I've been doing this for the past couple of months, but I haven't taken it too long yet (13mi). I used to eat a whole bagel and pb before running, then take a gel every 5mi for anything < 12mi. Now I'm heading out in empty (or just a coffee) and eating no gels while running. So far so good, but it'll be interesting as things get longer. I may take one gel on 16-20. We will see.A glycogen bonk just tells you that your HR was too high for the bulk of your run. If you go back a few pages and find Duck's metabolic rate testing, you'll see the cross over point where his body begins to burn more carbs than fat. Ideally, your long runs should spend more time before the COP than after.
I'm planning on getting tested soon at Rutgers. I expect my COP is pretty poor.
But isn't it more complicated than that? I was thinking the point of low-carb training was to improve your fat utilization percentage so you could run longer without bonking at a given intensity level (perhaps measured by heart rate).
You bonked because you exhausted your glycogen stores too quickly. Slow it down (lower HR) and you'll burn a lower percentage of carbs and more fat. Keep training with the lower carb intake and keep the pace slower so you don't burn through the glycogen as quickly. You'll be training your body to burn more fat. What was your avg HR for the run? More importantly, can you tell where you spent the bulk of your run? Sometimes the avg can be skewed by a really low hr at the start and give you a false impression that you're training in the right range.I'll bet my house that Duck's COP is way higher than mine because he's training at an ultra marathon pace, which keeps him in the lower HR and thus the higher fat:carb burn.
Correct, HR is just a form of biofeedback that indicates intensity, not the actual variable we are interested in. The higher the intensity (as indicated by HR), the higher the proportion of glycogen to fat burned. Now what that ratio is differs for each individual, and is determined by genetics (slow twitch v fast twitch muscle ratio), diet, and training. What's most interesting to me about HR is that it "figures in" or includes other stresses on the body besides the exercise intensity itself, things like work/life stress, diet stress, stimulant stress (caffeine), liver stress (those 3 nightcaps), overtraining, illness, lack of sleep, etc. While we'd like to ignore these as runners, the body doesn't differentiate very well and the HR will typically be impacted accordingly.
There are a couple of similar but different concepts floating around on the last two pages related to glycogen/fat utilization and carb consumption/timing. Let me put on my pseudo

and lay out my understanding:
Metabolic Efficiency Training: This is the concept of limiting and timing carb intake in the overall diet combined with primarily low intensity training in your Metabolic Efficiency Zone (MEZ), in an effort to increase the ratio of fat/glycogen burned at a given intensity, or increasing the Crossover Point (COP). A standard MET cycle would include 5-6 workouts for a minimum of 6 hours per week at MEZ, with some strides/intervals thrown in at the end of two of the sessions. The training makes up about 25% of the plan, with the diet being the most important factor. It's not low carb, per se, but more about replacing grains and starches with fruits, veggies, and legumes. Doing so does tend to lower the overall carb intake of course, and starches, sugars, and grains only come into play during and immediately following long (>2 hrs) or high intensity workouts (which really don't happen much in this phase outside of racing). The idea is to limit insulin spikes, which triggers the utilization of glycogen for fuel. This type of training actually increases the density of mitochondria in the cells that are responsible for fat oxidation, allowing the body to tap into fat stores more efficiently. Think of this as your base building cycle.
So for me, with a crossover point of 156 bpm, and a MEZ of 134-149, I'd spend my time in that 134-149 range, with some intervals thrown in twice per week. Of course where I am in my training I'm not going to be in an MET phase and will be adding more intensity to try and increase my speed at the various intensities.
Maffetone method: This is similar in concept to the above, but differs a bit in that it is solely based upon his 180 formula (180-age, with some adjustments based upon a few other factors), and there is no intensity at all as he believes it inhibits the fat burning adaptation. Dr. Maffetone himself will admit that this his method is more about health than performance, but will point to the success of Mark Allen and others as proof that followers of his method can reach elite levels. His formula worked pretty well for me as I've been using a 137-142 range for the past two years, which falls right in line with what my lab results showed. His method relies upon regular MAF tests to measure improvement (5-mile tests at your MAF HR), and only suggests periods of intensity once a plateau has been reached. But he, being health based, suggests eliminating other stresses as factors in the plateau first before adding intensity. He also suggests limited simple carb intake, and goes so far as to suggest relying upon the most natural race nutrition possible - things like diluted honey or fruit juice. Needless to say he's a little out on the extreme edges, but there are some great concepts involved.
Fasted runs: Most of the talk has focused on this, which to me can fit into any training philosophy. Obviously they fit into the two above, as we know that ingesting simple carbs prior to exercise will spike insulin and trigger a heavier reliance on carbs for that workout. While that can be a great thing for shorter, harder efforts, it would obviously inhibit the aerobic and fat burning adaptations that are the goals of the above methods. Fasted runs aren't necessarily carb-depleted runs, as unless you have severely limited carb intake in your diet we all have enough for about 2 hours of running. As several noted, most people do this without thinking about it by getting up and running in the morning with nothing but coffee or water. If you don't do any of these, I'd suggest you incorporate 1 or 2 a week up to 1 hour as I think they do show real benefit, imho. On the flip side doing all runs fasted may be performance limiting and stressful on the body. I probably err to much on that side by pushing out to 2-3 hour runs without taking on anything before or during other than water.
Carb depleted runs: Basically a more extreme version of the above where you either extremely limit carbs in the diet, push beyond 2 hours without intake, or run a 2nd time in a day after depleting and not replacing carbs earlier. This is also a forced bonk run. I'm not clear on the real physiological benefits of this as it seems the positives and negatives probably counteract each other, but I do think there can be some real mental benefits of learning how to push through the bonk. I liken it to the diminishing returns of a training run beyond 3-4 hours - after that point you're probably not getting much physiological benefit but you are gaining a mental one.
As I said, this is all my understanding of these concepts gleaned from 100s of hours of podcasts, reading
Metabolic Efficiency Training and
The Big Book of Training and Racing (Maffetone) and going to a few talks at the local running shop. But I'm by no means formally educated in any of this stuff.