Renal conservation of carnitine by infants and adults: no evidence of developmental regulation

Nutrient requirements for preterm infant formulas

Achieving appropriate growth and nutrient accretion of preterm and low birth weight (LBW) infants is often difficult during hospitalization because of metabolic and gastrointestinal immaturity and other complicating medical conditions. Advances in the care of preterm-LBW infants, including improved nutrition, have reduced mortality rates for these infants from 9.6 to 6.2% from 1983 to 1997. The Food and Drug Administration (FDA) has responsibility for ensuring the safety and nutritional quality of infant formulas based on current scientific knowledge. Consequently, under FDA contract, an ad hoc Expert Panel was convened by the Life Sciences Research Office of the American Society for Nutritional Sciences to make recommendations for the nutrient content of formulas for preterm-LBW infants based on current scientific knowledge and expert opinion. Recommendations were developed from different criteria than that used for recommendations for term infant formula. To ensure nutrient adequacy, the Panel considered intrauterine accretion rate, organ development, factorial estimates of requirements, nutrient interactions and supplemental feeding studies. Consideration was also given to long-term developmental outcome. Some recommendations were based on current use in domestic preterm formula. Included were recommendations for nutrients not required in formula for term infants such as lactose and arginine. Recommendations, examples, and sample calculations were based on a 1000 g preterm infant consuming 120 kcal/kg and 150 mL/d of an 810 kcal/L formula. A summary of recommendations for energy and 45 nutrient components of enteral formulas for preterm-LBW infants are presented. Recommendations for five nutrient:nutrient ratios are also presented. In addition, critical areas for future research on the nutritional requirements specific for preterm-LBW infants are identified.

Decreased carnitine biosynthesis in rats with secondary biliary cirrhosis

Carnitine biosynthesis was investigated in rats with secondary biliary cirrhosis induced by bile duct ligation (BDL) for 4 weeks (n = 5) and in pair-fed, sham-operated control rats (n = 4). Control rats were pair-fed to BDL rats, and all rats were fed an artificial diet with negligible contents of carnitine, butyrobetaine, or trimethyllysine. Biosynthesis of carnitine and its precursors was determined by measuring their excretion in urine and accumulation in the body of the animals. Four weeks after BDL, total carnitine content was increased by 33% in livers from BDL rats when compared with control rats, but was unchanged in skeletal muscle and whole carcass. The plasma total carnitine concentration averaged 29.0 +/- 4.1 vs. 46.4 +/- 7.3 micromol/L in BDL rats and control rats, respectively. Urinary total carnitine excretion was reduced by 56% in BDL rats as compared with control rats. Carnitine biosynthesis was significantly decreased in BDL rats (0.45 +/- 0.19 vs. 0.93 +/- 0.08 micromol/100 g body weight/d in BDL and control rats, respectively). The tissue content of free and protein-linked trimethyllysine, a carnitine precursor, and trimethyllysine plasma concentrations were not different between BDL and control rats. However, urinary trimethyllysine excretion was increased 5-fold in BDL rats and approximated glomerular filtration. In contrast, urinary excretion of butyrobetaine, the direct carnitine precursor, was decreased by 40% in BDL rats as compared with control rats. Trimethyllysine biosynthesis was not different, but butyrobetaine biosynthesis was decreased by 51% in BDL as compared with control rats. In conclusion, carnitine biosynthesis is decreased in BDL rats as a result of a defect in the conversion of trimethyllysine to butyrobetaine.

Carnitine homeostasis in patients with rheumatoid arthritis

Myopathy is a frequent finding in patients with rheumatoid arthritis (RA). Since carnitine is important for skeletal muscle energy metabolism, carnitine metabolism was investigated in patients with RA and myopathy. Muscle strength was estimated by determination of a muscle strength index (MSI) which is derived from isometric measurements of muscle strength at knees and elbows. Carnitine was determined by a radioenzymatic method and 3-methylhistidine by high-performance liquid chromatography. In comparison to control subjects, patients had a reduced MSI. Both the 24-h creatinine and 3-methylhistidine excretions were reduced in patients. The plasma carnitine pool was not different between patients and control subjects, except for a higher long-chain acylcarnitine concentration in patients. Urinary excretion of carnitine was decreased in patients, also after normalization for body weight. Accordingly, renal carnitine clearance and excretion fraction were both decreased in patients. Skeletal muscle free- and total carnitine levels were increased in patients, whereas the long-chain acylcarnitine content was markedly decreased. The total skeletal muscle carnitine content showed a negative correlation with the MSI and no association with disease activity. Carnitine deficiency does not explain reduced skeletal muscle strength in patients with RA. Decreased renal carnitine excretion in patients is most likely due to reduced carnitine biosynthesis, leading to more efficient tubular carnitine reabsorption for maintaining the carnitine body stores.

Renal handling of carnitine in ill preterm and term neonates

OBJECTIVE

To investigate the renal handling of carnitine in preterm and term ill neonates.

METHODS

We studied the fractional tubular reabsorption of carnitine and the proximal renal tubular function of infants in the first week of life who were receiving very little or no carnitine in their diets.

RESULTS

Mean plasma levels were low: total carnitine was 16.4 +/- 7.0 mumol/L, free carnitine was 9.2 +/- 5.0 mumol/L, and acylcarnitine was 7.2 +/- 4.1 mumol/L. The most premature group of neonates (gestation age, 26 to 31 weeks) had a fractional tubular reabsorption rate of free carnitine of 94.3% +/- 3.3%, which was lower than in the other two groups (98.1% +/- 2.4% for gestational age 32 to 36 weeks, p = 0.001; and 99.2% +/- 0.6% for gestational age 37 to 42 weeks, p = 0.002). In all patients the fractional tubular reabsorption of acylcarnitine was lower than that of free carnitine, indicating possible tubular secretion of acylcarnitine. It correlated with the total plasma carnitine levels (r = 0.53; p = 0.002). The fractional tubular reabsorption of free carnitine also correlated with gestational age (r = 0.60; p < 0.001).

CONCLUSIONS

Ill neonates have a fractional tubular reabsorption rate of free carnitine within the normal range. It increases with gestational age, and has the same maturation rate as the other known indexes of proximal tubular function.

Effect of acetyl-L-carnitine on forebrain cholinergic neurons of developing rats

It has been shown that the endogenous compound, acetyl-L-carnitine (ALCAR), acts in the brain as a metabolic cofactor in the synthesis of acetylcholine. In these studies, ALCAR was injected into the brain of developing rats every other day for the first three weeks after birth in order to assess its effect on forebrain cholinergic neurons. The results showed that intracerebroventricular (icv) administration of ALCAR causes an increase of choline acetyltransferase (ChAT) activity and of nerve growth factor receptor expression in the striatum. Biological assays of brain tissues revealed that the level of nerve growth factor (NGF) in the hippocampus also increases. The ability of brain cholinergic tissues to respond to exogenous administration of ALCAR is discussed.