Benefits
Heart health support, studied in heart patients
A pooled analysis of 13 controlled trials in 3,629 people treated after a heart attack linked L-carnitine with 27% lower all-cause mortality, 65% fewer ventricular arrhythmias and 40% less angina, but no change in heart failure or repeat heart attacks. The mortality result was borderline (P=.05), so the benefit is uncertain.
Less muscle soreness and muscle damage after exercise
Pooled results from seven randomized trials found less muscle soreness from right after exercise up to 96 hours later, and lower creatine kinase, myoglobin and LDH, markers of muscle damage, versus placebo. The benefit on damage markers was seen mainly in resistance-trained and untrained people.
Supports sperm motility in men with low sperm quality
Meta-analyses of randomized trials, mostly of L-carnitine combined with acetyl-L-carnitine, find better sperm motility in men with low sperm quality. Effects on sperm count and shape vary between reviews, and the largest review, rated low certainty, found no effect on pregnancy or live birth for any supplement.
Small reductions in fasting glucose and HbA1c
Pooled results from 41 randomized trials (2,900 adults) found L-carnitine lowered fasting glucose by about 3.2 mg/dL and HbA1c by 0.27% versus control, more so at 2 g/day or more for 12 weeks or longer. In type 2 diabetes, each 1 g/day lowered HbA1c by 0.16% and BMI by 0.37. Results varied widely between trials.
Muscle carnitine and exercise output when taken with carbohydrate
In a 24-week randomized trial of 14 healthy men, L-carnitine L-tartrate with carbohydrate twice daily raised muscle carnitine by 21%, cut muscle glycogen use by 55% during easy cycling, and raised work output 11% from baseline, while carbohydrate alone did not. This is a small trial.
Mechanism of action
Carrying fat into mitochondria
Carnitine shuttles long-chain fatty acids into mitochondria to be burned for energy. Muscle carnitine is hard to raise by mouth; in one trial it rose only when L-carnitine was taken with enough carbohydrate to raise insulin, and the extra carnitine spared muscle glycogen during light exercise, consistent with more fat use.
Gut bacteria convert carnitine to TMAO
Gut microbes break dietary carnitine down to trimethylamine, which the liver turns into TMAO. Omnivores made more TMAO than vegans after a carnitine dose, carnitine feeding increased atherosclerosis in mice, and high blood carnitine predicted heart events in patients only when TMAO was also high.
Blunting thyroid hormone action
Carnitine blocks thyroid hormone from entering cell nuclei. In a placebo-controlled trial in women taking high-dose thyroid hormone, 2 or 4 g/day eased symptoms of thyroid excess, which shows the effect is strong enough to matter clinically.
Clinical trials
Randomized, double-blind 24-week trial of 2 g L-carnitine L-tartrate plus 80 g carbohydrate twice daily versus carbohydrate alone (Wall et al. 2011, J Physiol).
14 healthy male volunteers, tested before and after 12 and 24 weeks.
After 24 weeks muscle total carnitine rose 21% with L-carnitine and did not change with carbohydrate alone. During cycling at 50% of maximum, muscle glycogen use was 55% lower; at 80%, muscle lactate was 44% lower. Work output in a 30-minute performance test rose 11% from baseline with L-carnitine and did not change in the control group.
Systematic review and meta-analysis of 7 randomized controlled trials comparing L-carnitine plus acetyl-L-carnitine with placebo (Zhang et al. 2020, Andrologia).
693 men with idiopathic oligoasthenoteratozoospermia (low sperm count, motility and normal forms).
Forward sperm motility, total motile sperm and forward motile sperm rose versus placebo, and pregnancies were more common (odds ratio 3.76). Sperm concentration, semen volume and abnormal forms did not differ. A later, larger review of supplement trials rated the evidence low certainty and found no effect on pregnancy or live birth.
Systematic review and meta-analysis of 13 controlled trials comparing L-carnitine with placebo or control after acute myocardial infarction (DiNicolantonio et al. 2013, Mayo Clin Proc).
3,629 patients across 13 trials.
L-carnitine was associated with 27% lower all-cause mortality (odds ratio 0.73, P=.05), 65% fewer ventricular arrhythmias and 40% less angina, with no reduction in heart failure or repeat heart attacks. The mortality finding sat at the edge of statistical significance, and the authors called for large modern randomized trials.