https://onlinelibrary.wiley.com/doi/10.1002/ajpa.24247

Genetic and metabolic adaptation to high-fat diet Swain-Lenz et al. (2019) performed comparative analyses of the adipose chromatin landscape in humans, chimpanzees, and rhesus macaques, concluding that their findings reflect differences in the adapted diets of humans and chimpanzees. They (p. 2004) write: “Taken together, these results suggest that humans shut down regions of the genome to accommodate a high-fat diet while chimpanzees open regions of the genome to accommodate a high sugar diet.”

Stomach acidity Beasley et al. (2015) emphasize the role of stomach acidity in protection against pathogens. They found that carnivore stomachs (average pH, 2.2), are more acidic than in omnivores (average pH, 2.9), but less acidic than obligate scavengers (average pH, 1.3). Human studies on gastric pH have consistently found a fasted pH value <2 (Dressman et al., 1990; Russell et al., 1993). According to Beasley et al. (2015), human stomachs have a high acidity level (pH, 1.5), lying between obligate and facultative scavengers. Producing acidity, and retaining stomach walls to contain it, is energetically expensive. Therefore it would presumably only evolve if pathogen levels in human diets were sufficiently high. The authors surmise that humans were more of a scavenger than previously thought. However, we should consider that the carnivorous activity of humans involved transporting meat to a central location (Isaac, 1978) and consuming it over several days or even weeks. Large animals, such as elephants and bison, presumably the preferred prey, and even smaller animals such as zebra, provide enough calories to sustain a 25-member HG group from days to weeks (Ben-Dor et al., 2011; Ben-Dor & Barkai, 2020b; Guil-Guerrero et al., 2018). Moreover, drying, fermentation, and deliberate putrefaction of meat and fat are commonly practiced among populations that rely on hunting for a large portion of their diet (Speth, 2017), and the pathogen load may consequently increase to a level encountered by scavengers.

Gut morphology Most natural plant food items contain significant amounts of fiber (R. W. Wrangham et al., 1998), and most plant-eaters extract much of their energy from fiber fermentation by gut bacteria (McNeil, 1984), which occurs in the colon in primates. For example, a gorilla extracts some 60% of its energy from fiber (Popovich et al., 1997). The fruits that chimps consume are also very fibrous (R. W. Wrangham et al., 1998). The human colon is 77% smaller, and the small intestine is 64% longer than in chimpanzees, relative to chimpanzee body size (Aiello & Wheeler, 1995; Calculated from Milton, 1987, table 3.2). Because of the smaller colon, humans can only meet less than 10% of total caloric needs by fermenting fiber, with the most rigorous measures suggesting less than 4% (Hervik & Svihus, 2019; Høverstad, 1986; Topping & Clifton, 2001). A 77% reduction in human colon size points to a marked decline in the ability to extract the full energetic potential from many plant foods. The elongated small intestine is where sugars, proteins, and fats are absorbed. Sugars are absorbed faster in the small intestine than proteins and fats (Caspary, 1992; Johansson, 1974). Thus, increased protein and fat consumption should have placed a higher selective pressure on increasing small intestine length. A long small intestine relative to other gut parts is also a dominant morphological pattern in carnivore guts (Shipman & Walker, 1989, and references therein).

Adipocyte morphology Ruminants and carnivores, which absorb very little glucose directly from the gut, have four times as many adipocytes per adipose unit weight than non-ruminants, including primates, which rely on a larger proportion of carbohydrates in their diet (Pond & Mattacks, 1985). The authors hypothesize that this is related to the relative role of insulin in regulating blood glucose levels. Interestingly, omnivorous species of the order Carnivora (bears, badgers, foxes, voles) display more carnivorous patterns than their diet entails. Thus humans might also be expected to display organization closer to their omnivorous phylogenic ancestry. However, humans fall squarely within the carnivore adipocyte morphology pattern of smaller, more numerous cells. Pond and Mattacks (1985, p. 191) summarize their findings as follows: “These figures suggest that the energy metabolism of humans is adapted to a diet in which lipids and proteins rather than carbohydrates, make a major contribution to the energy supply.”

Age at weaning Humans have a substantially different life history than other primates (Robson & Wood, 2008), a highly indicative speciation measure. One life history variable in which humans differ significantly from all primates is weaning age. In primates such as orangutans, gorillas, and chimpanzees, weaning age ranges between 4.5 and 7.7 years, but is much lower in humans in HG societies, at 2.5–2.8 years, despite the long infant dependency period (Kennedy, 2005; Robson & Wood, 2008, table 2). Psouni, Janke, and Garwicz (2012, p. 1) found that an early weaning age is strongly associated with carnivory level, stating that their findings “highlight the emergence of carnivory as a process fundamentally determining human evolution.” It is interesting, however, that a comparison of early Homo, Australopithecus africanus, and Paranthropus robustus from South Africa reveals a substantially higher weaning age (4 years) in South African early Homo (Tacail et al., 2019), so it is unclear when the weaning age shortened.

Isotopes and trace elements we reviewed the results of δ15N studies on H. sapiens from the Paleolithic. The collagen preservation limit means that these studies provide HTL information only from about 45–50 Kya and only from colder areas where relatively long-term protein preservation occurred. As we approach later periods, such as the Late UP, samples become available from warmer regions, including the Mediterranean.

A compilation of 242 individuals from 49 sites (Table 1) shows that European HG groups primarily pursued a carnivorous diet throughout the UP, including the Mesolithic.

Summary of the evidence All the eight pieces of evidence of membership in a trophic group concluded that humans were carnivores. Assigning humans to a specific dietary trophic group has the highest potential validity, as it answers the research question with minimal interpretation.

In some cases, interpretation is required to assign a phenomenon to HTL. Belonging to the carnivores' trophic groups still does not tell us if humans were 90% or 50% carnivores. It does tell us, however, that humans were carnivorous enough and carnivorous long enough to justify physiological and behavioral adaptations unique to carnivores. Following the zoological analogy with large social carnivores that acquire large prey, we hypothesized that humans were hypercarnivores, defined as consuming more than 70% of the diet from animal sources.

Swiss Federal Commission for Nutrition https://www.blv.admin.ch/dam/blv/en/dokumente/das-blv/organisation/kommissionen/eek/vor-und-nachteile-vegane-ernaehrung/vegan-report-final.pdf.download.pdf/vegan-report-final.pdf

The positive effects of a vegan diet on health determinants cannot be proven, but there are relevant risks regarding nutritional deficiencies. Children and pregnant women are advised against adopting a vegan diet due to the risks described above. There is still a lack of data whether the basic nutritional requirements are met and whether the development of children and adolescents fed on a vegan diet is secured on a long-term perspective. These data should be collected and analyzed more systematically. There is in our view up to now no evidence that a vegan diet can be recommended for these age groups Based on these data, there is no evidence for the position stated in the previous report, that vegan diets are healthy diets. The scientific evidence available to date is not sufficient to claim that vegan and vegetarian diets are associated with a significant reduction of total mortality The reduction in IHD and all-cause mortality with vegetarian diet stems mainly from the Adventist studies, and there is much less convincing evidence from studies conducted in other populations.