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How Gut Bacteria Impacts Obesity, Allergies, Happiness and More

By Gretel H. Schueller, August 7, 2014 - 11:54am

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A growing body of research has shown that the teeming populations of gut bacteria within us have evolved complex connections that can affect our body’s basic functions — from metabolism to sleep to mood.

Changes in the makeup of the gut bacteria in the human digestive system have been associated with a growing number of diseases.

It’s important to remember, though, that the science is still young and evolving.

Here we highlight the most intriguing of the cutting-edge studies.



Healthy Heart

Some of the many beneficial compounds that certain gut bacteria produce for us are carotenoids—antioxidants that are believed to protect against stroke and angina.

In a 2012 study in Nature Communications, researchers in Sweden compared the gut microbiome of stroke patients to that of healthy subjects and found that there were more carotenoid-­producing gut bacteria in healthy participants.

Stanley Hazen, M.D., Ph.D., and his team at the Cleveland Clinic have been looking at a variety of ways microbes play a role in heart disease.

For example, when certain gut bacteria metabolize lecithin (abundant in egg yolks) and ­carnitine (a compound in red meat), it boosts compounds in the blood that are linked to an increased risk of heart attack and stroke. The bacteria and the risk-causing compounds vanish even ­after eating those foods.

Allergies Denied


Researchers in Copenhagen reviewed the medical records and stool samples of 411 children for 6 years and found that those who had the least diverse colonies of gut bacteria as infants were more likely to develop some types of allergies.

Trim Weight


A growing body of studies indicates that obese people tend to have a much lower diversity of gut bacteria — up to 40 percent less — than people with a healthy weight. And gut microbes may be responsible for those lean or plump traits: several studies with mice have found that transferring gut microbes from obese mice (or from obese humans) into non-­obese mice, leads the non-obese mice to gain weight.

In turn, adding gut bacteria from lean mice into heavy ones causes the chubby mice to lose weight on a high-fiber, low-fat diet. Researchers believe different bacteria metabolize food differently, which could affect how much your body absorbs.

Lower Blood Pressure


A 2013 study in the Proceedings of the National Academy of Sciences suggests certain gut microbes may help lower blood pressure.

Researchers found that by-products produced by certain gut bacteria activated a specific kind of cell receptor in blood vessels that lowers blood pressure. More bacteria equals more by-products, which equals healthier blood pressure.

Fight Cancer


A robust and diverse gut microbiome may help certain chemotherapies work better. French researcher Laurence Zitvogel, M.D., Ph.D., and his colleagues observed that gut bacteria in mice encouraged immature immune cells in the lymph nodes to develop into tumor-targeting T-cells.

In contrast, when mice were treated with antibiotics (which wipes out bacteria) before begin­ning chemotherapy, the chemotherapy was less effec­tive. The researchers ­believe the gut bacteria help prime the immune system to respond to chemotherapy.

Some microbes may be associated with colon cancer. University of Michigan researchers exposed two groups of germ-free mice — essentially mice with sterile colons — to a known carcinogen.

One group then received gut bacteria from mice with colon cancer and went on to develop twice the number of tumors than the other group who got gut bacteria from cancer-free mice. The researchers narrowed down the microbial families associated with colon cancer and one included Prevotella.

People with inflammatory bowel disease are at higher risk of developing ­colon cancer than the general population. Researchers have thought that the main culprit is overactive immune cells, which release DNA-damaging molecules. 

Now new findings in Science suggest that overactive immune cells also may be causing an imbalance in your gut bacteria — encouraging E. coli strains that produce cancer-causing toxins.

Relaxed & Happy


There’s something to be said for “gut feelings.”

Gut bacteria produce hundreds of different neurotransmitters, including up to 95 percent of the body’s supply of serotonin, a mood and sleep regulator. Serotonin also controls movement within the intestines.

Our gut is said to be our “second brain” in part because the vagus nerve is a major communications highway that stretches from the brain to various points along the intestinal lining; communication travels in both directions.

One Lactobacillus species, for example, sends messages from the small intestine to the brain along this nerve: in a study led by John Cryan, Ph.D., a neuroscientist at University College Cork in Ireland, anxious mice were dosed with a proprietary strain of Lactobacillus rhamnosus.

Those rodents then had lower stress hormone levels and an increase in brain receptors for a neurotransmitter that’s vital in curbing worry, anxiety and fear. The effects were similar to those of Valium.

According to a 2011 study, when mice had this bacteria in their gut, they showed less depressive behavior. Whatever bacteria may be responsible for “feeling good,” it appears they can be acquired: a recent experiment moved gut bacteria from fearless mice into anxious mice. The new bacteria sparked a personality change, making timid mice more gregarious.

Autoimmunity Explained


Dan Littman, M.D., Ph.D., a microbiologist at the NYU School of Medicine, led a study that found an association between the gut bacteria Prevotella copri and the onset of rheumatoid arthritis, an autoimmune disease of the joints. While the connection was significant, it’s not clear which comes first: the bacteria or RA.

However, other animal studies, Littman says, have clearly shown that gut microbes play a role in causing autoimmune diseases. One of these, published in 2013 in the journal Science, showed 75 percent of female mice at risk of autoimmune type 1 diabetes were protected against the disease when they were given gut bacteria from healthy mice.

Manage Crohn’s


A large study out of Massachusetts General Hospital involving more than 1,500 patients recently reported a connection between gut microbes and Crohn’s disease (an inflammatory bowel disease that can affect any part of the GI tract, but typically the intestines).

In addition to having less diversity, Crohn’s patients had fewer bacteria known to quell inflammation and more bacteria that cause inflammation.

Interestingly, those who received the standard antibiotic treatment for Crohn’s had a microbe mix that was even more out of balance. Another study found that when Crohn’s patients were given a prebiotic fiber supplement each day, disease symptoms decreased significantly.

Related: Top Fiber-Rich Foods for Good Gut Bacteria

TAGS: Gretel H. Schueller, Food News Blog, Health, Nutrition

Gretel H. Schueller
Gretel H. Schueller is an award-winning journalist and book author. A graduate of New York University's Science, Health and Environmental Reporting Program, she’s put her master's in journalism to good use. While on assignment, she has eaten backyard weeds, harvested cactus buds in an Arizona desert and made goat cheese in Greece.

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