The short version
Carnitine ferries fatty acids into cells to be burned for energy, and almost all of it in the American diet comes from red meat, poultry, fish and dairy. A vegan diet supplies almost none of it by comparison. NIH's own numbers make the gap plain.
Primary source 01 / 07
A typical omnivorous diet provides about 24 to 145 mg carnitine daily for a person weighing 165 pounds. In contrast, a vegan diet provides about 1.2 mg carnitine
The body makes its own
That hundred-fold dietary gap sounds alarming until the second half of the picture shows up: carnitine is not just eaten, it is built in the liver, kidneys and brain from the amino acids lysine and methionine, and that internal supply line does not appear to depend on what shows up on the plate.
Primary source 02 / 07
Endogenous carnitine synthesis does not appear to be affected by dietary carnitine intake or carnitine excretion and is sufficient to meet the carnitine needs of healthy people
That is also why the federal government's own nutrition board looked at carnitine decades ago and declined to treat it like a vitamin.
Primary source 03 / 07
In 1989, the Food and Nutrition Board (FNB) of the National Academies of Sciences, Engineering, and Medicine concluded that carnitine is not an essential nutrient
What actually gets measured
One small trial put the question to a direct test: give vegetarians and omnivores the same carnitine supplement and see what changes. Performance held steady in both groups. Muscle stores did not.
Primary source 04 / 07
Carnitine supplementation had no significant effect on VO2max, blood lactate concentration, skeletal muscle energy metabolism, or physical performance in either the vegetarians or the omnivores. Carnitine supplementation increased skeletal muscle carnitine stores in the vegetarians, but not the omnivores, by approximately 13%.
That is the closest thing to a smoking gun in the deficiency argument: vegetarians had room to top up their muscle stores where omnivores did not, which fits with a lower starting point. It still did not translate into any measured difference in how hard either group could ride a bike.
The TMAO research runs the other way
If the story stopped there, carnitine would file neatly as a nutrient vegans get less of but do not need. A separate research line then complicated the deficiency framing from the opposite direction. Gut bacteria can turn carnitine into trimethylamine-N-oxide, or TMAO, a compound tied to artery disease, and a landmark 2013 study found the two diets do not process carnitine the same way.
Primary source 05 / 07
Omnivorous subjects are shown to produce significantly more TMAO than vegans/vegetarians following ingestion of L-carnitine through a microbiota-dependent mechanism.
The same study tied plasma carnitine to heart risk, but only conditionally.
Primary source 06 / 07
Plasma L-carnitine levels in subjects undergoing cardiac evaluation (n = 2,595) predict increased risks for both prevalent cardiovascular disease (CVD) and incident major adverse cardiac events (MI, stroke or death), but only among subjects with concurrently high TMAO levels.
NIH's own fact sheet folds that finding in, and is careful about how far to take it.
Primary source 07 / 07
This effect appears to be more pronounced in people who consume meat than in vegans or vegetarians. The implications of these findings are not well understood and require more research.
The honest bottom line
Neither half of this settles cleanly. The intake gap between vegan and omnivorous diets is real and large, and endogenous synthesis compensating for it is a claim resting on the body meeting an average need, not a guarantee for every individual. The TMAO research is early, observational in its human arm, and explicitly flagged by NIH as not well understood. Calling this Strong evidence in either direction would oversell what is actually two unfinished threads pointing in opposite directions.