The short version
Taurine is not built into proteins, but it is everywhere else in the body, and it is the compound the deficiency argument leans on almost as hard as carnitine.
Primary source 01 / 08
taurine is one of the most abundant amino acids in the brain, retina, muscle tissue, and organs throughout the body
Where it comes from, and the catch
Plants make none of it, so an animal-free plate supplies close to zero dietary taurine. The body has its own production line.
Primary source 02 / 08
The major route for the biosynthesis of taurine...is from methionine and cysteine via cysteinesulfinic acid decarboxylase (CSD)
The catch is that humans do not run that enzyme especially hard, a trait shared with an animal famous for needing taurine in its bowl.
Primary source 03 / 08
CSD levels are very low in cats, as well as humans and other primates...helps to maintain normal tissue concentrations of taurine
That low enzyme activity is why taurine sits in an odd classification: technically not required from food, but not fully made in-house either.
Primary source 04 / 08
taurine is often referred to as a “nonessential” amino acid, or more generously as a “conditionally essential” amino acid.
What the vegan studies actually measured
The most-cited data point is a small, decades-old comparison of vegan and omnivorous men.
Primary source 05 / 08
Plasma taurine levels and urinary taurine excretion were measured in 12 strict vegetarian (vegan) males who had maintained a vegan diet for 53 +/- 26 mo (SD) and in 14 male nonvegetarian control subjects.
Plasma taurine came out modestly lower in the vegans; urinary taurine, the more sensitive marker, came out far lower.
Primary source 06 / 08
Plasma taurine levels differed (45 +/- 7 vs 58 +/- 16 mumol/L, respectively).
Primary source 07 / 08
Prolonged absence of dietary taurine intake causes decreased plasma taurine and severely restricted urinary taurine output.
What that 1988 study did not find is any sign of illness in its 12 vegan subjects. It measured a biochemical marker, not a symptom, and neither it nor the decades of vegan nutrition research since have documented a taurine-deficiency disease in a healthy adult eating a varied plant-based diet.
What this does and does not prove
Even the review arguing hardest for taurine's importance is careful about where its deficiency evidence actually comes from.
Primary source 08 / 08
the results of studies in animal models suggesting that taurine may play a therapeutic role in the management of epilepsy and diabetes
That is the honest shape of the taurine question. The lower plasma and urinary numbers in vegans are a real, replicated finding, and the low activity of the human synthesis enzyme is real too. What is not established is that either one causes a clinical problem in a healthy adult. Where taurine's most severe deficiency effects are documented, cardiomyopathy, retinal damage, developmental harm, the evidence is animal work, mostly in cats and monkeys fed taurine-free diets outright, not a finding reproduced in vegan humans. A parent or a pregnant vegan with specific questions about taurine is better served asking a doctor or registered dietitian than reading a nutrient page.