There is a dearth of metabolic information, even for short-duration flights, without which changes in metabolic patterns to longer flights cannot be extrapolated. However, using scattered information, certain changes which may be encountered can be hypothesized. Decalcification of bone and changes in water-holding capacity of the body may be anticipated. It is also possible that changes in proportion of fat to lean body mass could be experienced and should be considered in nutritional planning. Nutritional requirements depend on size, particularly lean body mass, sex, physiological state, and individual metabolic rates. Therefore, individuals for space flight should be screened with these factors in mind if it is desirable to minimize food intake in long flights. The factors which influence the total nutritional requirements of the individual also influence his mental and physical responses to stress. Synthetic Foods The development of food materials other than those derived directly from animal or vegetable origin is of interest. Advantages of such diets may be low residue, ease of storage, rehydration, and manipulation. Experiments with chemically defined synthetic diet for humans have been carried out by Medical Sciences Research Foundation, San Mateo, Calif. The complete liquid diet is composed of required amino acids, fat, carbohydrate, vitamins, and minerals. A cubic foot of the diet (50 percent solids in H₂O) supplies 2500 calories per day for 1 month, and has been given a variety of artificial flavors. This synthetic diet has been fed to human volunteers for 6 months in a pilot study at the California Medical Facility, Vacaville, Calif., and the results are being reviewed. Schwarz Bioresearch, Inc., is studying the storage, stability, and packaging of chemically defined synthetic diets for human and animal flights. Food Production in Space Long-term feeding in space depends upon a payload of stored food unless food is produced during flight. If sufficient propulsive energy is available, the duration of missions using stored food may be quite long. However, in emergencies in which a mission lasts longer than planned, survival may depend on the ability to produce food extraterrestrially. Eventually it will be desirable or necessary to produce food beyond the confines of Earth.