Endurance fueling has three sources:

• Nutrition: during exercise, the body’s nutritional demands as a function of the duration of the event. Shorter events draw upon glycogen (carbohydrate) stores while longer-distance events utilize more fat sources.
• Hydration: 70 percent of the human body is water. Minimum water consumption during an endurance event should be 16 to 20 ounces per hour, and up to 34 ounces in austere conditions.
• Electrolytes: the minerals sodium, chloride, magnesium and calcium help to retain water across the body, including within muscles.

The body can retain approximately 1800 to 2200 calories of glycogen when fully fueled. Incidentally, pre-race fueling occurs at breakfast and lunch on the day before the race; eat a normal dinner that evening, followed by a normal breakfast the morning of the event. During intense exercise, the body can burn 600-1500 calories per hour, though it can only absorb 200 to 600 calories per hour. Based upon the longevity of an endurance event, refueling strategies must be flexible and call upon different sources of calories:

• For events up to 90 minutes: no additional calories are required because the primary fuel source of glycogen is sufficient. The hydration goal should be between 16 to 20 ounces of water per hour.
• For events between 90 minutes and four hours: glycogen stores will become depleted, and the body’s fueling transitions to free glucose and carbohydrate. Gels or honey should be the sources of additional calories, targeting 300 calories per hour, per 20 ounces of water. Additionally, electrolytes begin to have a larger role during this time domain.
• For events between four and 12 hours: as intensity decreases, the body is better able to digest additional calories. The body’s fueling moves from glucose to fat, and carbohydrate consumption from gels or honey must continue at the rate mentioned above in order to fuel fat burning. Electrolytes must be replaced at a rate of 500 to 2000 milligrams per hour.
• For events between 12 and 18 hours: protein must be added to account for approximately 10 percent of consumed calories. Fat begins to account for 20 to 35 percent of total burned calories. Electrolyte and water prescriptions remain identical to the four to 12-hour protocol. The body requires 300 to 600 calories per hour, in the percentages of 60 to 70 percent carbohydrate, 20 to 30 percent fat and 10 to 15 percent protein. Potassium stores should be replaced once every three to four hours via electrolytes or bananas.

Gel and honey guidelines:

• Within an endurance event, insulin is required to open the body’s cells to accept and convert energy from carbohydrates. The most effective way to accomplish this is via a gel or honey.
• If two different sugars are consumed (e.g., gel and honey, or gel and Gatorade, etc.), the stomach’s osmolality (the concentration of ions, such as sodium or potassium) increases, which will almost always result in sickness. In other words, the body must expel the stomach’s contents or slow down in order to aid digestion. We recommend choosing one carbohydrate/sugar source and consuming it with water.

Hydration highlights:

• For most athletes training and racing in time domains over 60 minutes, and in temperatures between 72 and 76 degrees, 16 to 20 ounces of water per hour is an adequate target. To establish hydration targets for higher temperatures, please visit this consumption calculator at: http://www.gatorade.co.uk/.
• Dehydration begins when two percent of water loss occurs. Less than a two percent water loss constitutes “manageable” loss; five to six percent loss results in sleepiness, headaches, nausea and numbness/tingling in the extremities; 10 to 15 percent loss results in loss of muscle control, impaired hearing and diminished vision; 15 percent loss results in death.

Electrolyte highlights:

• Electrolyte management is vital in endurance efforts, and both under- and over-prescription of electrolytes can be fatal.
• Hyponatremia (decreased sodium concentration in blood) is caused by sodium loss in the blood due to under-consumption of electrolytes. This is typically due to excessive intake of water. In extreme cases, this sodium-free blood travels to the brain, permeates brain cells, causes brain swelling and ultimately death.
• Hypernatremia (elevated sodium concentration in blood) can be caused by both over-consumption or under-consumption of water, but it is more commonly associated with the latter.
• Electrolyte guidelines:

Electrolyte	Role	Target Dose Per Eight Ounces of Water	Daily Performance Target
Sodium	Muscle contraction Nerve transmission	150 to 250mg	1500 to 4500mg
Chloride	Peak muscle function	45 to 75mg	45 to 75mg
Potassium	Muscle contraction Nerve transmission Glycogen formation	50 to 80mg	2500 to 4000mg
Magnesium	Muscle relaxation ATP production	20 to 30mg	400 to 800mg
Calcium	Bone health Nerve transmission Muscle contraction	10 to 15mg	1200 to 1600m

• Electrolyte management can be very personal and can vary dramatically from athlete to athlete. The only way to confirm electrolyte needs is to perform a sweat rate test:
  • Weigh yourself without clothes
  • Perform a 60-minute time trial at goal race pace in a temperature-controlled environment
  • Weigh yourself after the effort
  • Subtract one pound for every 16 ounces of water consumed during time trial
  • Once you have confirmed total weight loss, you can then correlate each pound lost with the following loss in electrolytes: 220 milligrams of sodium, 63 milligrams of potassium, eight milligrams of magnesium and 16 milligrams of calcium