L-carnitine is a nutrient factor synthesised by mammals and found mainly in muscle, kidney, liver and brain tissue. The total L-carnitine pool in mammals including humans is made up of L-carnitine as well as acylcarnitine esters of which acetyl-L-carnitine is the most common. L-carnitine from food is absorbed by active and passive transport through enterocytes and it shows a bioavailability of around 50 to 90 %. Studies on supplements show that absorption of the L-carnitine is passive and the absorption is only around 15 %. The non-absorbed L-carnitine is degraded by microflora in the intestinal tract. Acetyl-L-carnitine is partially hydrolysed to L-carnitine in enterocytes during absorption, although supplements can increase circulating acetyl-L-carnitine levels by around 45 % after 2 grams per day of acetyl-L-carnitine per day. Following supplements of L-carnitine and acetyl-L-carnitine, tissue levels rise but return to normal following around 12 hours suggesting daily intakes are required to elevate tissue levels.
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