Forget calories and exercise for a minute and focus on the latest weapon in the war against obesity: bacteria. There’s an entire community consisting of billions of microscopic organisms (microbes) that have made the human gut their home. Without these microbes we wouldn’t be alive today— they help us digest food, make some of the vitamins our body needs, influence endocrine functioning and enhance immune system defenses. But more recent research suggests that the collective population of bacteria in the gut, called microflora, can also keep us lean.
Bacteria Alone May Influence Weight
In 2006, a groundbreaking study published in the journal Nature uncovered a relationship between gut bacteria and weight. Scientists at Washington University in St. Louis found that lean and obese people had a very different makeup of gut microflora. And the microflora in obese people changed and more closely resembled their lean counterparts when they lost weight. In more recent years, scientists discovered the same phenomena in obese people after gastric bypass surgery— their gut microflora changed considerably. Yet these studies couldn’t identify which came first— does weight loss change gut microflora or, does gut microflora impact a person’s weight and ability to lose weight? To explore this topic in more detail, the same group of scientists conducted a study in a population they could control at all times— rodents.
Knowing obese and lean humans have a considerably different makeup of gut microflora, the scientists took four pairs of twins to rule out any differences in childhood environment and diet that could influence their microbial makeup. Each one had an obese sibling and one lean sibling, and researchers transferred their gut bacteria into the intestines of germ-free mice. The mice that received the bacteria from the obese humans gained more weight and fat than the group that received bacteria from the lean humans, despite no significant differences in food intake between the groups, which suggests that bacteria independently influence weight.
Next they put the obese and lean mice in the same cage and took advantage of a rather disgusting aspect of rodent life— mice eat each other’s feces and therefore share microbes. After dining on their lean counterpart’s poop, bacteria from the guts of the lean mice, specific Bacteroidetes strains, took up residence in previously empty areas in the guts of obese mice, their metabolism improved and they lost weight. Yet the reverse situation didn’t occur: lean mice remained lean. So, in addition to discovering that bacteria from obese humans led to weight gain in previously germ-free mice, they also discovered that transferring bacteria from lean rodents to obese rodents facilitated weight loss.
Taking the research a step further, they examined the effect of diet on microbial transfer. When the obese mice ate a low-fat, high-fiber diet, the beneficial transfer of microbes from the lean mice to the obese mice occurred. However, when given a more typical high saturated fat, low-fiber American diet, the beneficial microbes didn’t populate in the guts of the obese mice, which indicates that this transfer is diet dependent.
While the results of this rodent study suggest a low-fat, high-fiber diet combined with bacterial transfers may be the ticket to weight control, human studies have come to slightly different conclusions. In a study examining how diet affects weight loss in obese humans, scientists found equally beneficial effects resulting from a low-calorie fat-restricted diet and a low-calorie carbohydrate-restricted diet. Both diets resulted in significant increases in Bacteroidetes and proportional decreases in Firmicutes after one year.
Also, a 10-week calorie-restricted diet (10-40% reduction) combined with physical activity resulted in microflora-dependent differences in weight loss in obese adolescents. At the end of the intervention period, subject results were divided into two groups, a low weight-loss group (< 4.4 lbs with an average weight loss of 3.08 lbs) and a high weight-loss group (> 8.8 lbs with an average weight loss of 14.96 lbs).
Despite the significant between group differences in weight loss and changes in BMI in the high weight-loss group only, there were no differences in reported calorie intake, macronutrients or food groups between the low weight-loss group and high weight-loss group. However, prior to the dietary intervention, the low and high weight-loss groups had a significantly different makeup of gut microflora, which suggests that the bacterial makeup of their guts influenced their ability to lose weight.
At the end of the study, there were no significant changes in bacterial count for any of the analyzed bacterial groups in the low weight-loss group. However, gut microflora changed significantly in the high weight-loss group and some of these changes were diet dependent. Reduced carbohydrate content resulted in a decrease in beneficial Bacteroides fragilis. Likewise, a reduction in polyunsaturated fats, the kind found in nuts, seeds and oils made from nuts and seeds, decreased Lactobacillus group counts. And therefore, while gut microflora at the start of the study seemed to influence the adolescents’ weight loss, aspects like dietary intervention, decreasing calorie intake, increasing calories burned through physical activity and weight loss also led to changes in gut bacteria at the end of the study. In addition, they found that interactions between gut microbiota and bodyweight changes may be dependent on diet and lifestyle to different extents based on an individual’s microbiota makeup.
