Should my Fuel Change as I go Longer?
By Kris Walker MD

Ultra-endurance events, defined as events that last over 5 to 6 hours, are increasing in number and popularity.  These include 24 hour mountain bike races, ironman and half-ironman length triathlons, 100 mile trail runs and mountain bike races, and even multi-day events.  These competitions are challenging even to finish and require specific training and nutrition strategies.

A 70 kilogram athlete has about 1400 kilocalories of stored muscle glycogen, 320 kcals of liver glycogen, and 80 kcals of blood glucose.  During shorter races, this athlete may only require 150-200 kcals per hour to maintain blood glucose levels, relying on his glycogen stores for the remainder of his caloric needs.  During an ultra-endurance event, however, this athlete may burn over 6000 kcals and glycogen stores will become depleted.  Nutrition strategies must address this caloric deficit.  Other aspects of nutrition to consider during competition include the nutritional composition (carbohydrate, protein, fat) and vehicle (liquid, gel, solid), convenience and availability, palatability, digestibility, and tolerability. 

Carbohydrates are the cornerstone of fueling for endurance.  Studies have shown that carbohydrate ingestion during aerobic activities over 45 minutes improves performance.  It is even more important to consume carbohydrates during long efforts and it is recommended that 300-400 calories be consumed per hour.  It is thought that exogenous carbohydrate oxidation maxes out at about 1.0 to 1.1 gms per minute, or about 60 grams per hour and this rate of oxidation can be achieved by increasing carbohydrate intake to 1-1.5 grams per minute, or 60-90 grams per hour. The limiting factor of oxidation is probably related to intestinal absorption and/or delivery of glucose by the liver.  By mixing carbohydrate sources, the maximum rate of oxidation can increase to about 1.25 grams per minute. This may be due to increased capacity for intestinal absorption, potentially by utilizing more than one transporter to cross the intestinal membrane.  (Jeukendrup 2006, Jeukendrup 2000)

It is known that adding protein to carbohydrate in a ratio of about 1 to 4 post-workout will speed up glycogen repletion.  Some recent studies have also shown that a carbohydrate/protein combination during exercise can prolong time to exhaustion and improve performance.  Protein is difficult to digest during exercise, though, especially during exhaustive exercise.  Branched chain amino acids have been studied and shown to improve performance.  Several reasons may explain these benefits.   First of all, adding protein may simply increase calories and available fuel.  Branched-chain amino acids (BCAAs) may reduce central nervous system fatigue by competing with seratonin to cross the blood-brain barrier.  BCAA’s also may replenish Kreb’s cycle intermediates during gluconeogenisis.  The addition of protein and/or BCAAs to carbohydrates also has been shown to reduce muscle damage during exercise bouts.  (Saunders 2007, Romano-Ely 2006, Greer 2007, Saunders 2004, Ivy 2003)

What about fat? Fat can be used for fuel but only at relatively low exercise intensities.  Medium-chain triglycerides have been studied during exercise, as they are more rapidly metabolized  for immediate energy.  However, they have not been shown to spare glycogen during exercise and tend to cause gastric distress.  Dietary fat may replace calories after an event, but does not replenish or spare glycogen stores or provide effective fuel during high intensity exercise.  (Goedecke 2000, Horowitz 2000)

As far as the vehicle used to deliver the fuel, this is mostly a matter of personal preference.  Some people prefer liquid calories: they are easily absorbed, provide necessary fluid, and may provide needed electrolytes.  A 6-8% carbohydrate solution actually speeds fluid absorption.  However, overhydration is a concern with only taking liquid calories.  It is currently thought that the current recommendation of drinking  600-1200 milliliters of fluid per hour in events lasting longer than an hour is too high.  Ultra-endurance athletes should adhere to more conservative guidelines of 300-800 milliliters per hour, depending on body size, event type (cycling vs. running) and actual fluid losses.  Additional calories can come in the form of gels, energy bars, or JPF (just plain food).  (Peters 2003, Hew-Butler 2006, Noakes 2004)

This brings us to the next set of considerations, which are:  convenience, availability, and cost.  Gels and bars are convenient and easy to carry.  Real food is usually cheaper than gels and bars but may be less convenient to carry during an event.  The presence or absence of aid stations and knowing which nutritional products will be available is a vital part of planning for a race or event. 

