1. Introduction: Contained inside your gastrointestinal tract, more specifically your small and large intestine, is a collection of trillions of different microorganisms called the microbiota (Gut Microbiota Worldwatch, 2015). The main function of the gut microbiota is to break down hard-to-digest dietary fiber into absorbable parts such as short chain fatty acids (Lippincott, 2012). In addition, these microorganisms “de-conjugate bile acids, help us to synthesize vitamin K, some B vitamins and amino acids” (Radford, 2015). The importance of these microorganisms cannot be taken for granted or gone unnoticed. In fact, experts are now calling the gut microbiota its own organ due to the multiple metabolic processes in which it takes part (Gut Microbiota Worldwatch, 2015). The gut microbiota differs in humans, with only one third being consistent amongst all humans, while the remaining two-thirds of the microbiota varies for each individual (Gut Microbiota Worldwatch, 2015). Since the microbiota is heavily involved in digestion, diet naturally plays a role. Based on foods that an individual consumes, the profile of the microbiota in the gut can change (Kasai, 2015). Specific foods are prone to promote good gut bacteria while others will negatively change the composition to promote bad gut bacteria (Sonnenburg, 2014). On a global scale, Westernized countries generally consume a Westernized diet (high in salt, sugar, and fat) which negatively affects the composition of their gut microbiota (Magnussen, 2015). Even further, these countries have a high prevalence of obesity (Harvard School of Public Health, 2012), and therefore, we can conclude that consuming a Westernized diet correlates with a high prevalence of obesity in Westernized countries. Through our research, the effect of diet on the gut microbiota composition will be reviewed more thoroughly, concluding with tips on how to improve the composition of an individual's microbiota through dietary choices. Microbiota: A Deeper Look Inside The definition of a microbiota is an ensemble of microorganisms residing in an established environment (Gut Microbiota Worldwatch, 2015). Microbiota exists virtually everywhere that the human body is exposed to the external environment, but the most prominent microbiota humans possess is in the gastrointestinal tract (the gut) (Sekirov, 2015). The colon alone is estimated to contain over 70% of all the microbes in the human body (Sekirov, 2015). In the human gut microbiota, there are tens of trillions of microorganisms and bacteria. Within this large amount of microorganisms, there are about 1,000 different species of known bacteria with more than 3 million genes (Gut Microbiota Worldwatch, 2015). Although every human has a well-established gut microbiota by the time they become an adult, about two thirds of an individual’s gut microbiota is unique to that specific individual (Gut Microbiota Worldwatch, 2015). The gut microbiota is fairly stable once it is established, however there has been evidence to suggest that external forces, such as what you eat, can alter the community of gut microbes (Ursell, 2013). The human gut microbiota fulfils the same physiological functions among all humans, and is an integral component to our health. It can affect brain development, insulin production, appetite, metabolism, and inflammation of the gut (Hur, 2015). Some functions of the gut microbiota include digesting foods that the stomach and small intestine have not been able to digest, ensuring proper digestive functions, extraction of energy from foods, production of some vitamins, performing a barrier effect for the immune system, and combating aggressions from other microorganisms (Gut Microbiota Worldwatch, 2015, Ursell, 2013). Microbes in the gut microbiota also have the ability to increase the number of metabolic tools of the human gut, which allows digestion of many different foods and substrates (Ursell, 2013).. Microbiota and Food Choices A large body of research is emerging making connections between the quality and variety of one’s diet and one’s specific microbiota profile. When you eat, you are feeding good or bad bacteria of the microbiota. They, quite simply, eat the same thing that we eat. Foods high in prebiotics and probiotics are key players in maintaining the microbiota and thus, keeping it healthy (Sanders, 2015). Prebiotics are found in dense, fibrous vegetables; probiotics are found in fermented foods like yogurts, dairy, and sauerkraut (Sanders, 2015). Prebiotics act as nourishment for the microbiota. The beneficial bacteria in our gut are essential to breaking down the indigestible fibers. Probiotics work to replenish or diversify the microorganisms of the microbiota. It is important to have not only a large amount of good bacteria, but also diversity of different strains of bacteria (Mullin, 2015). Macromolecular content of the diet plays a role in the microbiota (Ericsson, 2015). The three macromolecules are fat, carbohydrate, and protein. Research shows that abrupt changes in the ratio of macromolecule intake plays a role in the rapid change in the microbiota (Ericsson, 2015). For example, a high carbohydrate/high fat meal will impact the microbiota differently than a