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You might be familiar with “calories in, calories out” (or CICO) as an approach to losing weight or maintaining a healthy weight. The idea is that if you eat less food than your body needs you will lose weight, and if you eat more food then your body needs you’ll gain weight. Seems simple enough, right? In reality, simply calculating your daily calories and filling your diet with foods that fit the calorie recommendations might not be the best approach. Is a calorie just a calorie, or does the type of food make a difference? The answer is complicated—thanks to the bacteria living in our gut.

While some evidence supports putting your body in a calorie deficit to cause weight loss, there’s more to the story. Cravings, hunger regulation, and how the body breaks down food to turn it into energy all have bigger roles in weight loss than the number of calories listed on nutrition labels. (1) When we eat, the enzymes in our mouths and digestive tracts break food down into smaller pieces. Only the smallest pieces are absorbed and utilized for energy. The undigested food is removed from the body. How our food interacts with our gut bacteria after a meal is the complicated part.

Gut bacteria play a role in the food we eat, and therefore how well we are able to maintain a healthy body weight. They might be involved in our cravings and the feeling of satisfaction we experience from eating certain foods, especially those with added fat and sugar. While the exact mechanism is still not understood, experts hypothesize that the variety and diversity of bacteria in the gut influence how the host reacts to certain foods. A particular food may be more satisfying or impact cravings for certain foods between meals. (2) Unfortunately, this is most commonly caused by the “bad bacteria,” which may create stronger cravings for and satisfaction from poor food choices like sweetened beverages and fried food. (2) As if cravings aren’t already difficult to ignore, our gut bacteria add a megaphone to their cries for that afternoon cookie you have been trying to break up with.

Gut bacteria are also involved in how food is digested and the utilization and creation of certain nutrients. It is well known by researchers that intestinal gut bacteria synthesize vitamin B12 and vitamin K2 in the body, but their role in carbohydrate, fat, and protein utilization is showing to be just as important. (3,4)

Because our bodies naturally lack the ability to break down complex fiber, some carbohydrates are hard for us to digest. This is why a cow is able to survive on a diet of grass, but humans are not. Cows have the necessary enzymes, not to mention several stomachs, to aid in the breakdown of complex fibers in grass. For humans, foods like whole grains and vegetable fibers are similar to grass because they contain indigestible fibers that the human digestive system cannot fully process. Although these fibers are indigestible to our enzymes, bacteria in the gut can break down complex fibers for further utilization. As bacteria break down fibers, they produce gases and something called short-chain fatty acids (SCFAs).

Short-chain fatty acids may reduce the risk of disease, and are important for maintaining the intestinal lining of the colon and stimulating leptin production. (5,6,7) Leptin is a hunger hormone that sends a “stop eating” signal to the brain when the stomach is full. This signal, if working properly, will help you avoid that uncomfortable overstuffed feeling. But, when there is an imbalance or a lack of good bacteria in the gut, leptin might not be able to do its job, leading to overeating—and eventually weight gain.

Although it’s not as well researched as carbohydrates, there is also an association between gut bacteria and protein synthesis and fat metabolism. Researchers found that gut bacteria affect protein metabolism by initiating the breakdown of proteins, as well as producing essential amino acids (the building blocks of protein). (8,9) While this doesn’t mean that we need to undermine the importance of dietary protein, it might mean that we need to focus on the health of our gut bacteria to ensure we are getting the most out of our post-workout protein shake.

Similar to protein, the association with gut bacteria and fat is not fully understood. A 2012 study found that the presence of certain microbes, specifically Firmicutes, in the gut stimulates the absorption of fat in zebrafish. (10) While this is not a human study, it does offer insight into how bacteria in the gut affect fat absorption, especially because levels of Firmicutes (in humans) have been found to increase with body mass index (BMI).

The relationship between weight gain and the bacteria in the gut is similar to the chicken and the egg debate. Is weight gain and obesity to blame for an unhealthy gut microbiome, or does our gut microbiome contribute to how quickly we gain weight? The simple answer is that they both affect each other. If the diet disrupts the gut microbiome, the gut microbiome will suffer and therefore be unable to help the body digest and metabolize properly. There is also a possibility that a damaged gut microbiome (due to illness, stress, intolerance, and medications like antibiotics) can weaken the population of gut bacteria and create imbalances, leading to disrupted hunger cues and poor nutrient utilization. (11)

