Carnitine metabolism and its application in parenteral nutrition

The Use of Carnitine in Pediatric Nutrition

Carnitine is synthesized endogenously from methionine and lysine in the liver and kidney and is available exogenously from a meat and dairy diet and from human milk and most enteral formulas. Parenteral nutrition (PN) does not contain carnitine unless it is extemporaneously added. The primary role of carnitine is to transport long-chain fatty acids across the mitochondrial membrane, where they undergo beta-oxidation to produce energy. Although the majority of patients are capable of endogenous synthesis of carnitine, certain pediatric populations, specifically neonates and infants, have decreased biosynthetic capacity and are at risk of developing carnitine deficiency, particularly when receiving PN. Studies have evaluated for several decades the effects of carnitine supplementation in pediatric patients receiving nutrition support. Early studies focused primarily on the effects of supplementation on markers of fatty acid metabolism and nutrition markers, including weight gain and nitrogen balance, whereas more recent studies have evaluated neonatal morbidity. This review describes the role of carnitine in metabolic processes, its biosynthesis, and carnitine deficiency syndromes, as well as reviews the literature on carnitine supplementation in pediatric nutrition.

Liver dysfunction and energy source: results of a randomized clinical trial

Controversy still exists regarding the role of the carbohydrate:fat ratio on liver function abnormalities associated with the administration of total parenteral nutrition (TPN). We designed a prospective clinical trail comparing standard carbohydrate-based TPN (8.5% amino acids, 50% dextrose, 7.5% of total calories from lipids) with an isocaloric lipid-based TPN (8.5% amino acids, 30% dextrose, 40% of total calories from lipids) in 43 patients exclusively receiving TPN > or = 2 weeks. Energy needs were calculated as basal energy expenditure x 1.5. The mean daily calorie intake for patients who obtained carbohydrate-based TPN (CHO) was 2227 kcal, whereas the lipid-based TPN (LIP-CHO) group achieved a mean of 2310 kcal. Patients with preexisting liver disease were excluded. There was no significant difference in age or diagnosis between the groups. We monitored total bilirubin, direct bilirubin, alkaline phosphatase, gamma-glutamyl transferase, lactic dehydrogenase, serum glutamic oxaloacetic transaminase, and serum glutamic pyruvic transaminase. Initial liver-associated tests did not vary significantly between groups. Group mean values after 2 weeks of TPN were significantly different for total bilirubin (1.5 mg/dL in the CHO group compared with 0.7 in the LIP-CHO group, p < .05) and direct bilirubin (0.8 mg/dL in the CHO group compared with 0.3 mg/dL in the mixed substrate group, p < .05). Differences in mean values between groups were also noted for serum glutamic oxaloacetic transaminase, serum glutamic pyruvic transaminase, and lactic dehydrogenase. In conclusion, this prospective trial reveals that the use of a balanced energy source TPN solution prevents the abnormalities in liver-associated tests commonly associated with TPN.(ABSTRACT TRUNCATED AT 250 WORDS)

Intestinal, pancreatic, and hepatic effects of gastrointestinal hormones in a total parenteral nutrition rat model

The adverse effects of long-term total parenteral nutrition (TPN) are well documented. Lack of gastrointestinal (GI) stimulation from oral feeding, reduction of GI hormone secretion, and interruption of enterohepatic circulation of bile may be found. TPN results in atrophy of the digestive system, intestinal bacterial overgrowth and translocation, liver cell damage, and gallstone formation. In addition, the increase incidence of sepsis of gut origin may lead to an increase in mortality. In some studies, results of the administration of GI hormones to patients receiving prolonged TPN suggest the possibility of reducing some of the adverse effects of long-term TPN. To evaluate the role of GI hormone in the prevention of adverse effects of TPN, we designed the following study: 50 young adult male Wistar rats, weighing approximately 200 g, were divided into five equal groups. All animals received identical TPN infusate for 7 days. GI hormone was added to the TPN infusate as follows: Group A (control) received no GI hormone, group B was given glucagon at a dosage of 330 micrograms/kg per day, group C was administered cholecystokinin 2 Ivy dog units twice a day, group D received secretin 2 clinical units twice a day, and group E was given both cholecystokinin and secretin at the dosages stated for groups C and D. Maintenance of mucosal brush-border hydrolase activity was found in group B. Neither atrophy of the pancreas nor hypoplasia of intestinal villi was observed in groups C and D. Group C showed improvement of liver function-associated tests, better weight gain, and acceleration of enterohepatic circulation of bile.(ABSTRACT TRUNCATED AT 250 WORDS)

