Nonalcoholic steatohepatitis (NASH) patients have reduced levels of choline in the liver.1 Choline metabolism is critical in maintaining normal liver function2 and is important in the synthesis of very-low-density lipoproteins from triglycerides (TAG) in the liver.3 Choline is also important for normal kidney and mitochondrial function.4 Choline deficiency will lead to an accumulation of liver TAG levels, resulting in elevated lipid levels. Furthermore, choline can be oxidised to betaine, which is an important methyl donor and participates in the methionine cycle in the liver. Methionine, together with lysine, forms L-carnitine in the liver and kidneys.
L-carnitine is critical for the transportation of long-chain fatty acids. NASH patients display abnormal parameters in liver function tests, such as elevated levels of aspartate transaminase (AST) and alanine transaminase (ALT);5 however, L-carnitine has been shown in several studies to improve liver function in NASH patients and prevent the progression of the disease.6–8
This study investigated what effect L-carnitine supplementation could have on liver metabolites. In this study, the authors hypothesised that L-carnitine could elevate choline in the liver through a regulation of betaine. Furthermore, they aimed to investigate liver function to understand how L-carnitine affects key liver enzymes.
A total of 16 healthy male Wistar rats (approximately 200 g) were treated daily with either saline (n=8) or L-carnitine (n=8; intra-peritoneal injections, 3 g/kg/day) for 2 weeks. Following treatment, body weight was measured and blood obtained, before euthanasia and extraction of liver tissue for metabolomic analysis. Liver tissue was crushed and prepared with previously described methods to separate the aqueous, lipid, and protein layers,9 and sent to the Department of Biochemistry at the University of Cambridge, Cambridge, UK, where liquid chromatography mass spectrometry was undertaken.
The L-carnitine-treated group had 15% lower body weight (p<0.01) and 55% reduced serum TAG levels compared to the saline-treated group. L-carnitine resulted in higher liver choline levels (47%; p=0.05) and reduced levels of betaine (24%; p=0.04). Alanine was elevated in the liver by a factor of 76.6, while oxaloacetate was elevated by a factor of 1.3 following L-carnitine treatment compared to saline treatment. Pyruvate, α-ketoglutarate, glutamate, and aspartate all stayed constant between the two groups (Figure 1).
Figure 1: Liver function measured in the saline (Sal) treated group and L-carnitine (Carn) treated group.
All data presented as mean+standard deviation, significant values presented as *p<0.05 and ****p<0.0001 Metabolites relevant for alanine transaminase and aspartate transaminase measured with metabolomics (μmol/gww).
Metabolism in the liver was modulated by L-carnitine. The liver is critical for choline metabolism, and studies have shown that NASH is associated with lower choline concentrations. This study demonstrated that some of the beneficial effects of L-carnitine could be mediated through increased liver choline levels, possibly by an elevation of betaine, which was reduced in the liver when treated with L-carnitine. In addition, L-carnitine improved the AST/ALT ratio, which is commonly used to assess liver damage, where an elevation signifies advancement of fibrosis.10 L-carnitine elevatedalanine levels considerably more so than it elevated oxaloacetate, forcing the AST/ALT ratio to reduce, and thereby improving liver function. Future studies should investigate liver enzymes and metabolites in NASH patients treated with L-carnitine, which could help elucidate the mechanism through which L-carnitine can prove beneficial toliver function and provide insights into novel therapeutics.
