DISCUSSION
In this study, we found that critically ill patients who received LC at a dose of 3 gr/day for 7-day had significantly reduced levels of inflammation markers and 28-day mortality in the intervention group compared with the control group. Lee et al reported that daily consumption of 1 g LC for 12 weeks significantly reduced levels of IL-6, CRP, and Tumor necrosis factor-α (TNF-α) in patients with cardiovascular diseases (13). Recent evidence has demonstrated that LC supplementation prevents oxidative stress in patients with coronary artery disease (CAD) by regulating and reducing lipid peroxidation and increasing antioxidant enzymes for scavenging free radicals and reactive oxygen species (ROS) (14-16). Furthermore, ROS and other radicals, can increase inflammation and up-regulate nuclear factor kappa-light-chain-enhancer of activated B cells )NF-Kβ( pathway (17-19), which is a transcription factor and regulator of multiple gene expression in immune-inflammatory responses and reduces gene expression of proinflammatory cytokines including IL-6, IL-1, CRP, and enhances gene expression of anti-inflammatory cytokines IL-10 (20-22). Consequently, LC could decrease inflammatory markers in patients with chronic inflammation including critically ill patients with the same mechanism. In their study, Shakeri et al. reported that consumption of 1 gr/day of oral LC resulted in a 29% decrease in baseline CRP and 61% decrease in IL-6 levels in hemodialysis patients (23). Dastan et.al supplemented cardiovascular patients with 3 g/day of LC for 5 days and reported a significant reduction in CRP levels (12). In recent years, two clinical studies have shown that supplementation with 20 mg/kg/day LC may decrease CRP levels in hemodialysis patients (11, 24). However, two other clinical studies, it has been shown that LC supplementation had no effect on the inflammatory markers, including CRP and IL-6, in obese healthy subjects (25, 26). This lack of effect in healthy people may be due to the fact that healthy subjects do not have high levels of inflammation, while in our study, all subjects had high levels of inflammation with CRP levels higher than 1 mg/dl and IL-6 levels higher than 1.5 pg.ml. In the present study, supplementation with LC resulted in a significant reduction in IL-6 and CRP levels in critically ill patients. Results of the previous meta-analysis showed a significant reduction in CRP due to LC supplementation in comparison to the control group . Recently, Haghighatdoost et al. conducted another systematic review and meta-analysis, which included 13 clinical trials to assess the effect of LC supplementation on inflammation and inflammatory markers (IL-6, CRP). Findings of this study showed that supplementation with LC led to a significant reduction in CRP and IL-6 levels compared with the control group (P = 0.001 and p=0.002 respectively) (28). The mentioned study showed that LC reduced inflammation in patients, especially in studies that supplementation continued for more than two weeks at a dose higher than 2 g/day. Although in our study, LC was used for one week, intervention among critically ill patients with high levels of inflammatory markers and using mega-dose of 3 gr/day might be the main reason causes similar findings of our study with the recent meta-analysis findings.
Pharmacological studies have reported that the bioavailability of oral dose of LC is low and it is absorbed only by 5 to 16%, so it has been suggested that doses higher than 2 g/day might have a greater effect (29, 30). Therefore, the additional effect of LC on the level of inflammatory markers can be dose and time-dependent. Besides, LC is able to regulate the Peroxisome proliferator-activated receptor gamma is a type of nuclear receptor (PPAR γ) pathway, which is a key factor in the regulation of oxidative stress pathways and liver inflammation (31). Another study showed that LC may be able to improve the inflammatory response of the liver by regulating the PPAR γ signaling pathway, which is a transcription factor for lipolytic genes including carnitine palmitoyltransferase 1 (CPT1) (32, 33). They also reported that SOFA score was significantly associated with LC deficiency in these patients, and no significant difference was observed in clinical outcomes, length of hospital stay between LC deficient patients and non-deficient patients. They did not mention any mechanism for the increase in SOFA score but one explanation could be due to the important role of LC in transporting long-chain fatty acyl CoA into the mitochondrial matrix for beta-oxidation by carnitine palmitoyltransferase 1 (CPT1) (10). LC deficiency disrupts B-oxidation of fatty acid in critically ill patients. Since most of the acyl-CoA in the body is used to bond with fatty acid, in case of depleted CoA pool, due to metabolic dysfunction in mitochondrial function, the multi-organ failure and mortality is increased (34). Jones et al. assessed the effect of high-dose LC supplementation in 250 sepsis patients. LC was administered three doses of 6 g, 12 g, and 18 g during the first 24 hours of admission. They reported that the SOFA score reduced in the first 48 hours and also treatment with carnitine was significantly decreased 28 days mortality. The effect of LC was dose dependent but the difference in LC did not statistically significant. The proposed mechanism for this finding could be that LC may decrease the metabolic effects of sepsis by promoting and increasing the fatty acids transportation into the mitochondria, and thereby reducing the inhibitory effects of acetyl-CoA on pyruvate dehydrogenase (35). Puskarich et al reported that supplementation with 12 gr LC led to a greater reduction in SOFA score and mortality compared to the control group in 31 sepsis patients. The mortality was reported 50% (8 of 16) in the LC group in the mentioned study while mortality occurred in 80% (12 of 15) of patients in the control group (36)(38). Chung et al. measured plasma levels of acylcarnitine in sepsis patients and evaluated its association with multi-organ dysfunction and mortality. They reported that plasma levels of acylcarnitine may reflect the severity of multi-organ dysfunction, inflammation, infection, and sepsis and may serve as a predictive biomarker for mortality. These findings suggested that carnitine deficiency and disorder in B-Oxidation of fatty acids lead to defects in lipid utilization by vital organs, especially the liver and kidney, which may lead to mitochondrial dysfunction in patients who died of sepsis (33). In the present study study, critically ill patients who were supplemented with LC reduced the duration of ventilation compared to the control group, and 37.5% of patients in the LC group were discharged from the ICU. Goetzmen et al, studied long-chain acyl-CoA dehydrogenase deficiency in relation to pulmonary surfactant dysfunction. They showed that fatty acid oxidation was increased in the lungs similar to the liver, and that the inhibition of fatty acid oxidation by acyl-CoA can lead to abnormal synthesis of surfactants (37). The main function of LC is the esterification and transfer of long-chain fatty acids into the mitochondrial membrane and production of energy, therefore, LC supplementation may improve clinical outcomes by decreasing inflammatory mediators in critically ill patients.
Although this is the first study investigating the effects of a mega dose of oral LC supplementation for seven days among critically ill patients, some limitations could be acknowledged. First, the present study was conducted in the general ICUs with the heterogenic patients. The second limitation was the duration of LC supplementation (7-day). We suggest that larger and longer intervention studies are required to document the effects of L-Carnitine on inflammatory markers and clinical status in ICU patients.