Athletes who are carb fueled (glycogen stores provide their energy) try to optimize their carbohydrate consumption. They carb up before events like marathons, in order to have their stores at a maximum. This pushes out the time to bonk to later in the race. They take goo packs with them in races and feed themselves carbs during longer events to keep up their energy sources.
Optimizing these strategies are key. But how fast can glycogen stores be refilled? From this article (Glycogen Replenishment After Exhaustive Exercise):
The body stores approximately 450-550 grams of glycogen within the muscle and liver for use during exercise. At higher exercise intensities, glycogen becomes the main fuel utilized. Depletion of liver glycogen has the consequence of diminishing liver glucose output, and blood glucose concentrations accordingly. Because glucose is the fundamental energy source for the nervous system, a substantial decline in blood glucose results in volitional exhaustion, due to glucose deficiency to the brain.
Turns out it takes a whole lot of carbohydrates to replenish these reserves:
One study reported that a carbohydrate intake totaling up to 550-625 grams per day was found to restore muscle glycogen stores to pre-exercise levels within the 22 hours between exercise sessions. The findings of this study were supported by second study in which a carbohydrate intake of 3100 kcal resulted in complete resynthesis of glycogen within 24 hours.
Turns out that the rate of replenishment is quantified as well:
Normally, 2% of glycogen is resynthesized per hour after the initial 2 hours immediately after exercise. With administration of 50 grams of carbohydrate every 2 hours, the rate rose to 5% per hour, but did not rise when additional carbohydrate was administered. Administration of .7grams per kg body weight every two hours is another strategy that appears to maximize the rate of glycogen resynthesis. There is also some evidence that even smaller loads (28 grams every 15 minutes) may induce even greater repletion rates.
So here’s the math I see. Suppose you are in an athletic event consisting of several competitions spread out over several hours. If these events are spread out an hour between each event then the most replenishment you could get would be 5% per hour. That doesn’t seem like all that much to me and wouldn’t necessarily justify carb loading.
Now, I wouldn’t do carb loading anyway. I’d rather be fat fueled even if it does mean my performance is slightly less than it could be.
I’d rather have all my toes to exercise with than be a bit faster and not have my toes.
So What’s the Problem?
The problem is the nature of the foods consumed during refeeds are simple carbohydrates which have a higher Glycemic Index. These foods are known to not only refill Glycogen stores but increase liver fat (Diabetes Obes Metab. 2017 Jan;19(1):70-77. Increased liver fat and glycogen stores after consumption of high versus low glycaemic index food: A randomized crossover study. Bawden S, Stephenson M, Falcone Y, Lingaya M, Ciampi E, Hunter K, Bligh F, Schirra J, Taylor M, Morris P, Macdonald I, Gowland P, Marciani L, Aithal GP.).
Compared with an LGI diet, a 1-week HGI diet increased hepatic fat and glycogen stores. This may have important clinical relevance for dietary interventions in the prevention and management of non-alcoholic fatty liver disease.
Refill the glycogen stores with cupcakes and you get a fatty liver. Get a fatty liver and you get diabetes.