Estimating glycogen usage from power meter data - Page 2

Glycogenolysis or the catabolism of glycogen is an energy consumption phase, glycogen resynthesis or the restoring phase isn't just catabolism in reverse. It's an entirely seperate process that requires among other things a ready supply of blood glucose, is typically aided by insulin and is part of the magic during the critical half hour refueling after a workout but goes on at a slower rate throughout the day. Just the fact that glycogen resynthesis rates are known to change from the first half hour to hour after exercise to lower rates later in the day makes it hard to believe a linear data fit from exercise data accurately predicts that rate.

Like I said, all I've done is look at the charts, but they're just fitted scatter plots of data taken during moderate to hard exercise. I'd be amazed if that data linearly extrapolates down to rest and accurately describes how your body engages seperate processes to refuel. It would be like taking automotive fuel consumption data collected while driving, fitting a line and predicting rates of refueling at the gas pump.

At least that's how it hits me, but again I'm just thinking out loud and don't claim to know this stuff anywhere near this level of detail.

-Dave

Well it might not hold much value but seeing as how we can estimate it using available data from a power meter, it would not hurt to investigate it!

Let me give you an example of how it could be useful. Just assume for a moment that the normal sedentary person needs, say, 150 grams of carbs daily for their base metabolic activity. Anything significantly more increases insulin and causes weight gain, fat storage, insulin resistance, etc. But, as active cyclists, we need more. How much more? One could argue that we need 150 grams + x where x is the amount of carbs necessary to top off glycogen stores (x will therefore always be some amount greater than the glycogen consumed during a ride due to metabolic inefficiency).

So given these two rides, planning your diet to recover for a ride the next day:

Ride A: 2000 kj = 2000 cal = 250 g carbs + 111 g fat
Ride B: 2000 kj = 2000 cal = 40 g carbs + 204 g fat

Ride A would require doubling or tripling your carb intake over the next day. In addition to the recovery drink or two, this would mean planning carb-heavy dishes for several upcoming meals.

Ride B might only require a recovery drink and an extra helping of potatoes at dinner.

Now I know there are a lot of assumptions thrown in there (assumptions I'm in no way qualified to make ) but I'm just throwing this out there as an example of how info esimated from a power meter could be used to make dietary choices for riders on a semi-low carb or paleo type diet.

Well since I don't buy into many of the underpinnings of low carb or paleo diets I don't see much value in carb restricted diets. So basically you're losing me at the outset.

IME, there are a lot more endurance athletes that hinder their training and adaptation via short and long term glycogen depletion than those that run into insulin resistance or excessive fat storage by eating a relatively high carb diet coupled with regular training.

But from a purely academic standpoint I agree it's an interesting subject. I just don't think it's that valuable to endurance athletes but if you're a fan of carb restricted diets and are looking for minimum daily carb intake levels then I can see your interest.

Good luck and keep an eye out for bonking, chronic glycogen depletion and symptoms of overtraining. And remember your brain only feeds on glucose and in addition to the estimates of glycogen and blood glucose burned during exercise you'll also want to get good estimates of your daily non-cycling rate of glucose and glycogen burn. Dial those in perfectly, measure your meals with great accuracy, time your carb ingestion so you don't exceed your ability to store that as glucose and glycogen and you may be able to safely stay at low carb levels without leading to problems. But IMO it's playing with fire for little gain....

-Dave

Whether or not you personally subscribe to this diet is irrelevant to whether estimating glycogen usage from a power meter could be a useful dietary planning tool.

In terms of "playing with fire", the paleo/primal eating patterns has worked for many atheletes. Nothing too crazy, just lots of veggies, nuts, fruit, meat, fish and some root veggies, whole grains and sports drinks when carbs are needed. I've been on several podiums in the US Cup Cat 1 cross country race series this year, all following Friel's "Paleo Diet For Atheletes" book. So I find your scare tactics against such a diet to be somewhat baseless and amusing given the number of endurance atheletes that are successfull with it. But thats another topic for another forum.

I do believe a PM is a useful dietary planning tool, total kj burned on a ride is very useful for planning my daily refueling. I just don't see a lot of added value in breaking calories burned down further. Attempting to single out the sugars burned only makes sense to me if I'm also trying to minimize carbs in my diet so yes, subscribing to a restricted carb dieting philosophy is relevent to this discussion. If you're not trying to restrict carbs and go with something like the common 70% carb recommendation for endurance athletes(see AIS link below) then there's not so much value in pinpointing the exact breakdown of the calories you burned during exercise.

Your approach of pinpointing sugars burned and using that to drive refueling strategies only makes sense if you believe you can accurately estimate glucose/glycgogen utilization for the rest of the day and also believe you can refuel in a way that translates all ingested carbs to stored glycogen or blood/liver glucose. Fail to do those two things and you're back to calories in minus calories out regardless of the macronutrient makeup so where's the value added?

Take an athlete with a daily BMR of 2000 Calories and they ride at an AP of 200 watts for 2 hours per day. That's roughly 1440 Calories per day on the bike of which some percentage comes from stored sugars, for the sake of discussion let's just say you have a method of determining that 1000 Calories came from stored sugars.

Well now you've got 1000 Calories worth of glycogen to replenish plus some percentage of that 2000 you burn just to make it through your day. How many of those come from sugar stores? Even if it's only 25% of the total that's still the difference between trying to replenish 1000 Calories of carbs per day and 1500, pretty big swing without a lot of tools to refine your estimate. IOW, the percentage of the day when we have PM data and tools like the IF chart posted above are a small percentage of our lives and to do what you suggest you'll want to account for more than just what happens on the bike.

As for the diets, sure folks have success with many approaches and many also find ways to modify diet templates to their own ends. See what the Australian Institute of Sport says about various diets. In particular note what they say about what athletes are actually doing in the context of the Zone diet. Sure it's not the paleo, but someone saying they follow a particular diet and what they actually do often differs.

Australian Institute of Sport :: Fact sheets

If you're going to make the argument that some high level athletes have had success perhaps even great success with a particular diet then you also have to acknowledge that far more athletes including very successful athletes have had success and even great success not following that diet. IOW, anectdotes of someone's success does not prove that the approach is superior, just different even if they have followed it to the letter which is hard to validate.

You may consider warning someone of bonking or chronic glycogen depletion a "scare tactic" but bonking happens all the time as does chronic depletion of glycogen levels and the poor training that results from it. As I said, manage it well enough and you can avoid those things but they happen all the time and if you intentionally restrict carbs you're increasing the odds of ending up there. Nope, not a given but again unless you buy into the "scare tactics" of insulin resistance and excessive fat storage that these diets promote then there's not much benefit for the added complexity and risk of fine tuning carb utilization estimates and refueling strategies.

Do whatever makes sense to you, but if you're asking for buy in or support for your approach you're not likely to find it here.

Still, interesting discussion even if I don't agree with the premise...

-Dave