Carnitine metabolism in human subjects. III. Metabolism in disease

Biomarkers of meat intake and the application of nutrigenomics

Objective dietary intake markers for meat would be useful to assess meat intake in observational studies and as compliance markers in dietary intervention studies. A number of compounds are specific to meat compared with most other dietary items but there is some overlap between protein rich foods. A number of single compounds have been analysed in urine, plasma, serum or hair samples in studies of their relationship to meat or total protein intake. Among potential markers of dietary meat intake are urea, creatine, creatinine, carnitine, carnosine, anserine, ophidine, 1- and 3-methylhistidine, and sulphate or sulphite. Anserine and 1-methylhistidine come close to being meat-specific markers but true quantitative biomarker may not exist. Modern profiling techniques are increasingly used to look for useful biomarkers or for constructing them from latent information in complex profiles. Metabolomics by NMR spectroscopy of urine has also been applied to search for meat intake markers. Studies on single compounds or metabolomics markers are shortly reviewed here.

Carnitine supplementation vs. medium-chain triglycerides in postburn nutritional support

The effect of dietary supplementation of carnitine on protein metabolism was studied in a burned guinea-pig model. Animals bearing a 30 per cent total body surface area burn were enterally infused with three isocaloric and isonitrogenous diets via gastrostomy feeding tubes for 14 days. Two diets contained safflower oil (long-chain triglycerides, LCT) and another diet contained medium-chain triglycerides (MCT) as their lipid sources (30 per cent of total calories as lipid). L-Carnitine was added to one of the two diets containing safflower oil. There were no significant differences in nitrogen balance, urinary excretion, serum albumin or transferrin among the three groups. However, the use of MCT in place of LCT appeared to increase liver weight and liver nitrogen. In this model, carnitine supplementation did not enhance the nitrogensparing effect of fat following burn injury.

Effects of L-carnitine loading on the aerobic and anaerobic performance of endurance athletes

L-Carnitine (L-C), a well known physiological carrier across the inner mitochondrial membrane of activated long chain fatty acids and acceptor of acyl groups from acyl-CoA, has been recently synthesised industrially. This has made it possible to study the effects of L-C loading (4 g X d(-1) by mouth over a period of 2 weeks) on the aerobic and anaerobic performance of 6 long distance competitive walkers. As a result of the treatment: 1) mean total, free and esterified serum L-C both at rest and shortly after completing a 120 min walk at about 65% of the individual maximal aerobic power (VO2max) were significantly increased; 2) VO2max increased 6%, from 54.5 +/- 3.7 (S.D.) to 57.8 +/- 4.7 m1O2 X kg(-1) X min(-1) (P less than 0.02); 3) blood lactate concentration (Lab) as a consequence of short bouts repeated exercise (series of 10, 15 and 20 jumps off both feet on a force platform) was unchanged; 4) heart rate, pulmonary ventilation, oxygen consumption, and respiratory quotient in the same conditions as for 1) were unchanged. It is concluded that, in trained athletes, as a consequence of L-C loading VO2max is slightly but significantly raised, probably as a result of an activation of substrate flow through the TCA cycle, whereas the lipid contribution to metabolism in prolonged submaximal exercise remains unchanged.

Carnitine deficiency

Carnitine is an essential cofactor in the transfer of long-chain fatty acids across the inner mitochondrial membrane. Carnitine is metabolized from lysine, trimethyllysine and butyrobetaine. Butyrobetaine undergoes hydroxylation in the liver, brain and kidney to form carnitine which in turn is transported via the plasma to the heart and skeletal muscle where it is important for allowing beta oxidation of fatty acids. Three clinical forms of carnitine deficiency have been described: myopathic, systemic and mixed forms. Carnitine deficiency results in accumulation of neutral lipid within skeletal muscle, myocardium and liver. Ultrastructurally, myofibrils are disrupted and there is an accumulation of large aggregates of mitochondria and lipid deposits within the skeletal muscle and myocardium. Carnitine therapy has been effective in the treatment of the myopathic and some cases of systemic and mixed forms. Several syndromes of secondary carnitine deficiency have been described; these may be secondary to genetic defects of intermediary metabolism and to other conditions, particularly following hemodialysis.

