The Preference For Sweetened Foods May Decline After Exercise

Background

Scientists are aware that there is a decrease in glycogen content in the liver and skeletal muscles following exercise. Prolonged exercise can lead to a decline in blood glucose concentration. Thus it is important to ingest carbohydrates after exercise in order to replenish and restore glycogen.

Fluid replacement is also crucial to avoid severe dehydration, and factors such as taste and flavor have been shown to be keys for successful rehydration. (It has been shown that athletes consumed greater amount of fluids that they prefer than fluids that they dislike.) It is known that tastes, which stimulate voluntary fluid intake during and/or after exercise, include saltiness and sweetness.

Fluids designed for physical exercise situations contain carbohydrates to supply energy. But after exercise, athletes have demonstrated a preference for salt, as well as a temporary increase in the perception of sweetness. Anecdotally, athletes occasionally quote that sweetness of a beverage that they prefer before exercise is too strong during as well as after exercise. These comments seem to support the idea that animals, too, may become sensitive to sweetness or the taste of carbohydrates following exercise. If this notion is true, then it could be a starting point for developing foods and supplements that meet the post-exercise taste and nutritional needs of the competitive female and male athlete.

A New Study

To test the hypothesis, a team of Japanese researchers has attempted to determine responses to sweetened fluids after exercise in male rats. The authors of “Preference for Sweetness May Decrease after Exercise in Rats,” are Koji Okamura, Kayo Ikeda, and Yachiyo Harada, all at the Exercise Nutrition Laboratory Graduate School of Sport Sciences, Osaka University of Health and Sport Sciences Osaka, Japan. They will present their findings at the American Physiological Society’s (APS) (www.the-aps.org) annual scientific conference, Experimental Biology 2004, being held April 17-21, 2004, at the Washington, D.C. Convention Center.

Methodology

The research consisted of two tests utilizing different concentrations of sucrose (sugar). Ten male rats were individually housed in a cage equipped with two bottles containing distilled water (W) or a sucrose solution (S, 0.4 percent, threshold for sweetness in study 1; or four percent, consumed before exercise, in study 2). The bottles were arranged enabling the rats to drink both water and the sucrose solution at will.

The rats ran on a treadmill at 30 m/min for two hours. Water and sucrose consumption was measured on the day of exercise (Ex) as well as before (Pre) and the following (Post) day for six hours with two hour intervals. No food was given during the six hour timeframe.

Results

The researchers found that:

* sucrose ingestion did not increase significantly in study 1 (where the 0.4 percent sucrose solution was available at the threshold for sweetness);

* the preference for 0.4 percent (threshold) sucrose solution showed no change after exercise; and

* sucrose ingestion decreased in study 2 (consumption of the four percent sucrose solution decreased after exercise).

The results raise the possibility that the sucrose concentration which rats favor changes after exercise, since the amount ingested did not change in 0.4 percent solution while it decreased substantially in the four percent solution.

Caveat

Scientists have known that opioid-receptor antagonists decrease intake of sweet solution in rats, as well as reduce highly preferred food and increasing intake of non-preferred food (standard laboratory chow). In human studies, opioid antagonists have been shown to reduce pleasurable food ratings and especially intake of foods with high preference ratings before administration of opioid antagonists. Exercise is also known to increase â-endorphin levels. While this study did not measure â-endorphin levels in blood, a possible rise in opioid peptide due to exercise might have led to the decreased intake of four percent sucrose solution which rats had preferred.

Conclusions

The authors conclude that preference for sweetness decreases after exercise and the sweetness level preferred before exercise is insufficient to restore exercise-induced anorexia in rats. Because early ingestion of carbohydrates is effective after exercise in recovering skeletal muscle glycogen expended by exercise, the production of foods with a desired sweetness level and carbohydrates would provide a significant step in offsetting exercise-induced anorexia. This would benefit all athletes who experience difficulties to consume enough nutrients to recover after exercise.

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The American Physiological Society (APS) was founded in 1887 to foster basic and applied science, much of it relating to human health. The Bethesda, MD-based Society has more than 11,000 members and publishes 3,800 articles in its 14 peer-reviewed journals every year.