How Gut Bacteria Affect Weight

The first inkling that bacteria could affect weight came out of animal husbandry back in the 1940s. That’s when researchers and ranchers discovered that antibiotics helped fatten livestock, and heavier livestock meant bigger profits. “But what if that meat is us?” asked the author of a New York Times article in 2014. Three studies conducted in the 1950s—which could not ethically be performed today— found that children and adults also gained more weight when they took antibiotics.ⁱ

Over the past few years, researchers have focused on two large families of intestinal bacteria called Fermicutes and Bacteroidetes. Both groups of bacteria play major roles in extracting nutrients from food. Thin people have a higher proportion of Bacteroides relative to Fermicutes. But in overweight people and animals, the number of Fermicutes bacteria increases and Bacteroidetes decreases.^{ii iii iv v} A similar pattern has been identified in people with insulin resistance and diabetes.vi vii Other studies have confirmed that certain eating habits favor one family of bacteria over the other, and that an increase in Bacteroidetes can lead to weight loss.

The current focus on intestinal bacteria and weight began in 2004, when researchers transplanted bacteria from obese mice to thin mice. Within two weeks, the thin mice gained 60 percent more body fat—without eating more food or exercising less.viii Interest in the research grew, and in 2007 Ruth E. Ley, PhD, currently at Cornell University, reported that obese subjects had larger populations of Fermicutes, a finding subsequently confirmed by researchers around the world. ^{ix x xi xii} In a small study, Ley found that 12 obese subjects had an increase in Bacteroidetes and a decrease in Fermicutes after they lost weight— resembling the pattern typically found in thin people.^xiii

More recent studies have shown that taking probiotics can in fact lead to weight loss. Peter J. H. Jones, PhD, of the University of Manitoba, Canada, tested two types of Lactobacillus probiotics on 28 overweight men and women. Everyone in the study ate the same diet, and the probiotics were consumed in yogurt. People consuming the Lactobacillus-rich yogurt lost an average of 3 to 4 percent of their body weight (equivalent to 10 pounds for a 250 pound person) during the 43-day study, while people eating regular yogurt lost only 1 percent of their body weight.^xiv

In the other study, Angelo Tremblay, PhD, of Laval University, Canada, and his colleagues asked 125 overweight men and women to follow a low-calorie weight loss and maintenance diet for 24 weeks. Half the group also took two capsules of a probiotic containing L. rhamnosus (commonly sold as a supplement) and the other half took placebos each day. Women taking the probiotics lost an average of 11.5 pounds during the study, twice the amount of weight lost by people in the placebo group.^xv 

What Disturbs Our Gut Bacteria?

Antibiotics, which destroy gut bacteria, may boost the risk of obesity. Researchers at the Children’s Hospital of Philadelphia, Pennsylvania, analyzed antibiotic use in 65,480 newborn infants through age five. They found that the earlier a baby received antibiotics, the more likely he or she would become obese within a few years. When infants and small children received four or more prescriptions for antibiotics, including one or more broad-spectrum antibiotics, they were 17 percent more likely to become obese by age five.^{xvi xvii} The lesson, whether you’re an adult or child, is to not take antibiotics unless they are absolutely necessary, and then to also take probiotics.

Eating a diet of refined sugars and other carbohydrates also promotes a bacterial imbalance known as dysbiosis, as do artificial sweeteners. Just recently a team of researchers from Israel found that, in both people and laboratory animals, the consumption of soft drinks with saccharin, sucralose, or aspartame led to increases in blood sugar levels, creating an effect no better than that of sugary beverages. But the underlying mechanism was a surprise: The artificial sweeteners led to changes in more than 40 different types of gut bacteria, including a reduction in Lactobacillus reuteri (another common probiotic supplement).

The researchers, led by Eran Elinav, MD, PhD, of the Weizmann Institute of Science, found that people who consumed artificial sweeteners were more likely to have metabolic syndrome, including more abdominal obesity, higher fasting blood sugar, and higher HbA1c levels (a measurement of glucose in the blood). In an experiment with five men and two women who did not usually consume drinks or foods with artificial sweeteners, four of the seven subjects developed “significantly poorer glycemic responses” in less than one week after consuming the sweeteners.^{xviii xix}

Putting the Research into Practice

In a small study conducted at Harvard University, researchers found that changes in eating habits can alter the composition of gut bacteria within days, with a high-protein diet reducing the numbers of Fermicutes (the bacteria associated with obesity).^xx Highfiber diets, which are rich in the prebiotics gut bacteria use as food, promote a more diverse population of gut bacteria—something that’s good for everyone.^{xxi xxii xxiii} Such habits point to a Paleo-style diet with quality protein, fiber-rich vegetables, and very few refined carbohydrates.

Based on the research so far, probiotic supplements can also shift and help maintain a healthy population of gut bacteria. Although Lactobacillus, L. rhamnosus, and L. reuteri are available as supplements, some of the probiotics used in studies are not yet available as supplements. Therefore, the best advice is to take a probiotic supplement that provides a diverse selection of bacteria. This is especially important if you have taken antibiotics or other drugs like acid-blockers, or have made a habit of consuming soft drinks or a highly processed, low-fiber diet. Lactobacillus and Bifidobacteria bacteria, found in the majority of probiotic supplements, help foster an environment that supports the growth of other beneficial species. 

Not too long ago, we thought that our gut bacteria simply helped us digest food and synthesize trace amounts of a few vitamins, but research continues to reveal the wonderful world of the human microbiota and its incredible role in health, including helping to maintain a healthy weight.

References available on request.