Thread by @fleroy1974

Are human bodies adapted to eating #meat and other animal source foods? And how do we know?

🧵 with summary of @aleph2020 info 👇
aleph-2020.blogspot.com/2019/05/animal…

Animals thrive best on diets resembling the ones to which they are physiologically adapted; it would be unlikely that Homo sapiens constitutes an exception to this principle. Any discussion on the healthiness of meat & other animal source foods should at least address this.

Homo sapiens emerged with the anatomical & physiological equipment of a habitual rather than facultative meat eater. Those who argue that the human diet is naturally herbivorous based on a phylogenetic relationship with apes overlook key divergences that occurred during evolution

1⃣ Anatomical adaptations

Substantial intake of animal source foods during most of the human evolutionary window led to cranial-dental and intestinal morphological changes, human erect posture, reproductive characteristics, longer lifespan, and brain expansion.

The gut microbiome changed substantially towards an adaptation to higher meat/fat intake. Anatomical and genetic changes are suggestive of ecological adaptations that involve the pursuit/hunting of animals (endurance running, heat loss, vision, breathing, throwing, clubbing).

The shift from fibrous plants to animal source foods, 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.

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).

To protect against meat-borne pathogens, the human stomach evolved into one of the most acidic in the animal kingdom, similar to carnivores & scavengers.

Adaptation led to a small intestine of 56% of total gut volume and shrinkage of the large intestine (fermentative capacity) to 20%; the inverse situation of what is found in apes. Both human intestinal area & length are closer to cats & dogs than to herbivores or even baboons.

According to the 'expensive tissue hypothesis', an increase in brain size was made possible by an overall reduction in the size of the energy-consuming gut & the supply of energy/nutrients via animal source foods (e.g., iron, zinc, vitamin B12, choline, docosahexaenoic acid, fat)

Vitamin B3 has been explicitly cited as a key brain-trophic element in animal source foods. 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.

2⃣ Metabolic adaptations

Due to consumption of animal source foods - in the absence of coprophagy - humans lost the ability to absorb vit B12 produced by gut bacteria. This may also explain the preferential absorption of haem iron over ionic forms in humans but not herbivores.

Higher dependency on choline, most abundant in animal source foods, is seen in comparison to other primates, as well as a lower potential to produce taurine from amino acid precursors & to convert alpha-linolenic acid into the biologically important long-chain fatty acids EPA/DHA

Humans may not be able to make sufficient DHA for normal infant brain development. The human brain has not only a particularly high requirement for energy, but also for DHA and arachidonic acid (AA). Plant-only diets would have been unable to deliver enough preformed DHA/AA.

Adaptation to the eating of animal source foods can also be inferred from a comparison of the age at weaning of herbivores, omnivores, and carnivores. For humans, an early age was enabled by a switch from maternal milk to nutrient-dense meat, marrow, organs, and fats.

Overall, human energy metabolism is adjusted to diets dominated by lipids and proteins.