First I should say I am not a Sports Scientist, Coach, etc.  What I understand here is just my own reasearch from the web hopefully made simplistic.

Most people talk about weight loss and dieting and see the two as connected and the be all and end all.  I think it is easier to understand if you talk about energy.

Your body needs energy for everything you do and you get that energy through food.  So very simplistically if you take in less energy than your body requires you will loose weight and vice versa.  Your body though turns your food into two types of energy; fat and carbohydrate.

Both of these energy sources are used in different ways.  Fat is a very slow release energy and even skinny people have enough fat reserves to keep their body going for a long time.  Carbohydrate is a very fast burning energy but your body does not have much of it at any one time.

If we now apply this to exercise then if you are doing some high intensity exercise (such as sprinting) then your body will immediately use the fast burning carbohydrate.  This works very well but can’t be sustained for very long.  If we are doing a very low intensity exercise (such as walking) your body will use the slower burning fat.  This is why there is the concept of the “fat burning zone” at low intensity workouts.  The problem for somebody using this zone purely for the benefit of weight loss is that fat is such an efficient enrgy source that for a long period of exercise you have had to supply little  of your fat energy and hence minor weight loss.

From an endurance perspective as we do a long distance cycle we want our body to use the fat efficently to keep going for a long period of time.  At the same time we also are hoping to go faster.  What we therefore have to do is balance the energy use between fat and carbs.  Ideally we want to raise the level from which our body will cross over from fat burning to carb burning.  We achieve this by doing base training.

To do a base training program we have to work with either heart rate zones or power meter zones which is a topic in its’ own right.  Google will help.

The best example I found of it in practice was a description from a triathlete somewhere on the web.  He was competing at a very high level but always found that in the final run section he was going as fast as he could yet those around him looked more comfortable.  He then assessed his training and started using a heart rate monitor (this was an old article and HRM training was new).  He began a program of running in the “fat burning zone”.  His running pace went up from 5 1/2 mins/km to 8 1/2 mins/km.  Over a period of time though his times came down until he was slightly faster than when he started, the difference was his heart rate was now still low.  This meant he now could run his final run stage at a comfortable pace, faster than before, and if needed still had fast burning carbs remaining for a competitive sprint.

What the triathlete had done was raised the level at which his body burned fat before converting to using carbs.

What needs to be considered here is he is only running for approximately 30 mins.  When we do an event like the Etape we will potentially be going for 6-10 hours depending on your ability.  Your body does not carry enough energy to do that so you will have to take carbohydrate drinks and energy bars with you that can be converted quickly from food to energy.

The reverse of this can now be applied for training.  If you want to loose weight and get up the mountains easier then getting a good base will help you achieve this faster.  You have raised the level at which your body uses fat as a fuel source.  You can now increase the intensity of your training rides but burn fat and not carbs.  You are now capable of going on longer training rides and hence burn more of that hard to get rid of fat!  Again there is a balance to be found here.  As I said previously you can not manage a long event in this way or you will “Hit the wall” as used in marathon terms or “bonk” in cycling.  This is basically your body has run out of energy and needs food and rest to get going again.

My personal level last year I found I could do a 2-2 1/2 hour ride on just water.  This meant no extra fuel coming in and I was fat burning.  In training for the Etape I went from 15 1/2 stone to 12 1/2.  It makes a huge difference on the hills.