Plant foods need special preparation to be safe, which shows we aren't designed to eat them
Needing to cook a food doesn't show we aren't suited to it. Humans have cooked for hundreds of thousands of years, and meat needs cooking for safety too. The beans that cause trouble raw are made safe by ordinary boiling, which is how everyone already eats them.
Several staple plant foods, such as beans, some grains and cassava, contain compounds that are harmful when eaten raw or badly prepared. People have been hospitalised after eating undercooked kidney beans. Soaking, boiling and fermenting exist because our bodies can't neutralise these compounds on their own. So a diet built on these foods depends on knowledge many people lack, and it isn't as simple or as safe as 'just eat plants'.
Needing preparation is not evidence of being unsuited to a food. If it were, the same test would rule out meat. Raw chicken carries salmonella and campylobacter. Undercooked pork can carry parasites. Raw minced beef causes outbreaks of E. coli. Cooking makes meat safe in the same way it makes beans safe. Applied evenly, the argument proves that humans aren't 'designed' to eat most of what any diet contains.
Cooking is part of human biology, not a workaround for it. Our ancestors have used fire for a very long time, and one leading account of human evolution holds that cooking shaped our small guts and large brains. A species that evolved alongside cooking is not showing a design flaw when it cooks.
The toxins in question are real, and the examples are real. Red kidney beans contain a lectin that causes acute illness when they are eaten raw or undercooked. That is a good reason to boil them properly, or to buy them tinned, already cooked. It is not a reason to avoid them. Soaking, boiling and fermenting are ordinary kitchen knowledge, not specialist detoxification.
Most plant foods need no special treatment at all. Fruit, most vegetables, nuts and seeds are routinely eaten raw. The foods that do need care are a short and well-known list.
The passage itself also cuts against the claim. It notes that dogs and cats are harmed by some plants because they detoxify them less well than we do. That is an observation that humans do have substantial capacity to handle plant compounds, mainly through the liver.
None of this bears on the ethical question. Knowing to boil a kidney bean is a matter of cooking competence, and it doesn't make the harm involved in animal farming necessary.
- If plants need processing to release their nutrients, we aren't designed to eat them
Different wording, same argument. The response above answers all of them.
Applying a principle generally while claiming an exemption for one's own case, without giving a relevant reason for the exception.
How it applies here: The arguer treats 'needs cooking or preparation to be safe' as proof we aren't designed to eat plant foods, but doesn't apply the same test to meat, which also needs cooking to avoid serious foodborne illness. No reason is given for exempting it.