LCHF and Endurance Sports

I recently started a LCHF ketogenic diet and the results are amazing. For 30 years, I have struggled with my glucose levels and rarely ever saw an A1C below 8.0. In fact, over the past two years my A1C started to rise - likely due to increase insulin resistance causing higher sugars.

LCHF is a game changer. I have been on it for one month and have achieved glucose control that I haven’t seen in over 30 years.

I am an avid cyclist and plan to do my first “Century” ride in September. I am struggling with how to fuel the ride. From my understanding, a cyclist should be able to go much further on LCHF. The body has about a 90 - 120 minute store of carbs, but an unlimited store of fat to produce ketones for fuel. So, in theory, after about 90 minutes, a cyclist should be fueling with carbs. On LCHF - in theory - the cyclist should be able to go the distance without refueling - if it is steady state cardio.

The problem - a long ride is rarely steady state cardio. Once you hit those hills, and your thighs are screaming in pain from the lactic acid, you’ve hit anerobic excercise which is fueled by glycogen stores. On LCHF, those glycogen stores are depleted.

Here’s the question. For those who are on LCHF, and doing endurance sports, do you fuel with carbs? Do you fuel with something else? Or, do you just go the distance fueling exclusively with onboard fat stores?

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It really depends on whether your goal is to finish or if you want to compete and finish faster and improve your PR.

There is an inverse relationship between the rate different fuel sources can produce ATP, and the total amount of each that is available (i.e. the distance you can travel with each).

Yes, the distance you can travel using fat for fuel aerobically is greater than what you can get from carbs stored as muscle glycogen, but the rate that fat metabolism can produce ATP is much slower than you can get from muscle glycogen.

Fat metabolism produces ATP 2.5 times slower than burning muscle glycogen aerobically, and 5.8 times slower than burning muscle glycogen anaerobically.

But since the distance you can travel using only muscle glycogen is limited, things like this depend on your speed/intensity and the duration of the race.

Ultimately if your goal is to just finish, you can do it with fat metabolism alone (low carb). If you want to finish faster, you need to use all the fuel sources, which means you should carb up. Not just on race morning but during parts of your training cycle so that you can put in some higher intensity sessions.

Fuel systems speed - fastest to slowest:

  • Substrate level phosphorylation - PCr + ADP + H -> ATP + Cr
  • Stored carbs - Anaerobic glycolysis - Glucose -> Pyruvate -> Lactate
  • Stored carbs - Aerobic carb metabolism - Glucose -> Pyruvate -> CO2 + H2O
  • Stored fat - Aerobic lipid metabolism - Fatty acid -> Acetate -> CO2 + H2O

Fuel systems duration - longest to shortest:

  • Stored fat - Aerobic lipid metabolism
  • Stored carbs - Aerobic carb metabolism
  • Stored carbs - Anaerobic glycolysis
  • Substrate-level phosphorylation