Just how much energy do you have at your disposal for a long run like the Ridge Run? The formulas here are simplifications of more sophisticated accurate formulas, but they work well for planning energy needs.
First lets breakdown your energy reserves and define some terms. This blog post is going to get a bit technical and will appeal to true running nerds.
2o calories per pound body weight
Glycogen or muscle sugar is your body’s quick and easy to access stored energy reserves. Glycogen is the fuel source your body first reaches for, especially during high intensity efforts. Unfortunately, it is quite limited in amount.
To estimate your glycogen energy reserves just multiply your body weight (in pounds) by 20 calories per pound to get total energy in calories.
An example of a 150 pound person:
150 Pounds * 20 Calories per Pound = 3000 Calories
This formula applies to your full glycogen storage. To get to this level, requires proper nutrition and copious refueling with carbohydrates especially after activity. Lack of rest and inadequate refueling can significantly reduce your glycogen reserves.
4000 calories per pound body fat
Body fat represents a large source of energy reserves. Unfortunately, it is not quick and easy to access. There is 4000 Calories of energy in each pound of body fat. Obviously, we have many pounds of body fat and 10s of thousands of Calories of available energy from fat.
Using 10% (a very lean fit male) as a body fat content by weight, a 150 pound man would have:
150 Pounds * 10% Fat * 4000 Calories per Pound Fat = 60,000 Calories available.
Fat is the fuel source your body utilizes at greater percentages during lower intensity efforts where the rate of energy expenditure is lower. Your body’s ability to use fat as a fuel improves with long endurance style training.
Energy Usage Running
1 calorie per pound body weight per mile
A conservative (generous) simple formula for energy required to run a mile is about 1 calorie per pound of body weight per mile. For a 150 pound runner like myself this would represent 150 calories. The energy expenditure is about the same regardless of the speed you are running. Obviously, there is a speed that is your most efficient. Shuffling along at a painfully slow speed or racing at near your maximum uses a bit more energy, but not enough to make significant changes to the 1 calorie per pound per mile formula.
Energy Usage Climbing Uphill
1 calorie per pound body weight per 1000 feet of vertical gain
A conservative simple formula for energy required for climbing is about 1 calorie per pound of body weight per 1000 feet of climbing. This is in addition to the energy required running the distance over which you gain or climb.
For example a 150 pound person running 1 mile at steep 15% grade would require:
150 + 150 * ( 0.15 * 5280 / 1000 ) = 150 + 119 = 269 Calories
Energy Needs for the Ridge Run
The Ridge Run is about 20 miles long and climbs 7000 feet. The energy needs for a 150 pound person is:
( 150 Pounds * 20 Miles * 1 Calorie Per Pound Per Mile ) + ( 150 Pounds * 7000 Feet * 1 Calorie Per 1000 Feet ) = 3000 + 1050 = 4050 Calories
4050 Calories is provided by only 1 pound of body fat if you could use just fat during the Ridge Run. 4050 Calories is more than the glycogen reserves of 150 pound person, 3000 calories. You cannot complete the Ridge Run on glycogen reserves alone. In fact, if you use up your glycogen reserves, you bonk (become exhausted and have to slow down significantly).
The question becomes how do you train your body to use fat as a fuel to spare your limited glycogen reserves? And is there anything else you can do to spare your glycogen?
One thing you can do is supplement your energy needs by taking in calories (food) during the race. You can safely ingest about 100 calories per hour. More than that can cause stomach distress or decrease performance as energy (blood) is diverted from running to assimilating the food. As the intensity of activity increases, the ability to take in food decreases.
Using the 150 pound runner again as an example and utilizing 100 calories of food per hour during a 5 hour Ridge Run will add another 500 calories. Figuring on safely depleting 2/3, or 2000 of 3000 glycogen calories yields 2500 calories out of 4050 calories. This leaves 1550 calories that must be supplied by body fat. This in nearly 40% of your energy needs. This is a significant fraction. Reaching this level fat burning requires adaption that occurs over long term endurance training.
Doing long runs of 2 hours or more is probably the best way to train your body to use your body fat as a fuel. Remember, the Ridge Run is a third longer duration and more significant effort than a marathon and demands the requisite training.