Skipping breakfast

I used to skip breakfast in the mistaken belief it would help me lose weight and sessions done would increase my "fat burning". In fact it doesn't for the reasons below.

The average person has 450-550g of glycogen stored. If depleted this will be replenished as a priority by any carbs digested from food (and to a lesser extent from fat reserves). Only after it is replenished will extra carbs be stored as fat. Each g of glycogen provides about 4kcal worth of energy.

Glycogen is stored in 2 places
- 80% Locally in muscles, prime use is to provide energy for exercise. Glycogen will only be available for the muscle where it is stored
- 20% Is stored in the liver. The prime purpose is to provide energy for non-muscular body functions that require glucose, the most important of which is brain function. As a secondary function it will provide a top up to muscles whose glycogen is starting to get depleted. (Note: this happens before the glycogen level in the muscle hits zero, at around 25-30% left.)

Assuming your glycogen stores are full on kipping down, following a night's sleep you will wake with
- Practically all of your muscle glycogen still available (assuming you have have not been involved in some serious "interval training" with your partner :) )
- 50% of your liver glycogen available (the rest will have been used to provide the energy the brain needs to create your dreams/nightmares)

So if you went to bed with 500g of glycogen stored you will wake with 450g.

Even taking into account the fact that not all the muscle glycogen is available for training this is still allows a lot of exercise, very roughly there is enough to allow a 70kg rider to climb Alpe D'Huez one and half times flat out without feeding.

If you are not going flat out then you will be able to go for an even longer as fat provides an additional source of energy at intensity levels less than flat out (around 30% at endurance pace).

Skipping breakfast will however make training sessions FEEL harder. This is because on waking your blood sugar level will be low and, since the brain needs sugar to work at its best, this will make you feel rough. Missing breakfast and going straight into training will aggravate this. Worse, if your session is quite long and you don't eat during it after a while your liver glycogen will start to get seriously challenged

Skipping breakfast means that the liver glycogen is level only 50% and this will continue to decline. It will decline even quicker if some of it is needed to help out muscles whose local glycogen levels are getting low. Once the liver level hits around 10% the brain signals that things are getting serious. Pushing past this point without eating will be extremely tough regardless of how fatigued your muscles are or how much  muscle glycogen you have left. Push too hard and you will collapse big time. All this is very stressful but has very little benefit in terms of physical fitness, though it may make you mentally tougher.
 
So bottom line is that skipping breakfast really has little impact in any sense that will affect your fitness unless the subsequent training session is very brutal and/or long.  You will only by training in a really "fasted" state if you eat and train in a way that stops your body replenishing glycogen (i.e. running a serious calorie deficit and/or following a very low carb diet). Even if you do this best advice is to have eat a small amount of high GI carbs before exercising first thing in the morning and, if planning to ride more than a couple of hours, eat a few more during it just to keep brain ticking over.