Fat as Fuel

My first experience with training fat burning was when I started training for ultra running last year, leading up to Kepler specifically. The plan was to incorporate some long runs into my programme that were completed first thing in the morning and before I had consumed any carbohydrates that day. The point behind this is that the body starts the day preferentially using fat instead of carbohydrate and if you train in this state you can improve your body’s ability to use fat as a fuel for exercise. Once you have consumed significant carbohydrate, such as through an oat-based breakfast, your body will be less interested in tapping into fat as a fuel, and instead will prefer to use carbohydrate. Up to that point in time I had spent almost every session in my life in high-carb mode, the only exception being towards the end of some long runs when I’d hit the wall (run out of carbs).

My first low-carb session, about a year ago, was after eggs and avocado for breakfast, because I was too scared to have nothing! I knocked out 60 minutes with nothing unusual. Normal pace, normal heart rate. This was actually a big revelation. I knew I would survive with no carbs for breakfast, but I expected that removing the carbs would impair my performance in some way. I did a few more, then even some 90 minutes sessions with no breakfast at all and still found no reduction in running or cycling speed.

It wasn’t all smooth sailing though. The first sign of trouble involved an easy 2-hour session before breakfast following an easy 1-hour session after dinner the previous night. The after dinner session’s purpose was to burn some carbs such that the carb tank was not fully topped up going into the morning session. The morning session of 2 hours was enough to burn through whatever carbs my body was still relying on. I could feel the wall looming just as I finished the 2 hours and I shuffled into the shower shaky and spaced out. This gave me a feel for how big my carb tank was and how strongly (weakly) I could be fuelled on the fat tank. The answer was that my carb tank was moderate to large and my fat fuelling system needed improvement.

This first bout of low-carb training definitely caused me to relax my attitude around always training with a full tank. More accurately, it proved to me that my single-tank model was not the right way to think about the human engine. Instead of thinking of fat as a reserve storage for extreme famine, I began to think about it as one of two tanks that could be online simultaneously, although I remained unconvinced that this was relevant for anything shorter than a moderate ultra-marathon.

A year on I’m training for Kepler again and pushing my body’s ability to burn fat once more. I’m now doing 5 to 6 sessions before breakfast per week, and sometimes a long session after a low carb breakfast. This adds up to about 6 hours of training before breakfast, which will increase towards 9 hours over the next month. The seventh day is my long run day, where I’m eating low-carb foods like avocados, nuts, seeds, eggs, leafy vegetables, mushrooms and olive oil for breakfast.

I’ve just finished week 3 of my training block towards Kepler and I think I have something worth reporting already. The 10.5 hour week (12 hours including strength and conditioning) was rounded off with 2 hours of training on Saturday, a very early dinner, a late, low-carb breakfast on Sunday, and then my long session after midday. I chose to ride on this occasion as I’m still recovering from surgery, but the intensity is the same. I packed bars and gels ready for an epic bonk. Surely this was a recipe for another masochistic struggle. I smashed out close to 3 hours at a steady pace (harder than an easy pace) with no sign of slowing. Completely to my amazement I just kept cranking the whole time. To knock off such a solid session with minimal fatigue, 23 hours after I last put a grain of rice in my mouth was extremely surprising. And let’s just remember that this is week 3 after 5 weeks of minimal exercise post surgery. All my training paces are well down as I would expect, but there is a level of endurance that I did not expect to see so soon.

So I’m definitely keeping this routine going and looking forward to sharing how far I can push these long sessions without carbs. I’m not expecting to change my race day fuelling strategy, but if I can bring my fat tank online as a significant second source of fuel then I should be able to run faster without emptying my precious carb tank as rapidly as in the past.