Some people swear by the boost they get from energy drinks, but experts say the products may have no effect on tired muscles. Instead, they may work on your brain. British researchers had volunteers on stationary bikes rinse their mouths with either a sugary energy drink or an artificially sweetened one and then conducted brain scans. The subjects who tasted sugar had activity in the reward and pleasure center of the brain; the others didn't. That brain surge, the researchers believe, creates an expectation of more sugary fuel to come and prods the body to push on. Another researcher found that distance runners select higher treadmill speeds after they've tasted sugar. An energy drink, it turns out, may be more a metabolic pep talk than a real jolt.