The researchers chose 60 infertile men between the ages of 20 and 40 to take a combination of L-carnitine and L-acetyl-carnitine or a placebo for six months.
The group, led by Dr Andrea Lenzi, reported in the medical journal Fertility and Sterility, that two months after the patients had completed their course of supplements, the men who had taken L-carnitine and L-acetyl-carnitine had increased sperm concentration and movement.
The most significant improvements in sperm motility, both forward and total, were observed in men who had the lowest levels of moving sperm when the study began.
The researchers noted that four spontaneous pregnancies were achieved during the study by men who had taken the combination therapy.
"Combined treatment with L-carnitine and L-acetyl-carnitine was effective in increasing sperm motility, especially in groups with lower baseline levels (of moving sperm)," they concluded.Many food supplements and dietary products across Europe contain L-carnitine.
L-Carnitine, a vitamin-like nutrient, occurs naturally in the human body and is essential for turning fat into energy. It is frequently used as a dietary supplement by physically active people to help with post-exercise recovery.
Lonza, which claims to be the world's largest manufacturer of L-Carnitine, has said that extensive scientific research shows the supplement promotes cardiovascular health and that other studies suggest the nutrient may be useful in weight management.