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“I do not like vegetables”

“Yes but it is good for your health”

We all probably had to deal with our kids refusing to eat vegetables. We tried to convince them that it is good for their health because it brings vitamins. The proper explanation is actually more complex than only vitamins but it requires to have a good understanding of the digestion physiology.

For years, we have been told that most of the digestion occurs in the stomach and the beginning of the small intestine and then nutrients are absorbed in the rest of the intestine tract. But it is now confirmed that a major part of the digestion takes place as well in the colon and it is important to understand the differences between these two different processes.

First, it is critical to remember that in adult animals, the supply of energy comes at 75% as result of the digestion occurring in the stomach and up to 25% is the result of the fermentation happening in the colon. Therefore, nutritionists who are looking at optimizing the cost of energy supplied to monogastrics needs to pay attention of what is happening in the colon.

Direct vs Indirect digestion

In the stomach and duodenum, the digestion is direct. The nutrients provided through the diet are cut in small pieces by the enzymes and then transported to the liver where they will used as such. It is a direct utilization of the nutrient.

The nutrients that pass the small intestine and arrive in the colon, will encounter a large population of bacteria who will use them as source of energy to grow and multiply. As results, these bacteria will produce acetate, propionate or butyrate. These acids will pass in the blood and go to the liver who they will be transformed into glucose. It is an indirect process where the diets feed the bacteria who will in return feed the animal.

Enzymatic versus Fermentation

The mechanism of utilization between digestion in the upper part of the gut and fermentation in the lower part is sensibly different. The digestion is enzymatic. It will use endogenous enzymes produced by the animals or exogenous enzymes produced by some probiotics or additives supplied in the diet. The fermentation is actually a process involving bacteria. The microbial composition of the gut microbiota varies across the digestive tract. In the stomach and small intestine, relatively few species of bacteria are generally present. The colon, in contrast, contains the highest microbial density recorded in any habitat on Earthwith up to 1011 cells per gram of intestinal content. These bacteria represent between 300 and 1000 different species. However, 99% of the bacteria come from about 30 or 40 species. As a consequence of their abundance in the intestine, bacteria also make up to 60% of the dry mass of feces.

Over 99% of the bacteria in the gut are anaerobes. It is estimated that these gut flora have around a hundred times as many genes in total as there are in the human genome. Many species in the gut have not been much studied outside of their hosts because most cannot be cultured.

The balance between all these populations is very fragile. Some bacteria are strict anaerobic, some others are facultative aerobic. Some bacteria are cellulolytic (ferment fibers) and some others are proteolytic (they ferment proteins). To optimize the capacity of the colon to supply energy to the liver through production of acetate, propionate and butyrate, we need to ensure a healthy balance of all these populations. That depends on the nutrients that will reach the colon. If we provide too much undigested protein, that will develop the proteolytic population (E.Coli, Clostridium). If we do not provide enough fermentable fibres, that will depress cellulolytic population as firmicutes. If stress is too high, that will lead to the passage of oxygen from the blood to the gut and will depress the strict anaerobic bacteria. Nutrition is a key driver to maintain proper gut flora balance and health. It is important to remember that gut inflammation is one of the major consumer of energy and it needs to be avoided at maximum if we want to promote better animal growth.

Digestibility vs Fibre

For the last 50 years, the focus of nutritionists was on the upper part of the tract concentrating on the enzymatic digestion. We wanted to be sure that all the nutrients supplied in the Swine and Poultry diets were as digestible as possible meaning that they should get into the blood as early as possible in the tract. We wanted to minimize any nutrients that would not be absorbed in the duodenum or ileum. That would be seen as waste. But we never consider that we were then depraving the 1011 bacteria in the colon from any nutrient.

It was not that critical at that time as we were still using AGP. Antibiotics were distributed in constant flow to animals. These antibiotics were disturbing the fermentation process by depressing the colon flora but there were at the same time maintaining acceptable gut health by preventing any harmful bacteria to develop.

Since the ban of AGP, the situation changed. Nutritionists need to learn how to manage the gut flora without antibiotics and only through diet management. It is in return an opportunity for the nutritionists to get the best output as possible by feeding correctly the colon flora and stimulating the production of volatile fatty acids available for the liver.

By principle, the gut flora get fed with the nutrients that have not been absorbed in the upper part of the tract; what we call the undigested nutrient. It turns out that these are not waste for everybody. All the undigested polysaccharides that are the main source of energy for the bacteria are grouped under the generic named of fibres. The exact definition of this fibre family is NSP or Non-Starch Polysaccharides. We are excluding the starch that is supposed to be easily digested and absorbed in the duodenum and ileum. The other polysaccharides that are not digested will be used by bacteria as source of fermentation in the colon (not all of them) and lead to a production of glucose by the animal liver. This fibre family contains different chemical components as pectin, betaglucan, MOS (for Mannan-OligoSaccharides), hemicellulose, cellulose or chitin. The benefits of these components are multiple but their main mode of action is the supply of energy to the colon gut flora.

Therefore, in addition of optimizing digestibility in the upper part of the tract, nutritionist must now take into account the requirement for the lower part of the tract. To do so, we will need to add new nutrients in formulation matrix to maximize the supply of energy to gut flora. These new nutrients should track the supply in the diet of a proper mix of fibres and minimize the supply of undigested proteins.

Carnivorous, Herbivorous or Omnivorous

For Carnivorous, the situation is simpler as meat is more digestible. All the nutrients come from the digestion in the upper tract. Their colon is not much developed. They have a very acid stomach and very little requirement of fibres. It is the perfect example of the direct digestion system.

For Herbivorous, the mechanism is mostly indirect. Herbivorous can be polygastric as the cow or monogastric as horse or rabbit but in both cases, we provide them a diet to feed the bacteria population who will in return feed the animals by producing VFA.

But as it is explained in the name, the omnivorous like Swine and Poultry have a dual system, 75% direct like carnivorous and 25% indirect like ruminants. It makes the job of Swine and Poultry nutritionist a little more complex but it should be rewarded by healthier animals and faster growth.

To come back to my story about my kids, my answer to them is that we have 2 stomachs, one that can be fed with meat and starch but another one that can eat only vegetables. I agree that this summary is a little too simplistic but at least it is good enough to make my kids finish their vegetables.


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