(Protein just kicks around in the middle - let's park him for now)
Tradition says: eat carbs, a lot of them, they're your most effective source of fuel.
HFLC says: you can do just as well (better?) using fat as a primary fuel.
Who is right?
It's really difficult to discern.
Dr Timothy Noakes
Most recently I listened to the Velonews Podcast "Inside Ketogenic and High Fat Diets".
Click here to listen.
They interviewed Dr. Timothy Noakes - who wrote "The Lore of Running" which praised carbs... only to in recent years do a 180 degree turn and support a HFLC/Keto regimen. It is a fascinating listen.
Fat: More Energy, More Stored
One undisputable fact is that you have tens of thousands of calories of fat available to you on your body, vs only a couple thousand tops of carbohydrates. So fat has to play some role when you get into longer endurance sports.
Studies have shown that fat-adapted athletes (those who have been on low carbs for 6-8 weeks, generally) can metabolize 2.5 times more fat per hour. I did some quick math and it comes out to 600kCal based on some studies. That's a very big chunk of what I need, just from the fat I already have too much for.
Performance ... Wither Efficiency?
This is where the crux of the debate is. The good thing about carbs - they are stored in your muscles and are ready to go. Even when you ingest glucose - it goes straight to the blood stream, it's metabolized easily and quickly.
So can fat, which has a longer metabolic pathway from need to burn, compete?
Some studies have suggested "No". This article does a pretty good job looking at one of the studies - during a training camp, athletes who switched to a HFLC diet failed to gain any performance, vs the regular carb athletes.
Hold on, says Dr Noakes. Those athletes weren't already fat adapted, so they were going through the adaptation while taking on hard training - maybe the results would have been different if they were already fat adapted!
It's hard to tease out for sure. If I were an Olympian with lofty performance goals I would stick to tradition until the science is clearer. Noakes works with those folks, though, so maybe not!
I have been doing intermittent fasting for about 11 weeks now, which isn't exactly the same as HFLC but at the same time it kind of is. Low carbs, relying on fat. When I do eat I have been lowering carbs, although not religiously.
What have I noticed during fasted runs/rides?
Higher heart rate for sure - about 10bpm higher for equivalent running pace, poking up into the 180bpm range often with 175bpm average. For me that's crazy territory, 180bpm is close to "sprinting to the finish".
Noakes dismisses this - sure you need more oxygen to burn fat, so what?
My results somewhat back this up - my heart rate was high enough that I should have been completely wiped after 5km, but I did 12km with hills... so could be that my heart needs to beat faster, but am I necessarily less efficient? Will I run out of energy and be done?
I'm going to keep chugging and monitoring it. My goals aren't the same as an Olympian, I want to lose weight (and I'm down a whoooole bunch!) and be healthy, performance is really secondary, so I can afford to take a risk on performance.