Glycogen depletion rates/guidelines

[ktw]

Does anyone have any information on how long it takes for glycogen depletion from endurance exercise? I've read several times that depletion does not occur from typical weight training, but I have been doing a lot of long distance cycling and want to get an idea of how much I should recarb after depending on timeframe and distance. Also, I was wondering if there is a point during an endurance w/o when it starts to happen so I can switch from just water to a carb solution.

 

[t-bone2]

Depends on a lot of factors: how CHO loaded you are, what the CHO sources are (i.e. glucose, sucrose, maltose, maltodextrins and amylopectin are oxidised at high rates. Fructose, galactose and amylose have been shown to be oxidised at 25 to 50% lower rates), other sources availabe for oxidation (PRO, FAT), etc.

Having said that, there were a couple of studies citing that:

1. Using (stable) isotope methodology has shown that not all carbohydrates are oxidised at similar rates. (As noted above.)

2. In fasted subjects, it was found that CHO oxidation rates do not exceed 1.0 to 1.1 grams/min, with the limiting factor more likely to be the intestine or liver rather than at the muscular level.

3. The liver may provide glucose to the bloodstream at a rate of about 1 gram/min by balancing the glucose from the gut and from glycogenolysis/gluconeogenesis.

4. Peak rates of glucose oxidation occur approximately 75 to 90 minutes after ingestion and are unaffected by the time of glucose ingestion during exercise. Rates of oxidation also appear not to be influenced to a major extent by the use of different feeding schedules.

 

[ktw]

So for cycling I would want to try to be carb loaded 90 minutes or so before riding and then use a fast digesting source like dextrose, glucose, etc. during and right after. Anything that clears the stomach fast is an issue. Waxy Maize is out because the leg pumps would be terrible.

 

[t-bone2]

It depends on the duration of your cycling.

Peak rates of glucose oxidation occur approximately 75 to 90 minutes after ingestion. So if you CHO loaded 90 minutes or so before riding, you'd be peaking right at the beginning of the event. It seems like you would ideally like to peak later in the event.

Also from one of the studies was the following:

Combinations of multiple transportable CHO may increase the total CHO absorption and total exogenous CHO oxidation. Increasing the CHO intake up to 1.0 to 1.5 g/min will increase the oxidation up to about 1.0 to 1.1 g/min. However, a further increase of the intake will not further increase the oxidation rates.

So it is not only CHO source, but rate/qty of refeed over the duration of the event. Also, from an abstract (I don't have access to the full study) on a study concerning nutritional recommendations for competing in triathlons is the following:

Depletion and dehydration are likely causes of fatigue, yet hyponatremia has been highlighted as a major concern during such events. As a consequence, triathletes are recommended to evaluate fluid losses during practice sessions and develop personal fluid replacement programs to ensure fluid balance. With regard to dietary preparation there are new methods of glycogen supercompensation, recommendations for improving fat oxidation while maintaining endogenous glycogen stores, and evidence aligned to the benefit of consuming combined carbohydrate intake during the race to increase exogenous carbohydrate oxidation rates

So, energy is also derived from the oxidation of FAT. While CHO is stored in both liver and muscle glycogen, FAT is stored (for energy) in intramuscular triglycerides. As the intramuscular stores are the primary sources of energy during exercise, then FAT loading also has a role to play.

Remember that these are guidelines. You are going to need to figure out what works for you through trial and error.