This post is part of an ongoing series about my learning process as I train to become a personal trainer.
Today is the third part of three posts about nutrition. In the first post, I looked at the basic divisions of the three macronutrients. In the second post, I go into the roles of the macronutrients in the body. In this final post, I look at the ways in which the macronutrients are digested.
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Digestion of the macronutrients
I’ve read a great deal on the internet about the complex reactions that result in the release of energy at the cellular level but I’ve never actually seen anyone address how you get from food through to the basic building blocks of the macronutrients (i.e. glucose, amino acids, fatty acids and glycercol) on a fitness blog. Probably, because the process is so basic.
The underlying process for the digestion of all the macronutrients is hydrolysis. Hydrolysis is a mechanism in which organic (carbon-based) molecules are split into simpler forms that the body can deal with more easily. In essence, hydrolysis is simply “adding water”.
- Carbohydrate: complex carbohydrates such as starch are hydrolysed to form monosaccharides, predominantly glucose.
- Lipids (fats): lipids are hydrolysed to form glycerol and fatty acids.
- Protein: protein is hydrolysed to form amino acids.
Once the macronutritents are broken up into their respective building blocks, we then have to consider how they are then treated by the body in order to produce energy. Breaking the macronutrients up into the basic elements occurs before absorption through the gut wall. It is performed by means of various enzymes.
Once the macronutrients are broken up into glucose, amino acids, fatty acids and glycerol, however, they can be absorbed and are then variously processed.
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Carbohydrate (glucose)
This pathways down which glucose goes in order to be metabolised are quite complicated and it took me a while to get my head around them. In fact, there is no guarantee that I have actually got my head around them!
Essentially, though, glucose is converted firstly into pyruvate by a process called glycolysis. This process releases a small amount of energy in the form of ATP. After this, the pyruvate can be metabolised either by anaerobic or aerobic respiration. Here is a diagram:
The diagram shows the various pathways that carbohydrate can follow in being converted to energy. I’m going to do some posts about anaerobic and aerobic respiration as part of my self-education on energy systems, so I have only mentioned that briefly here. These are the key steps:
- Glycolysis: this process takes glucose, adds the energy from turning 2 ATP into 2 ADP and creates 4 ATP (from 4 ADP) and pyruvate.
- Oxidative phosphorylation: the pyruvate from glycolysis is pumped into mitochondrion. One CO2 molecule and one H2 molecule are removed from the pyruvate to produce an acetyl group, which joins to an enzyme called CoA to form acetyl-CoA. Once acetyl-CoA is formed, two processes can occur, aerobic or anaerobic respiration.
- Aerobic: if oxygen is present, the mitochondria will perform aerobic respiration on the acetyl-CoA through the citric acid cycle.
- Anaerobic: if oxygen is not present, lactic acid fermentation of the pyruvate molecule will occur.
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Lipids (fatty acids and glycerol)
Lipids can be metabolised by almost all of the tissues of the body. Most notably, they cannot be metabolised by the brain. This fact is often belaboured when discussing carbohydrates.
- Gluconeogenesis: Glycerol can be converted to glucose by the process of gluconeogenesis (see below).
- Beta Oxidation: Fatty acids pass through the Beta-Oxidation pathway. This is a four-phase pathway, where the fatty acids undergo dehydrogenation, hydration, dehydrogenation and finally cleavage by thiolase, yielding acetyl-CoA and a fatty acid that has now been shortened by two carbons (acyl-CoA). This fatty acid is then recycled through the Beta-Oxidation pathway until it has been fully metabolised. The acetyl-CoA then enters the citric acid cycle.
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Protein (amino acids)
Protein, in the form of amino acids, is processed by the body through a process known as gluconeogenesis. This is an 11-step metabolic pathway that results in the generation of glucose from non-carbohydrate carbon substrates such as lactate, glycerol, and amino acids. Many of the reactions are the reverse of those found in Glycolysis.
As a by-product of the anaerobic respiration process, lactate is transported back to the liver where it is converted into pyruvate by the Cori cycle before entering the process of gluconeogenesis. Pyruvate can then be used to generate glucose. Glycerol and amino acids can enter the process of gluconeogenesis directly.
