NATICK, Mass. (Sept. 26, 2019) -- Bacteria live all around us. They live in the soil, in our food and even inside our bodies, particularly in our digestive tracts. These trillions of bacteria that live in the human gut, better known as the gut microbiome, are not merely passengers. A growing amount of scientific evidence has shown that the gut microbiome communicates with many other parts of our bodies, affecting our physical, mental and general health.Scientists from the U.S. Army Research Institute of Environmental Medicine (USARIEM) are exploring the complex relationship between the gut microbiome, health and operational readiness. Research by Army and civilian scientists is beginning to show that the gut microbiome reacts to stress, influences responses to stress, and can be shaped by diet. These findings suggest that interactions between diet and the gut microbiome may be factors in mission success.Soldiers inevitably experience changes in physical activity, environment, diet, and sleep patterns during operations. These changes may affect the health and diversity of their gut microbiomes, and they may increase gastrointestinal symptoms and intestinal permeability, also known as gut leakiness. Gut leakiness is a condition that is influenced in part by the gut microbiome, in which the intestinal walls weaken and allow waste products to leak into the bloodstream. These factors could ultimately compromise Soldier health, readiness and lethality.During operations, Soldiers often shift from eating their regular diets to eating military rations, particularly the Meal, Ready-to-Eat (MRE). The MRE contains similar amounts of carbohydrates, fat, protein and fiber as the average American diet, and the vitamin and mineral content is designed to meet Soldier nutritional requirements. However, unlike most diets, the MRE needs to withstand rough conditions and exposure to the elements while maintaining a three-year shelf life. As a result, commercially sterile, highly processed items and no fresh foods are used.The Surgeon General's current policy allows the MRE to be consumed as the sole source of subsistence for up to 21 days. Earlier studies demonstrated that consuming the MRE for 21 days had no negative effects on Soldier nutrition status. However, the question of whether subsisting solely on MREs could cause gut leakiness or other gastrointestinal symptoms and change the gut microbiome had not been considered.It turns out that the MRE does not appear to have negative effects on gut health and has only a small impact on the gut microbiome community. USARIEM recently published these findings in the Journal of Nutritional Biochemistry in a report that concluded, "The MRE ration diet alters fecal microbiota composition and does not increase intestinal permeability."Dr. Phil Karl, the principal investigator, explained that the study's purpose was to understand how military rations interact with the gut microbiome and impact various measures of gut health like gut leakiness. To do this, the scientists wanted to separate the effects of the MRE diet from any other factors that could potentially affect the gut microbiome in a stressful operational environment."Usually, when someone is eating an MRE, they aren't sitting at their dining room table to enjoy a meal," Karl said. "MREs are often consumed in the field, where Soldiers can be operating in hot, cold or high-altitude environments and performing strenuous activity while not getting enough sleep. Psychological stress is also common. All of these factors can affect the gut microbiome and gut function."We found that the MRE does not increase gut leakiness, does not appear to negatively impact gut health, and has only subtle effects on the gut microbiome in people eating the MRE while going about their normal daily lives."According to Karl, the scientists conducted the study in response to growing evidence demonstrating that the gut microbiome is highly reactive to diet, essential to overall health, and might play a role in cognitive and physical performance. Another reason they did the study was because of Soldier anecdotes of gastrointestinal issues while eating the MRE in operational environments. Whether those issues were due to the MRE or to other factors present in operational environments had not been studied.The need for the study became even more apparent after Karl was part of a team that conducted a nutrition study in 2016 at USARIEM's high-altitude research laboratory in Pikes Peak, Colorado. One purpose of that study was to see how a high-altitude environment might affect the gut microbiome and gut leakiness. The study volunteers experienced gastrointestinal discomfort and increased gut leakiness while at high altitude, along with minor changes in their gut microbiomes. Volunteers in that study also ate a diet mainly consisting of MREs."After that result, we wanted to next determine whether the high-altitude environment, the MREs, or their combination were responsible for the findings," Karl said.To address the question, the scientists collected data from 60 volunteers. Half of the volunteers committed to consuming only MREs for 21 days, and the other half continued their regular diet for the entire study. Both groups visited USARIEM's lab at the Natick Soldier Systems Center three times weekly. The scientists collected blood, urine and fecal samples from all of the volunteers before, during and after the study. They also administered questionnaires to see if any of the volunteers were experiencing gastrointestinal symptoms like gas, diarrhea, constipation or bloating.The scientists analyzed the samples in the laboratory to look at how gut microbiome composition in the MRE group changed relative to the control group. Gut microbiome compositions in the control group remained the same throughout the study. Meanwhile, scientists found less lactic acid bacteria in the MRE group's samples while they were eating MREs. After looking at records of what the volunteers were eating before and after the 21 days of MREs, Karl suspected the decrease was a result of eliminating fermented foods from the diet.MREs do not contain fermented yogurts and cheeses, which, for most people, are the primary dietary sources of lactic acid bacteria. Despite these differences, volunteers who ate the MREs reported few, minor differences in gastrointestinal symptoms and did not experience an increase in gut leakiness compared to the control group.According to Karl, these study results strengthen the theory that Soldiers experience gut leakiness and gastrointestinal symptoms during operations due to psychological and environmental stress, rather than the MRE diet.This fall, the research team is preparing to expand upon the 2016 Pikes Peak study. Their goal is to determine if gut leakiness at high altitude can be reduced or prevented with the help of a dietary intervention designed to nourish a healthy gut microbiome.The Army has spent decades making field rations nutritious and safe. USARIEM nutrition scientists constantly work with food developers at the Combat Capabilities Development Command Soldier Center to find new and improved ways to provide Soldiers with the nutrients they need to perform optimally in operational environments. Karl explained that this ongoing gut microbiome research is part of that mission. Understanding how the gut microbiome behaves in future battlefields, and how diet can be used to promote a healthy and resilient gut microbiome, will enable the Army to develop gut microbiome-targeted nutrition interventions that improve health and readiness."By having a better understanding of how the gut microbiome is impacted in an operational environment, and the role of diet in that response, we will be able to develop foods that can help make the gut microbiome, and, as a result, the Soldier, more resilient to operational stress," Karl said. "Doing so will improve Soldier health, readiness and, ultimately, lethality."