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In a new study appearing in the journal Nutrition in Clinical Practice, researcher Rosa Krajmalnik-Brown and her colleagues at the SwetteCenter for Environmental Biotechnology at Arizona StateUniversity's Biodesign Institute in collaboration with John DiBaisefrom the Division of Gastroenterology at the Mayo Clinic, reviewthe role of gut microbes in nutrient absorption and energyregulation. According to Krajmalnik-Brown, "Malnutrition may manifest aseither obesity or undernutrition, problems of epidemic proportionworldwide. Microorganisms have been shown to play an important rolein nutrient and energy extraction and energy regulation althoughthe specific roles that individual and groups/teams of gut microbesplay remain uncertain." The study outlines the growth of varied microbial populations --from birth onwards -- highlighting their role in extracting energyfrom the diet. The composition of microbial communities is shown tovary with age, body weight, and variety of food ingested; as wellas in response to bariatric surgery for obesity, use of antibioticsand many other factors. Based on current findings, the authors suggest that therapeuticmodification of the gut microbiome may offer an attractive approachto future treatment of nutrition-related maladies, includingobesity and a range of serious health consequences linked tounder-nutrition. Micromanagers The microbes in the human gut belong to three broad domains,defined by their molecular phylogeny: Eukarya, Bacteria, andAchaea. Of these, bacteria reign supreme, with two dominantdivisions -- known as Bacteroidetes and Firmicutes -- making upover 90 percent of the gut's microbial population. In contrast, theAchaea that exist in the gut are mostly composed of methanogens(producers of methane) and specifically by Methanobrevibactersmithii -- a hydrogen-consumer. Within the bacterial categories however, enormous diversity exists.Each individual's community of gut microbes is unique andprofoundly sensitive to environmental conditions, beginning atbirth. Indeed, the mode of delivery during the birthing process hasbeen shown to affect an infant's microbial profile. Communities of vaginal microbes change during pregnancy inpreparation for birth, delivering beneficial microbes to thenewborn. At the time of delivery, the vagina is dominated by a pairof bacterial species, Lactobacillus and Prevotella . In contrast, infants delivered by caesarean section typicallyshow microbial communities associated with the skin, including Staphylococcus, Corynebacterium , and Propionibacterium . While the full implications of these distinctions are stillmurky, evidence suggests they may affect an infant's subsequentdevelopment and health, particularly in terms of susceptibility topathogens. Diet and destiny After birth, diet becomes a critical determinant in microbialdiversity within the gut. Recent research indicates that microbialpopulations vary geographically in a manner consistent withregional differences in diet. Children in rural areas of BurkinaFaso for example showed much more abundant concentrations ofBacteroidetes compared with their cohorts in Italy, a findingconsistent with the African children's plant-rich diet. While microbiomes appear to have adapted to local diets, changes ineating habits significantly alter composition of gut microbes.Variations in macronutrient composition can modify the structure ofgut microbiota in a few days -- in some cases, a single day.Studies in mice show that changing from a low fat, plantpolysaccharide diet to a Western diet high in sugar and fat rapidlyand profoundly reconfigures the composition of microbes in the gut. Another modifier of gut microbe composition is gastric bypasssurgery, used in certain cases to alleviate conditions of seriousobesity. In earlier work, the authors found that the post-surgicalmicrobial composition of patients who underwent so-called Roux-en-Ygastric bypass was distinct from both obese and normal weightindividuals. "Obesity affects more than a third of adults in the U.S. andis associated with a raft of health conditions including heartdisease, stroke, type 2 diabetes and certain forms of cancer,"says Dr. John DiBaise. The authors further note that concentrationsin the blood of lipopolysaccharides derived from gut bacteriaincrease in obese individuals, producing a condition known asmetabolic endotoxemia. The disorder has been linked with chronic,systemic, low-level inflammation as well as insulin resistance. Energy harvest In the current review, the cycle of microbial energy extractionfrom food, involving hydrogen-producing and consuming reactions inthe human intestine, is described in detail. Short chain fattyacids (SCFAs) are a critical component in this system. During thedigestive process, fermentation in the gut breaks down complexorganic compounds, producing SCFA and hydrogen. The hydrogen iseither excreted in breath or consumed by 3 groups of microorganismsinhabiting the colon: methanogens, acetogens and sulfate reducers. Research conducted by the authors and others has demonstrated thathydrogen-consuming methanogens appear in greater abundance in obeseas opposed to normal weight individuals. Further, the Firmicutes --a form of acetogen -- also seem to be linked with obesity.Following fermentation, SCFAs persist in the colon. Greaterconcentration of SCFAs, especially propionate, were observed infecal samples from obese as opposed to normal weight children.(SCFAs also behave as signaling molecules, triggering theexpression of leptin, which acts as an appetite suppressor.) While it now seems clear that certain microbial populations helpthe body process otherwise indigestible carbohydrates and proteins,leading to greater energy extraction and associated weight gain,experimental results have shown some inconsistency. For example,while a number of studies have indicated a greater prevalence ofBacteroidetes in lean individuals and have linked the prevalence ofFirmicutes with obesity, the authors stress that many questionsremain. Alterations in gut microbiota are also of crucial concern for theone billion people worldwide who suffer from undernutrition.Illnesses resulting from undernutrition contribute to over half ofthe global fatalities in children under age 5. Those who do surviveundernutrition often experience a range of serious, long-termmental and physical effects. The role of gut microbial diversityamong the undernourished has yet to receive the kind ofconcentrated research effort applied to obesity -- a disease whichhas reached epidemic proportions in the developed world. Exploiting microbes affecting energy extraction may prove a usefultool for non-surgically addressing obesity as well as treatingundernutrition, though more research is needed for a fullunderstanding of regulatory mechanisms governing the delicateinterplay between intestinal microbes and their human hosts. Dr. Krajmalnik-Brown and colleagues at the Biodesign Institute andMayo Clinic are currently in the second year of an NIH-funded studyto better understand the role of the gut microbiome in the successor failure of surgical procedures performed to treat obesityincluding the Roux-en-Y gastric bypass, adjustable gastric band andvertical sleeve gastrectomy. 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