The journey of food in your gut

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The past decade has shed light on the role of the gut microbiome in human’s overall health. Gut microbiome pertains to the collective bacteria, viruses, and fungi that live in our gut. It has become clear that our health is intertwined with our microbes.

Think of it as a very complex highway system, with traffic going back and forth at high speeds on many different levels, directions and junctions. Our body affects the community living in our gut, and these microbes regulate different functions in our body.

Microbial communities throughout the digestive tract.

Your digestive tract is one of the main contacts of your body to the outside world — all the food that you eat passes through it. Throughout the gastrointestinal tract reside different communities of microbes.

The digestive tract starts from the mouth, which is responsible for grinding the food into smaller chunks. Also the digestion of starch starts in the mouth. Mouth is home for about 20 billion microbes.

Next, food moves through the stomach, which is responsible for partial degradation of proteins. As the stomach is a very acidic environment, there are about 1000 times less microbes than in the mouth.

Then, food moves forward to the small intestine where degradation of macronutrients is finalized. Here the macro- and micronutrients also get absorbed to the bloodstream. The environment here is a lot friendlier than in the stomach, so it is home to about the same amount of microbes that live in the mouth.

However, the digestion is not finished in the small intestine. Food, or what’s left of it, moves forward to the large intestine. This is a perfect environment for microbes to live in. The food moves at a slower pace and it’s full of nutrients for microbes. Although most of the nutrients get absorbed already in the small intestine, there are compounds — the dietary fibers — that our own digestive enzymes are not able to degrade.

Fibers make their way to the end part of the large intestine — called colon. Here resides about 10 thousand more microbes than in the mouth and small intestine and more than million times more microbes than in the stomach. This is found to be the most densely and diversely inhabited part of the human body and most likely the highest microbial density recorded in any habitat on the Earth.

The microbes, living in our gut, compensate for what our own digestive enzymes cannot degrade. They help to increase the availability of energy from food. Without microbes we would get about 10–15% less energy from our diet.

But it’s not only energy. While degrading the fibers, gut microbes produce different molecules called metabolites that regulate essential functions in our body. For example, these metabolites strengthen the gut barrier — the only defence wall that prevents pathogenic microbes, toxins and other potentially harmful particles from entering the bloodstream and getting access inside our organism.

In addition, gut microbes produce vitamin K and group B vitamins, as well as modulate metabolism. It has been found that the gut microbiome composition differs between lean and obese individuals. This is well illustrated in studies conducted with mice who have been raised in completely sterile conditions so that they lack microbes. When transferring the gut microbiome of lean or obese human to such mice, then the mice quickly adopt the metabolic state similar to humans. Those getting the microbes from lean individual stay lean and those getting the microbes from obese person become obese.

Doesn’t it seem simple, that we could just get the microbes from lean people and become lean ourselves? Well, I’m afraid, I have to disappoint you. Yes, we could get a great push towards, but the food we eat influences who stays alive and permanently inhabits our gut.

Studies have shown that diet can make some shifts in the gut microbiome as quickly as 24 hours. However, to make a lasting change, you must eat a healthy diet consistently.

So, the communication between our gut microbes and digestion is two-way, as the gut microbes affect how our metabolism works and the food we eat affects the composition of that community living in the gut.

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