In this MR study, we conducted a large-scale human genetic analysis to explore the causal relationship between serum metabolites and asthenia. There were four metabolite pathways associated with asthenia: “Porphyrin and chlorophyll metabolism,” “Valine, leucine and isoleucine biosynthesis, Caffeine metabolism,” “Ether lipid metabolism,” “Valine, leucine and isoleucine degradation.” Seventeen blood metabolites were observed to have a causal role in the asthenia risk. Genetically determined lower serum plasma levels of five metabolites were associated with higher asthenia risk: bilirubin Z, myristoleate, N-acetylalanine, threonate, and 1-methylxanthine. Additionally, the other twelve blood metabolites were detected to present a significant relationship with an increased risk of asthenia development, including arachidonate, kynurenine, lysine, octadecanedioate, oleoylcarnitine, palmitoleate, stearoylcarnitine, thymol sulfate, valine, 3-dehydrocarnitine, 4-methyl-2-oxopentanoate, 4-vinylphenol sulfate. These findings selected the underlying causal effects of metabolites on asthenia. As far as we know, this study is the first MR analysis merging metabolomics with genomics to uncover the pathophysiology of asthenia. This study supplies innovative conceptions of the prevention and treatment of asthenia.
The reason for asthenia is correlated with nervous systemic, endocrine, immune disorders, and abnormal metabolisms26,27. From the point of central mechanism, the monoaminergic systems and abnormal 5-hydroxytryptamine metabolism were widely studied and considered as the potential mechanism of asthenia syndrome28. The morphological perspective of asthenia syndrome manifests as dysfunctions of the ascending reticular system, leading to the separation of cortical and subcortical structures. On the peripheral mechanisms aspect, the energy metabolism of muscle cells has an important influence on skeletal muscle activity. The impairment of carnitine transfer into mitochondria may result in a disorder of oxidative ability and a deficiency of phosphocreatine resynthesis. This phenomenon has been verified in asthenia syndrome29,30. Nowadays, several studies have advocated conducting suitable individualized therapy to correct the pathogenesis of asthenia syndrome. Substances that improve brain function and muscle energy may be feasible approaches for asthenia syndrome.
More and more researchers support agents influencing the condition of acety1-L-carnitine as a metabolic treatment that plays an important role in asthenia31,32. Carnitine was regarded as a supplement effectively addressing the issue of asthenia33. Carnitine is a kind of similar vitamin that stimulates fat into energy. As a carrier for transporting fatty acids, carnitine transfers the medium and long-chain fatty acids from the extracellular mitochondrial membrane to the intramembrane, which causes oxidation to create energy. Patients with carnitine administration will experience alleviation of asthenia in the mind and muscles. Carnitine also crosses the blood-brain barrier and addresses neurohypoplasia by protecting mitochondrion and nerve stimulation, contributing to treating asthenia syndrome through a central mechanism. This study suggests 3-dehydrocarnitine as a risk factor for asthenia. 3- dehydrocarnitine, the precursor of L-carnitine productions, performs an intermediate role in carnitine degradation. A previous study reported that 3-dehydrocarnitine administration elevated fatty acid metabolism in a mouse model with a deficiency of L-carnitine transporter. This caritine precursor plays a potential advantage in sustaining energy34. The positive effect of 3-dehydrocarnitine on asthenia seems contrary to our study result. However, this study does not deny the protective role of 3-dehydrocarnitine on asthenia. This study supported the causal association of 3-dehydrocarnitine with asthenia. At the same time, we also indicated that excessive 3-dehydrocarnitine may lead to adverse effects on asthenia. 3-dehydrocarnitine has a role in abnormal glucose metabolism, manifesting as fatigue, lack of concentration and energy, etc. Moreover, excessive carnitine likely produces cardiovascular, nervous, and gastrointestinal tract-related adverse effects, such as insomnia, excessive dreaming, anxiety, chest tightness, shortness of breath, increased heart rate, etc. The causality between carnitine and asthenia provides an insight that carnitine deficiency may be a potential cause of asthenia. As long as excessive supplementation is avoided, carnitine is a promising method for patients with asthenia due to a lack of carnitine or abnormal muscle energy and nervous function.
Among all organ systems, the liver occupies the central position for pathogens. Some metabolism factors induce asthenia by releasing energy from the liver, muscles, and brain35. Bilirubin is a heme decomposition from hemoglobin in aging red blood cells, which exists and transports in the form of the bilirubin albumin complex. The liver has the function of absorbing, transferring, and excreting bilirubin. This study discovered that lower levels of bilirubin Z are causally associated with increased asthenia risk. Bilirubin Z, located in the liver, performs the function of combining with bilirubin. There are two kinds of albumen in the liver: Z and Y. Bilirubin entering the liver needs to combine with Z and Y albumens to excrete to the intestine by bile. Z and Y albumens are responsible for binding and transporting bilirubin. The reduced levels of bilirubin Z downgrade the content of the bilirubin albumin complex to result in increased levels of bilirubin in serum. This phenomenon probably allows for hyperbilirubinemia occurrence. Due to the toxicity characteristic, excessive bilirubin in the serum tends to impair the brain and nervous system, inhibit oxidation of linoleic acid and phospholipids, and frequently create jaundice and liver injury. Excessive bilirubin-related adverse effects often manifest a common symptom as asthenia. Therefore, this study indicated the role of bilirubin metabolism in asthenia by the potential causal association of bilirubin Z with asthenia risk.
This study concluded a causal impact of higher arachidonate levels on increased asthenia risk. Arachidonic acid (AA), a polyunsaturated fatty acid, is a precursor of prostaglandins and thromboxanes. AA maintains normal growth, cellular function, and immune response. AA-induced inflammation can activate the immune system to protect against infectious events. However, excessive levels of AA will boost the inflammatory response easily, leading to various adverse outcomes, such as tissue injury, angiogenesis, oxidative stress, and so on. Therefore, the imbalance of AA levels could contribute to the damage that influences the pathogenesis of many chronic diseases, including cardiovascular, liver, cancer, and so on36,37. Previous researchers have reported that higher AA intake elevates the inflammatory response by improving the production of pro-inflammatory eicosanoids. The levels of AA always is positive for myocardial infarction risk38,39. Asthenia syndrome has several characteristics, including a chronic inflammatory condition40. Consequently, this finding indicated that inflammatory-related metabolites deserve to be given more emphasis by patients with asthenia. It can be a signal for asthenia in the early detection.