So I’ve been getting lots of questions about how to fuel best on race day and what to think about. Fuelling has evolved in the past decade or so and I feel like a triathlon dinosaur so I’ve done a bunch or research and had discussions with a few of my favorite and most respected peeps on the scene.
The latest trend is that it’s best to take in as much carbohydrate as possible. The reasoning for this as I understood is that carbohydrate is the most efficient fuel and so the bigger ratio of carbohydrate to fat being burned for energy, gives the athlete higher performance I was a little skeptical of this because I wasn’t sure if you burn more carbohydrate because there is higher levels of sugar in your blood. I looked into this as much as I could and didn’t find any data to confirm that. It seems that the primary driver of fat:carb metabolism is simply the intensity of exertion and that higher blood sugar levels will be stored however briefly as glycogen in the muscles.
That hypothesis is dubious, and even if you burned a little higher percentage of carbohydrate because of higher blood sugar the performance increase would be minimal. The performance benefit of burning carbs compared to fat is apparently up to 7%. So if you are burning 10% more carbs at a given intensity because of higher blood sugar the performance benefit would be less than 1% (10% of 7%)
The graph above shows the relationship between carb/fat metabolization and intensity. It doesn’t say which percentage of threshold pace the 3m/s is but I am assuming over 90% as it’s a goal marathon time for the athlete. As you can see the further you move into higher intensity the ratio of carbs to fat metabolized goes up exponentially. You will notice the red carb curve looks a lot like a lactate curve. This is because carb metabolism follows lactate levels very closely and therefore as physical intensity increases, so does carb metabolism, fat metabolism tapers off quickly.
In an Ironman competition an athlete might burn up to 50% of their energy fat stores because the lower intensity one trains/race at the higher percentage of fat is utilized. At threshold intensity for instance, you might be burning 80% carbohydrate. This makes Half Ironman or marathon nutrition interesting as well where many athletes are pushing somewhere near 90% of threshold and therefore roughly 75% carbs to 25% fat. Though I have heard of some athletes burning up to 50% fat during a half ironman distance and surely can vary in the other direction to 85-90% carbs depending on the athlete and the intensity they can race at. These disparities can be genetic or trained (an athlete who trains a very high amount of low intensity and ingests small amounts of carbs may have a more efficient fat burning engine) and will depend on race day intensity (aggressive racing vs. conservative racing) .
These rules of thumb can be useful but can vary greatly from athlete to athlete. Carb metabolism also increases with every surge in intensity (burning matches), adrenaline levels, and can depend on how much zone 2 training the athlete as done and thus how efficiently they burn fat.
Here are some calculations that show needed carbohydrate as fuel during ironman.
Body storage: 2000 cal*
Carb metabolism during race: if 800** calories per hour are being burned, we can assume that approximately 400 calories per hour is from fat and 400 from carbs to keep it simple.
We’ll estimate a 10hr competition also for simplification:
400×10= 4000cal carbs during the entire race
4000-2000 (body storage) = 2000cal needed as fuel during the race.
2000/10hours = only 200 cal (50g of carbs) per hour
It is well shown that in the first two hours of competition there is a higher percentage of carbs burnt compared to fat so we’ll take that into consideration.
*this body storage can be increased by several hundred calories with a carb rich breakfast and pre race gel or carb drink. Body storage can vary athlete to athlete and be well over 2500cal (600+g of carbs)
** 800 calories per hour is the high end of how much calories you will burn on race day. Bigger stronger athletes will burn close to this but smaller athletes might burn as little as 600 cal/hr.
I was going to end this blog by stating that my belief was that sticking to the old school 300cal (75g carbs) per hour is the best and most secure (lower chance of stomach distress) fuel plan for a well paced competition because it should be sufficient to get the energy you will need on race day given the amount of carbs you will burn and how much blood glucose and glycogen is stored in your body to start with.
However, I have since been enlightened by my former coach and Sports Physiologist Susanne Buckenlei (who also happened to have won the Norseman Extreme Triathlon several times) and will be doing a part two to this blog next week because she had a very interesting and surprising take on this topic.
