Microbiota – How Do Beneficial Gut Bacteria Affect Obesity or Being Overweight?

Obesity has tripled in prevalence over the past 40 years. Obesity is associated with diabetes, cardiovascular disease, stroke and certain types of cancer. Physical inactivity, medication use, genetic factors and especially inappropriate diets contribute to the development of obesity.

Our eating habits are important in shaping the gut microbiota / balance of beneficial bacteria. In recent years, both laboratory and human studies have shown that the gut microbiome plays an important role in obesity.

Good–Bad & Guilty–Innocent

Microbiota research initially showed that, in people with obesity, the microbiota contained high levels of Firmicutes and low levels of Bacteroidetes.

In mice prone to obesity, it was also claimed that a higher Firmicutes-to-Bakteroidetes ratio had genetic causes. After weight loss, the Bacteroidetes-Firmicutes ratio was observed to return to normal.

However, more recent studies did not support this. It was even suggested that increased Bacteroidetes was more common in the microbiota of people with obesity. These differing results may reflect the more restricted conditions of the early studies and advances in laboratory techniques and methodology in newer studies. It should also be remembered that the Bacteroidetes-Firmicutes ratio is not a sensitive measure of obesity.

Does Obesity Disrupt the Balance, or Does an Imbalance Cause Obesity?

Many studies have investigated whether changes in the gut microbiota contribute to obesity or whether obesity changes the gut microbiota. In one study, stool samples were collected from 49 infants at 6 and 12 months of age and when they reached 7 years. Those who were overweight at age 7 were found to have high levels of Staphylococcus aureus and low levels of Bifidobacteria in their intestines.

The Concept of Gut Bacterial Richness

A recent important study in humans extensively investigated the metabolic functions of the microbiota. The study found that people with low gut bacterial richness / diversity more frequently had obesity, insulin resistance, fatty liver and inflammation. In addition, participants with low microbiota diversity were shown to have a strongly proinflammatory (inflammation-promoting) microbial profile. Under the influence of these bacteria, researchers observed (a) a reduction in butyrate-producing bacteria; (b) increased mucus degradation, potentially impairing the barrier function of the intestinal mucus layer; and (c) increased oxidative stress.

An increase in gut microbial richness was observed in patients with low bacterial richness who followed an energy-restricted diet. At the same time, the clinical symptoms of patients with low and high bacterial richness also improved. However, only a limited effect on improvements in inflammatory parameters was observed.

These study findings should not be regarded as an absolute rule. Not all participants with low bacterial richness were obese. Similarly, not all participants with high bacterial richness were lean and healthy.

Examples of proinflammatory bacteria reported to be increased in people with low bacterial richness include Ruminococcus gnavus and Bacteroides species. Examples of anti-inflammatory bacteria observed to be increased in people with high bacterial richness include Faecalibacterium prausnitzii, Bifidobacterium and Lactobacillus species.

The Effect of Weight Loss on the Microbiota

Bariatric operations performed to treat people with obesity who have a high risk of death (morbid obesity) appear to cause changes in the gut microbiome.

Bariatric surgery (obesity surgery) was followed, in association with the resulting weight loss, by an increase in Bacterioides thetaiotaomicron, which is usually reduced in people with obesity, and a decrease in elevated serum glutamate levels.

Another bariatric operation, Roux-en-Y gastric bypass (RYGB), was observed to increase Proteobacteria and decrease Firmicutes. After laparoscopic sleeve gastrectomy (LSG), an increase in Fecalibacterium prausnitzii, which has anti-inflammatory effects, was also observed.

In the figure below, commonly used types of bariatric surgery can be seen

Conclusion

These studies support an association between the gut microbiota and obesity. In general, it is possible to say that proinflammatory (inflammation-promoting) bacteria occur more frequently in the intestines of people with obesity, while anti-inflammatory (inflammation-reducing) bacteria occur less frequently.

Until recently, we thought that eating habits were the main cause of obesity. However, recent microbiota studies have brought a different perspective to obesity and raise the following question: "Does our microbiota change because of obesity, or do we become obese because of our microbiota?"

Thanks to technological advances, we understand the microbiota better every day and are moving closer to answering this question. Nevertheless, it is clear that more human studies are needed to answer this important question.

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