Fat not burned until after 90 minutes?!?

jtintx

This is a quote from the University of Illinois McKinley Health Center web site:

"Your body utilizes three sources of energy; carbohydrates, proteins, and fats. They are utilized in that order. Fat is the last energy source your body uses and this doesn't occur until after 90 minutes of exercise. By exercising at a greater intensity (more than the fat burn button has you working at) you can burn more calories in a shorter period of time. You will also benefit your cardiovascular system. The more calories you burn the more fat stores you use."

I've been learning a lot about exercise lately but this info doesn't seem right to me. Any exercise physiologists or experts out there that can verify or debunk this?

jtintx

This is a quote from the University of Illinois McKinley Health Center web site:

"Your body utilizes three sources of energy; carbohydrates, proteins, and fats. They are utilized in that order. Fat is the last energy source your body uses and this doesn't occur until after 90 minutes of exercise. By exercising at a greater intensity (more than the fat burn button has you working at) you can burn more calories in a shorter period of time. You will also benefit your cardiovascular system. The more calories you burn the more fat stores you use."

I've been learning a lot about exercise lately but this info doesn't seem right to me. Any exercise physiologists or experts out there that can verify or debunk this?

songbyrdsweet

You're right, this is incorrect.

Barring exercise lasting just a couple seconds (it uses only ATP and creatine), you will always use both glycogen and fatty acids, just in different ratios, which depend on your intensity, not duration. At rest or very, very low intensity, about 70% of your energy needs come from fatty acid metabolism and 30% from carbohdyrate (glucose) metabolism. At 80% of your VO2Max (really high intensity), you're only going to get about 25% of your energy needs met by fatty acid metabolism. At about 65% VO2Max, you're running at about a 50/50 split.

Highly trained individuals are better at using fat for energy, so they can use more fat at higher VO2Maxes. This has a glycogen-sparing effect which allows for greater duration and intensity.

MyaPapaya75

Hmm good info it kind of seems right to me I do about 2hrs of excercise a day and havent hit any plateaus and I have a consistent weight loss...its very interesting to say the least i will have to look up more and perhaps ask a dietician or nutritionist....thks

jtintx

You're right, this is incorrect.

Barring exercise lasting just a couple seconds (it uses only ATP and creatine), you will always use both glycogen and fatty acids, just in different ratios, which depend on your intensity, not duration. At rest or very, very low intensity, about 70% of your energy needs come from fatty acid metabolism and 30% from carbohdyrate (glucose) metabolism. At 80% of your VO2Max (really high intensity), you're only going to get about 25% of your energy needs met by fatty acid metabolism. At about 65% VO2Max, you're running at about a 50/50 split.

Highly trained individuals are better at using fat for energy, so they can use more fat at higher VO2Maxes. This has a glycogen-sparing effect which allows for greater duration and intensity.

Umm...thanks for replying...but can you put that in layman's terms? I'm very interested in what you have to say but don't understand what you wrote....sorry for being dense.

songbyrdsweet

I should also add that the duration theory is a pretty old-school approach. It sounds like they're either trying to keep it really simple or they just haven't changed it in 10 years. :laugh:

After about 90 minutes of moderate/high intensity work, you'll start to see a decrease in glycogen stores without supplementation, but at normal carbohydrate intake for an athlete, it'd take more like 2+ hours to get a total depletion. And even then the glucose will just be produced. You can't use JUST fat or JUST glucose for any extended period of time because the two energy systems use what the other doesn't need.

songbyrdsweet

You're right, this is incorrect.

Barring exercise lasting just a couple seconds (it uses only ATP and creatine), you will always use both glycogen and fatty acids, just in different ratios, which depend on your intensity, not duration. At rest or very, very low intensity, about 70% of your energy needs come from fatty acid metabolism and 30% from carbohdyrate (glucose) metabolism. At 80% of your VO2Max (really high intensity), you're only going to get about 25% of your energy needs met by fatty acid metabolism. At about 65% VO2Max, you're running at about a 50/50 split.

