Carnitine is a nutrient that plays an important role in fatty acid absorption and mitochondrial function. It is synthesized endogenously from lysine and methionine and is also found in meat and dairy products. Deficiencies can be caused by genetic disorders, malnutrition, malabsorption and kidney dialysis, and affect the heart, skeletal muscles, liver, nerve, and endocrine functions. Carnitine supplements are used to enhance physical performance and to treat fatigue, cardiovascular disease, diabetes, obesity, chronic fatigue syndrome, liver disorders, and cancer.

A meta-analysis determined that supplementation improves body weight and BMI in overweight and obese individuals ⁽⁵⁵⁾. It also alleviated muscle damage in resistance trained and untrained groups ⁽⁵⁶⁾, muscle wasting in patients with pemphigus vulgaris ⁽⁶¹⁾, and prevented cardiovascular disease in hemodialysis patients ⁽²⁴⁾. Long-term supplementation has been correlated with improved myocardial mechanical performance, reduction in ventricular arrhythmias and increased exercise tolerance ⁽⁷⁾, and higher levels of carnitine were associated with lower risks of cardiovascular events and recurrent stroke after ischemic stroke ⁽⁶³⁾. Carnitine also improved oxidative and inflammatory markers in patients with cardiovascular disease ⁽⁶²⁾, but did not have any impact on blood pressure ⁽⁶⁶⁾ or reduce risk of death or heart failure in patients with anterior acute myocardial infarction ⁽³⁶⁾.

In critically-ill patients, supplements improved nutritional status indicators, which have a key role in prognosis of diseases ⁽⁶⁴⁾ and adjunctive use improved symptoms of autism spectrum disorder in children and adolescents ⁽⁶⁵⁾.

Preliminary findings also suggest improvement in symptoms of chronic fatigue syndrome  ⁽²²⁾ and physical performance in patients undergoing dialysis for end-stage renal disease ⁽⁹⁾, but data on benefits against fatigue associated with multiple sclerosis are inconclusive ⁽²¹⁾. Some trials reported enhanced physical performance ^(17) (20), and improved aerobic capacity and exercise tolerance ⁽³²⁾ whereas others yielded mixed results ^{(37) (38) (39)}.

L-carnitine used by itself or in combination with clomiphene citrate may help treat idiopathic male infertility ⁽²⁵⁾ and reduce lower urinary tract symptoms in those with benign prostatic hyperplasia ⁽⁶⁷⁾ although it did not improve sperm count or motility ⁽⁴⁰⁾. An antioxidant formulation containing L-carnitine was also ineffective in improving semen parameters or DNA integrity ⁽⁵⁷⁾. In women with polycystic ovary syndrome, supplementation may reduce body mass ⁽⁶⁸⁾, and improve mental health parameters and biomarkers of oxidative stress ⁽²⁾.

Small studies in oncology settings found supplementation to improve nutritional status and quality of life in pancreatic cancer patients ⁽²⁷⁾, and to reduce vismodegib-associated muscle spasms ⁽⁵²⁾. L-carnitine by itself ^(15) (23) or in combination with Coenzyme Q10 ⁽³³⁾ may also relieve chemotherapy-related fatigue, reduce fatigue in both younger hypothyroid patients receiving levothyroxine, and in thyroid cancer patients who have hypothyroidism, post-surgery ⁽⁴¹⁾. But it did not improve fatigue in patients with invasive malignancies ⁽²⁸⁾. Confirmatory data are also needed to determine if chemo-induced carnitine insufficiency increases malaise or fatigue ^(58) (59). 
Current guidelines do  not recommend L-carnitine for cancer-related fatigue ⁽⁶⁹⁾.

In prostate cancer patients initiating androgen deprivation therapy (ADT), a low-carb diet reversed ADT-induced metabolic abnormalities which included reductions in acylcarnitine  ⁽⁶⁰⁾. 

Acetyl-L-carnitine, an ester derivative available as a supplement, is often used as a neuroprotective agent. It may help reduce diabetic neuropathy ^(42) (43) and improve cognition in patients with severe hepatic encephalopathy ⁽⁴⁴⁾. In elderly patients with dysthymic disorder, it was found comparable to fluoxetine ⁽⁴⁵⁾, but ineffective against Alzheimer’s disease ^(46) (47).
In cancer settings, acetyl-L-carnitine increased chemotherapy-induced peripheral neuropathy (CIPN) ⁽³⁵⁾, which persisted at the 2-year follow-up ⁽⁵³⁾. Current guidelines do not recommend this supplement for preventing CIPN ⁽⁵⁴⁾.