Originally Posted by rjkresse
From what I can gleen, exercising at a lower HR burns fat, while exercising at a higher HR burns glycogen stored in the muscles and the liver.
It's not quite so simplistic. There's a ratio of fat-to-carb calories that's burnt. People often confuse the
percentage of fat-burning with
total-calories of fat burned. In general, the higher the intensity up to your LT, the higher the total calories burned from both fat and carbs. Fat-calories also goes up with intensity, just not as much as glycogen. It really depends upon your level of fitness and condition. Let's compare a beginning-rider vs. an elite athlete:
BEGINNER RIDER
50% max-HR = 500 Cal/hr = 75/25% fat/carbs =
375 fat-calories/hour
65% max-HR = 650 Cal/hr = 60/40% fat/carbs =
390 fat-calories/hour
80% max-HR = 800 Cal/hr = 25/75% fat/carbs =
160 fat-calories/hour
ELITE FIT ATHLETE
50% max-HR = 750 Cal/hr = 75/25% fat/carbs =
560 fat-calories/hour
65% max-HR = 1000 Cal/hr = 70/30% fat/carbs =
700 fat-calories/hour
80% max-HR = 1250 Cal/hr = 60/40% fat/carbs =
750 fat-calories/hour
90% max-HR = 1400 Cal/hr = 25/75% fat/carbs =
350 fat-calories/hour
Basically, the more fit you are, the more fat/hour you can burn at a higher pace. Your energy and cardiovascular systen becomes more efficient at converting fat to ATP for burning, without resorting to anaerobic fermentation and phospho-creatine at high-effort levels. So the more fit you become, the more total fat-calories you burn at a higher-intensity, even though the percentage is lower compared to a lower effort.
The latest studies appear to show that total-weight loss is more related to total-calories burnt in the workout regardless of whether it's fat or carbs. If you burn off 3000 calories in a workout and eat only 2500 calories a day, you'll lose about 1-lb fat/week regardless of the intensity. You can ride easy for 6-hours a day or ride really hard for 2-hours. The weight-loss will end up being the same. What happens is that after the ride, your body will convert fats into glycogen to replenish your energy stores. So it doesn't matter if you burned up that fat during the ride, or if it's converted later to be used on the next ride, it's still gonna be used up at one point or another to make up the calorie deficit.
Originally Posted by rjkresse
When we run out of carbs and glycogen during a training ride, event, or race, we run out of energy. We simply can't go anymore. This is called as bonking... unless you're in Australia and bonking is something else much more enjoyable altogether.
We can replace glycogen by eating carbs (complex are better) during exercise.
We also have a window of time after exercising to restore glycogen to the muscles and the liver.
Do I have this right so far? Does the body turn stored fat into glycogen?
Looks good so far. You have about a 10-15 minute window to eat carbs right after ride. About 1.5gm/kg body-weight is about optimum. If you wait longer than this, the body will disassemble perfectly good muscle to convert into glycogen to replenish your energy stores. This is the prefered source rather than fats. So you cannot starve yourself to make your body burn fat, it'll take apart muscle instead. Fat-catabolism is initiated at the same time and is triggered by low blood-sugar. Glucagon is then secreted to signal the body to convert fats into glycogen (through glycerol->glucose) But this process is much slower than converting muscle & protein, so you have to eat carbs right after a ride.
Originally Posted by rjkresse
Are fat and glycogen linked together in a cycle, or are they two separate energy sources?
They are linked somewhat. Here's a summary of the processes:
The bottom-line is that the citric acid cycle (Krebs cycle) is what generates the ATP for muscle contractions. It is fed acetyl-CoA from either carb or lipid metabolism. The carb pathway is much quicker and can allow the kreb cycle to generate more ATP/second than the lipid pathway.
* So that's why as intensity picks up, a larger percentage of energy comes from burning carbs. But the amount generated by lipid metabolism also increases as well, just not as quickly.
Here's how they are linked (a closer look at the fatty-acid spiral):
Notice that the fatty-acid spiral also generates acetyl-CoA for the kreb's cycle, but it initially requires the input of ATP. Where does this ATP come from? It comes from carb metabolism! So you cannot burn fats without having carbs to burn as well! This is a double-whammy when you bonk, because once you go through your carbs, you have nothing left to ignite the lipid pathway either. Your body shuts down carb & lipid metabolism and resorts to burning proteins and muscles for energy.
So that's why it's so important to replenish your carbs during long rides, once you bonk, that's it, you pretty much gotta stop unless you want to tear your muscles apart. About 250-300 cal/hour is the maximum your body can absorb, so you gotta start eating early if you're doing a long ride at high intensity (to burn as many calories/hour per workout).
Another catch is that in the intestines, in order absorb a single glucose molecule, you have to have an exchange of a sodium ion. You sweat away about 1000mg of sodium per hour, so you have to make sure you get electrolytes as well on the ride. You can end up with a triple-whammy when you bonk if you run out of carbs and electrolytes. You've got a stockpile of fats you can't burn because there's no carbs to initiate the process. Then you can't absorb carb scraps you've been picking off the ground because you've got no sodium... heh, heh...
no wonder cyclists who bonk look like zombies!
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* Slight error in this summary chart. Fructose is not entered into glycolysis direclty. It has to go through the liver and get converted from a 5-carbon fructose into a 6-carbon glucose first. Then that glucose is fed into glycolysis.