Background L-carnitine (LC) is used as a supplement by recreationally-active, competitive and highly trained athletes. This systematic review aims to evaluate the effect of prolonged LC supplementation on metabolism and metabolic modifications. Methods A literature search was conducted in the MEDLINE (via PubMed) and Web of Science databases from the inception up February 2020. Eligibility criteria included studies on healthy human subjects, treated for at least 12 weeks with LC administered orally, with no drugs or any other multi-ingredient supplements co-ingestion. Results The initial search retrieved 1024 articles, and a total of 11 studies were finally included after applying inclusion and exclusion criteria. All the selected studies were conducted with healthy human subjects, with supplemented dose ranging from 1 g to 4 g per day for either 12 or 24 weeks. LC supplementation, in combination with carbohydrates (CHO) effectively elevated total carnitine content in skeletal muscle. Twenty-four-weeks of LC supplementation did not affect muscle strength in healthy aged women, but significantly increased muscle mass, improved physical effort tolerance and cognitive function in centenarians. LC supplementation was also noted to induce an increase of fasting plasma trimethylamine-N-oxide (TMAO) levels, which was not associated with modification of determined inflammatory nor oxidative stress markers. Conclusion Prolonged LC supplementation in specific conditions may affect physical performance. On the other hand, LC supplementation elevates fasting plasma TMAO, compound supposed to be pro-atherogenic. Therefore, additional studies focusing on long-term supplementation and its longitudinal effect on the cardiovascular system are needed.

中文翻译：

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左旋肉碱补充剂的光明面和阴暗面：系统评价

背景 左旋肉碱 (LC) 被休闲活跃、有竞争力和训练有素的运动员用作补充剂。本系统评价旨在评估长期补充 LC 对代谢和代谢改变的影响。方法 自 2020 年 2 月成立以来，在 MEDLINE（通过 PubMed）和 Web of Science 数据库中进行了文献检索。资格标准包括对健康人类受试者的研究，口服 LC 治疗至少 12 周，不使用药物或任何药物。其他多成分补充剂共同摄入。结果初始检索检索到1024篇文献，经纳入排除标准最终纳入11项研究。所有选定的研究都是对健康的人类受试者进行的，补充剂量从每天 1 克到 4 克不等，持续 12 或 24 周。LC 补充剂与碳水化合物 (CHO) 相结合可有效提高骨骼肌中的总肉碱含量。24 周的 LC 补充剂不会影响健康老年女性的肌肉力量，但显着增加了肌肉质量，改善了百岁老人的体力耐力和认知功能。还注意到补充 LC 会导致空腹血浆三甲胺-N-氧化物 (TMAO) 水平的增加，这与确定的炎症和氧化应激标志物的改变无关。结论 在特定条件下长期补充 LC 可能会影响身体机能。另一方面，LC 补充剂会提高空腹血浆 TMAO，这种化合物应该是促动脉粥样硬化的。所以，