"So how do pigs get their essential amino acids if they can't be fed any animal protein ? Eating just carbohydrates must be as bad for pigs as it is for humans ? And of course we are not just what we eat, but what we've fed what we eat "
Quite right Jane! Commercially manufactured pig feed has essential amino acids added. Most of the protein in pig feed is from soya.
Spot on
There are 20 amino acids that pigs need to make pig protein that makes pig muscle, and hense pig meat!
Of these 20, the pig can make 11, which leaves 9 needed in the feed (essential amino acids)
In British feeds, we know which ones are likely to run out from each source feed (eg soya, wheat), and the order they’re likely to run out in.
The first one to run out is Lysine, which is why most feed has it added.
Next, Threonine, and most feed has pure threonine added
Next Methionine, then tryptophan, then valine, then isoleucine. The other three aren’t likely to run out.
In the wild these amino acids would be derived from meat - mostly "dead stuff" that pigs find, but some from worms, and grubs.