Highly trained individuals are better at using fat for energy, so they can use more fat at higher VO2Maxes. This has a glycogen-sparing effect which allows for greater duration and intensity.

Umm...thanks for replying...but can you put that in layman's terms? I'm very interested in what you have to say but don't understand what you wrote....sorry for being dense.

You're not being dense, I'm just too wordy.

Basically, you will always use both glucose and fat, just in different amounts depending on how hard you're working. If your exercise is really intense, you'll use more glucose and less fat. If it's less intense, you can use more fat. But like I said, the two different systems rely on one another because their by-products have to be recycled. It's about intensity, not duration.

Also, people who work out a lot are better at using fat for energy, so they can use more fat even when they're working hard. This helps save some glucose so they can keep up both duration and intensity.

thejarviclan

I lost over 100 pounds by exercising twice a day for 30-45 minutes at a time. I think that's living proof that you can burn fat before the 90 minute mark.

jtintx

You're right, this is incorrect.

Barring exercise lasting just a couple seconds (it uses only ATP and creatine), you will always use both glycogen and fatty acids, just in different ratios, which depend on your intensity, not duration. At rest or very, very low intensity, about 70% of your energy needs come from fatty acid metabolism and 30% from carbohdyrate (glucose) metabolism. At 80% of your VO2Max (really high intensity), you're only going to get about 25% of your energy needs met by fatty acid metabolism. At about 65% VO2Max, you're running at about a 50/50 split.

Highly trained individuals are better at using fat for energy, so they can use more fat at higher VO2Maxes. This has a glycogen-sparing effect which allows for greater duration and intensity.

Umm...thanks for replying...but can you put that in layman's terms? I'm very interested in what you have to say but don't understand what you wrote....sorry for being dense.

You're not being dense, I'm just too wordy.

Basically, you will always use both glucose and fat, just in different amounts depending on how hard you're working. If your exercise is really intense, you'll use more glucose and less fat. If it's less intense, you can use more fat. But like I said, the two different systems rely on one another because their by-products have to be recycled. It's about intensity, not duration.

Also, people who work out a lot are better at using fat for energy, so they can use more fat even when they're working hard. This helps save some glucose so they can keep up both duration and intensity.

Is this why it is recommended to take in some "calories" when exercising for more than 90 minutes? To replace glucose?

songbyrdsweet

You're right, this is incorrect.

Barring exercise lasting just a couple seconds (it uses only ATP and creatine), you will always use both glycogen and fatty acids, just in different ratios, which depend on your intensity, not duration. At rest or very, very low intensity, about 70% of your energy needs come from fatty acid metabolism and 30% from carbohdyrate (glucose) metabolism. At 80% of your VO2Max (really high intensity), you're only going to get about 25% of your energy needs met by fatty acid metabolism. At about 65% VO2Max, you're running at about a 50/50 split.

Highly trained individuals are better at using fat for energy, so they can use more fat at higher VO2Maxes. This has a glycogen-sparing effect which allows for greater duration and intensity.

Umm...thanks for replying...but can you put that in layman's terms? I'm very interested in what you have to say but don't understand what you wrote....sorry for being dense.

You're not being dense, I'm just too wordy.

Basically, you will always use both glucose and fat, just in different amounts depending on how hard you're working. If your exercise is really intense, you'll use more glucose and less fat. If it's less intense, you can use more fat. But like I said, the two different systems rely on one another because their by-products have to be recycled. It's about intensity, not duration.

Also, people who work out a lot are better at using fat for energy, so they can use more fat even when they're working hard. This helps save some glucose so they can keep up both duration and intensity.

Is this why it is recommended to take in some "calories" when exercising for more than 90 minutes? To replace glucose?

Well it's not just calories, but carbohydrates specifically. It's hard enough to complete digestion while exercising, but with protein or fat it'd take so long that you might be done exercising before the nutrients would become available. Plus getting energy from fat or protein takes longer. Glucose is pretty much usable once it exits the stomach, and in liquid form that happens rather quickly.