 The objective of this study is to determine the impact of consuming MREs as the sole source of subsistence for 21 days on gut bacteria community composition and gut health. Up to 80 free-living adults will be randomized to consume their usual diet or only MREs for 21 consecutive days. MREs will be provided by the Military Nutrition Division, US Army Research Institute of Environmental Medicine USARIEM. Fecal, urine and blood samples will be collected periodically before, during and after the intervention to measure gut barrier integrity, gut bacteria community composition, and markers of gut health, inflammation, and nutritional status. 