Inside your intestines reside trillions of microorganisms that collectively make up your gut microbiome—and can weigh up to a typical 3 pounds—but far from being a source of undue weight, your gut microbes might be the key to staying slim and fit.

Gut bacteria have been well-studied for their important role in regulating normal immune and intestinal barrier function. Now growing research is improving our understanding in how microbiota affect energy balance including regulation of insulin and fat storage (1-3). These findings might lead to new, promising methods for supporting weight management in the future.

The relationship we have with our gut microbes is thought to be mutually symbiotic, or that their existence within supports us being their hosts (2). We rely on them for a variety of functions that includes crowding out potentially harmful bacteria, acting as part of our immune systems, and through the fermentation of undigested parts of food helping to produce vitamins, short-chain fatty acids, and creating a protective mucus lining of our intestines.

What You Eat Affects Your Microbes

Our hungry gut microbes may, in fact, extract up to half of the calories of the foods we eat. Although it can depend heavily on the types of foods in our diet. A diet that is rich in raw and unprocessed fruits and vegetables will contain a higher amount of prebiotic digestion-resistant starches (such as oligosaccharides) that are more likely to reach the large intestine, where a greater majority of our gut microbes are present (1).

Conversely, a typical Western-style diet that is comprised of processed foods that are high in fat, refined carbohydrates, and sugars not only makes us more likely to absorb caloric energy—leading to weight gain—but presents little nourishment to gut microbes.

The types of microbes in your gut, and their diversity, also plays a role in how food is digested and absorbed. Consequently, these factors can also play a role in the likelihood of how the gut microbiome affects energy balance and weight management.

In studies in mice, for example, the ratio of certain types of microbes—Bacteriodetes and Firmicutes—are found to regulate fat storage and weight gain (1). In obese mice and mice fed a Western-style diet, scientists observed an increase in abundance of Firmicutes with a concomitant decrease in the abundance of Bacteriodetes (1). The reason is thought to be related to how enzymes produced by these microbes affect the metabolism of both lipids and carbohydrates.

Several studies in humans have also found associations between low levels of the bacterial genus Bifidobacterium with being overweight or obese. Similarly, some levels of bacteria, such as Clostridium, reduced in proportion with weight loss.

These associations can only provide clues into the complexity that gut microbes might contribute to weight management. Based on the existing evidence, it’s difficult to reach conclusions on how interventions targeting gut microbiota could be used to deliver changes to the gut microbiome that would be favorable for bodyweight. It’s also far too early to blame obesity on any single bacterial species in a person’s gut microbiome.

Diet and Microbe Diversity Matters

Gathered from the latest scientific evidence, one bit of advice is clear, however: we should all strive to consume diets that encourage microbial diversity for better health and weight management. This recommendation doesn’t involve adhering to any specific eating plan so long as that it includes plenty of high-fiber and a variety of resistant starches that act as prebiotic foods for microbes.

The reason is that studies have consistently shown that a Western-style high-fat-and-sugar diet that is low in fiber and resistant starches is associated with a reduction in microbial diversity (1). It also favors the types of microbes that don’t ferment complex carbohydrates and fibers efficiently. Conversely, following a diet that’s rich and diverse in fruits and vegetables can rapidly reverse losses of microbial diversity (1, 4). Such as diet can also lead to a greater abundance of microbes that more efficiently ferment fibers and digestion-resistant starches.

The end-goal is not only calorie control and weight management. A rich and diverse diet that supports microbial diversity is also associated with improved digestive and long-term overall health.

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  1. Stephens Rw, Arhire L, and Covasa M. Gut microbiota: from microorganisms to metabolic organ influencing obesity. Obesity. 2018 May; 26(5): 801-809.
  2. Xu J, Gordon JI. Honor thy symbionts. Proc Natl Acad Sci USA 2003;100:10452‐10459.
  3. Dhurandhar EJ, Keith SW. The aetiology of obesity beyond eating more and exercising less. Best Pract Res Clin Gastroenterol 2014;28:533‐544.
  4. David LA, Maurice CF, Carmody RN, et al. Diet rapidly and reproducibly alters the human gut microbiome. Nature. 2014 Jan 23; 505(7484): 559-563.