Why Animal Based Nutrition?

It's estimated that the average human eats about 1.5 g of naturally-occurring plant-produced toxins such as phytoalexins, lectins, and oxalates, to name just a few. Vegetarians consume far more. These compounds have various functions but are there to harm predating insects and mammals in various ways.  Lectins damage the intestinal lining to slowly kill mammals and deny them absorption of nutrients. Oxalates bind up nutrients and form spiky crystals that cause damage all over the body including in the skin and thyroid. Oxalates are also involved in kidney stone formation. Plants do not want to be eaten!

Our last common ancestor between plants and animals existing somewhere around 1.5 Billion years ago, and since then they have taken different biochemical pathways and use very different molecules to grow and propagate. There is little to no evidence that humans can use plant chemicals like polyphenols and anti-oxidants in their own biochemistry. In fact, it seems to really screw up our biochemistry when they are present to any meaningful degree. Some plant chemicals look similar to animal molecules, but end up not working very well for us. Beta-carotene vs retinol, vitamin K1 vs K2, vitamin D2 vs D3, and alpha-lipoc acid vs DHA are all examples.

Plants have specific compounds that bind up nutrients to reduce or even eliminate their bioavalablity to predating animals. Phytic acid and oxalates bind up minerals such as iron, magnesium,calcium, phosphorus selenium, copper, and zinc.  Oysters are a great source of zinc.  But, if they are eaten with beans, half the zinc is bound up, and if eaten with a corn tortilla, almost all of the zinc is bound up and no longer bioavailable. Vegetarians tend to be deficient in zinc, copper and especially selenium.

Of course, but it also appears that they evolved along with the plants and developed coping mechanisms to deal with those toxic chemicals.  Ruminant herbivores have 4 chamber stomachs and primate herbivores have short small intestines, a large cecum and long large intestines to allow for fermentation of plant products, and what actually gets absorbed is free fatty acids as a by-product of the fermentation process.  But, it appears that humans really switched to primarily eating animals about 2 - 2.5 million years ago.  Most of the absorption of nutrients happen in our long small intestines, and we have short large intestines that are not adapted for fermentation.  In fact, fermentation in our guts leads to pain, bloating and gas. And, in some people even lead to auto-intoxication from alcohol production in our own intestines. Not much fun.