Your body contains trillions of bacteria.

The majority of these bacteria are located in your intestines.

Gut bacteria play several important roles in your health, such as communicating with your immune system and producing certain vitamins.

Your gut bacteria can also affect how different foods are digested and produce chemicals that help make you feel full. As a result, they can affect your weight.

This article explains how your gut bacteria affect your weight and what foods promote healthy gut bacteria growth.

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Trillions of bacteria and microorganisms live on your skin and in your body (1, 2).

In fact, there are likely more bacterial cells in your body than human cells.

It is estimated that in a 154-pound (70-kg) man, there are around 40 trillion bacterial cells and only 30 trillion human cells (3).

Most of these bacteria live in a part of your large intestine called the cecum.

There are hundreds of different types of bacteria in your intestines. While some may cause disease, most of them carry out essential tasks to keep you healthy (4).

For example, your gut bacteria produce certain vitamins, including vitamin K, and communicate with your immune system to help your body fight off infection (5, 6).

They also influence how you digest certain foods and produce chemicals that help make you feel full. Therefore, your gut bacteria may influence your weight (7, 8).

Summary Your body contains more bacterial cells than human cells. These bacteria are primarily located in your intestines and carry out essential tasks to keep you healthy.

Since your gut bacteria line your intestines, they come into contact with the food you eat. This may affect what nutrients you absorb and how energy is stored in your body.

One study examined the gut bacteria in 77 pairs of twins, one of whom was obese and one of whom was not.

The study found that those who were obese had different gut bacteria than their non-obese twins. In particular, obesity was associated with lower gut bacteria diversity, meaning there were fewer types of bacteria in the gut (9).

Other studies have shown that if the gut bacteria from obese people are put into mice, the mice gain weight. This suggests that gut bacteria could affect weight (10, 11).

This may be due to the effect of bacteria on the digestion of different foods.

For example, humans can’t digest fiber but certain gut bacteria can. By digesting fiber, these gut bacteria produce a number of chemicals that benefit gut health and possibly promote weight loss (12).

For example, a number of studies have shown that people with high fiber intake have a lower weight, which may be due to the role that gut bacteria play in digesting fiber (13, 14, 15).

A recent study found that the ratio of two types of bacteria in your intestines may determine how much weight you lose when given a particular diet.

These two bacteria are Prevotella, which digests fiber and carbohydrates, and Bacteroidetes, which people who eat more animal protein and fat have more of (16).

In this study, 62 people were given a high-fiber, whole grain diet for 26 weeks. Those who had more Prevotella in their intestines lost 5.1 pounds (2.3 kg) more body fat than those with more Bacteroidetes in their intestines (17).

Your gut bacteria also digest certain antioxidants found in plants known as flavonoids, which may help prevent weight gain (18).

Finally, your gut bacteria can influence how dietary fats are absorbed in the intestines, which may affect how fat is stored in the body (19).

Summary Your gut bacteria may affect your weight by influencing how different foods are digested in your body. Dietary fiber is digested by certain species of gut bacteria, which may aid weight loss.

Inflammation occurs when your body activates your immune system to fight infection.

It can also be caused by an unhealthy diet. For example, a diet containing too much fat, sugar or calories can lead to elevated inflammatory chemicals in the bloodstream and fat tissue, which may contribute to weight gain (20, 21).

Your gut bacteria play an important role in inflammation. Some species produce chemicals like lipopolysaccharide (LPS), which cause inflammation when they pass into the blood.

When mice are given LPS, they gain as much weight and have similar increases in blood sugar and insulin as mice fed a high-fat diet (22).

Therefore, certain gut bacteria that produce LPS and cause inflammation may contribute to weight gain and insulin resistance.

A study in 292 people found that those who were overweight had lower gut bacteria diversity and higher levels of C-reactive protein, an inflammatory marker in the blood (23).

However, some species of intestinal bacteria may reduce inflammation and prevent weight gain.

