How the microbiome contributes to obesity

February 25, 2019

The gut microbiome plays a large role in how we eat, from when we feel full to what foods we crave.  Now that we understand some of the science behind the interaction between our guts and the microbes, scientists are seeing a link to obesity.  There are three parts to the link: 1) extracting energy from food and how it is stored in the body, 2) protecting against weight gain, 3) dysfunction and toxins in the gut.

Energy from Food

Some bacteria, like Firmicutes, are better than others at extracting energy from the food we eat.  They produce specific enzymes that digest otherwise indigestible types of carbohydrates known as dietary polysaccharides.  The bacteria also ferment these oligosaccharides into digestible forms known as monosaccharides and short chain fatty acids (SCFA).  SCFAs play an important role in metabolism and can either contribute to or protect against obesity.  If the conditions are right, SCFAs will help you use up the extra energy the bacteria produced by triggering metabolic pathways that expend a lot of energy.  Under other conditions, SCFAs will activate the pathways that lead to the conversion of energy to fat.

Preventing Weight Gain

The ratio of different types of bacteria in the gut may play a role in obesity.  Studies correlated high levels of Bacteroidetes and low levels of Firmicutes with healthy body weights.  When the ratio flipped with high levels of Fimicutes, body weight increased.  This may be because Bacteroidetes help us process food in a way that increases the energy we absorb from it.  When levels of Bacteroidetes are high enough for us to absorb all of the energy produced from food, there isn’t extra energy to store as fat.  That is one of the hypotheses anyway.  Not all studies reached the same conclusions, so we need to wait and see what future studies conclude.

Gut Dysfunction and Toxins

Some types of bacteria like Akkermansia muciniphila are part of a balanced microbiome in a healthy gut.  These bacteria degrade mucin, a key component of mucus.  Too much mucus promotes the growth of harmful bacteria and makes it difficult for your body to absorb nutrients and energy through the intestine.  Too little mucus puts the intestinal lining at risk and reduces the space for the good bacteria to live.  Other types of bacteria trigger an immune response in the gut by producing compounds like lipopolysaccharide, peptidoglycan, and inflammatory cytokines.  These compounds and other toxins produced by the bacteria lead to increased intestinal permeability, otherwise known as leaky gut.  Increased permeability means that compounding are crossing over into the blood stream that should have passed through the gastrointestinal tract without being absorbed.

As scientists learn more about the links between the microbiome and obesity, there may be ways of preventing and treating obesity by altering the microbiome.  One additional caveat is that our microbiomes are changing all of the time.  As we grow older, our microbiome changes.  As we exercise more or less, our microbiome changes.  As we change our diet, our microbiome changes.  With dietary changes, the microbiome is measurably different in as little as 48 hours.  This makes it difficult for researchers to narrow in on exactly what the best microbe based therapy might be, but it is great news for potentially speedy treatments!  It also explains why dietary changes, like the low FODMAP diet, can make a significant difference within a few days for patients with irritable bowel syndrome.

If you’re interested in the scientific details, check out the following scholarly article.

Stephens RW, Arhire L, Covasa M. Gut Microbiota: From Microorganisms to Metabolic Organ Influencing Obesity. Obesity (Silver Spring). 2018 May;26(5):801-809.

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