Rapid spectrophotometric determination of plasma carnitine concentrations

A spectrophotometric enzymatic assay for plasma carnitine concentrations has been automated on the Monarch 2000. Prior to the assay, each plasma sample was divided into three fractions, ie, free carnitine, acid-soluble carnitine and total carnitine, in order to determine the concentration of both free and esterified carnitine. Using the method developed by Tachikawa et al (Seikagaku 56:998, 1984), each of the samples was then chromatographed on an anion exchange resin to eliminate those compounds which could adversely impact the accuracy of the enzymatic assay for carnitine. After the completion of these preparatory steps, 32 specimens were assayed in less than 16 min on the Monarch 2000 with a high degree of both accuracy and precision. The assay was linear over a wide concentration range (5.0-80 mumol/liter), with the lower limit of sensitivity being 5.0 mumol/liter. The coefficient of variation (CV%) of the within run precision was 2.1%, 2.8%, and 6.7% for the determinations of free carnitine, acid-soluble carnitine, and total carnitine, and 17.4% and 27.8% for the calculated values of short-chain and long-chain acylcarnitines, respectively. The CV% of the between run precision for the same fractions was 6.5%, 2.7%, 3.8%, 14.2%, and 14.3%, respectively. When authentic L-carnitine was added to the plasma, the mean recovery rate was 94.7 +/- 11.0%. Reference values were determined using plasma obtained from 40 healthy adult volunteers.

Comparison of medium and long chain triglyceride metabolism in intensive care patients on parenteral nutrition

The metabolic effects of an intravenous lipid emulsion containing medium chain triglycerides (MCTs) were studied in eleven critically ill, ventilated patients receiving parenteral nutrition. The effects were compared in a cross-over study with those of a conventional emulsion containing only long chain triglycerides (LCTs). The lipid was well tolerated, but there were differences in the metabolic effects with a significantly greater increase in the plasma concentrations of glycerol and ketones during MCT/LCT infusion compared to LCT. The plasma concentration of non esterified fatty acids was also higher. This fell rapidly post infusion. Since non esterified fatty acids and ketones are readily available energy sources for tissues lipid emulsions containing MCT may prove valuable for catabolic, critically ill patients.

Serum carnitine levels in normal individuals

Serum carnitine levels have been measured in 178 samples from 75 normal volunteers. We report a wide range of values (10-70 mumol/liter and 8-74 mumol/liter for free and acetylated carnitine, respectively) and a distinct difference between the ranges for males and females (p less than 0.001). There is also substantial, seemingly random fluctuation in any one individual's levels, when measured serially over several weeks.

Total parental nutrition using conventional and medium chain triglycerides: effect on liver function tests, complement, and nitrogen balance

Conventional long chain triglyceride (LCT) was compared with a new emulsion containing 50% medium chain triglyceride (5% MCT/5% LCT) in a randomized cross-over trial of 10 days duration. Plasma concentrations of albumin, prealbumin, the complement components C3 and C4, and prothrombin times measured daily at 8 am, before lipid infusion, showed no progressive change during the 10 days of the trial, nor in each separate 5-day period when LCT or MCT/LCT was infused. Aspartate transaminase and alkaline phosphatase activities were similar over the two periods. There was a significant increase (compared with preinfusion levels) in C3 and C4 levels after 5 hr of either lipid infusion. Nitrogen balance was improved, and plasma bilirubin levels were lower on the regimen containing MCT/LCT.

Use of a lipid containing medium chain triglycerides in patients receiving TPN: a randomized prospective trial

Lipid emulsions which contain long chain triglycerides (LCTs) provide a valuable energy source for patients requiring total parenteral nutrition (TPN). We have investigated the use of a new lipid emulsion containing both long and medium chain triglycerides (MCTs) in a randomized prospective trial. Sixty patients received TPN including 500 ml of either 20 per cent Lipofundin S (LCT) or Lipofundin 10 per cent MCT/10 per cent LCT for at least 6 days. Patients with renal or hepatic impairment, or major trauma, were excluded from the study. The MCT/LCT emulsion was found to be as safe and as effective a source of calories as LCT but the differences in metabolic parameters did not differ significantly between the two groups of patients. A lipid emulsion containing MCTs may have important advantages for seriously ill patients, but appears to have no obvious advantages for the majority of patients receiving TPN who are not severely stressed.

