Carnitine intake

Individuals with HMG-CoA lyase deficiency are particularly susceptible to carnitine deficiency. With restriction of red meats and dairy products, dietary carnitine intake is quite low. Carnitine is also synthesized endogenously from the modified, methylated lysine resides of various proteins free trimethyllysine is released when the protein is degraded. Since the therapy for patients with HMG-CoA lyase deficiency must minimize their endogenous protein catabolism, they also have limited availability of trimethyllysine for carnitine synthesis. 

Historically choline, inositol and carnitine have been considered to be part of the vitamin B complex. However, for the general population there has been no demonstration of a dietary need for these agents and also for none of them has there been a therapeutic role established. Vitamins of the B family are found in many food ingredients like in yeast, in meat, in dairy products and also in eggs and grain cereals and separate vitamin B deficiencies are unlikely to occur. Excessive intake of these vitamins is eliminated in the urine because of the fact that they are water-soluble. 

Carnitine, L-3-hydroxy-4-(trimethylammonium)butyrate, is a water-soluble, tri-methylammonium derivative of y-amino-jS-hydroxybutyric acid, which is formed from trimethyllysine via y-butyrobetaine [40]. About 75% of carnitine is obtained from dietary intake of meat, fish, and dairy products containing proteins with trimethyllysine residues. Under normal conditions, endogenous synthesis from lysine and methionine plays a minor role, but can be stimulated by a diet low in carnitine. Carnitine is not further metabolized and is excreted in urine and bile as free carnitine or as conjugated carnitine esters [1, 41, 42]. Adequate intracellular levels of carnitine are therefore maintained by mechanisms that modulate dietary intake, endogenous synthesis, reabsorption, and cellular uptake. 

Subjects who are maintained for prolonged periods by total parenteral nutrition are obviously wholly dependent on what is provided in the nutrient mixture, normally with no contribution from intestinal bacteria. A great deal has been learned from such patients, including the essentiality of the amino acid histidine, and evidence that endogenous synthesis of taurine (Section 14.5.3) and carnitine (Section 14.1.2) may not be adequate to meet requirements without some dietary provision. However, for obvious ethical reasons, such patients have not been subjected to trials of graded intakes of vitamins, but are generally provided with amounts calculated to be adequate and in excess of minimum requirements. 

Fatty acids are the major fuel for red muscle fibers, which are the main type involved in moderate exercise. Children who lack one or the other of the enzymes required for carnitine synthesis, and are therefore reliant on a dietary intake, have poor exercise tolerance, because they have an impaired ability to transport fatty acids into the mitochondria for /S - oxidation. Provision of supplements of carnitine to the affected children overcomes the problem. Extrapolation from this rare clinical condition has led to the use of carnitine as a so-called ergogenic aid to improve athletic performance. 

Patients who survive the initial event have developmental delay, hypotonia or hypertonia, and variable cognitive and neurological sequelae that are linked to the number and severity of acute episodes. Early detection seems to improve the prognosis treatment is based on restriction of precursor amino acid intake, carnitine, and cofactors—biotin or cobal-amin—as indicated. 

ObiciS, FengZ, Arduini A, Conti R, Rossetti L. 2003. Inhibition of hypothalamic carnitine palmitoyltransferase-1 decreases food intake and glucose production. Nat Med 9 756-761. 

Little evidence of sustained positive outcomes after carnitine supplementation exists. However, decreased plasma carnitine concentrations have been reported in neonates within 2 weeks of inadequate enteral intakes of carnitine-containing infant formula. Thus intravenous carnitine supplementation is generally reserved for patients receiving sole PN support for more than 2 weeks. 

An individual with a deficiency of an enzyme in the pathway for carnitine synthesis is not eating adequate amounts of carnitine in the diet. Which of the following effects would you expect during fasting as compared with an individual with an adequate intake and synthesis of carnitine ... 

Fig. 21.3 (a)Comparison of energy and protein intake, maternal and birth weight, and carnitine supplementation in two pregnancies in a woman with mild propionic acidemia. (b) Estimated fetal weight (EFW) and abdominal... 

Higher than normal intakes of MCTGs have become attractive to the athlete since the MCFAs that make them up can be absorbed faster, go into portal circulation, and enter the mitochondria without carnitine. It is well known that increased consumption of MCTGs can increase circulation of ketone bodies. Provided the liver has ample calories, the rapid metabohsm of MCFA can result in the production of ketone bodies (P-hydroxybutyrate, acetoacetate, and acetone) or an increase in free fatty adds (FFAs) in drculation. These ketones and FFAs can then be used by the muscles for energy. How well this works will be discussed in Section 3.4. 

As mentioned previously, the diet can provide a significant amount of carnitine, approximately 50% (100 to 300 mg/day) in the form of dther free carnitine or short-and long-chain FAs. This is true for individuals who consume large amounts of beef, pork, and lamb. In addition, this intake is suffident to maintain normal carnitine homeostasis. Vegetarians who consume less than 0.5 /body weight/day must rely on endogenous production of carnitine to maintain homeostasis. " ... 

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