & Lieberman 2004; Lieberman et al. 2006; Lieberman & Bramble 2007; Ruxton & Wilkinson 2011; Glaub et al. 2017Holowka & Lieberman 2018], heat loss [Lieberman 2015; Halsey & Brice 2020], vision [Kobayashi & Kohshima 2001], breathing [Bramble & Carrier 1983; Carrier 1984; Franciscus & Trinkaus 1988], throwing [Darlington 1975; Isaac 1987; Knüsel 1992; Watson 2001; Young 20032008; Roach 2012; Roach et al. 2013; Roach & Lieberman 2014; Roach & Richmond 2015; Kuhn 2016; Lombardo & Deaner 2018a,b], and clubbing [Young 20032008]. Some of these traits and changes can be traced back to Homo erectus [Ruxton & Wilkinson 2011; Roach & Richmond 2015; Pontzer 2017; Hora et al. 2020; Domínguez-Rodrigo et al. 2021], others perhaps even earlier [Isaac 1987; Ungar 2012]
 
The shift from fibrous plants to the inclusion of substantial amounts of ASFs, together with the use of tools, paralleled a decrease in teeth size and jawbones, a reduction in chewing muscles, and weaker maximum bite force capabilities [Teaford & Ungar 2000; Zink & Lieberman 2016]. Homo molars gained steeper slopes and more relief, also suggestive to an adaptation to meat eating [Ungar 2004]. Starting with Homo erectus, humans developed smaller molars and began to spend a lot less time on feeding than would be predicted from body mass and phylogeny with other apes (only 5% instead of a predicted 48% of daily activity in Homo sapiens) [Organ et al. 2011]. As a protection towards meat-borne pathogens, the human stomach evolved into one of the most acidic in the animal kingdom, similar to carnivores and scavengers [Beasley et al. 2015]. Adaptation also led to a small intestine comprising 56% of total gut volume and a shrinkage of the large intestine (and therefore fermentative capacity) to a mere 20%; which is the inverse situation of what is found in apes [Milton 2003]. The surface area of the human digestive tract is surprisingly small [Helander & Fändriks 2014], both human intestinal area and length being closer to cats and dogs than to herbivores or even baboons [Henneberg et al. 1998].
 
According to the 'expensive tissue hypothesis', an increase in brain size was made possible - under selective pressure for more cognitive capacity - by an overall reduction in the size of the energy-consuming gut [Aiello & Wheeler 1995], as well as by the supply of energy and nutrients via ASFs (e.g., iron, zinc, vitamin B12, choline, docosahexaenoic acid, fat, cholesterol) [Chamberlain 1996; Mann 1998; Gupta 2016]. Nicotinamide (vitamin B3) has been explicitly cited as a key brain-trophic element in ASFs [Williams & Dunbar 2013; Williams & Hill 2017a,b]. The exceptionally high energy needs of the brain may also be the reason why humans - infants in particular - have higher body fat than non-human primates [Kuzawa 1999; Cunnane & Crawford 2003]. While the brain of an adult primate consumes <10% of the total resting metabolic rate, this amounts to 20-25% in the case of anatomically modern humans [Leonard et al. 2003].

Metabolic adaptations

Due to the enduring consumption of ASFs - and in the absence of coprophagy - humans have lost their ability to absorb vitamin B12 produced by gut bacteria [Domínguez-Rodrigo et al. 2012]. This may also be an explanation for the preferential absorption of haem iron over ionic forms in humans but not in herbivores [Henneberg et al. 1998; Mann 2007]. Likewise, a higher dependency on choline, most abundant in ASFs, is seen in comparison to other primates [Domínguez-Rodrigo et al. 2021]. Moreover, a reduction is seen in the human potential to produce taurine from amino acid precursors [Chesney 1998; Mann 2007] and to convert alpha-linolenic acid into the biologically important long-chain fatty acids eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) [Stark et al. 2015; Hodson et al. 2018; Pignitter et al. 2018; Greupner et al. 2018]. Humans, therefore, may not be able to make sufficient amounts of DHA for normal infant brain development [Cunnane et al. 2007]. The human brain has not only a particularly high requirement for energy, but also for DHA and arachidonic acid (AA). Even in the unlikely case that a high enough caloric density would have been available from nuts and seeds, plant-only diets would have been unable to deliver enough preformed DHA and AA [Cordain et al. 2001]. 
 
Adaptation to the eating of ASFs can also be inferred from a comparison of the age at weaning of herbivores, omnivores, and carnivores [Psouni et al. 2012]. For humans, an early age was enabled by a switch from maternal milk to nutrient-dense meat, marrow, organs, and fat [Kennedy 2005]. Overall, human energy metabolism is adjusted to diets dominated by lipids and proteins [Pond & Mattacks 1985; Finch et al. 2004Swain-Lenz et al. 2019].

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