In today's Globe and Mail Alex Hutchison wrote an article on the merits of training / exercising 'empty.'
This is a method I have been advocating for 25years.
Essentially it calls for exercising on an empty stomach 3-6hours away from the last meal.
This is an easy route for your body to directly access fat stores for fuel. This is the lipolysis pathway. (lypolytic pathway)
With normal blood sugar levels and slightly depleted glycogen stores (this is the storage form of carbohydrates)
your body will fire up the enzymes for fat metabolism immediately. If you have fueled with sugar or gel packs the blood sugar rises, and the body will choose the sugars for fuel rather than go searching for fat for fuel. I believe that fuelling with constant supply of carbohydrates is inefficient and will cause many people to actually gain weight rather than loose fat because there is a constant demand for higher blood sugar and no burning of fat.
Like any other part of training this take will take practise to master. The more often you do not fuel for short workouts the more efficient the enzymatic pathway will become allowing you to go longer and longer durations without 'fuel.' You will still need fluids rich in electrolytes but not sugar.
As an endurance athlete I have cycled from Toronto to London Ontario (240km) on the back roads with nothing more
than 3-4litres of electrolyte drink rich in sodium, potassium, magnesium, calcium and chloride. These allow the muscles to
continue to contract with force and not become too saturated with plain water. Too much plain water leads to HYPONUTRIMIA.
Too much water not enough electrolytes and muscles slow their contractions. A drink rich in electrolytes also allows the body to access fat for fuel and not depend on carbohydrates / sugars.
The lipolytic pathway is interesting and far more detailed than outlined here. If you have further questions feel free to comment. Recreational runners would do themselves a great service to stop feeding on starch and start eating for high nutrient, normal portion sizes and let the body figure out where to get its' fuel from.
Like your mother always said, "don't swim with a full belly." Well don't exercise with a full belly either. The only competitors loading up at the pasta dinner are the recreational runners. The elites eat a balanced meal of protein, fat and some starch based on their lean body mass only.