Before taking a long bike ride on a hot summer day, have some watermelon: The juicy fruit may ward off muscle pains. Researchers report that people who drank watermelon juice before exercising felt less sore the next day than those who drank a pink placebo beverage (J. Agric. Food Chem. 2013, DOI: 10.1021/jf400964r). They also found that cells absorb the presumed active ingredient, L-citrulline, more readily from unpasteurized watermelon juice than from plain water spiked with the compound, suggesting the natural source is the optimal delivery medium.
L-citrulline is an uncommon amino acid that, until recently, hasn’t gotten a lot of attention, says Encarna Aguayo of the Technical University of Cartagena in Spain. Scientists now recognize that L-citrulline has antioxidant properties and may enhance athletic performance, she says. For example, studies have shown that L-citrulline in supplement form accelerates removal of lactic acid from muscles, allowing for more intense training and faster recovery. Watermelon is one of the few natural foods with an abundance of L-citrulline, so Aguayo wanted to test whether the fruit’s juice could function as a sports drink.
First, the researchers purchased 10 seedless watermelons from a local market in Cartagena and juiced the fruit. They pasteurized some of the juice by heating it to 80 ˚C. The rest was left unpasteurized. Pasteurization kills harmful pathogens, but it also can “reduce the bioavailability of many bioactive compounds, reducing the food’s functional properties,” Aguayo says.
Aguayo and her team measured the concentration of L-citrulline in the watermelon juice with high-performance liquid chromatography and mass spectrometry and then adjusted the concentration to a standard 0.685 mM. They also spiked water with the amino acid at the same concentration. The researchers then added the solutions to cultures of human colon cancer cells. After eight minutes, the researchers measured how much L-citrulline remained outside the cells. They found that cells bathed in the unpasteurized watermelon juice had absorbed 19% of the L-citrulline, while absorption from pasteurized juice and spiked water never got above 13% and 12%, respectively. Better absorption should lead to a more potent benefit, Aguayo says.
Next, the researchers recruited seven men who regularly played sports but were not competitive athletes. The men participated in an exercise test on three separate days. They cycled for 11 minutes on a stationary bicycle, alternating between periods of intense pedaling and rest. Before getting on the bike, each participant drank half a liter of one of three beverages: watermelon juice with 1.17 g of L-citrulline, watermelon juice enriched to have 6 g of L-citrulline, or a pink, fruity drink without watermelon or L-citrulline. On the day after the trials in which the men drank either type of watermelon juice, they reported essentially no leg soreness. Meanwhile, after the trials when they imbibed the placebo, they reported feeling sore.
The take-home message from this study, according to Thomas Swensen of Ithaca College, is that it’s better to drink unpasteurized watermelon juice than to take an L-citrulline supplement. Swensen says the next step would be to see if drinking the juice helps athletes perform better the next day because they are less sore. Aguayo plans to tweak the concentration of L-citrulline in watermelon juice to determine the optimal concentration for the prevention of muscle fatigue.