Appeal to Nature
Logical fallacyTreating 'natural' as evidence of 'good', or 'unnatural' as evidence of 'bad'. Whether something occurs in nature tells you nothing about whether it is beneficial or right: hemlock, malaria and infant mortality are all perfectly natural.
An everyday example
“This remedy is made entirely from plants, so it must be safer than the pharmaceutical one.”
The same move in this debate
“Eating meat is natural, so there is nothing wrong with it.”
Where this appears
Treats the natural or evolved condition as the standard of what is acceptable, which it is not in any other domain of medicine or ethics.
Presents an anatomical fact as a justification, as though what our bodies can process determined what we are entitled to do.
Offers naturalness as the justification. The scale, density and genetics of the current cattle population are in any case human products, so the premise is doubtful before the inference is even assessed.
The argument assumes that a diet is healthy because it is the animal's natural one, and that eating 'naturally' protects against chronic disease. It then treats that assumption as settling what humans should eat, instead of looking at evidence about human health.
The argument treats getting nutrients from whole animal foods as better in itself than getting them from a supplement or fortified food. It never says why a B12 tablet is worse than B12 that reached the animal through supplemented feed. Its unspoken premise is that the food route is more natural, and that being more natural makes it superior.