Despite the fascinating conclusions drawn from this study, the authors pointed out that they can’t be completely sure that the kids in the low weight-loss group really adhered to the diet and accurately reported their dietary intake, both very common confounding issues in weight-loss intervention studies in obese humans.
Change Your Diet
Because we are years away from a targeted pharmacological or dietary approach to selectively increasing specific types of bacteria in the gut, changing your diet is the best approach right now. And, what you eat can help feed as well as populate gut microflora. In particular, prebiotics are specific types of fiber that selectively stimulate the growth of beneficial bacteria in the gut. Natural sources of various prebiotic fibers include apples, berries, onions, leeks, garlic, whole grains, wheat, legumes, asparagus and other plant foods. In addition, some packaged foods contain inulin or oligosaccharides, both of which do triple duty by increasing the fiber content of a food while lending a sweet flavor and stimulating the growth of good bacteria. Look for both of these under the ingredient list.
In addition to consuming greater amounts of prebiotics, you may also want to consider probiotics— live bacteria that can temporarily populate your gut. However, choosing probiotics is complicated because there are numerous types, each with a different function in the body. And many strains and species don’t have good research to back their use while some studies used mixtures of probiotics, making it difficult to determine which ones had an effect or if there was a synergistic effect among the different types. So, before you start popping probiotic supplements, know which specific strain and species you need for the intended effect. For instance, there are probiotics on the market that decrease the duration of infant diarrhea, decrease the duration of antibiotic-associated diarrhea, decrease constipation and support immune health. If you are looking for immune support, choosing a probiotic strain and species that helps with infant diarrhea probably won’t do much for you. And although probiotics are considered safe for most people, talk to your physician first— especially if you’ve been seriously ill or you have a compromised immune system.
If you are looking for an alternative to probiotic supplements, choose yogurt. Live strains of bacteria are added to fluid milk to make yogurt. However, not all yogurts are created equally— so look for one that says “live cultures” or has a “Live and Active Cultures” seal on it from the National Yogurt Association, because the bacteria needs to be alive to be beneficial. Top your yogurt with whole grains and you’ll end up with an excellent meal that not only feeds the bacteria in your gut but also populates bacteria there as well. Other foods that naturally contain probiotics include unpasteurized sauerkraut, miso soup (made from fermented soybean paste), fermented soft cheese like Gouda and kefir (a fermented milk drink).
Weight control is multi-faceted, and research is just beginning to uncover the complex relationships between diet, gut microflora and weight. And though poop transfers aren’t completely out of the question in humans— physicians actually perform these now in people suffering from Clostridium difficile (C. diff) bacterial infection, targeted drug or dietary interventions will likely be the wave of the future. Until then, focus on eating a diet that promotes a healthy gut and if you have any weight to lose, it’s possible that you may also lose a few pounds in the process. In particular, a high-fiber diet will help feed the bacteria in your gut and including different types of probiotic rich foods will continually populate your gut with beneficial bacteria.
Ridaura VK, Faith JJ, Rey FE, Cheng J, Duncan AE et al. Gut microbiota from twins discordant for obesity modulate metabolism in mice. Science 2013;341:6150.
Turnbaugh PJ, Ley RE, Mahowald MA, Magrini V et al. An obesity-associated gut microbiome with increased capacity for energy harvest. Nature 2006;444:1027-1031.
Bisanz JE, Reid G. Unraveling how probiotic yogurt works. Sci Transl Med 2011;3:106.
Dhurandhar NV, Geurts L, Atkinson RL et al. Harnessing the beneficial properties of adipogenic microbes for improving human health. Obesity Reviews 2013;19:721-735.
Delzenne NM, Neyrinck AM, Bäckhed F, Cani PD. Targeting gut microbiota in obesity: effects of prebiotics and probiotics. Nat Rev Endocrinol 2011;7(11):639-46.
Furet JP, Kong LC, Tap J et al. Differential adaptation of human gut microbiota to bariatric surgery-induced weight loss: links with metabolic and low-grade inflammation markers. Diabetes 2010;59:3049-3057.
Ley RE, Turnbaugh PJ, Klein S, Gordon JI. Microbial ecology: human gut microbes associated with obesity. Nature 2006;444: 1022–1023.
Santacruz A, Marcos A, Warnberg J et al. Interplay Between Weight Loss and Gut Microbiota Composition in Overweight Adolescents. Obesity 2009;17:1906–1915.