3 Ways Gut Bacteria Affect Body Weight

3 Ways Gut Bacteria Affect Body Weight

There was a time when most people thought of bacteria only as microscopic organisms that cause infections. Boy, have times changed! Now we know our guts are lined with trillions of gut bacteria that help keep our digestive tract and immune system healthy and balanced. In fact, 70% of your immune system lies in your gut and is referred to as GALT, gut-associated lymphoid tissue. These “friendly” probiotic bacteria have a variety of potential health benefits. In fact, gut flora, the bacteria that make up your intestinal tract, also referred to as your microbiome, likely impact your body weight. Let’s see how these tiny organisms, visible only with a microscope, collectively impact your body weight.

Appetite Control

Can you blame your cravings on gut bacteria? It seems far-fetched but not necessarily. There’s growing evidence that gut bacteria increase your cravings for the very foods that best support their growth. If you happen to have bacteria in your gut that thrive on sugar, guess what you might crave? Sugary foods, of course. How might bacteria exert their effects? By influencing “signaling molecules,” proteins released by cells that allow cells to communicate with each other.

The desire to eat is partially regulated by appetite hormones that feedback to the brain and by brain chemicals called neurotransmitters, as well as by a nerve that extends all the way from the brain to the intestinal tract called the vagus nerve. It’s your vagus nerve that controls how quickly food moves through your stomach and intestinal tract. Experts believe one way gut bacteria influence appetite, food cravings, and what you choose to eat is by interacting with the vagus nerve.

As Athena Aktipis, Ph.D., co-founder of the Center for Evolution and Cancer with the Helen Diller Family Comprehensive Cancer Center at UCSF points out gut bacteria have the ability to alter nerve signals through the vagus nerve and produce chemicals that alter how food tastes – all in the name of getting the nutrients they need. Who knew bacteria could be so manipulative?

Inflammation, Gut Bacteria, and Obesity

Another way experts believe gut bacteria contribute to obesity is through their effect on the immune system. Gut bacteria have the power to alter the immune response. Remember how 70% of your immune system lies in your gut? If gut bacteria get out of balance or the wrong strains gain a foothold, your immune system can go into overdrive, triggering inflammation and a host of metabolic disturbances such as insulin resistance that leads to metabolic syndrome and obesity.

Nutrient Absorption

Another way gut bacteria may impact weight is by influencing nutrient absorption. As Claire Fraser, a professor at the University of Maryland School of Medicine points out some bacteria are more efficient at breaking down and processing the food you take in. These uber-efficient bacteria make it easy for you to absorb more nutrients and more calories from the food you eat. If you have super-efficient bacteria you might process the full 300 calories from a 300-calorie meal while someone with less efficient bacteria may only absorb 260 calories. This is a case where you want less efficient bacteria, although you may take in fewer nutrients as a result.

Keeping a Healthy Balance

Now that you know the importance of gut bacteria and having a healthy microbiome, what can you do to encourage healthy gut bacteria to live in your gut, ones that are less likely to cause weight gain? Start by making smarter food choices. What you eat affects what bacteria flourish in your gut and it’s not as simple as just eating yogurt.

Do you eat a Western diet? When mice consume a typical Western diet, high fat, and sugar, their gut flora changes into strains that are “obesogenic ” particularly a strain called Firmicutes – it happens fast too, within a day. Even more interesting, when researchers transferred stool from obese humans to mice bred to be free of this gut bacteria, the mice became obese. This suggests that what you eat influences the strains of gut bacteria you have. Based on animal studies, you can change your gut bacteria to less desirable forms quickly by eating the wrong diet.

People who are less healthy or obese also tend to have a less diverse population of gut bacteria. Interestingly, research links eating a Western diet with fewer gut bacteria diversity. This isn’t surprising since the Western diet is low in fiber, the material gut bacteria feed on and contains few fermented foods, a source of probiotic bacteria. Bacteria love to break down fibrous plant material that we can’t completely digest.

The Possibility of Fecal Transplants

Is it possible to get the “good” bacteria you need for weight control?  With growing evidence that gut bacteria play an active role in body weight, researchers are looking into the possibility of doing fecal transplants to transfer bacteria from lean people to those who are obese. You may not be a candidate for a fecal transplant nor would you necessarily want one. Take a less radical approach – change your diet.

As mentioned, a diversity of gut bacteria linked with overall health. To keep your microbiome as diverse as possible is to eat more fiber-rich foods, including fruits, legumes, vegetables, and whole grains, as well as fermented foods like yogurt with active cultures, miso, kefir, and fresh sauerkraut. Only take antibiotics when you really need them. Antibiotics quickly disrupt the balance inside your gut, which is why so many people get diarrhea when they take them, and it can take months to restore balance. If you are forced to take antibiotics, consider taking a probiotic supplement along with it.

The Bottom Line

Behind the scenes, in the dark recesses of your gut, the gut bacteria that make up your microbiome may be subtly, or not so subtly, influencing your appetite and metabolism. Your best defense at this point is to encourage a diversity of bacteria to take up residence in your intestinal tract by eating fiber-rich, whole foods and fermented foods. Maybe one day, you’ll be able to take a supplement with the exact microbes you need for healthy body weight. Until then, feed your gut plenty of whole foods and fiber and enjoy more fermented foods.



Nutrition Review. “How Gut Bacteria Affect Your Mind and Appetite to Influence Which Foods You ‘Choose’ to Eat” August 16, 2014.

Scientific American. “How Gut Bacteria Help Make Us Fat and Thin” June 1, 2014.

Live Science. “The Microbes in Your Gut May be Making You Fat” December 13, 2013.


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