Exercise boosts sugar’s taste

From Washington, D.C., at the Experimental Biology 2004 meeting

People who work out regularly often report a feeling of euphoria after exercise. Wondering how exercise affects other sensations, scientists at the Osaka (Japan) University of Health and Sport Sciences evaluated taste thresholds in competitive runners. The researchers report that the volunteers’ sensitivity to sweetness increased greatly after their endurance training.

Koji Okamura and his colleagues recruited six male and five female athletes for taste tests immediately before and after their 2-to-3-hour training sessions. On each of three test days, the athletes fasted for at least 3 hours before the first taste test. The researchers gauged the runners’ sensitivity to sweetness by successively placing on their tongues paper dots soaked in sucrose solutions of increasing strength.

The athletes’ threshold for sweetness sensitivity varied from day to day but was consistently lower after the exercise than before. In a typical set of tests, a volunteer would begin recognizing sweetness at a 1 percent sucrose concentration before exercise but at just a 0.45 percent after a workout.

Rats also showed a heightened sensitivity to sweets after exercise, Okamura reported. The researchers tested this by giving the animals access to both distilled water and sugar water. Because rats always choose the sugar water, the researchers pegged the animals’ sensitivity threshold at the lowest sucrose concentration at which the animals would begin preferentially choosing sugar water.

The researchers found that rats forced to exercise for 2 hours tasted sucrose at lower concentrations than sedentary rats did. However, Okamura and his colleagues also discovered a surprising limit to the animals’ preference for sugar water.

Rats offered a sugar solution that had been concentrated to 10 times the animals’ preexercise sensitivity threshold drank 40 percent less liquid after exercising than after being sedentary. The effect carried over to the day after exercising.

Okamura concludes that despite the animals’ desire for the sugar, their exercise-enhanced taste sensitivity rendered the sugar solution too sweet to take in large doses.