L-Carnitine is a compound produced by the body from lysine and methionine. It can be acetylated to produce Acetyl-L-Carnitine (ALCAR), which is similar but crosses the blood-brain barrier more efficiently. L-Carnitine is best known for its involvement in the mitochondrial oxidation of long-chain fatty acids.
It's found in food and is most prevalent in meat, and beef in particular.
A notable improvement in sperm quality has been found with supplementation of high doses. It may modestly reduce fatigue in elderly people with low muscular endurance but its effects on athletes during physical activity aren't well-supported. L-Carnitine may slightly limit muscle damage during resistance exercise. Minor beneficial effects on blood glucose and insulin sensitivity have been noted in people with diabetes and healthy people alike, but much more research is needed to confirm this.
When it comes to fat-burning, studies on ALCAR in isolation do not show very good results. Some studies have found minor fat loss, which is typically attributed to increased physical activity due to increased energy levels.
A few studies have noted a benign adverse effect of 'odd smell', which is said to be due to the formation of trimethylamines; it has occurred at a frequency of 4%.
Probably only if it helps to reduce fatigue and that leads to an increase in physical activity. It's possible that very low carnitine levels could lead to impaired fatty acid oxidation, but for most people, there isn't a notable increase in resting fatty acid oxidation when taking L-carnitine.