There are compounds in animal foods you can't get anywhere else
Most of them the body makes itself, which is why no deficiency syndrome has been identified in vegans for any of them. Where supplementing helps, the supplement is synthesised rather than taken from an animal. "Found only in animal foods" and "needed from animal foods" are different claims, and this argument needs the second one.
Taurine, carnosine, anserine, creatine, carnitine and a range of bioactive peptides occur in animal tissue and are absent or negligible in plants. Several have identified physiological roles, and a vegan diet removes all of them at once rather than one at a time. Saying that costs nothing assumes we have already worked out everything that matters, which is a strong assumption for a field that is still characterising these compounds.
The list is real. Taurine, carnosine, anserine, creatine and carnitine genuinely are concentrated in animal tissue and genuinely are low or absent in plants. The argument turns on what follows from that, and the step it needs is the one it does not make.
For each of them, the body is the main source rather than the diet. Taurine is synthesised from cysteine and methionine; vegans carry lower plasma levels and no deficiency state has been identified in healthy adults. Cats cannot make their own and die without it, which is where much of the alarm around this one comes from, and humans are not cats. Creatine is synthesised at roughly a gram a day. Carnosine is built from beta-alanine and histidine, and beta-alanine supplements raise muscle carnosine without any animal involved. Carnitine is made from lysine and methionine, and clinical deficiency is a genetic or medical condition rather than something that happens to people who stop eating beef. Anserine has no established human requirement at all.
"Bioactive peptides" is not a nutrient but a category, and plant foods have their own. Listing the ones concentrated in animal foods while passing over sulforaphane, the polyphenols or the fermentable fibre that feeds the gut is not a survey of the evidence, it is a survey of one side of it.
The logical step worth naming: being found only in a food is not the same as being obtainable only from it, and neither is the same as being required. Plenty of compounds are unique to particular foods. Uniqueness on its own establishes nothing about whether anyone needs them, and the argument quietly treats the three as interchangeable.
- Animal foods contain compounds found nowhere else
- There are unique bioactive compounds in meat
- Plants simply do not contain taurine or carnosine
- You cannot replace what is only in animal tissue
Different wording, same argument. The response above answers all of them.
Using a single word in two different senses within one argument, so that a conclusion appears to follow when it does not.
How it applies here: Slides between three different claims: that a compound is found only in animal foods, that it can only be obtained from them, and that it is required from them. Only the first is established, and only the third would matter.
Presenting only the evidence that supports a conclusion while leaving out evidence that tells against it.
How it applies here: Lists the compounds concentrated in animal foods and passes over those concentrated in plants, presenting one side of a comparison as though it were the whole of it.