meal with a balance of protein, carbohydrate, and fat (Magnusson, 2015). Within 72 hours after a meal, the microbiota is already adapting and manipulating itself to break down the food (Ericsson, 2015). This served an evolutionary purpose: when our ancestors ate something that their body had not been introduced to before, the microbiota would manipulate itself to produce the type of bacteria that would be able to break it down (Ericsson, 2015). Holly Schneider Meredith Carothers Rachel Brown Caroline Thomason Factors influencing the human Microbiota (Biome Onboard Awareness, 2015) Picture of Microbiota (Ed Bites, 2015 ) Bacteria Composition The bacteria composition of an individual’s gut may be analyzed through tests, with an example being a fecal matter analysis (Kasai, 2015). From this test, researchers have been able to identify certain bacterial composition trends that differ in obese and lean individuals. The biggest difference found between the microbiota of an obese individual, in comparison to a lean individual, is their ratio of two phylum of bacteria; Firmicutes and Bacteroides (Kasai, 2015). It has been discovered that obese populations generally contain a higher Firmicutes to Bacteroidetes bacteria ratio in their gut (Kasai, 2015). Bacteroidetes are characterized as bacteria that can digest polysaccharides that the human body is not able to digest. The bacteria then internalizes the energy from the breakdown of the food and does not present the host with the absorbable energy (Kasai, 2015). Firmicute bacteria are characterized by their ability to digest polysaccharides that the body is not able to digest, and produces short chain fatty acids that the body can reabsorb and use to promote the formation of adipose tissue (Kasai, 2015). Both of these bacteria are able to ferment fiber, but the Bacteroidetes use the energy for their own metabolism while the Firmicutes supply the host with the energy they break down (Kasai, 2015). This small difference in bacterial metabolism changes their effect on the host; Bacteroidetes prevent extra calorie absorption from high fiber foods and help to prevent obesity, while Firmicutes increase energy absorption when high fiber foods are consumed, increasing the caloric intake of the individual (Kasai, 2015). As a result of the metabolism in the Firmicute bacteria, an obese individual adds more calories from fat to their diet even when they are consuming the recommended fruits and vegetables. Implications of Food Choices This area of research has greater implications for the diets of Americans and other Westernized countries. A Westernized diet is high in sugar, salt, and saturated fat (Magnusson). When one’s diet includes these three components, their microbiota has shown to be dysbiotic and predisposes individuals to disease (Sonnenburg, 2014). A dysbiotic microbiota simply means that the ratio of negative and positive bacteria is off. The creation of a dysbiotic microbiota can be described through the metabolism of high fat diets (Beil, 2015). Fat is hard for certain organisms to break down and use for food (Beil, 2015); thus, a diet high in fat will select for only the microbes that can break them down and reduce the type of microbes that cannot break fat down (Beil, 2015). Therefore, a dysbiotic microbiota will arise from a high fat diet selecting for bad bacteria (Beil, 2015). Inflammation is a related effect of an unhealthy microbiota. When a microbiota is regularly introduced to a high fat/high sugar diet, the gut bacteria do not have quality sources of fuel (Beil, 2015). Evidence suggests that the gut bacteria may be more likely to “eat” the mucosal lining of the intestines which in turn causes small, microscopic tears in the lining of the intestines (Beil, 2015). The gut organisms then slip through these tears and set off an immune response, which then causes a specific gut inflammation response (Beil, 2015). Anti- inflammatory foods can restore the inflamed gut and begin the healing process (Beil, 2015). These foods contain Omega-3 fatty acids such as fatty fish and nuts, which work to aid in healing an inflamed gut (Beil, 2015). Tips for improving the microbiota: Include prebiotics from fibrous vegetables in your diet to “feed” the microbiota Consume probiotics through fermented foods, dairy, or supplementation to replenish and diversify the microbiota Eat less of a Western Diet template to promote a healthy microbiota (including processed foods and foods high in fat, sugar, and salt) Eat foods high in Omega-3 fatty acids, such as fish and nuts, to reduce inflammation in the gut Since different microbes thrive on different types of food, the microbes that are currently flourishing in the microbiota send signals to eat more of the types of foods that nourish them specifically (Zimmer, 2014). This implies that our diet has the potential to make us crave more of the same foods we have been eating (Zimmer, 2014). Our microbiota can communicate with one another by releasing chemicals such as serotonin and dopamine (Zimmer, 2014). These signals are picked up by the human nervous system that lines the digestive tract, and the nervous system then directly communicates with the brain to process the signals (Zimmer, 2014).