For decades, scientists have known that the human body is host to trillions of commensal microbes, bacteria, yeast, and other organisms living in our bodies that confer benefits and cause no harm (Ciorba, 2012). The human gut is home to so many bacteria that bacterial cells outnumber host cells by 10 times. Different strains of bacteria offer different benefits. Bacteria may assist with digestion, immune function, and even production of some vitamins. Byproducts of bacterial fermentation may even provide 10% of a person's energy needs. Interestingly, poor nutrition, antibiotic use, and other factors may kill off these helpful bacteria causing a variety of problems. Only recently has the relationship between the microbiota of the gut and obesity been studied (Backhed et al., 2004).

Some scientists argue that the fetus is sterile and that, through the process of natural childbirth, breastfeeding, and introduction of foods, the gut becomes populated with a healthy microbial flora (Dai & Walker, 1999). The types of bacteria living in the gut can be grouped into two subcategories: Bacteroidetes and Firmicutes. Observations in both animals and human research suggest that leaner individuals have more Bacteroidetes and a lower ratio of Firmicutes/Bacteroidetes than subjects who are affected by obesity (Sanchez, Panahi, & Tremblay, 2014). Studies also show that dietary changes influence these levels and that changing the ratio of these types of bacteria in the gut influences food intake, appetite, and body composition (Sanchez, Panahi, et al., 2014). Microbes that are purposely introduced to the gut for health benefits are called probiotics. This column provides a brief overview of what are probiotics, as well as related prebiotics and synbiotics, and how they affect obesity as well as safety, recommendations, and food sources.

Probiotics are live microorganisms that are thought to impart health benefits to the host (National Center for Complimentary and Integrative Health, National Institutes of Health, 2016). They may be supplemented in the host via fermented foods or via dietary supplements. Probiotics must survive transit through the gastrointestinal tract and reach the intestine alive to impart these benefits (Ciorba, 2012). Fermented foods rich in probiotics include kefir (fermented yogurt drink), yogurt, miso, tempeh, and sauerkraut. Not all yogurts with live cultures are considered probiotic foods as many contain insufficient microbes to impart health benefits; on the other hand, some yogurt is specifically supplemented with additional bacteria and is deemed probiotic food.

Prebiotics can be thought of as the "fertilizer" for the gut microbiota. Prebiotics are the nondigestible fibers or foods that promote the growth of the gut flora and help it to thrive. However, not all fibers are prebiotics. Examples of prebiotics are inulin and oligosaccharides (Niness, 1999). Foods naturally high in prebiotics include artichokes, onions, garlic, leeks, legumes, chicory root, peaches, nectarines, bananas, soy beans, asparagus, grapefruit, and watermelon. Chicory root and inulin are also used to increase fiber levels in processed foods such as high-fiber snack bars and pastas. Finally, synbiotics are combinations of probiotics and prebiotics that work together synergistically (Saez-Lara, Robles-Sanchez, Ruiz-Ojeda, Plaza-Diaz, & Gil, 2016).

In a 6-month, randomized controlled clinical trial of 225 volunteers in Finland, diets supplemented with a probiotic (Bifidobacterium animalis ssp. lactis 420) decreased body fat mass and body weight versus placebo (Stenman et al., 2016). Supplementing with a probiotic and a prebiotic synergistically decreased fat mass and increased lean body mass versus placebo. Participants showed positive improvement in waist circumference in those treated with probiotic (with or without prebiotic) and decreased energy intake in the probiotic groups (with or without prebiotic) after 6 months.

Sanchez et al. in Canada randomized a group of adult men and women to receive a probiotic supplement (Lactobacillus rhamnosus CGMCC1.3724 [LPR]) with prebiotics (to help improve survival of the bacteria) twice daily versus a control group treated with placebo (Sanchez et al., 2014). Both groups were placed on energy-restricted diets (500 calories less than each individual's measured resting energy expenditure and predicted energy for activity). After 12 weeks of restriction, subjects were given a new meal plan for weight maintenance and were followed for an additional 12 weeks. Interestingly, the group as a whole did not lose weight, but there was a distinct difference in the men and women in the study group versus their counterparts in the control group. The women lost a significant amount of weight and kept losing weight in the subsequent 12 weeks after the probiotic was ceased versus female controls. In men, there was no significant difference between the control and treatment groups. This suggests a positive weight loss benefit for women taking this particular probiotic, with potentially lasting effects for sustained weight loss.

Prebiotics alone have shown some positive effects promoting healthier weights. In another Canadian randomized controlled clinical trial, subjects were randomized to receive a placebo or prebiotic fiber (oligofructose) 21 g per day (Parnell & Reimer, 2009). After 12 weeks, participants had statistically significant weight loss in the treatment group, whereas controls gained weight. The weight loss was primarily fat loss and came from the trunk region of the body. Furthermore, the treatment group reduced their caloric intake and experienced decreases in ghrelin (a gut hormone that stimulates appetite) and increases in PYY (gut hormone that increases satiety).

These studies along with numerous related research suggest promising effects with supplementing the diet with probiotics and prebiotics. Mechanisms proposed to explain these beneficial effects include decreasing insulin resistance, improved carbohydrate metabolism, increasing leptin levels, decreasing ghrelin levels, increasing PYY levels, and improving gut barrier function (Saez-Lara et al., 2016; Stenman et al., 2016).

In summary, probiotics and prebiotics, suggested in numerous studies, have a positive, albeit small, effect on improving body composition, although more research is needed to determine how they work and what barriers may keep them from working. Probiotics are generally considered as safe to use with most populations and have few complications. However, exceptions to this rule would include critically ill and immune-compromised patients and those with indwelling central vein catheters or possibly cardiac valve disease as there have been a few case reports of systemic infections and endocarditis (Doron & Snydman, 2015). Although prebiotics are safe to use in most populations, there are some individuals with certain gastrointestinal conditions that may not tolerate large amounts of prebiotic fibers. In addition, it is important to keep in mind that probiotics do not always colonize the gut; therefore, to see lasting effects, one needs to continue eating the probiotic foods or taking the probiotic supplements indefinitely (Ciorba, 2012), while eating a diet rich in prebiotic fibers to provide the "fertilizer" to help feed these microbes needed nutrients. Finally, probiotics are a dietary supplement, which means that they are not closely regulated by the Food and Drug Administration for quality and purity; the Food and Drug Administration relies on manufacturer's quality control testing.

Eating a diet rich in probiotic foods and prebiotics may help you and your patients maintain a healthy gut microflora and a healthy weight.

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