Energy and protein requirements of general surgical patients requiring intravenous nutrition

General surgical patients require intravenous nutrition either because their gastrointestinal tract is blocked, too short or inflamed or because it cannot cope. Such patients can be grouped into four nutritional/metabolic categories: normal and unstressed; normal and stressed; depleted and unstressed; depleted and stressed. The energy requirements of patients in each of these groups vary according to their energy expenditure. Normally nourished and stressed patients have the highest energy expenditure and therefore require the highest energy input (45-55 kcal.kg-1day-1). Other groups of patients rarely require more than 40 kcal.kg-1day-1. Energy can be given mainly as dextrose although calories needed above 40 kcal kg-1day-1 should be given as fat (unless lipogenesis is desirable). In very stressed patients high rates of glucose infusion can themselves constitute a metabolic stress and fat may play a bigger role as a calorie source. For long term feeding, 1 litre of 10 per cent fat emulsion should be given weekly to avoid essential fatty acid deficiency. The level of nitrogen intake required to maintain a positive nitrogen balance is a lot higher in surgical patients than the suggested recommended dietary allowances for normal subjects. It is dependent not only on the nutritional and clinical state of the patient but also on the levels of energy and nitrogen intake given. When energy intake is below energy needs, normally nourished patients cannot retain nitrogen, although depleted patients can. When energy intake exceeds energy needs, both normally nourished and depleted patients retain nitrogen at levels of nitrogen intake ranging from 250 mg kg-1day-1 (depleted and unstressed) to over 400 mg kg-1day-1 (stressed). Depleted patients can maintain a positive nitrogen balance at lower levels of calorie and nitrogen intake than normally nourished patients and in this respect are analogous to a growing child. In all surgical patients, energy and nitrogen intakes can be manipulated to provide for a controlled maintenance or restoration of either wet lean tissue and/or fat. There is little place for protein sparing therapy or the use of insulin and anabolic steroids to promote nitrogen retention in surgical patients requiring intravenous feeding.

Carnitine deficiency with hyperbilirubinemia, generalized skeletal muscle weakness and reactive hypoglycemia in a patient on long-term total parenteral nutrition: treatment with intravenous L-carnitine

Low levels of plasma carnitine and reduced urinary carnitine excretion with persistently elevated plasma bilirubin levels, reactive hypoglycemia and generalized skeletal muscle weakness are described in a patient requiring long-term total parenteral nutrition (TPN). Intravenous administration of L-carnitine at 400 mg/day for 7 days and subsequently a maintenance dose of 60 mg/day corrected the plasma carnitine deficiency and reactive hypoglycemia and was associated with a return to normal plasma bilirubin levels and a restoration of skeletal muscle strength.

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.

Urinary carnitine excretion in surgical patients on total parenteral nutrition

Urinary free and total carnitine excretions were measured in 41 normal adults and seven surgical patients on fat-free total parenteral nutrition for 8 to 45 days. The means (+/-SEM) of urinary free and total carnitine excretion in normal adults were 162 +/- 19 and 328 +/- 28 micrometers/days, respectively. All of the patients exhibited protein-calorie malnutrition with a mean carnitine intake of 11.6 +/- 1.5 micrometers/day. Under this stringent carnitine economy with the adequate supply of lysine and methionine, urinary total carnitine excretion significantly reduced to 127 to 162 micrometers/day. This probably reflects the carnitine biosynthetic rate. However, during the periods of operation and/or infection, urinary total carnitine excretion significantly increased 2- to 7-fold that of normal levels. Significant positive correlation was found between the two forms of urinary carnitine and total nitrogen excretions. Serum free and total carnitine levels in patients were significantly higher than normal adults. Such findings can be explained by the endocrine responses to the stress phenomenon and indicate a catabolic response of skeletal muscle in which most of the body carnitine resides. This can impair their carnitine status.