Bifidobacteria and Akkermansia are beneficial species of bacteria that help maintain a healthy gut barrier and prevent inflammatory chemicals from passing from the gut into the bloodstream (24).

Studies in mice have found that Akkermansia can reduce weight gain and insulin resistance by reducing inflammation (25).

Similarly, when mice were fed prebiotic fibers to help increase Bifidobacteria in the gut, weight gain and insulin resistance decreased without affecting energy intake (26).

This is a relatively new area of research. Therefore, it is still not clear how gut bacteria affect inflammation and weight in humans.

Summary Certain types of gut bacteria are essential for maintaining a healthy gut barrier and preventing inflammation, which can contribute to weight gain.

Your body produces a number of different hormones that affect your appetite, including leptin, ghrelin, peptide YY (PYY).

Some studies have shown that different bacteria in the gut can affect how much of these hormones are produced and whether you feel hungry or full (27, 28).

Short-chain fatty acids are chemicals that are produced when certain species of gut bacteria break down fiber. One of these is known as propionate.

One study in 60 overweight adults found that taking propionate for 24 weeks significantly increased levels of the hormones PYY and GLP-1, both of which influence hunger.

People who took propionate also had reduced food intake and reduced weight gain (29).

Other studies have shown that prebiotics supplements, which contain compounds that are fermented by gut bacteria, can have a similar effect on appetite (30).

People who ate 16 grams of prebiotics per day for two weeks had higher levels of hydrogen in their breath. This indicates gut bacterial fermentation, less hunger and higher levels of the hormones GLP-1 and PYY, which make you feel full (31).

Summary Your gut bacteria can produce chemicals that can help make you feel full. By affecting your appetite, your gut bacteria may play a role in your weight.

A number of different foods are good for gut bacteria, including:

  • Whole grains: Whole grains are grains that haven’t been refined. They're high in fiber, which is digested by healthy gut bacteria like Bifidobacteria and may aid weight loss (32).
  • Fruits and vegetables: Fruits and vegetables contain many different fibers that are good for gut bacteria. Eating an assortment of plant-based foods can improve gut bacteria diversity, which is linked to a healthy weight (33).
  • Nuts and seeds: Nuts and seeds also contain lots of fiber and healthy fats, which help support the growth of healthy bacteria in the gut (34).
  • Polyphenol-rich foods: These include dark chocolate, green tea and red wine. The polyphenols in these foods can’t be digested alone but are broken down by beneficial gut bacteria, promoting the growth of good bacteria (35).
  • Fermented foods: Fermented foods include yogurt, kombucha, kefir and sauerkraut. They contain beneficial bacteria like lactobacilli and can minimize other disease-causing bacteria in the intestines (36).
  • Probiotics: Probiotics aren’t necessary all of the time, but they can help restore healthy gut bacteria after an illness or course of antibiotics and may even aid weight loss (37).

On the other hand, eating some foods in excess may harm gut your bacteria, including:

  • Sugary foods: A diet high in sugar can stimulate the growth of certain unhealthy bacteria in the gut, which may contribute to weight gain and other chronic health disorders (38).
  • Artificial sweeteners: Artificial sweeteners such as aspartame and saccharin reduce beneficial bacteria in the intestines, which may contribute to high blood sugar (39).
  • Foods containing unhealthy fats: Healthy fats such as omega-3s support beneficial bacteria in the intestines, whereas too many saturated fats may contribute to the growth of disease-causing bacteria (40, 41).
Summary Whole grains, fruits, vegetables and fermented foods all support healthy gut bacteria, while too many sugary foods, artificial sweeteners and unhealthy fats can be bad for your gut bacteria.

Your body contains trillions of bacteria that influence your health in many ways.

Your gut bacteria can affect how your food is digested, how fat is stored and whether you feel hungry or full.

Thus, a healthy gut bacteria may be important for maintaining a healthy weight.

High-fiber foods like whole grains, fruits, vegetables, nuts and seeds can all promote healthy gut bacteria.