Serum L Carnitine Levels Are Associated With First Stroke in Chinese Adults With Hypertension Stroke L Carnitine Oral Bioavailability NOW l Carnitine Supplement L carnitine levels decrease with age, contributing to aging and a decline in mitochondrial function. The list doesn't stop there. L carnitine supports: Joint health L carnitine has been shown to support joint comfort If you are like a lot of people, you may have never heard of carnitine but this important nutrient is critical for our body's ability to optimally burn fat required for energy.