The most important consideration when it comes to nutrition is not to try new things on race day. You do not want to find out that something upsets you stomach in the middle of a race that you have spent months training for.
Given this, you need to try different pre run fuelling strategies in advance of your race.
For shorter distance training runs I often do not even eat breakfast. The problem for me is that if I eat breakfast then I need to wait for it to digest sufficiently for it to actually help. This usually means waiting a minimum of 45 minutes which depending on your schedule could be a deal breaker. As such, if I am running less than 5 miles I tend not to eat breakfast at all. Your results may vary but I find that this is also a good way of training - somewhat depleted. It mimics those final miles of longer distance races.
For harder workouts (speed sessions for example) I find that a small breakfast is needed - a strong coffee, a banana, and an hour long wait usually does the job. Again, trial and error. Training for a race is as much about trying and testing your nutrition options as it is hitting the roads.
If you are doing a long run for example then having breakfast is vital (in my opinion). Bonking/'Hitting the Wall' is a genuine concern for marathon runners whereby they simply run out of energy and cannot go on. There is a bear minimum amount of energy that one requires to run and over a long distance there is a much higher likelihood that you will deplete any available stores.
The general consensus is that carbohydrates are the most important fuel source for running.
That said, for longer distance endurance events many argue that alternate fuel sources are appropriate as there is less need for the 'quick' energy that good carbohydrates provide. Some ultra distance runners for example swear by the Ketogenic diet which is premised around using fat for fuel and intentionally minimising carbohydrate intake.
I have personally tried and tested all sorts of different approaches to breakfast (and nutrition more generally) and personally find that a carbohydrate heavy diet is necessary for optimal performance. For breakfast I have settled on a fairly simple option: Oatmeal. I eat oatmeal near enough every day and as such my body (I assume) knows exactly what to expect. Eating well is difficult enough as it is and I personally find that eating the same breakfast daily means that I have one less nutrition based decision to make each day.
Oatmeal is carb heavy and has a high glycaemic index. That means that the energy contained within can be digested and used efficiently without putting unnecessary strain on your digestive system. I often mix in a banana for flavour or eggs (apparently this is strange) to add some protein (for muscle repair) and creaminess.
Bananas are high in Potassium, Magnesium, and dietary fibre. All three are important for optimal body function - energy metabolism and muscle development. Other fruits and 'oatmeal toppings' provide similar nutritional benefits whilst providing a little variety. Some people swear by peanut butter for example.
Whilst selling the value of carbohydrates (above) I think it is important to note that not all carbohydrates are a good breakfast option. For example energy gels (that many use during races) often times contain raw sugars and maltodextrin. These sugars have an extremely high glycaemic index and are absorbed almost instantly. If you eat energy gels for breakfast or load your oatmeal with sugar it may well result in an immediate increase in blood sugars but it may also mean that when you start running an hour later those energy sources are no longer available. The quick manipulation of blood sugar levels may result in you feeling tired - exactly what you do not want.
If you browse the web for appropriate runners breakfasts you will find articles like the following:
It soon becomes apparent that both Oatmeal and Bananas regularly appear on these lists. My view is that (assuming that you like both oatmeal and bananas) you keep it simple.
Previously I have tried fuelling on cereals and milkshakes/smoothies. With the former I found the high processed sugar content of most cereals to be an issue and also found that the milk (which i had with the cereal) left me bloated and unable to run. Bloating is also an issue with milkshakes, and I find that whilst fruit sugars are better than processed sugars the amounts that you find in a medium sized fruit smoothie are not ideal.