What's the fuel source - HR zone training vs. RPE

(Ben Kirchner) #1

During high intensity exercise, the body utilizes glucose as a fuel source. At lower intensities, the body it utilizing fat as a fuel source. However, there is some grey area here…

“High intensity” exercise - determined by heart rate, or more rate of perceived exertion(RPE)? Example - You do a few ALL OUT sprints and elevate your heart rate close to your max HR. You then slow it down to a jog. Your heart rate may still remain 85%+ during this jogging period, but a fit individual can recover enough to maintain a conversation relatively quickly. Even if the HR remains elevated at a high level.

Or in another case, you may start a long run at a light pace. Your heart rate is 60-70% for much of the early going, but after awhile at the same pace, your heart rate elevates to 80-90%, but you’re still able to hold a conversation.

What is the fuel source in these circumstances? If you’re going all out and struggling to utter out more than a couple words a time, clearly you’re utilizing glycogen. If you are able to speak without much difficulty, but your heart rate is elevated, are you utilizing glycogen or fat?

There is also a grey area of exactly when the body chooses glycogen over fat. I see many resources that universally say ~60% you’re utilizing fat, and close to max exertion, you’re utilizing glycogen. However, I haven’t seen much on what you’re utilizing in the middle. Say 75% of max HR or max exertion/RPE.

(Bunny) #2

I only see two questions and the rest is seemingly instructional or informational.

I will give you my personal opinion:

Human skeletal muscle does not prefer glucose or fatty acids for fuel at all, it prefers non-essential amino acids as the primary source of fuel, more specifically the kind that reside inside muscle tissue itself for fuel; so that gets burned up (catabolized tissue mass or loss) first without adequate essential amino acid protein intake (balance?); then glycogen is utilized as the secondary back up unit; then after that, the third backup unit is utilized which are ketone bodies.

It takes time to convert sugar/carbs or protein into glycogen to put back into the muscle or liver so the muscle has to burn itself (eat itself) for fuel that is why it is so difficult to deplete glycogen storage in muscle or liver so unused circulating glucose has somewhere to go.

If you have stored glycogen in the liver and the muscles need glycogen it has to convert it back into glucose and release back it into the circulatory system then back into glycogen to be stored into the skeletal muscle, that is of course if your not eating or drinking more dietary glucose/carbs/sugar to throw into the mix and once again overloading the system and excess glucose just floats around in your blood stream?

That is why people who are fat and exercise excessively cannot burn body fat because they are burning up the very endocrine organs (skeletal muscle) that they need more of to prevent storage of glucose into fat or lipid droplets that get stored in adipose cells to begin with and cannot oxidize dietary carbohydrates immediately after ingesting or burn body fat while at rest.

Bottom line you need more dietary protein not carbs or fat.

I’m starting to see the light why people are going carnivore!

And another thing that is extraordinary is protein releases stored glycogen from the liver and skeletal muscle without even exercising or lifting a finger when you eat a little tiny bit of it, like 2 or 3 grams by itself, no carbs or fat.

(Ben Kirchner) #3

I think we’re getting away from the question, and focusing more on the conversion of glycogen.

Let’s simplify it and say someone on a ketogenic diet is running. Having little to no carbs to fuel from, during high intensities, the body will create glucose from gluconeogenesis (conversion from protein to glucose). While staying in certain zones may produce more ketones to get into a deeper ketogenic state.

One of my questions is - are we defining high intensity by heart rate or rate of perceived exertion (RPE)? Example case when you hit a sprint, then return to a light jog, but heart rate remains elevated.

My other questions is what is the fuel source when you’re in the middle of light/moderate intensity and high intensity? If w’re running off fat and ketones around ~60%, and running off glucose at 85%+, what is the fuel source when we’re at around 75% of max? Is it a black and white thing? A mixture of both glucose and fat?

(Bunny) #4

Are you using a Borg Rating Scale because of a heart condition? Just curious!

Phinney, Bailey & Volek on fuel sources:

“…How the body shifts its primary energy source from carbohydrates to fats and ketones is anything but simple. This process, which we have named ‘keto-adaptation,’ starts within days but takes a considerable period of time to fully develop. And even after it is complete, the result is not an absolute exclusion of glucose from the body’s fuel supply. Rather, the need and use of glucose is dramatically reduced, while at the same time pathways that salvage products of partially metabolized glucose (e.g., pyruvate and lactate) for recycling into fuel and other beneficial metabolic intermediates become more finely tuned. The result is maintenance of normal blood glucose and muscle glycogen levels that can be sustained without the need for dietary carbohydrate intake. …” …More

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(Give me bacon, or give me death.) #5

The studies performed by Prof. Jeff Volek and his students, previously at the University of Connecticut and now at Ohio State University, address some of these questions. Ketones are a good source of energy for endurance performance, but glucose is needed for explosive performance, which is why serum glucose rises when we exercise. The respiratory quotient is what they use to determine what percentage of energy expenditure is from glucose and what percentage is from fatty-acid metabolism. In keto-adapted athletes, the fat-burning phase lasts considerably longer before glycolysis becomes necessary.

