Duke Medicine labsters believe the popular supp carnitine, a compound critical for energy production, can aid muscle stamina by activating a special gene to boost the energy output of mitochondria. The study was done on mice—next up, human guinea pigs.

Supplement Guide: Carnitine >>>

Some background: Carnitine (and L-carnitine) helps the body break down fat and turn it into energy. It also increases the activity of certain nerve cells in the central nervous system.

While not a true amino acid, carnitine is a crucial protein building block that is naturally produced by the body in the liver and kidneys. Guys looking to get more can find it in fish, poultry, red meat, and some types of dairy—or, of course, through over-the-counter supplements.