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Cooking, processing and the microbiota, their importance in digestion

How are nutrients released during digestion?
This figure shows how food structure leads to different amount of nutrients available for absorption. Large structures (bottom left) have nutrients trapped within the cell wall, some of which will be released for digestion as cells aredamaged in processes
© Quadram Institute

How are nutrients released during digestion?

Does cooking and processing affect the nutrients available to us and what food constituents reach the colon and our microbiota?

These questions will be answered in this step.

How are the nutrients released? In the video you watched earlier you learned about the processes that need to happen to release nutrients from inside foods so that they can be absorbed and used by the body. The macronutrients (protein, starch and fat/lipids) are too big to pass through the intestinal wall and must be digested by enzymes into their smaller molecules in order to pass through the gut lining. In the case of plant-based foods, If the cell wall stays intact, those nutrients are kept within the cells, and since the digestive enzymes cannot reach them, they are not available for digestion and absorption.

However, when the cells are broken or damaged, then the nutrients are available to be digested and absorbed. This is achieved naturally by chewing our food. Mechanically grinding food between our teeth will break up the food into particles and damage some cells, making some of the nutrients available for digestion.

But did you know that cooking and processing will also break open the structure of plant tissue? In fact, the more extensive the processing the greater the nutrient availability will be further up the intestine. The cooking method has a big impact on its digestibility, and therefore how far down the gut it goes before it is absorbed (small intestine) or broken down by our gut microbiota (large intestine).

In raw starch granules (remember the raw kidney beans from the last section?), amylose and amylopectin chains densely packed and this makes it difficult for the enzyme amylase to access and digest it, which means a very slow digestion. Although we eat some raw starch (eg uncooked oats, underripe bananas, etc), most of the starch in the diet has been cooked before eating.

For example, when starch is cooked through a process involving heat and moisture (e.g. baking a loaf of bread, boiling potatoes) the granules swell and become less ordered, so that the glucose chains are more spread out and easier for the amylase enzyme, in the upper small intestine, to digest, leaving less ‘resistant starch’ for the microbiome.

When starch is cooled after cooking, some opened up amylose and amylopectin glucose chains can re-associate again, which makes it difficult for the amylase to work. This process is called retrogradation and it increases the amount of resistant starch in cold foods.

Pulses such as beans, chickpeas and lentils that have been cooked whole still contain many intact plant cells, where the nutrients are trapped inside the cells, and can help to deliver resistant starch to the colon.

Whole grains/seeds are milled to produce flour, as the particle size is reduced, the tissue fractures and intracellular components are released, reducing the amount of dietary fibre reaching our gut microbiota in the colon.

What food constituents reach our large intestine (otherwise known as colon) and what happens there? Science is still finding out! Fibre/cell wall polysaccharides are certainly major components. Research has shown that sweetcorn, lettuce leaves, bean cells, bran (outer layers of wheat), carrots, nuts and mushrooms have been observed in samples taken from the end of the small intestine (known as ileal effluent) from ileostomy patients. As we learnt, cell wall ‘fibre’ is not digestible in the upper gut and so the nutrients inside these cells are carried further down into the colon, ready for the gut microbes to work on.

The main functions of the colon are as an ‘anaerobic fermenter’ and for re-absorption of water and electrolytes. Within the colon there are ‘primary degraders’, which include multiple bacteria from the Bifidobacterium spp and Bacteroides spp, as well as Ruminococcus brommii. These ‘primary degraders’ have specialised enzymes that break down fibre. Another set of bacteria belonging to the Firmicutes spp, the ‘secondary degraders’, use the glucose produced by the ‘primary degraders’ to produce a short-chain fatty acid, butyrate. Butyrate is the major source of energy for the human colon cells.

What can we conclude from this step? The nutrient composition information on food labels tell us about the nutritional ‘potential’ of the food, but we need to understand the food structure, digestion and absorption to more accurately predict its nutritional value or health effects. The structure of food and the processing of our meals affects the availability of ‘food’ for our gut microbiota, which in turn can affect our health.

© Quadram Institute
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The Human Microbiome

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