- Slower, unfortunately. At least at first.
- Fat oxidation requires more oxygen than glucose oxidation. Thus, you’ll need to get/process more oxygen to extract the same amount of energy
- Using fat your aerobic system is going to have to work harder at the same pace. Or you’ll need to slow down.
I’m a big fan of HFLC eating. Since I started about a year ago, I can see a ton of benefits: I’m leaner, my recovery is faster, I sleep better, I feel better.
I’ve definitely seen the difference HFLC makes on my long run fueling. I used to walk out the door on those super long run days with an apocalyptic survival ration of Gu and Gatorade. Not only was it bulky, but it was constantly yo-yoing me through the sugar highs and subsequent crashes. Now, I just bring a sleeve of Nuun and maybe an almond butter pouch. That’s it. Talk about freedom!
However, I noticed that after I went low-carb my pace got significantly slower at the same HR. I mean, MAJORLY slower. When I would try to pick up the pace, even a tiny bit, I’d have to downshift immediately to try and get my heart under control. Very frustrating.
And if I hit a stretch of uphill? Forget about it – most times I had to walk to keep my HR from skyrocketing.
What the heck was going on?
What I found out was that when you ask your body to burn fat – fat oxidation – your body will do it, but it wants more oxygen in return. About twice as much oxygen*.
That would be fine if you had an oxygen tank in your pack. But since your supply of O2 is limited by how much available air your little lungs and kool-aid pumping heart can process, a higher oxygen demand means heavier breathing and an elevated HR. Bottom line: the exact same energy demand (say, the same pace), is going to be harder.
Here’s the kicker. When your HR goes up too high (from all of that oxygen demand), you’re going to kick yourself right out of fat oxidation into glucose oxidation. If your heart is beating like crazy your body is going to figure that you’re currently being chased by a hungry, saber-toothed tiger and not calmly padding across the savannah – and it’s going to go into anerobic/panic mode to save you. Which defeats the entire purpose or being a fat-adapted, keto-cranking monster.
So, what to do?
Well, gang – I don’t know yet. For the past few months I’ve been doing speed work and it’s definitely making me a little faster at a lower HR. And on really challenging long runs (such as 100-milers), I’ve been adding carb-based fuel back into the mix, which seems to help. But I don’t have a definitive answer yet. Whatever I figure out, I’ll post it in “Will Keto Make You Faster (Part 3)”
* Essentially, glucose comes with it’s own oxygen. Fat shows up empty-handed. So the oxygen needed for oxidation has to be supplied by you.
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