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Why does low-intensity exercise rely more heavily on fat metabolism than high intensity exercise?

Answered on August 19, 2014
Created April 25, 2012 at 1:15 PM

Why is low-intensity exercise (walking, jogging, cycling) fueled by fat but higher intensity exercise is fueled by carbohydrates/glycogen/glucose?

Everything says that less fat is burned during high intensity activity but is that an absolute or relative measurement? I'm just confused about why your body's metabolism process of choice would be intensity-dependent instead of dependent on available materials (whereas it makes a lot of sense why a different metabolic system is used when oxygen isn't available). So what causes the switch?

I've been poking around the internet to answer this. I've either found websites with about.com levels of complexity talking about %max heart rate to burn the most fat or highly technical papers discussing triglyceride lipolysis and plasma-derived substrate oxidation that are above my pay grade. Professional and amateur biochemists alike, can you chime in with something in-between?

Fd70d71f4f8195c3a098eda4fc817d4f

(8014)

on October 01, 2012
at 08:11 PM

Never thought of it this way, but you're right. You wouldn't want to use your relatively limited supply of glycogen to fuel all your activity (like just walking, sitting upright, digesting, blinking, and all the other low-level "stuff" our bodies do all day. Theoretically you'd want to save that precious glycogen for the rare all-out effort. Good point! ---> "From an evolutionary standpoint, stored glucose (glycogen) is a precious commodity, used only for fight or flight. Burning fat as much as possible would be advantageous."

531b053b68e92ac509fc1544f88dc103

(1205)

on April 25, 2012
at 06:05 PM

Can you elaborate on that?

913872e599d0b8e120ac325ec45749dc

(95)

on April 25, 2012
at 05:30 PM

This is true, which is why % of heart rate starts to become at best a guideline. The old, "make sure you can carry out a conversation out loud" is simple enough to allow most people to stay just below their anaerobic threshold which should be 80% but changes as you're in better shape.

6996bca5d2417dbb2a361c50f7f687f0

(45)

on April 25, 2012
at 04:56 PM

When people who are in better shape exercise, they burn a more favorable fat:glucose ratio (as compared with obese subjects). Is this because they have stronger hearts/bigger lungs, so they can get more oxygen to their cells?

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6 Answers

5
913872e599d0b8e120ac325ec45749dc

(95)

on April 25, 2012
at 03:31 PM

Oxygen usage is what causes the switch. Once you cross the anaerobic threshold (out of breath, maybe 80% of maximum heart rate for most people), your body doesn't have the same saturation of oxygen (hence the reason you're out of breath trying to catch up). Once past this threshold glucose/glycogen and eventually creatine are needed in increasing quantities. You're really looking at how the body uses different percentages of fuel based on how hard you're cranking the engine. This is why the CKD or TKD diets can work so well by making sure the right fuel is in the tank at the right time for the right activity.

4
F0a3e3f17d9a740810ac37ff2353a9f3

(3804)

on April 25, 2012
at 03:39 PM

What Mike said.

Both glucose and fat can be metabolized aerobically, but only glucose can be metabolized anaerobically. As intensity outpaces the oxygen supply, cells switch to glucose for fuel. It's what allows us to exercise above VO2 max.

From an evolutionary standpoint, stored glucose (glycogen) is a precious commodity, used only for fight or flight. Burning fat as much as possible would be advantageous.

Fd70d71f4f8195c3a098eda4fc817d4f

(8014)

on October 01, 2012
at 08:11 PM

Never thought of it this way, but you're right. You wouldn't want to use your relatively limited supply of glycogen to fuel all your activity (like just walking, sitting upright, digesting, blinking, and all the other low-level "stuff" our bodies do all day. Theoretically you'd want to save that precious glycogen for the rare all-out effort. Good point! ---> "From an evolutionary standpoint, stored glucose (glycogen) is a precious commodity, used only for fight or flight. Burning fat as much as possible would be advantageous."

4
Ae8946707ddebf0f0bfbcfc63276d823

(9402)

on April 25, 2012
at 01:26 PM

Isn't it because you need more oxygen to burn fat and there is less oxygen available during high intensity exercise?

913872e599d0b8e120ac325ec45749dc

(95)

on April 25, 2012
at 05:30 PM

This is true, which is why % of heart rate starts to become at best a guideline. The old, "make sure you can carry out a conversation out loud" is simple enough to allow most people to stay just below their anaerobic threshold which should be 80% but changes as you're in better shape.

6996bca5d2417dbb2a361c50f7f687f0

(45)

on April 25, 2012
at 04:56 PM

When people who are in better shape exercise, they burn a more favorable fat:glucose ratio (as compared with obese subjects). Is this because they have stronger hearts/bigger lungs, so they can get more oxygen to their cells?

2
64433a05384cd9717c1aa6bf7e98b661

(15236)

on April 25, 2012
at 02:08 PM

Burning carbs is less efficient but it can be done faster. Fat burning gives you a whole lot more bang for your buck, but it's a slower process. How much slower depends on the individual and how they're adapted. There are certain enzymes that control the speed of fat burning that can be upregulated or downregulated depending on what you use more of.

1
5b0ddeb09a8cbae9b9d8fb2ed8f8c06a

on September 29, 2012
at 04:30 PM

high intensity exercises -time duration of exercise is low ,heart rate high,mainly use glucose and glycogen(this break to glucose) as main fuel.why is this because high intensity exercises are done mainly by fast skeletol muscle fibers and they have adopted to supply quick energy.so they contain high glycolytic enzymes and much glycogen.relatively low intensity exercises done by slow fibers in our skkeletol muscles and adopted to burn fat for fuel.so it contain high mitochondrial amount and good blood supply.in marathon runners theses fiber amount is high and sprinters muscle have high fast fibers -thank you -anurudda munasinghe -Bsc human biology,Msc (munasinghe74@yahoo.com)

1
C3bc92e6b5eba45dc55f43ac3c70cc25

on April 25, 2012
at 04:56 PM

Oxygen combines with protien, fats & carbs make ATP.

When it comes to low intensity exercises our aerobic energy system produces ATP at a slower rate but uses a "Pas as you go" system from oxygen. Fatty acids molecules are large. So they require more oxygen than is needed for carbohydrate breakdown.

531b053b68e92ac509fc1544f88dc103

(1205)

on April 25, 2012
at 06:05 PM

Can you elaborate on that?

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