FAT

Fueling for Endurance

BY MARK TWIGHT

First, let's use "more than 120 minutes" to define the beginning of "endurance" and leave the duration beyond that open. The longest I have personally gone is 63 hours more or less non-stop, and 11 days straight in a "stage" type event where recovery between periods of 10-14 hours of effort occurred. For the purpose of this article I restrict the effort to a single, steady-state event of two to 24-hour duration.

Endurance efforts are largely fueled by fat oxidation. To be sure, fats require the background catabolism of carbohydrates for conversion but (in well-trained, endurance-adapted athletes) at levels of work 70% of MVO2 or below the predominant source of energy is fat. Those less adapted to endurance will fuel mainly with fat up to about 50% of MVO2.

Matteo Giglio, Dylan Freed and Mark Twight gassing up on the west face of the Castore (13,654') during the Trofeo Mezzalama ski mountaineering race, April 21, 2005
photo by Anna Toretta

For efforts at greater than 70% of MVO2 the primary source of fuel is carbohydrates. Many mixed, circuit workouts force the athlete to operate at 100% of MVO2, and even higher during interval sessions. The body naturally chooses the fuel source most appropriate to the level of effort demanded by the brain with no regard for how long the supply will last. If the effort is hard enough that carbs are fuel of choice the gas is going to last for roughly two hours. Once you have gone through the readily available carbohydrates (stored glycogen, blood glucose) the body automatically switches to alternative sources of energy production, all of which yield less energy than carbohydrate conversion. So it's up to your brain to manage the fuel inventory. I believe how your body chooses its fuels may be manipulated not only through daily diet but also by matching that diet with particular types of exercise. My belief is both backed and contradicted by scientific study - kudos to some scientists and too bad for the others because athletes on the ground (and on the podium) have proved the theory again and again.

Again, I am talking about endurance effort for which this type of dietary and training manipulation is effective. Such practice may work for sprint or middle distance type efforts but I do not have enough experience or interest to comment.

My interest is primarily endurance, secondarily power-endurance and I eat the fuels I need in quantities relative to the length of the effort they fuel. I don't need a lot of carbs to fuel a short, intense circuit in the gym - it just doesn't last that long. I do need to eat a reasonable quantity of protein to aid recovery from effort of any type and as a prophylactic against muscle catabolism during long effort. More than anything, for what I do (2-12 hour efforts) I need to eat a lot of fat. I jokingly say that I eat a 40-30-200 diet but it is close to the truth.

Dietary fat intake has been shown to improve fat metabolism, and increased fat metabolism spares muscle glycogen (E.V. Lambert, Speechley et al, 1994, Helge, Richter et al, 1996, Goedecke, Christie et al, 1999, E.V. Lambert, Goedecke, 2001). Of course it does; because, to a point, the body will tend to "go after" what ever it is accustomed to getting a lot of and if I eat nothing but fat then my body will either eventually learn to fuel activity from only fat or I'll go the way of the dinosaur.

The main reason to train your body to oxidize fats at a higher percentage of your MVO2 is that carbohydrate conversion produces acid (talking on a cellular level here) while fat conversion does not produce acid. Even carb conversion at aerobic levels of effort produces acid but the body can process it at the same speed it is being produced.

The athlete with more efficient fat conversion moves at a faster pace when fat is the main energy source. In the 1989 Hawaii Ironman Mark Allen ran a 2:40 marathon (the fastest recorded in the event) following 5:30 of hard effort. No human has enough carbs left on board to sustain such intensity for 8 hours. In a state of carbohydrate-depletion, his rate of work in the marathon depended on an ability to oxidize fat at a rate of 1.15g/minute, which is roughly 50% higher than most national-class athletes can achieve. An average level of fat oxidation (.76g/minute) would have produced a much lower rate of work and a time for the marathon of 3:30.

To learn what percentage of fat is being burned at what heart rate requires a stress test where the respiratory quotient or respiratory exchange rate is measured. The quantity of CO2 exhaled divided by the quantity of O2 inhaled provides a fairly accurate picture of the percentages of energy sources being used. Typically an RQ of .85 shows a 50-50 burn of fat and glucose. One of the guys who trains in our gym from time to time tested last year and hit .85 at an HR of 64, showing that he was (at that time) adapted to burning predominantly glucose. He is an ultra-endurance athlete but had been living at high altitude (over 12,000') and climbing higher for three months and we believe the glucose preference was due to hormonal changes caused by reduced O2 i.e. a stress reaction. However, it may have been due to low fat intake or perhaps he was getting a cold when he tested. No matter because, by manipulating his diet (more fat) and training at low HR for long duration shifted this number over the course of several months. Last test he was hitting that same RQ at much higher HR, which means he is now burning more fat at a higher percentage of MVO2 which means he has a greater pool of energy reserves available, is producing less acid at higher HRs, etc. On top of that foundation he stacked a lot of intervals and his last e-mail from overseas suggests great success in sport application of this fitness.

In our experience, the trick to becoming more fat efficient is to eat more fat (be careful to balance Omega 6 and Omega 3 fatty acids) and include a good deal of training at fat-fueled heart rates; this is shockingly low for most people meaning that it will not feel like useful training. From time to time do what we call "depletion days" where the effort is long enough and fueling lean enough that the body must access fat reserves in order to keep going. These adaptations are not immediate in athletes whose bodies are - through genetics and/or training - more efficient at using glucose as an energy source.

At Gym Jones we recommend that endurance athletes use a hybrid program that combines short, high-intensity circuits and power lifting to improve power and whole body integrity with longer duration, lower intensity efforts because these train fat metabolism, which, when accompanied by appropriate diet improves long-term energy production, giving one the ability to "go forever."