Intestinal or Gut Bacteria and Obesity: A Possible Connection
Obesity and Gut Bacteria
Obesity is a complex condition that may have multiple causes. It's becoming frighteningly common in several parts of the world, including North America. The condition increases the risk of serious health problems, including heart disease, strokes, osteoarthritis, type 2 diabetes, and some types of cancer. Obesity can be a serious drain on public health budgets as communities try to treat the problems that it causes.
Scientists have recently discovered that certain gut bacteria may contribute to—or even cause—obesity. Gut bacteria live in our intestine. The majority inhabit our large intestine, or colon (the longest part of the large intestine). Many intestinal bacteria are very helpful to us; some seem to have neutral effects on our lives; and a few are harmful.
The discovery that gut bacteria may affect body weight could have enormous repercussions in the treatment of obesity. In the future, it may be possible to manipulate the gut environment and its living contents to help normalize weight.
In 2016, more than 1.9 billion adults, 18 years and older, were overweight. Of these over 650 million were obese.— WHO (World Health Organization)
Definition of Obesity
There is a difference between being overweight and being obese. The definition of obesity is usually based on a number called the body mass index, or BMI. This number is derived from a person's height and weight. There are online calculators which enable a person to discover their BMI. A link to one of them is provided in the "References and Resources" section at the end of this article.
For adults, the significance of the BMI number is as follows:
- less than 18.5 = underweight
- 18.5 to 24.9 = normal weight
- 25.0 to 29.9 = overweight
- 30.0 or higher = obese
The numbers can be a helpful indication of a person's weight category, but this isn't always the case. For example, athletes often have increased muscle mass. This mass will increase their weight and give them a higher BMI than they would normally have. Seniors often lose muscle. This may reduce their weight and BMI, even though they may have an unhealthy amount of fat in their body.
Worldwide obesity has nearly tripled since 1975.— WHO (World Health Organization)
Gut Bacteria and Health
Scientists say that we have about ten times more bacterial cells in our body than human ones. The bacterial cells are smaller than ours. Bacteria live on our skin and in body passages that are connected to the outside world, such as the respiratory tract and the gastrointestinal tract.
Intriguing research is showing that some of our gut bacteria affect our lives in important ways. They make vitamins that we use, including vitamin K, break down some of our food, reduce the amount of feces that we make, and fight harmful bacteria. Some are thought to boost the activity of our immune system, regulate cholesterol metabolism, or reduce inflammation.
The Human Microbiome Project is a major effort to discover and categorize all of the microorganisms that live in and on our bodies in health and in disease. Identifying these bacteria may have great practical importance. This is especially true with respect to gut bacteria, since they seem to be so important in our lives.
The Human Microbiome
A Possible Role of Methanobrevibacter in Weight Gain
Researchers at the Cedars-Sinai Medical Center in California have identified one gut microbe that appears to be indirectly related to weight gain. The microbe is named Methanobrevibacter smithii. Despite its name, it belongs to a group of organisms known as the archaea. The members of this group were once considered to be bacteria. They have many similarities to bacteria but also have some important differences.
The researchers examined exhaled air samples from 792 people with respect to the hydrogen and methane content. They found four results:
- normal levels of each gas
- a higher concentration of hydrogen than normal
- a higher concentration of methane than normal
- a higher concentration of both gases compared to the normal values
The researchers found that people who had a high concentration of both hydrogen and methane in their breath had a significantly higher BMI and a significantly higher percentage of body fat than the other people in the study.
Scientists say that most of the methane made in our gut is produced by Methanobrevibacter smithii. This microbe absorbs hydrogen released by bacteria and then uses the hydrogen to produce methane. Some of the hydrogen and methane enters our bloodstream and is exhaled by our lungs.
An environment with a high hydrogen content is harmful for some gut bacteria. The researchers think that by absorbing hydrogen from the gut environment, Methanobrevibacter is helping bacteria that release hydrogen to survive and to increase in number. These bacteria may then break down more food, giving themselves and us more nutrients. As time passes, our increased absorption of nutrients could lead to weight gain.
The researchers are now performing experiments in which Methanobrevibacter is eliminated from the gut with targeted antibiotics. They hope to discover how the removal of the microbe affects the processing of food in the gut.
Methanobrevibacter and Weight Increase in Children
In another research project, a team of scientists in the Netherlands examined the feces of 472 children between the ages of six and ten. They found that a higher concentration of Methanobrevibacter smithii in the feces (and therefore presumably in the gut) was associated with increased weight, a higher BMI score, and being overweight.
The Dutch researchers say that when Methanobrevibacter removes hydrogen, the rate of bacterial fermentation of carbohydrates and polysaccharides increases. The fermentation process produces substances that humans use.
It should be noted that although multiple researchers have found that Methanobrevibacter smithii appears to contribute to weight gain, some have found that it seems to cause weight loss instead. The study of microbe effects on our body is complex. Many processes and interactions take place in the human body, not all of which are understood. In addition, different people may have a different collection of gut bacteria. The experimental conditions and protocol can also affect the results of a research project.