Several mouse studies have reported that transferring the gut bacteria from an obese subject to a lean subject can cause weight gain in the lean subject. (12) Although human studies replicating the methods in these mouse studies have not been conducted, researchers have been able to differentiate between the microbe composition of obese individuals and that of lean individuals. This might mean that if obese individuals work to change the composition of their gut microbiomes, they may also be able to alter their ability to break down fiber-rich foods and be more aware of their fullness and hunger cues. (13)

As you can see, there is still a lot to learn about the relationship and various jobs of bacteria related to metabolism and utilizing foods. But what is unquestionable is the importance of maintaining a lifestyle to support the health and stability of our gut bacteria when setting weight-loss goals or starting a new health plan. Ready to start your gut health journey? Check out this article: Three Simple Ways to Take Care of Your Gut.

About the Author

Registered Dietitian Nutritionist Rachel Stuck has a background in culinary arts and nutrition counseling. Rachel takes a positive approach to nutrition: she avoids recommending restrictive diets and instead focuses on helping people choose foods that promote health and well-being. She is passionate about empowering and assisting Ixcela members as they develop their unique, gut-healthy lifestyles.

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Resources

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2. Alcock, Joe, et al. “Is Eating Behavior Manipulated by the Gastrointestinal Microbiota? Evolutionary Pressures and Potential Mechanisms.” BioEssays, John Wiley & Sons, Ltd, 8 Aug. 2014, onlinelibrary.wiley.com/doi/full/10.1002/bies.201400071.
3. Martens, J H, et al. “Microbial Production of Vitamin B12.” Applied Microbiology and Biotechnology, U.S. National Library of Medicine, Mar. 2002, www.ncbi.nlm.nih.gov/pubmed/11935176.
4. Conly, J M, and K Stein. “The Production of Menaquinones (Vitamin K2) by Intestinal Bacteria and Their Role in Maintaining Coagulation Homeostasis.” Progress in Food & Nutrition Science, U.S. National Library of Medicine, Oct. 1992, www.ncbi.nlm.nih.gov/pubmed/1492156.
5. Ríos-Covián, David, et al. “Intestinal Short Chain Fatty Acids and Their Link with Diet and Human Health.” Frontiers in Microbiology, Frontiers Media S.A., 17 Feb. 2016, www.ncbi.nlm.nih.gov/pubmed/26925050.
6. Tan, Jian, et al. “The Role of Short-Chain Fatty Acids in Health and Disease.” Advances in Immunology, U.S. National Library of Medicine, 2014, www.ncbi.nlm.nih.gov/pubmed/24388214.
7. Xiong, Yumei, et al. “Short-Chain Fatty Acids Stimulate Leptin Production in Adipocytes through the G Protein-Coupled Receptor GPR41.” Proceedings of the National Academy of Sciences of the United States of America, National Academy of Sciences, 27 Jan. 2004, www.ncbi.nlm.nih.gov/pubmed/14722361.
8. Macfarlane, G T, et al. “Protein Degradation by Human Intestinal Bacteria.” Journal of General Microbiology, U.S. National Library of Medicine, June 1986, www.ncbi.nlm.nih.gov/pubmed/3543210.
9. Morowitz, Michael J, et al. “Contributions of Intestinal Bacteria to Nutrition and Metabolism in the Critically Ill.” The Surgical Clinics of North America, U.S. National Library of Medicine, 1 Aug. 2011, www.ncbi.nlm.nih.gov/pmc/articles/PMC3144392/.
10. Science, Carnegie. “Gut Bacteria Increase Fat Absorption.” Gut Bacteria Increase Fat Absorption | Department of Embryology, Carnegie Science, emb.carnegiescience.edu/news/gut-bacteria-increase-fat-absorption.
11. Krajmalnik-Brown, Rosa, et al. “Effects of Gut Microbes on Nutrient Absorption and Energy Regulation.” Nutrition in Clinical Practice : Official Publication of the American Society for Parenteral and Enteral Nutrition, U.S. National Library of Medicine, 27 Apr. 2012, www.ncbi.nlm.nih.gov/pmc/articles/PMC3601187/.
12. Bell, David S H. “Changes Seen in Gut Bacteria Content and Distribution with Obesity: Causation or Association?” Postgraduate Medicine, U.S. National Library of Medicine, 16 Oct. 2015, www.ncbi.nlm.nih.gov/pubmed/26474235.
13. Perry, B, and Y Wang. “Appetite Regulation and Weight Control: the Role of Gut Hormones.” Nature News, Nature Publishing Group, 16 Jan. 2012, www.nature.com/articles/nutd201121.