Carbo Loading, A Perspective from the Fastest Man in the Gym

The fastest member of Crossfit Inception and I were talking, as gym going friends are want to do, about physical related activities and he brought up a concern about my advice against trying to carbo load. 

I don’t think I really ever tried to dissuade anyone from consuming a larger than normal amount of carbohydrate before a race, but I do think that the mechanism of carboloading is overemphasized.

First, let me say that there is far more that we agree on than disagree on. This argument may seem like arguing over who is better looking: Jessica Alba or Jessica Biel. Actually this argument is probably a lot less interesting.

So we agree that the consumption of a pre-race meal containing dense carbohydrates prior to a race is vital.

We also agree that the human body has a finite potential to store glycogen. BC pointed out something I had not considered: that humans can increase their glycogen storage capability through training.

This makes a lot of sense to me. Almost everything is trainable. If athletes can increase their VO2 max,flexibility, and red blood cell count than why couldn’t they increase their readily available (not converted into adipose fat) glycogen storage. Since all of the glycogen not stored in the body is in the muscle cells, and you can increase the number of muscle cells you have, this also makes sense.

The other thing we agree on is the importance of peri workout (during the race) fueling to replace these glycogen stores.

Where we begin to differ is on what fuel source the body is using for what.

I think that since the body is operating in the oxidative metabolic pathway and the fuel for that pathway is fat, than the body can and is functioning independent of its sugar stores. There are several bodily functions (brain operations to name one) that requires glycogen and can only use glycogen. Processes like gluconeogenesis can convert proteins found extant in the body into that sugar.

I also think that there it would be impossible to provide a steady enough source of fuel to fuel an activity of that duration. If you think of the race like a plane flying across the Atlantic, I do not think that it would be possible to mid-air refuel a human with enough sugar to complete an event lasting over three or four hours. Sugar is a fast burning and inefficient fuel source.

I think that the benefit of a source of mid-race simple sugar (in the form of a goo or even better product which is a different post) is that it save the body a step.

That quickly digested sugar keeps the body from having to resort to gluconeogensis to provide sugar to the brain. I know that every marathon that I have done, I’ve felt a lot better after taking a goo packet. Maybe it was psychological, but I think it was also because my brain had the sugar it needed to keep me from hitting a wall.

The other benefit of mid-race replacement of sugar is that your fast twitch muscle run only on sugar, and this will differentiate between having enough “kick” to sprint it into the finish or having someone pass you in your finisher photo.

So back to carbo loading. We both agree on the importance of doing it for hydration, that it can be a trained or timed thing, and it should be done.

I do not see a scenario where doing it prior to a race would be harmful, especially if you are primarily a fat burning individual. Where it becomes an issue is for endurance athletes who are on a chronic high carb diet and use primarily sugar to fuel their activities. For them, that extra reserve of glycogen that they think they are storing is just another in a series of insulin spiking meals.

BC:  The article looks good overall. I think the biggest point of disagreement is fuel sources. Ninety-nine percent of runners will tell you that carbs are the most efficient fuel source, especially for distance running. Most running resources will cite fat requiring more oxygen to become usable as the reason for this. I am no scientist, and cannot give a more definitive answer than that.
Here is an article that goes over most of what I go by with regard to carbo loading: You will see variances on how much sugar your body can store, but there is general consensus on how to execute safe carbo-loading for endurance races. 

Finally, I will say this has worked for me. I have experienced marathons where I definitely hit the wall, and flamed out badly, and others where I was able to cruise all the way to the finish and walk around the next day like nothing happened. I think the combination of conditioning and proper carbo-loading helped. Hit me up if you have anymore questions/comments.


Here is my take on the article. Definitely written by someone way smarter than me. I want to highlight a few passages because I wrote my first piece before reading this. A lot of the points are the same, but it does not really address the Crossfit Endurance proposition that if you make your body able to function on fat, than you will expend less carbohydrate during your event, thus saving more glycogen for your brain and muscles.

“Remember also that muscle glycogen is committed to be used by muscle and cannot assist in maintaining blood sugar levels. Therefore should no additional carbohydrate be ingested during prolonged exercise, the task of maintaining blood glucose levels rests firmly on the liver’s glycogen stores and gluconeogenesis (the manufacturing of glucose from plasma amino acids). Oxidation of blood glucose at 70-80% VO2 max is about 1.0 g/min or about 60 g/hour. Therefore it can be predicted that even with full glycogen stores, a less conditioned athlete’s liver will be depleted of its carbohydrate within and hour and three quarters of continuous moderate intensity exercise.”

So there are really two ways around this issue, you either keep topping off or find a different fuel. If you think of your body as a hybrid solar/gas powered car, there are some functions that only gas (glycogen) can do and some that both can do (fat is solar). All are in agreement that the muscles can only go on gas, but the air conditioning, radio, GPS, etc can all run on solar power. This is like your body. All of your non-musclar organs need to keep going while you are doing your race. These organs can use either sugar or fat. So the more fat you can make them burn, the more sugar you save for the muscles, the better use of space you are making in your gas tank.

“For an untrained individual consuming a high carbohydrate (75%) diet, glycogen stores may increase up to 130g and 360g for liver and muscle respectively for a total storage of 490g.For an athlete training on a daily basis consuming a normal diet (45% carbohydrate), glycogen levels approximate 55g and 280g for liver and muscle respectively yielding a total of 330g. However, should this same well-conditioned athlete consume a high diet (75% carbohydrate), their total carbohydrate reserves may soar up to 880g with approximately 160g stored in the liver and 720g in the muscle.”

This makes perfect sense to me as an adaptation and mechanism.

“Firstly, glycogen storage is associated with a concomitant storage of water. It is estimated that every gram of glycogen stored is associated with about 2.7g of water. Therefore, a well-conditioned athlete with total glycogen stores approaching 800g will find their body weight about 2kg heavier at the start of the race. This increased body weight will have implications on running economy and performance at least near the beginning of the event when energy reserves will be high.”

I don’t really know that an extra 2kg will really have that detrimental of an effect on running efficiency, especially when you compare it to the effects of dehydration on running.

“Another drawback to carbohydrate loading if performed incorrectly is gastric/intestinal upset. Very large amounts of ingested carbohydrate can affect the osmolarity of the intestine. In other words, carbohydrates (especially simple/processed sugars) in the intestine draw water into the gut by osmosis affecting the water balance and may cause intestinal upset and diarrhea.”

I have experienced this on several occasions. Not a lot of fun.

I am at a loss how to wrap up this huge post.

I was unaware of the exact quantities, how, and the why behind effectively increasing your glycogen stores prior to a race, but it is fascinating and something I will recommend to endurance athletes in the future. I suspect that people who eat a lower carbohydrate diet will need to adjust to protocol a little bit. I think that they may get more for less with carboloading, but I see only good that could happen for following this as part of your taper for a race week.

Maximizing the use of fat stores, which are larger in your body than glycogen stores, will allow you to most efficiently fuel your muscles with glycogen during an endurance activity. No matter how big your gas tank, if you can use a near unlimited fuel source for your trip you will be able to get more miles out of it.