[Serum levels and urine excretion of L-carnitine in patients with normal and impaired kidney function]

The influence of age, sex, and renal function on serum levels and urinary excretion of free carnitine was studied in 187 subjects. Sixty-one subjects with normal renal function (creatinine clearance greater than 100 ml/min) showed a serum carnitine level of 72.2 +/- 23.2 mumol/l. The carnitine values of males (76.8 +/- 23.3 mumol/l, n = 39) were higher (p less than 0.05) than those of females (64.0 +/- 21.0 mumol/l, n = 22). Carnitine levels did not correlate with age. Values in patients with normal renal function did not differ from serum carnitine levels in healthy controls (74.7 +/- 17.5 mumol/l, n = 49). The mean urinary carnitine excretion per day was 163.5 mumol (range 63.7-419.6 mumol) in patients with intact renal function. Extreme impairment of glomerular filtration rate (creatinine clearance less than 20 ml/min) resulted in higher carnitine concentrations in serum (108.9 +/- 39.4 mumol/l, n = 18, p less than 0.05), lower carnitine elimination per day (78.5 mumol, range 14.5 - 424.3 mumol, n = 18, p less than 0.05) and a decreased carnitine clearance (0.8 ml/min, range 0.2 - 3.8 ml/min). These data together with earlier results obtained in dialysis patients suggest that carnitine metabolism in renal failure is altered by reduction of both endogenous carnitine biosynthesis and renal carnitine clearance.

Carnitine in human nutrition

The oxidation of long-chain fatty acids is carnitine-dependent. Indeed, only when they are bound to carnitine, in the form of acyl-carnitines, do fatty acids penetrate into the mitochondria to be oxidized. To meet the need for carnitine, animals depend on both endogenous synthesis and an exogenous supply. A diet rich in meat supplies a lot of carnitine, while vegetables, fruits, and grains furnish relatively little. Although it has a low molecular weight and acts at low doses in a vital metabolic pathway, carnitine should not be considered a vitamin, but rather a nutritive substance. Indeed, it seems that the diet of the adult human need not necessarily furnish carnitine: the healthy organism, given a balanced nutrition (sufficiently rich in lysine and methionine), may well be able to meet all its needs. Furthermore, it seems that a reduction of the exogenous supply of carnitine results in a lowering of its elimination in the urine. However, dietary carnitine is more important during the neonatal period. The transition from fetal to extrauterine life is accompanied by an increased role of lipids in meeting energy needs. This change is accompanied by a rise in the body of the levels of carnitine, which is mainly supplied in the maternal milk. Finally, this review briefly surveys the illnesses in which a dietary carnitine supplement proves useful.

Recurrent hypoglycemia associated with glutaric aciduria type II in an adult

Repeated episodes of hypoglycemia accompanied by elevated serum concentrations of free fatty acid without ketosis, fatty infiltration of the liver, hepatic dysfunction, and proximal myopathy in a 19-year-old woman, prompted us to analyze her urine for organic acids. Greatly increased quantities of glutaric acid, ethylmalonic acid, dicarboxylic acids with six to 10 carbons, and isovalerylglycine were consistently found in her urine. The ability of cultured skin fibroblasts from the patient to oxidize [1(-14)C]butyrate and [2(-14)C]lysine was reduced. These urinary and in vitro findings indicated defective activity of several acyl coenzyme A dehydrogenases, including glutaryl, isovaleryl, and butyryl coenzyme A dehydrogenases -- establishing a diagnosis of glutaric aciduria Type II. Carnitine concentrations in the skeletal muscle and liver were moderately reduced, but carnitine deficiency was considered a secondary biochemical abnormality. Although glutaric aciduria Type II has previously been described only in a neonate, the disease must be considered in the differential diagnosis of hypoglycemia in adults.

Plasma carnitine and cholesterol levels in monkeys and pigeons: species/breed differences and the influence of gender and diet

1. Total plasma carnitine and cholesterol were studied in two breeds of pigeon and four species of sub-human primates. 2. The levels of plasma carnitine and cholesterol differed within species and between species and were influenced by diet and gender. 3. In the animals studied, pigeons had the highest levels of plasma carnitine (greater than 120 nM/ml) and Squirrel monkeys (Saimiri sciureus) had the lowest levels (ca. 10 nM/ml). 4. Linear regression analysis of plasma carnitine and cholesterol in the monkeys and pigeons indicated no strong correlation between these parameters.