How quickly can the body restore energy reserves mid-ride/post-bonk, and how do we optimise this?

Question

I'm interested in how we can maximise restoring energy stores (mainly glycogen, but I'm deliberately being vague) either between closely spaced rides or on long rides.

I ask in the context of a couple of tough rides at the weekend: Friday 277 km solo into a serious headwind, Sunday 324 km with a group (they were all stronger than me and some of them were better rested; we had a bit of a headwind at times but not severe). In between was a rest day, and a well-fed one at that.

I got to a stage on Sunday's ride where I was clearly running on pretty much empty: even gentle climbs were almost impossible, and a snack of 100–200 kcal of mostly carbs would perk me up for a few minutes, starting a few minutes after I ate it but then it would wear off. A meal of several hundred kcal (again lots of carbs) with a bit of sitting around had a similar effect but lasted a little longer. I'd obviously overdone it: still depleted from Friday ride, trying to keep up with a fast group in the morning and missing a planned stop due to torrential rain, and nowhere open to have a food break out of the weather. This also meant I ran out of pocket snacks (at least those compatible with rain gear) towards the end and had to stop to get food out of a pannier.

There's literature on refilling glycogen stores over a period of several hours (example paper) but not much about shorter timescales, and how do we translate this to a post-bonk situation when you need to carry on against the clock? Is it better to have a long break, (perhaps eat, rest, eat again, go), or is it better to try to keep stuffing the carbs in while going along slowly? Is there a difference if there's a known hilly stretch?

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This is related to my old question Fuelling multi-day riding (carb loading/replacement) but this time I'm thinking of what to do when reserves are gone mid-ride – the multi-day aspect of last weekend is more like background, so this is about a much faster timescale.

Answer 1

I'm interested in how we can maximise restoring energy stores (mainly glycogen, but I'm deliberately being vague) either between closely spaced rides or on long rides.

On a ride

Here your ability to actually rebuild glycogen stores are compromised as your body is trying to fuel the activity not rebuild your stores.

Exercise actually alters your physiology, for example the blood flow to the gut is restricted. As such, severe or exhaustive exercise actually inhibits gastric emptying and interferes with gastrointestinal absorption. This makes it difficult to absorb food, why easily digestible foods such as gels were invented.

Your insulin response is also greatly reduced during exercise as your body is trying to keep your blood sugars elevated so it can create high energy molecules (e.g., ATP) through glycolysis which in-turn are used to power your muscle contractions.

As long as you continue to exercise your body will prioritize creating energy over rebuilding stores, so rebuilding or recovering from a bonk while riding will always be severely compromised. The short answer is on a ride DO NOT let your glycogen stores get depleted. The general rule of thumb is eat something easily digestible carbohydrates at least every 45 minutes. The idea is you want use ingested carbohydrates to elevate blood glucose levels, rather than relying on your glycogen stores, which can be depleted relatively quickly (e.g., within 2 hours)

In-between Rides

Here refueling within the first 4 hours post-ride can be critical:

Finally, take advantage of the glycogen repletion window that facilitates muscle glycogen repletion in the 4 hours immediately following vigorous exercise. During this time, any carbohydrates you eat are converted into muscle glycogen at up to 3 times the normal rate. And this window of opportunity closes fairly quickly as some data suggests there is a 50% fall in this super charged repletion rate by 2 hours with a return to the normal repletion rate by 4 hours post exercise. (Ivy JL et al,J Appl Physiol 1988 Apr;64(4):1480-5).

[...]

After this initial 4 hours surge, muscle glycogen stores are replenished at a rate of approximately 5% per hour. And while it may require up to 48 hours for complete muscle glycogen replacement following a 2 hour ride, for all practical purposes glycogen stores are almost completely rebuilt in the first 24 hours post event.

[...]

