Optimized Fat Metabolism – To Carb or Not to Carb

Part 2: You Are Going to Hit the Wall…

…unless you build a bonk-proof body. Ah, the bonk. The dreaded “wall” that is coming (for you marathoners it’s at about mile 20-22) and is basically the term for running out of energy. But don’t fear! You can stave off the wall by applying a few of these tricks, but first let’s talk about carbohydrates and energy.

Quick biology lesson: the body utilizes three macronutrients to power your muscles; carbs, fats and proteins. Enzymes then convert those into adenosine triphosphate (ATP), which basically transports chemical energy within cells for metabolism.

Carbs are stored in our muscles and liver in the form of glycogen. For super high intensity (think: sprinting) down to medium intensity (tempo training runs) carbs are the fastest and most accessible fuel source; here’s the downside though- your muscles have no choice but to start burning glucose in the absence of oxygen (anaerobic metabolism) which causes the dreaded by-product lactic acid (that’s the burn in your muscles). As all that lactic acid continues to build up in your muscles and blood they inactivate the enzymes that govern glucose metabolism causing you to do one of two things: slow down OR bonk.

So the average human body, with its 2,000 calorie supply of glycogen is ready to do about 2 hours of really hard running OR it can slow down a little bit and learn how to switch itself over to fat metabolism. Fat provides a much bigger ATP return and is by far our most plentiful fuel supply, with more than enough energy already in and on our bodies to power us along through a 100-mile race, a full Ironman or a double century bike ride. Since fat is burned in a carbohydrate oven, small concentrated sources of carbs are needed to kick-start the metabolizing of fatty acids.

Okay, after all that science we can get down to brass tacks. To carb or not to carb, that is the question.

You already know my answer, and it’s based partially off the following experience…

I ran some shorter trail races in the past year (a 7-miler, a 9.2 mile race and a 27k) and for all three of these, I did use primarily carbs before and during these activities. The pace I ran for the 2 short ones were definitely both at the upper end of my aerobic threshold. I went all out on those (and had some left in the tank after) because they were pretty short runs and I knew I could hammer it the whole way.

The 27k race had about 3,500 feet of climbing and I raced using a combination of anaerobic sprinting (feel the burn!) and was at the upper end of my aerobic capacity for the duration of said race. I narrowly avoided a bonk (only because cramping was a bigger issue and that issue forced me to slow down; but I felt woozy and strange and I’m sure that was an impending bonk) because I did two things wrong; started off at an unsustainable pace AND underestimated that I’d need to burn more fat- I had carbo loaded the days leading up to that race and didn’t rely as heavily on my fat stores as I did in say a marathon or a 50k. I should’ve treated treated this race like an ultra.

Let’s look a little deeper, then. Why in a 16.8 mile race did I almost bonk when I’ve never even come close to bonking in a 50-miler or a 50k? What was at play in my body that day?

I think a few things jump out; one- I had switched my body out of fat-burning mode (or nutritional ketosis; more on that later) and into a glucose dependent state; I’ve already discovered that I’m pretty insulin resistant (self-diagnosed, but all of the significant physical markers were there: Insulin resistance- Signs and Symptoms) so I may have “overdosed” on the sugars. Two- I had more or less felt absolutely physically great since early March when my body became fat-adapted to running aerobically. In the 2 days prior to this race, and I’m blaming the sugar here, I had some mood swings, slept shitty and generally was hungry the whole time.

Says Peter Defty of Vespa:

“When you do the math regarding fasting blood sugar in a human, this works out to amount to 1 teaspoon of sugar, as glucose in circulation, [just] one teaspoon! This is normal and this is where your body prefers your glucose levels to be. Blood sugar is VERY tightly regulated. So, say someone eats a whole wheat bagel. Basically they just dumped 8 to 10 teaspoons of glucose into their blood stream when the body likes to have [just] 1 teaspoon. The body deals with this by secreting insulin so this toxic level of glucose can get back down to fasting levels and do so quickly.

But, just as importantly, to help promote glucose use high levels of insulin suppress [both] fat burn via beta-oxidation in the cells and the production and use of ketone bodies for brain and nervous system function. On the receptor sites of adipose (fat) tissue insulin functions to promote fat storage and strongly inhibits the release of fat. These are the immediate “unintended consequences” of concentrated carbohydrate consumption. There are many other possible “unintended consequences” that can crop up over time like intestinal issues, insulin resistance, weight gain, energy swings, heart disease, cancer, non-alcoholic fatty liver disease, metabolic syndrome, etc.”

Basically, I wrecked my body for that day and wasn’t able to race it like I could, my body was storing fat instead of releasing as energy like I had taught it to the months prior. Since I’m happy with my placement and time in that race and was using it as a training run for the Skyline 50k I’m glad I learned what not to do (as in: DON’T DEVIATE FROM THE PLAN 2 DAYS BEFORE A RACE). I still couldn’t help feeling at the end of that race that something just wasn’t right. It took a few days of proper eating to get my body back to feeling okay, and besides all the soreness (I usually don’t have that much muscle fatigue and/or soreness, even after a 50-miler) my stomach was kind of destroyed as well.

So there you go. I’ve learned what works for me as an endurance athlete. What works for you?


Will from An Ultra Runner’s Blog “Burn Fat for Fuel? An Interview with Peter Defty of Vespa Power”

Ian Torrence “Your Ultra-Training bag of Tricks: Unbonk!”

Sara Latta “Hitting the Wall” Sept/Oct 2003 issue of Marathon & Beyond magazine

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