A strategy for promoting food intake during space flight.
Stinnett, TS; Keaton, AA; Davis, GR.
Department of Biology
Wofford College, Spartanburg, SC.
In the microgravity of space, body fluids no longer tend to pool in the lower extremities. Consequently astronauts experience head congestion, especially during the first few days in space. This phenomenon, known as space adaptation syndrome, is associated with nausea, dizziness and a loss of appetite. Food intake is reduced in astronauts. For long-term space missions, it will be important to promote adequate food intake by astronauts. However, experiments involving human subjects, especially those attempting to simulate weightlessness, present many obstacles. Therefore, we have studied food intake in rats under simulated microgravity using the hindlimb unloading model developed by NASA scientists. This animal model has been used extensively to study alterations in physiology that occur during space travel (see Morey-Holton & Globus, 2002). For hindlimb unloading, rats are suspended from a harness attached to the tail such that the hindlimbs bear no weight yet the animal can move freely about the cage. This arrangement mimics the fluid shifts experienced by astronauts.
Like humans, rats will consume a given type of food until they reach satiety and then reject additional amounts of that same food. However, if offered a second meal consisting of a more palatable food (such as dessert), a satiated rat will consume more food. This behavior, called sensory specific satiety, can be readily observed in “buffet” restaurants where humans tend to overeat if offered a wide variety of palatable foods. Thus, sensory specific satiety may contribute to expanding population of overweight individuals in developed countries.
In this study we used the hindlimb unloading model to determine if rats under conditions of simulated microgravity exhibit sensory specific satiety. We hypothesized that hindlimb unloaded rats will consume Froot Loops® after “getting full” on rat chow. Rat chow and Froot Loops® have similar caloric density but different sensory properties (taste, smells, and texture.)
Seven male Sprague-Dawley rats had free access to chow and water except on test days. In counterbalanced experiments, rats were hindlimb unloaded or simply housed in the test cages (horizontal control) for seven days. Following 14-18 hour food deprivation on Days 2 and 7 (assure the rats were hungry,) rats were allowed to eat to satiety on a first meal of chow for 90 minutes. Subsequently, a second meal of either rat chow or Froot Loops® was available for 30 minutes. Food intake was measured in grams for both meals. We found hindlimb unloaded rats ate about half as much rat chow as the control rats during meal one, which parallels the reduced appetite observed in astronauts experiencing space adaptation syndrome. When meal two consisted of rat chow, neither the horizontal control rats nor the hindlimb unloaded rats consumed more rat chow (which is characteristic of sensory specific satiety.) When meal two consisted of Froot loops®, hindlimb unloaded animal and their horizontal counterparts did consume this more palatable food, again indicative of sensory specific satiety under simulated microgravity. However, hindlimb unloaded rats ate only about half as many Froot Loops® as the horizontal control animals. The same pattern was observed after 7 days of hindlimb unloading. Our results indicate that rats have a reduced appetite under conditions of simulated microgravity, but food intake can be increased by offering a more palatable food as a second meal.
Our findings suggest that sensory specific satiety, a phenomenon that is responsible in part for overeating on Earth, can be employed to promote food intake during space flight. Based on our results, we predict that food intake by astronauts can be boosted by enhancing the variety and palatability of foods. Apparently at least some of the neural mechanisms underlying sensory specific satiety on Earth also operate in microgravity. In future experiments, we will attempt to gain a more complete understanding of the neural mechanisms responsible for sensory specific satiety by pharmacological manipulation of brain regions suspected of participating in this phenomenon.
Support: South Carolina Space Grant Consortium grant #520552