41 million children under the age of 5 were overweight or obese in 2016.— WHO (World Health Organization)
A Possible Role of Enterobacter in Weight Gain
Scientists at Shanghai Jiao Tong University in China have discovered that another bacterium may affect obesity. They placed a morbidly obese man on a special diet that was designed to change the composition of his gut flora by altering the pH in the environment. The diet consisted of whole grains, non-digestible carbohydrates, probiotics, and traditional Chinese medicines. The man weighed 175 kg (almost 386 lbs) at the start of the diet. He lost 51 kg (about 112 pounds) during the diet, without exercising.
The researchers found that at the start of the diet a member of the genus Enterobacter was the most abundant bacterium in the man's gut. At the end of the diet (which lasted for twenty-three weeks), the bacterium was almost undetectable in his gut.
The scientists wanted to determine if the change in the gut flora arose because the man lost weight or because the removal of Enterobacter was at least partly responsible for his weight loss. They fed Enterobacter from the patient's gut to some mice but not to others. All the mice were given a high-fat diet. The mice that had the patient's bacterium in their gut gained significantly more weight than the mice without the organism, suggesting that the bacterium can cause weight gain.
A common saying in biology is that correlation doesn't necessarily mean causation. Just because there is an association between two changes in the body doesn't necessarily mean that one change is causing the other. Understanding the true relationship is one of the challenges of research.
Bacteria and Metabolic Syndrome
Gut Bacteria Linked to Inflammation and Obesity
More bacteria than Methanobrevibacter and Enterobacter are suspected to play a role in obesity. In fact, researchers at the University of Maryland School of Medicine have identified twenty-six species of bacteria that they suspect are linked to inflammation, obesity, and metabolic syndrome.
Metabolic syndrome is a collection of factors that increase a person's risk for cardiovascular disease, strokes, and type 2 diabetes. To be diagnosed with metabolic syndrome a person must have at least three of the following conditions:
- abdominal obesity (a large waistline)
- high blood pressure
- high fasting blood sugar (or blood glucose)
- high blood triglycerides
- low HDL cholesterol (the good type of cholesterol)
Although they are not part of the criteria for diagnosing metabolic syndrome, a person with the condition often has a high blood level of LDL cholesterol (the bad cholesterol), insulin resistance, and widespread inflammation.
The researchers found that the twenty-six bacteria were common in people who were obese or had signs of metabolic syndrome but were much less abundant in healthy people. More studies are needed to discover whether the bacteria cause obesity or are present as an effect of the obesity.
Other Bacteria Linked to Obesity
Researchers have discovered that certain types bacteria can cause obesity in mice. Most intestinal bacteria belong to two groups—the Firmicutes and the Bacteroidetes. At least in mice, the Firmicutes are associated with obesity and the Bacteroidetes with weight loss.
Scientists have taken samples of the intestinal microbiome from obese mice and placed them in the intestine of lean mice which have no intestinal bacteria. As a result, the lean mice have developed extra fat deposits. Researchers have also transplanted the gut bacteria from lean mice into mice with no gut bacteria. This causes no change in the weight of the recipient mice. All of the mice in the experiments were given identical food containing the same number of calories.
What is true for mice may not be true for humans, but it often is. To complicate matters, however, researchers have noticed that when obese people lose weight the proportion of Bacteroidetes to Firmicutes in their gut increases. The decrease in weight may have triggered this change.
Despite the suspicion that gut bacteria contribute to obesity, the conventional recommendations for losing weight are still important. These include eating a healthy diet and exercising for at least 150 minutes a week. Obese and very overweight people should always seek medical advice before starting an exercise program.
A Potential Problem With Yo-Yo Diets
The term "yo-yo" diet is used to describe the situation in which a person loses some weight on a diet, returns to their former diet, regains some weight, and then returns to the weight-loss diet in an attempt to remove the weight that has just been gained. Some people follow this pattern repeatedly. Research suggests that there is a special problem with the pattern in terms of gut bacteria.
In 2016, some Israeli scientists reported that mice on a yo-yo diet gained more weight than mice not on a special diet but ingesting the same number of calories over the same time period. In addition, the mice on a yo-yo diet had a less diverse collection of bacteria in their gut. It's often been noticed that obese humans have a lower diversity in their gut bacteria than lean people. When intestinal bacteria from the yo-yo dieters were transplanted into mice that were not on a special diet, the animals experienced an additional weight gain beyond what would be expected from their food intake.
Genes, Diet, Gut Diversity, and Weight Loss
In 2017, researchers at the University of Nottingham published an interesting report in the International Journal of Obesity. The research team studied 1,632 women (but no men, which may have been a limitation of the study). All of the women had a twin involved in the project. In about half of the cases, the twin was an identical one. The fact that identical twins are genetically identical enabled the researchers to discover the role of genes in weight determination.