The final three considerations:  palatability, digestiblity, and tolerability are perhaps the most important.  Palatability is important:  you definitely do not want to be dreading your next gel or energy bar.  Digestibility is even more important and goes hand in hand with tolerability.  The last place that you want to find out that your new energy bar gives you diarrhea is during a 100 mile trail race.  NEVER try a new product or nutrional strategy during a race.  Again, NEVER try a new product during a race.  This is very important.  Your nutritional strategy should be planned and practiced in training BEFORE you try it in a race.  Practice under simulated race conditions as much as possible in terms of temperature, intensity, and duration. Your training should incorporate experimentation with different nutritional strategies and products and lead to a solid nutrional plan for race day.

In summary, fueling during an ultra-endurance event should consist of 60-90 grams of carbohydrates per hour, preferably from a mixture of sources.  Adding branched-chain amino acids in a ratio 1:4 can delay fatigue, improve performance, and reduce muscle damage.  Ingestion of fat is not recommended.  Fluids, gels, and solid foods may be combined as tolerated by individual athletes.  Nutritional strategies should be developed and refined during training, not during an important competition. 

REFERENCES

Goedecke JH, Clark VR, Noakes TD, lambert EV. (2005). Effects of medium-chain triacylglycerol and carbohydrate ingestion on ultra-endurance exercise performance. Int J Sport Nutr Exerc Metab, 15(1):15-27.

Greer BK, Woodard JL, White JP, Arguello EM, Haymes EM.(2007). Branched-chain amino acid supplementation and indicators of muscle damage after endurance exercise. Int J Sport Nutr Exerc Metab,17(6):595-607.

Hew-Butler T, Verbalis JG, Noakes TD. (2006) Updated Fluid recommendation: Position Statement From the International Marathon Medical Association. Clinical Journal of Sports Medicine,16(4): 283-292.

Horowitz JF, Mora-Rodriguez R, Byerley LO, Coyle EF.(2000). Preexercise medium-chain triglyceride ingestion does not alter muscle glycogen use during exercise. J Appl Physiol,88(1):219-25.

Ivy JL, Res PT, Sprague RC, Widzer MO.(2003). Effect of a carbohydrate-protein supplement on endurance performance during exercise of varying intensity. Int J Sport Nutr Exerc Metab,13:388-401.

Jeukendrup AE, Jentjens R.(2000). Oxidation of carbohydrate feedings during prolonged exercise: current thoughts, guidelines and directions for future research. Sports Med,29(6):407-24.

Jeukendrup AE, Moseley L, Mainwaring GI, Samuels S, Perry S, Mann CH. (2006). Exogenous carbohydrate oxidation during ultraendurance exercise. J Appl Physiol,100:1134-1141

Noakes T. (2004). Sodium ingestion and the prevention of hyponatraemia during exercise. British Journal of Sports Medicine, 38;790-792.

Peters E. (2003) Nutritional aspects in ultra-endurance exercise. Current Opinion in Clinical Nutrition and Metabolic Care, 6(4):427-434.

Romano-Ely BC, Todd MK, Saunders MJ, Laurent TS. (2006). Effect of an isocaloric carbohydrate-protein-antioxidant drink on cycling performance. Med Sci Sports Exerc,38(9):1608-16. 

Saunders MJ, Kane MD, Todd MK. (2004). Effects of a carbohydrate-protein beverage on cycling endurance and muscle damage. Med Sci Sports Exerc, 36(7):1233-1238.

Saunders MJ, Luden ND, Herrick JE. (2007). Consumption of an oral carbohydrate-protein gel improves cycling endurance and prevents postexercise muscle damage. J Strength Cond Res,21(3):678-84.