Carnitine Metabolism in Vegetarians
A study by Karlic et. al.  found that a vegetarian diet has a significant impact on a gene regulating carnitine metabolism. Carnitine is an amino acid (protein constituent) and a conditionally essential nutrient that plays a vital role in energy production and fatty acid metabolism. A “conditionally essential” nutrient is one that can be manufactured in the body, but the requirements of individuals might exceed dietary intake during specific disease states. Carnitine not obtained from food is synthesized in the body from two essential amino acids, lysine and methionine. Carnitine is found in higher levels in meat products as it is present in high levels in muscle tissue.
Vegetarian diets therefore contain less carnitine, and also often contain more carbodydrate than omnivorous diets as sources of concentrated vegetable proteins are not so readily available as animal proteins.
The study found increased expression of a gene  called Organic Cation Transporter 2 (OCTN2) in vegetarians which caused elevated levels of OCTN2 in cell membranes, compensating for lower carnitine levels obtained from the diet. Thus a vegetarian lifestyle has an impact on fat metabolism causing a remarkable stimulation of carnitine uptake.
The bioavailability of L-carnitine varies due to dietary composition. Bioavailability of L-carnitine in vegetarians who are adapted to low-carnitine diets is higher (66% to 86% of available carnitine) than regular red-meat eaters adapted to high-carnitine diets (54% to 72% of available carnitine). Carnitine influences carbohydrate metabolism. Abnormal carnitine regulation is implicated in complications of diabetes mellitus, cardiomyopathy, obesity, endocrine imbalances and other disorders. 
According to The Blood Type Diet and The GenoType Diet, individuals with a particular genetic characteristic and the associated metabolic consequences may be recommended to reduce the amount of red meat in their diets. This may be due to specific disease susceptibility and/or reduced ability to digest and metabolise red meats. Some of the consequences of increased carbohydrate intake in these individuals may be compensated for by the natural epigenetic effect of lowered carnitine intake on the gene that enhances the concentration of this nutrient and resultant increased bioavailability.
1. Karlic H, Schuster D, Varga F, Klindert G, Lapin A, Haslberger A, Handschur M: “Vegetarian Diet Affects Genes of Oxidative Metabolism and Collagen Synthesis.” Ann Nutr Metab 2008;53:29-32. Pubmed 18772587
2. OMIM OCTN2
3. Flanagan JL, Simmons PA, Vehige J, Willcox MDP, Garrett Q: “Role of carnitine in disease.” Nutrition & Metabolism 2010, 7:30 doi:10.1186/1743-7075-7-30