Nutrition 1: The Digestive System

So, we came to the end of my long series of posts about training as a personal trainer.  However, before I move on, I do have a few more posts to make about nutrition and weight management.

There will be a bit of overlap with the basic nutrition posts I did as part of my general course (see my posts on the basics of macronutrients, the roles of macronutrients and the respiration of macronutritents) but not a huge amount.  This series is more concerned with the big questions of what diet is appropriate, why it is appropriate and whether certain “fad” diets are healthy or not.

I will do my best not to get too wound up but two things persistently bothered me about the material:

1. There was a lot of material in my course that flew in the face of a lot of research that I have read; and
2. The material was presented as fact rather than conjecture even where the statements made were clearly hypotheses.

Having said that, there are a few general sections at the beginning, starting with a discussion of the digestive system.  I’lltry not to disagree with them too much.

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My course notes

The digestive system is basically everything from chewing up food, to swallowing it, eating it and getting energy from it.  Here’s a little diagram to show its basic components.

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• Mouth – the mouth is where we ingest food and carries out the process of mastication and also performs some digestion.  The tongue performs several functions, including assisting with mastication, moving food to the oesophagus and taste.  The tongue has taste buds that can be mapped to specific regions where we experience sweet, sour, bitter and salty sensations.  The mouth also produces saliva, which carries amylase, which is an enzyme that digests carbohydrate.
• Oesophagus – the oesphagus is a short, muscular tube leading to the stomach.  Rhythmic contractions (called peristalsis) in the smooth muscle walls drive the food onwards.
• Stomach – an empty stomach is only about 50ml in capacity but it can stretch to up to 4L.  The stomach is where protein is digested and requires a high acidity for the enzyme pepsin to work. Acid is secreted according to the action of the homone gastrin, which is stimulated by thought or sense of food.  Mucus on the stomach wall protects the lining from the acid but where this is breached it can cause ulcers.  Interestingly, only alcohol and certain drugs such as aspirin are absorbed directly through the stomach wall.  Everything else passes into the small intestine for absorption. 
• Small intestine – the small intestine is divided into three parts, as follows:
  1. Duodenum – the partly digested food from the stomach arrives here and is immediately covered in alkaline solution, a bicarbonate made by the pancreas.  Further digestion of protein and carbohydrate occurs here.  Also, the bile duct delivers bile (made from cholesterol), which emulsifies fat and lipase begins to digest it.
  2. Jejunum – most digestion occurs in the duodenum and most absorption occurs in the jejunum.  Hence, the jejunum has many deep folds and is covered with small, hair-like projections called cilia to create a very large surface area for absorption.  Cilia are supplied with blood capiliaries and lymphatic capiliaries called lacteals.  Emulsified fat droplets are absorbed into the lymphatic system via these lacteals and eventually enter the blood stream.
  3. Ileum – most of the absorption occurs in the jejunum so the ileum’s job is mainly to complete any absorption that has not been done and to reabsorb the bile salts.
• Large intestine - the large intestine is in fact smaller than the small intestine because it’s a lot shorter but it does have a larger diameter, hence the name.  The principal function of the large intestine is the eliminate the indigestible part of the food that we eat.  Fibre makes it easier for the large intestine to eliminate the food waste as it gives the gut something to push against.  Lactose that has not been digested by the body is fermented at this point and can cause flatulence and diarrhoea.

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My thoughts on the digestive system

The Mouth

You’d think I would be able to agree with the course notes on the simple subject of a mouth but no!  What could I disagree with?  Well, it’s a basic concept in zoology that you can learn about an animal by observation and the interesting observation about human teeth is that they do not appear specialised.  Therefore, biologists and nutritionists have typically inferred that this means we have adapted to an omnivorous diet.

This, certainly, is the standpoint of my course notes, which state that our dentition is “unspecialised”.  An interesting phrase, which implies to me that they are not adapted to any specific purpose.  However, that doesn’t qute ring true for me.  After all, chimpanzees have adopted a similarly omnivorous diet, with lots of fruit and nuts and the occasional monkey.  But they have some pretty useful teeth, as you can see in the photograph below.  I mean, how cool would it be to have tusks like that!

 Richard Wrangham explains How Cooking Made us Human 

What elses can I disgaree with?  Well, it’s not set in stone that the tongue has specific areas for tasting sweet, salty, bitter and sour sensations, as this article makes clear.

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The Stomach

I’m going to save my comments on the stomach until I’ve had a chance to read Robb Wolf’s new book The Paleo Solution, as Anna tells me it will help me piece together some of my thoughts on this subject. 

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The Small Intestine

I read an interesting post recently by Charles Poliquin, which noted that oestrogen is excreted through the large intestine having been converted into another substance in the liver.  Charles notes that some bacteria can reverse this conversion, causing the oestrogen to go back into circulation, thereby exposing the body to more oestrogenic effects.

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The Large Intestine

You’ve probably seen the quote “death begins in the colon” quite a lot on the internet, although most sites that refer to it seem to be trying to sell the process of colonic irrigation… Despite who you may have seen it attributed to, the quote actually comes from a famous Russian scientist, Elie Metchnikoff.

Metchnikoff did pioneering work on immunology and was the creator of the hypothesis that white blood cells enveloped bacteria and destroyed them.  Previously, people believed that white blood cells were actually harmful and spread disease throughout the body.  His work led to further research into bacteria and gut health and led to the development of fermented diary products being used in the treatment of the immune system and probiotics.  You can read more about Metchnikoff in this remarkable character study.

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I’m going to stop there in the interests of common decency.