A hybrid engine can run on more than one fuel. Hybrid cars run on electricity (from a battery) or gasoline (which charges the battery).
Turns out the human body is a hybrid engine. We can run on carbohydrates or fat. We can also run on Protein but it’s not efficient as a fuel (Glycogen Replenishment After Exhaustive Exercise).
The ability to switch between fuel sources is key to the ketogenic diet. Someone who is not fat fueled takes days to fully make the switch between fuel sources. But it turns out we are always running on a hybrid system regardless of whether or not we are ketogenic. For instance, overnight we are fasting until we eat and the carbohydrates in our system are depleted.
Similarly in exercise we switch from the various fuel systems in our body. Athletes on High Carb (HC) and Low Carb (LC) diets were put onto a treadmill and run for hours. These curves show the metabolic flexibility of the LC diet.
The top graph is the rate that fat is oxidized (burned). The LC diet provides fat as a fuel very quickly and at a much higher level than the HC diet.
The bottom graph is the rate of carbohydrate oxidation. The LC diet doesn’t have the carb repositories (glycogen stores) that the HC diet has so they don’t get much energy from glycogen stores. However, unlike the HC diet the LC diet provides consistent amount of carb energy throughout the workout.The HC diet has a high level at the beginning but drops over time which results in the characteristic bonk of the long distance runner.
Note that even the HC diet eventually requires body fat for fuel. It just doesn’t have instant access to the fat like the LC diet.
What I Observe In the Gym
I do CrossFit for exercise. CrossFit is commonly viewed as a glycogen intensive sport (J Physiol. 2013 Sep 15; 591(Pt 18): 4405–4413. Muscle glycogen stores and fatigue. Niels Ørtenblad, Håkan Westerblad, and Joachim Nielsen).
A Typical CrossFit Workout
Today’s Workout of the Day (WOD) was:
That’s 21 minutes of Box Jumping, Push-ups, Burpees, Back Squats, Hang Cleans, and Up-Downs (Burpees without pushups). That’s fairly short (not like running a marathon. It also includes 3 sets of 30 Wall balls. Quite a good workout.
Why Do CrossFit?
Note, I said I do CrossFit for exercise – not for sport. I don’t care what my exercise times are at CrossFit. I care if I get an effective workout. And I measure effectiveness by the accepted standard of heart rate .
Heart Rate as Metric of Effectiveness of a Workout
From the Mayo Clinic Website:
Gauging intensity using your heart rate
Another way to gauge your exercise intensity is to see how hard your heart is beating during physical activity. To use this method, you first have to figure out your maximum heart rate — the upper limit of what your cardiovascular system can handle during physical activity.
The basic way to calculate your maximum heart rate is to subtract your age from 220. For example, if you’re 45 years old, subtract 45 from 220 to get a maximum heart rate of 175. This is the maximum number of times your heart should beat per minute during exercise.
Once you know your maximum heart rate, you can calculate your desired target heart rate zone — the level at which your heart is being exercised and conditioned but not overworked.
The American Heart Association and the Centers for Disease Control and Prevention recommend a general target heart rate of:
Moderate exercise intensity: 50 to about 70 percent of your maximum heart rate
Vigorous exercise intensity: 70 to about 85 percent of your maximum heart rate
My Own Results
Here’s my heart rate as recorded by my Samsung Gear Sport watch:
The first part (0-30 minutes) was a chipper (the coach tells you what to do) which included a whole lot of stuff (pushups, jumping jacks, forward lunges, PVC work, sit-ups, squats). That got my heat rate as high as 160 near the end. The rest period then let my heart drop to “normal” range. Then the WOD…
I am 57-years old. To calculate max heart rate the standard way is to subtract your age from 220. So my max heart rate is 163. I hit that rate during the workout and I felt it. I had to stop and breath/rest at spots.
At the end I had spent most of my time at 85% (138 in my case) or more of my max heart rate. But I had energy left to clean up and started cleaning up immediately. It wasn’t that I didn’t work out hard. It’s that I am not glycogen dependent. (Heart Rate Calculator).
The different oxidation rates of carbs and fat may provide the reason why I can work out at a high heart rate but not be depleted at the end of the workout.
Both oxidation charts are in grams but remember that a gram of fat contains 9 calories of energy and a gram of carbohydrates contains 4 calories of energy. And the repositories of glycogen are in the hundreds of grams total. That’s a calorie repository of around 1600 calories, give or take. And it starts to drop quickly over time.
The repositories of fat are in the many thousands of grams. If you have just 22 lbs of fat, that’s 10 kg of fat or 90,000 calories available. Fat energy stays pretty constant. That may explain why I can keep moving after the WOD ends.
I work out for the express purpose of depleting my glycogen stores.