Ever since I experienced the first miserable bonks on the bike (i.e., ran out of fuel and could barely move my legs to get home) and also saw my first unflattering test results from the Stanford Human Performance Lab in 2009, I’ve tried to figure out how to make myself bonk proof.

What if you could move your “hitting the wall” point from 5 hours to 80 hours of running or riding a bike – wouldn’t that be amazing!? The advice I got was: you need to do plenty of training in your fat burning zone to become an efficient fat burning machine for long endurance events. I disagreed. I had done plenty of that (relatively speaking) and was left with NO fat burning zone anywhere.

Endurance athletes have several performance or effort limiters (lactate, fuel, heat, muscular endurance, etc.) and some scientists still argue whether eventual slow down is caused by our brain (i.e., central governor) or some peripheral limitation, such as running out of fuel in the working muscles. Either way, for practical purposes, I’ve personally found that the following measurable limiters determine most of your performance in a race:

< 2h race (Half-marathon, Sprint & Olympic distance Triathlon):

Lactate/Anaerobic threshold i.e. who can go hardest until your muscles burn and breathing becomes tough

i.e. who can go hardest until your muscles burn and breathing becomes tough 2h+ race (Marathon, Half-Ironman, Ironman):

Fuel limitations i.e. who bonks last

The reason for the fuel problem is quite simple: Our “carb tank” carries up to 2 hours worth of heavy exercise fuel, but even the leanest healthy athlete carries more than 24 hours worth of fuel in his “fat tank”. So the question is: how do you become a fat burner to be able to go longer and faster in a race lasting more than 2 hours? (Of course, in addition, you want to be energy efficient in general, and have a large cardiovascular engine to burn any fuel that might be available)

THREE TESTS: From a Sugar burner to a Fat Burner

What did Stanford human performance lab found out about me in 2009? I was tested using my own time-trial bike on a Computrainer with a “gas exchange” tube in my mouth after an over-night fast without breakfast. I had been following what I thought was the “healthiest diet” i.e. super low-fat and consequently super high carb diet for more than ten years and I already had 5 years of triathlon training under my belt leading into this test:

I couldn’t burn more than 200Cal/hr from fat after a few minutes

At the “comfortable” efforts of 250-300W I was burning 900-1000Cal of carbs per hour, which would mean bonking in 2-3hrs (assuming a 2000Cal glycogen fuel tank) even if I managed to eat some.

In short, I was a highly efficient sugar burning machine, who could ride a bike at 300 Watts without much effort, but burning all sugar, and at least theoretically hitting a wall half-way an Ironman bike ride at that effort.

For those who aren’t familiar with bike “watts”, here are two reference points:

The very best professional triathletes typically average their Ironman races at 270-290 watts (or 3.7 – 3.9Watts per body kg for most, but the less hills and corners, the less the power-to-weight matters)

At 300 watts, a cyclists with decent aerodynamics moves at ~25mph on a flat course (calculator)

My second, comparable test (same bike, Computrainer, testing equipment and test protocol) is after a three month “pretty high fat”, “moderate carbohydrate” experiment, during which I replaced almost all sugar and added large quantities of nuts, oil and avocado. There was no meaningful difference in my training; and certainly no increase in easy workouts in “fat burning zone”.

Now my fat burning had more than doubled to more than 400Cal/hr. The only visible change in training or lifestyle that I could point to, was my diet.

This spring, a week after winning the Wildflower Long Course triathlon amateur race, I completed my third substrate utilization test. I went into the Wildflower race 20 pounds overweight, with very limited triathlon training and also decided to cut my typical race time eating (= all carbohydrate calories) by a massive -60% – which I knew was a huge risk. Yet, I won the race and my time was as close to the best professional athletes as ever (~8% in overall time). I also felt that I could have kept running at the end. I knew that something had changed for the better to be able to produce that performance.

My third test was done with a KORR device, again using Computrainer and my own time-trial bike. I also used Powertap to double check Computrainer’s power measures.

The results were pretty unexpected. Now my fat burning peaked at close to 750Cal/hr and fat utilization was still contributing 50%+ of the energy at 300W. I had more than tripled my fat utilization from the bottom values. I would guess that this change is part of the reason why I was able to maintain my strong effort at Wildflower triathlon, even though I cut my race-time eating by 60% and came into the race overweight and undertrained.

BURNING FAT: So What and How?

So what’s the significance of becoming a fat burner in endurance sports? Like with most things in life, if you can go longer and harder, it’s usually better. My progress to a better fat burner is clear in the chart below:

I had pushed my “hitting the wall” moment* from 5.6hrs to almost 90hrs at a comfortable 200 watts. And now I could race a full Ironman (8+ hrs) at almost 300 Watts, where as earlier I would have hit the wall just after 2 hours at that effort. To me, this is as close to bonk-proof as it gets.

*) For these calculations I assumed that the total glycogen stores are 2000Cal to begin with and could be supplemented by 250Cal/hr by eating. You can eat more, but it is questionable whether your body oxidizes much more than 250Cal of digested carbs. Maybe ~100Cal more but it wouldn’t change the calculations much either.

What were the exact changes in making this transformation (as a N=1 experiment) in the third test:

Exercise: No significant change to my knowledge. Definitely no increased volume. If something, more shorter and high-intensity workouts. Diet: No sugar, no processed carbohydrates. Roughly ~15% of total daily calories from carbohydrates in the 5-6 months leading up to the last test, most of which came from vegetables and nuts. Supplements/sports products: none during training, unless it was a more than a 3-4hrs workout (=which for me is less than once a month) and even then I tried to stick to real food, such as bananas, cashew nuts, etc.

I completed all <3hour workouts with only water, without compromising my hard interval workout performance at all.

Although my N=1 experiment makes no science, I would recommend that if you want to become a bonk-proof endurance athlete, you might want to consider diet first, then adjustments to your training regime. Doubling or tripling fat burning abilities sounds like something that would take 10 years of endurance training, but I did it in months by changing diet.

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As a final note about the test result accuracy: the substrate utilization results are quite sensitive to the equipment and preparation, since the test equipment measures small amounts of inhale and exhale gases. Therefore I tried to use the same protocol for all three tests (over-night fast, 5 minute steps in test, etc.) and same type of Computrainer and time trial bike. Additionally, it is also possible that once you become a good fat burner, the results are even more favorable to fat burning if the test is performed after a 1-3hour “warmup” (simulating the second half of a race), and not just after an overnight fast.