After an athlete switches to a ketogenic diet, endurance generally returns within six to eight weeks, but explosive performance takes quite a bit longer to recover. However, a recent study by Prof. Volek’s team, conducted on athletes who had been keto-adapted for two years or longer, showed that they had glycogen levels indistinguishable from those of carb-eating athletes.

(Ben Kirchner) #6

These are informative. However, I’m trying to get a better idea on he fuel source during exercise, at different intensities. Ketones are a good source of endurance performance, which aligns with us primarily using fat/ketones at lower intensities, which can be sustained for much longer. Short, explosive intensities can be sustained for very short periods, relying on glucose. Therefore, slow jog - fat fueled All out sprint = glucose fueled.

This still leaves the question - at what point do we switch from fat to glucose, or vice-versa? What is our fuel at 75%? What is our fuel when we go from sprint to slow jog, but our heart rate remains elevated from the prior sprint?

@atomicspacebunny - I do not have a heart condition. I’m simply curious of the fuel sources during these stages of exercise intensities.

(Bunny) #7

Adrenaline will make your heart rate stay elevated along with blood PH, the heart and brain need more oxygen to sustain current exertion. How traumatized are you mentally, emotionally in addition to physical exertion?

Leucine and lysine is interesting as lower methionine turns off IGF-1 which is vital to future metabolic stability (muscle protein synthesis) but it accelerates the aging process.


[1] Biochemistry. 5th edition Section 30.4 Fuel Choice During Exercise Is Determined by Intensity and Duration of Activity: ”…The running of a marathon (26 miles 385 yards, or 42,200 meters), requires a different selection of fuels and is characterized by cooperation between muscle, liver, and adipose tissue. Liver glycogen complements muscle glycogen as an energy store that can be tapped. However, the total body glycogen stores (103 mol of ATP at best) are insufficient to provide the 150 mol of ATP needed for this grueling ~2-hour event. Much larger quantities of ATP can be obtained by the oxidation of fatty acids derived from the breakdown of fat in adipose tissue, but the maximal rate of ATP generation is slower yet than that of glycogen oxidation and is more than tenfold slower than that with creatine phosphate. Thus, ATP is generated much more slowly from high-capacity stores than from limited ones, accounting for the different velocities of anaerobic and aerobic events. *ATP generation from fatty acids is essential for distance running. However, a marathon would take about 6 hours to run if all the ATP came from fatty acid oxidation, because it is much slower than glycogen oxidation. Elite runners consume about equal amounts of glycogen and fatty acids during a marathon to achieve a mean velocity of 5.5 m/s, about half that of a 100-meter sprint. How is an optimal mix of these fuels achieved? A low blood-sugar level leads to a high glucagon/insulin ratio, which in turn mobilizes fatty acids from adipose tissue. Fatty acids readily enter muscle, where they are degraded by β oxidation to acetyl CoA and then to CO2. The elevated acetyl CoA level decreases the activity of the pyruvate dehydrogenase complex to block the conversion of pyruvate into acetyl CoA. Hence, fatty acid oxidation decreases the funneling of sugar into the citric acid cycle and oxidative phosphorylation. Glucose is spared so that just enough remains available at the end of the marathon. The simultaneous use of both fuels gives a higher mean velocity than would be attained if glycogen were totally consumed before the start of fatty acid oxidation. …” …More</small)

[2] “…Essential and nonessential amino acids are degraded to products that can be metabolized for energy. All amino acids are able to form glucose (glucogenic) except for leucine and lysine that can form acetoacetate and are, thus, uniquely ketogenic. …” …More

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(Ben Kirchner) #8

The ansswer actually lies in the Respitory Exchange Ratio (RER). Everyone is different, soa true VO2 max test woudl need to be done to measure at what intensity your body is burning the most fat (most overall fat, not highest ratio of fat). Without the test, we can go by the general knowledge of you’re fueling from both sources of fat and glucose, with the fat being higher ratio at lower intensities, and glucose being at higher intensities. As you become more fat adapted, you can be able to aid this ratio, burning more fat even at higher intensities. This can be seen and explained here:

(Give me bacon, or give me death.) #9

The studies by Jeff Volek’s team, as I mentioned, contain much relevant information. As do the many studies Prof. Volek did jointly with Dr. Stephen Phinney on fat-adapted and carb-adapted athletes. If you are uninterested in looking into the primary literature, why not go on YouTube, and search for their lectures? Both of them have given presentations at events sponsored by Low Carb Down Under (Dr. Rod Taylor of Australia and Dr. Jeffrey Gerber of the U.S.), various Ancestral Health symposia, and other low-carb events. Dr. Phinney has given a number of lectures discussing his and Prof. Volek’s data, and his descriptions of what happens to the O2 max and the respiratory quotient in fat-adapted athletes are interesting, even to a non-athlete such as myself.