After nine years, the researchers found that the women who had a diet that was rich in fibre were less likely to have gained weight than other women, even if the other woman in the comparison was an identical or non-identical twin and had eaten roughly the same number of calories. In fact, they found that genetics was responsible for less then than half of an observed weight change. In addition, the researchers found that woman who had maintained the same weight over the nine-year period or who had lost weight had a more diverse collection of gut bacteria.
Diet can affect both weight and gut bacteria. The researchers acknowledge that they have only shown a correlation between the composition of the gut microbiome and weight and haven't actually proven that gut bacteria can affect people's weight. They point out that studies in mice support this idea, however. It's interesting that some of the unique bacteria in the lean women in the experiment have been found to contribute to weight loss or maintenance in mice. Transferring gut bacteria from one person to another is not considered appropriate (except in very special health situations and with special precautions), so we can't confirm the role of the "lean" bacteria in humans as has been done in mice.
Antibiotics and the Gut Microbiome
More evidence supporting the link between weight and the gut microbiome comes from the use of antibiotics. Using broad-spectrum antibiotics (those that kill multiple species of bacteria) either repeatedly or continuously can reduce gut bacteria diversity. This change can be accompanied by weight gain.
Some relevant and interesting discoveries have been reported in Berkeley Wellness, an online publication of the University of California, Berkeley. Links to the original research articles are given in the report, which is referenced below.
- A study published in a 2015 edition of the Nature journal described an analysis of 2010 national prescription information. The researchers found that by the time an average child in their sample had reached the age of two, he or she had received "close to three" courses of antibiotics. By the age of ten, an average child had received ten courses of antibiotics.
- A study published in 2014 in JAMA Pediatrics examined the health records of 64,000 children in Philadelphia. The children who had been treated by antibiotics at least four times by the time they were two years old had a greater risk of becoming obese by the age of five. The relationship was strongest if broad-spectrum antibiotics were prescribed.
- A study published in the International Journal of Obesity in 2014 followed 400 pregnant women as well as their offspring up to the age of seven. The researchers found that the women who had taken an antibiotic in the second or third trimester of pregnancy had a significantly higher chance of having an obese child.
It's important that a child or an adult takes a prescribed antibiotic. The medication can save lives. It would be best if a doctor could prescribe a narrow-spectrum antibiotic whenever possible, however. This kills targeted bacteria instead of the general bacteria population. It would also be good if antibiotics of any type were prescribed only when necessary.
What Are Probiotics?
The Causes and Potential Treatments of Obesity
While inappropriate food choices and lack of exercise can cause weight gain, there is a growing suspicion that the causes of obesity are more complex. Some researchers believe that obesity is caused or influenced by fundamental changes in the body. One of these changes may be the composition of the intestinal microbiome.
There are two possible reasons for the observed link between gut bacteria and obesity. Specific gut bacteria could be the cause of obesity. On the other hand, obesity may produce conditions that favour the presence of the bacteria. Research results obtained so far suggest that in at least some cases the presence of specific bacteria contributes to obesity. This is an exciting observation because it could open the door to new treatments for the condition.
Certain bacteria may be able to prevent obesity by destroying harmful bacteria or by keeping their population under control. Other bacteria may change the environment in the gut, making conditions unfavourable for the growth of the harmful bacteria.
"Probiotics" are bacteria and yeasts that are thought to have health benefits. In the future, it may to possible to give people obesity-fighting bacteria in the form of a probiotic supplement. Dietary components may also be used to change the gut environment in a helpful way. Another possibility is that targeted antibiotics may be able to destroy harmful bacteria. Research is ongoing.
A Healthy Diet and a Good Exercise Routine Are Important
It's well known that a bad diet and lack of exercise often lead to weight gain. Anybody who is overweight or obese should modify their diet and exercise routine if these need improvement. It would be wonderful if the addition of certain bacteria to the intestine could help the process of weight loss, though. In the near future we will hopefully have new ways to help obese people. Obesity is a problem that we need to solve.
References and Resources
Just enter your height and weight into this handy National Heart, Lung, and Blood Institute calculator and it will quickly display your BMI.
- Obesity and overweight fact sheet from the World Health Organization
- Microorganisms detected via breath test linked to body mass, fat accumulation from the EurekAlert news service
- Gut colonization with Methanobrevibacter smithii is associated with childhood weight development from the Obesity journal, Wiley Online Library
- Enterobacter and weight gain from New Scientist
- Gut bacteria linked to obesity and metabolic syndrome identified from the Medical Xpress news service
- Bacteria and obesity in mice from the NIH (National Institutes of Health)
- Yo-Yo dieting and gut bacteria play a role in long-term weight gain from The Conversation
- Antibiotics, obesity, and the gut microbiome from the University of California, Berkeley, via Berkeley Wellness
- Information about the Human Microbiome Project from the NIH
This content is accurate and true to the best of the author’s knowledge and does not substitute for diagnosis, prognosis, treatment, prescription, and/or dietary advice from a licensed health professional. Drugs, supplements, and natural remedies may have dangerous side effects. If pregnant or nursing, consult with a qualified provider on an individual basis. Seek immediate help if you are experiencing a medical emergency.
Questions & Answers
© 2013 Linda Crampton