Understanding the Digestive System

Last week I introduced the idea that no one diet works for everyone.  I also proposed that gut health may be a factor in this fact.  Today I am introducing you to the digestive system so we are all on the same page about how that system operates.

Where do we start?

We’re going to start with the steps that occur from the time you put food in your mouth until the time it comes out the other end.  Your mouth is actually the first digestive organ.  As soon as you think about or smell food, your salivary glands are activated.  This process triggers the release of enzymes, water and mucous.  Their job, along with the teeth is to start the process of breaking down food into small enough pieces to travel down the esophagus.  When you chew your food thoroughly, you expose more of the surface of your food to the enzymes, helping them break the food down.

An enzyme, amylase, is released by the salivary glands which begins to break starches down into sugars.  Starches are an often forgotten source of sugar in the diet.  Thorough chewing also helps the stomach by sending it a product that is easier to digest.  Additionally, the chewing of your food gives your stomach time to prepare for its arrival.

You see, in preparing to accept food, the stomach lowers its pH down to around 2, creating a very acidic environment. Neutral pH is around 7, lemon juice and vinegar are around 2, and battery acid has a pH of 0.  Regarding stomach acidity; we are taught to believe that acid reflux is due to an excess of stomach acid, when in fact, it is more commonly due to a lack of stomach acid.  This is then exacerbated by over-the-counter medications that further decrease stomach acid.  Alternatively, there may be a problem with the lower esophageal sphincter or even excessive carbohydrates in the person’s diet.  Here’s a link to an article on acid reflux for more information.

Yes, I said sphincter.  We have more than one sphincter.  A sphincter is a ring of muscle found at the opening of each tube along the digestive tract.  Every organ along the way – stomach, small intestine and large intestine, has a sphincter at each end.  You can think of the sphincters as gateways or doors that open and close when it is time to pass content to the next organ.

Next stop – the stomach

Once in the stomach, the gastric juices (hydrochloric acid) continue to break down food and kill off any unwanted bacteria from the food.  The main activity going on at this point is the digestion of proteins and the release of a substance called intrinsic factor.  Intrinsic factor will be used later on in the small intestine to absorb vitamin B12.  Vitamin B12 is one of the essential nutrients our bodies cannot make on their own, and must be obtained through food and healthy digestion.  Proteins are broken down into amino acids, which our bodies use to build the proteins created by our cells and used to build muscle and tissue.  Fun fact:  DNA is made up of proteins, which are made up of amino acids.  The proteins we eat are not taken in “as is.”  The proteins we eat are broken down into their constituent amino acids so they can be used by our cells.

How long does this process take, you ask?  Food spends several hours in the stomach.  Carbs cycle out the fastest, after around two hours.  Protein is next, taking between 2-4 hours to cycle out.  Fat takes the longest to break down and leaves the stomach after around four hours.

The resulting product of this broken down food is released 2-3 teaspoons at a time into the small intestine.  That’s a tablespoon or less at a time.  This is another reason to take small bites and chew your food well before swallowing.  You should chew each bite until it is liquid, before sending it down the esophagus for the following reasons:

  • Aids the stomach with digestion by needing to do less work and having time for the brain to receive the message necessary to send in hydrochloric acid;
  • Gives the brain time (up to 20 minutes) to receive the message that you are full, allowing you to feel full with less food;
  • Reduces the risk of obesity and metabolic disease (linked to heart disease, stroke and diabetes).

Digest the rest

Once in the small intestine, food (called chyme now) is further processed, with the bulk of carbohydrate and fat digestion occurring.  The small intestine is also where most of our nutrient absorption takes place, like the B12 mentioned above.  This stage is also where the pancreas, liver and gallbladder step in to lend a hand.

The acid is further neutralized with bicarbonate from the pancreas.  (The stomach initially releases sodium bicarbonate towards the end of its process in order to keep it from digesting itself.)  In the small intestine, enzymes come in from the pancreas and wall of the small intestine to break down the different macronutrients – fat, protein and carbohydrates – from the food.  More amylase is produced to break down carbs, protease continues to break down proteins into amino acids, and lipase breaks down fat into fatty acids and glycerol.

The job of the liver is to produce bile, a digestive enzyme that helps with the break down of fats.  Bile helps to break up the clumpy fat molecules to make their nutrients more water-soluble and, thereby, more absorbable.  Bile is stored in the gallbladder, and called upon as needed.  The chyme is moved around like clothes in a washing machine, while the small intestine works to absorb the nutrients and minerals.

Grand finale

The final step in the digestive process is to take out the garbage, so to speak.  The large intestine, also referred to as the colon, is the receptor of what’s left.  This is where all of those bacteria live; the gut flora I talked about last week.  The bacteria go to work on indigestible fiber, transforming these leftovers into vitamins, minerals and fatty acids in an attempt to let nothing go to waste.  Having enough fiber in your diet is critical to the movement of these leftovers through and out of the colon for final elimination.

My goal this week was to give you an inside view of the complex machinery that is the digestive system.  Although this has been a greatly simplified version, I hope you gained some useful insight into the workings of the digestive system.  Next week we will pull the covers down on the age-old saying, “you are what you eat,” by looking at how nutrients are absorbed during digestion, and the hormones that are involved in the digestive process.

Yours in Wellness,


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