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Wednesday 20 June 2012

Training Zones for not quite Dummies.

There are lots of attempts to explain Heart Rate Zones.  Most try and come up with a simple explanation of what your body uses for fuel but by over-simplifying the explanation this tend to create confusion and assumptions (and marketing tools for nutrition companies).

When looking at fuel systems it is best to look at 3 basic questions and then equate those to your training goals.

1.  How much of the fuel source exists

2.  How quickly is it converted to energy by the body

3.  How long does it take to reload or refill this source.

But first lets start with a basic explanation of energy production. A muscle contraction is created using 3 types of fuel - Adenosine-5'-triphosphate (ATP), Creatine Phosphate (CP) and adenosine diphosphoribose (ADP) so at any final point in energy production these need to be produced.

These can be sourced from 4 different sources

1. ATP itself.
2. Glycogen
3. Glucose
4. Body Fat.

So going back and applying our original three questions.


ATP.


1. How much of the fuel source exists.

Not much - about 10 to 12 seconds worth

2. How quickly is it converted to energy by the body

Incredibly quickly - it's already present in the muscle

3.  How long does it take to reload or refill this source.

Around 120 seconds although it is mostly (81%) refueled within 60 seconds.

So ATP is primarily used for very short explosive outputs such as lifting weights.

Glycogen.


Glycogen is converted to ATP through a process called Glycolysis. It is stored within the body in the liver and muscles.


1.  How much of the fuel source exists

If your glycogen levels are 100% topped up you may have as much as two hours worth of Glycogen stored in the liver and muscles.  This level is rarely the case though so it is more like 1:40:00 - 1:50:00

2.  How quickly is it converted to energy by the body

Very quickly - it is already in the muscles and gets in the blood stream quickly via the liver

3.  How long does it take to reload or refill this source.

Glycogen is created from Glucose through a process called gluconeogenesis.  This can take from several hours to days.  The obvious down size if that if you run out of glycogen (commonly called bonking) it will take sometime to reload these stores.  Reloading of Glycogen is commonly what is referred to as Carbo Loading.  The levels of muscle glycogen measured after 24 hours of Carb Loading do not significantly increase if this load time is increased so extended Carb loading sessions arent really necessary and can cause a feeling of bloating.

Glucose.


Blood glucose is controlled through Insulin (lowering blood sugar) and Glucagon (raising blood sugar).  Glucose is converted to ATP through glycogenesis.  It should be noted that there are only 3 different 'natural' sugars - glucose, fructose and galactose.  Any other sugar that you ingest - eg maltodextrin, requires conversion by the body.  The maximum amount of any of these 3 sugar types that your body can convert is about 15g per 15 minute period

1.  How much of the fuel source exists

Not a lot.  In a 75kg person with normal blood sugar levels there is only about 5g of glucose in the blood.

2.  How quickly is it converted to energy by the body

Very quickly - the issue is actually how quickly your body can convert ingested carbs into blood sugar

3.  How long does it take to reload or refill this source.

See comment above - you can reload about 60g an hour which is only about 270 calories worth..

Body Fat


Body Fat is converted to ATP through a process called Lipogenesis.  Body fat is also by far the most abundant fuel supply in the body.

1.  How much of the fuel source exists

A LOT.  1g of fat realises 9 calories.  And incredibly lean Tour De France still has 60(kg) x 0.06(BF) x 9000 (calories per gram) = 32,400 calories. which is enough for 32 hours of hard riding.

2.  How quickly is it converted to energy by the body

Not very quickly

3.  How long does it take to reload or refill this source.

Given the volume in even a lean athlete this isnt really an issue.

For those that like pictures..



So your body burns fuel based on most to least abundant (fat to stored ATP) balanced by the rate at which it can be converted.  This is governed by exertion levels which are measured, as a correlation, by heart rate. Your body, at low levels of exertion, uses Body Fat. As energy requirements (exertion) increases your body switches to blood sugar and then, as requirements increase, glycogen.

Most diagrams show this as a  stacked bar with clear lines between the zones but it doesnt quite work that way.  It is more like

There are two very important take aways from this graph

1.  Your body, even when burning fat, needs a small amount of carbohydrates.  Think of an internal combustion engine - it primarily runs on petrol but still needs a spark for consumption.


2.  At the highest level of exertion your body is using Glycogen which is limited (around 1-2 hours) and takes an extended time to refuel so any form of exercise that extends greater than 1:45:00 (Marathon, Olympic - Ironman Triathlon) CANNOT use glycogen as the primary fuel source.  Of course the flip side is that for shorter events like a Half Marathon or Sprint Distance Triathlon you CAN use glycogen as your fuel source

BTW - The issue with heart rate is that it is an approximation of what fuel source is being used.  The actual percentages can be determined using an exercise metabolic test which involves a mask over the face to measure the amount of oxygen being consumed but also what the 'exhaust' products are.  Each fuel source creates different bi-products so measuring these can determine

a) what fuel source is being used - Fat, Glucose, Glycogen.

b) what amount in calories is being burnt.  In other words - fuel consumption..








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