• The rate of glycogen repletion is highest in the first 2 hours and then begins to fall off.
• Is more better? Although the rate of CHO incorporation into muscle glycogen storage begins to fall at 2 hours, there is a maximum repletion rate in the range of 1.5 grams of CHO per kg body weight during the first 2 hours post exercise. Thus a strategy of taking all your CHO replacement in the first few hours is not going to work.

http://www.cptips.com/recvry.htm

Summary

There are physical limitations to how fast we can rebuild stores. The optimal strategy is not to let your glycogen stores get depleted if you are planning on completing back-to-back endurance bouts.

A lot of this comes down to strategies:

• Nutrition, before, during and after;
• Not digging too deep on earlier rides, or too early in the final ride (i.e., pacing);
• Minimizing your effort where ever possible:
  • Skipping pulls in the group;
  • Never letting gaps form between you and your daft; and
  • Not running a pannier (these are essentially parachutes), instead try a frame bag or a larger saddle bag, as the latter has very little drag.

Answer 2

Some consideration needs to be made of what we mean by "bonk". I'm going to say that it refers to the relatively sudden onset of severe fatigue, often accompanied by dizziness, "foggy brain", and/or muscle pains (not simple soreness). You're not just tired, you're really tired.

When you exercise without more or less simultaneously consuming carbohydrates to maintain your blood sugar, eventually the blood sugar level will begin to drop and other mechanisms must kick in to provide an ongoing supply of energy. The liver stores glycogen which it will, in this situation, convert to glucose and inject into the bloodstream. The muscles likewise store glycogen and will convert to glucose for their own use. And the liver will convert fat to glucose.

But the rate that these conversions can occur is limited (and, in particular, varies from one person to another, based both on genetics and on the degree of muscle training the individual has undergone). When these mechanisms fall short then the muscles will directly convert fat to energy (and the liver will do this as well). But this conversion (especially in the muscles) is inefficient and "dirty" (like an oil refinery belching black smoke), most importantly because it produces ketones as a byproduct. (In theory the muscles can burn ketones instead of glucose, but they do it less efficiently, with even more byproducts.)

The problem is that when you experience reasonably severe ketosis (high ketone levels) it really messes with your body. It's similar in symptoms and chemistry to a hangover, and will not quickly go away just because you eat something (though you need to beware that high ketone levels suppress your appetite, so it's important to make an effort to eat). It will generally take 4-8 hours of rest/moderate activity (plus food and fluids) to recover from moderate ketosis, and you could well end up in the ER with a severe case.

So it's important to prevent full-out ketosis and, if necessary, stop and rest when you sense the potential for this condition.

Answer 3

Not all bonks are created equal and there's no silver bullet to tackling them, but recovery can often be reasonably fast. I'm mostly talking about my own experiences and some of the other long-distance riders I've discussed the topic with have a clearly different routine for refueling. In order to avoid bonking and recover faster when you have the misfortune of hitting that wall, you should find a routine suitable for you.

When you plan on doing multiple centuries, I'd recommend carrying three sorts of food items:

1. Something that fills your stomach a bit, but not too much, like pastries. Depending on the intensity and length of your ride, enjoy one every hour or two, maybe even three. Do avoid products with substantial amounts of fat and/or meat as they tend take their time in the digestive track and reduce the rate at which other nutrients are absorbed.
2. Potato chips or other somewhat snackables that you actually enjoy munching on. This helps to keep you eating at a more constant rate and gives you some much-needed salt for sweating. This is also the reserve you should use to regain motivation whenever you feel the ride is too much.
3. Fast-absorbing sugars in the form of energy gel or candy. While producing extraordinary amount of waste and stickiness, energy gel packs also offer the quickest bonk recovery I've experienced.

Absolutely no nutritional science to back this one up, but I'd estimate I often feel the effects ten minutes after consuming a 30g-ish serving of gel. I find it important to keep riding at a lower intensity until it kicks in so you don't lose the heat, but if the bonk is bad enough to keep you from riding at any intensity, your only option may be to take a break. Have some more of those fast sugars, but don't be afraid to enjoy a small meal if you find a place that offers those.

If you need to get somewhere, longer breaks are a bad idea. As soon as you're confident in your ability to continue riding, do so, but take it easy and pay special attention to refueling. Proper recovery always takes more time than you'd think.