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How does the body use fat as energy?(11 posts)

How does the body use fat as energy?PODIUMBOUNDdotCA
Aug 16, 2002 3:07 PM
Just a quick question of how the body uses fat as energy. I was once told the body uses carbs, then protein and finally fat as a fuel source. How true is this?

re: How does the body use fat as energy?CFBlue
Aug 16, 2002 3:18 PM
I've been looking at this question myself and came up with these links ear;ier today

something that attempts to debunk the above

there is a lot more out there if you want to spend a lot of time
re: How does the body use fat as energy?up_hiller
Aug 16, 2002 4:18 PM
the body never burns JUST protein or JUST fat or JUST carbs to release energy. it's always some combination of the three, but the proportion of each macronutrient being burned varies with workout intensity and availability.

at high intensities, carbohydrate sources (glycogen stored in muscles and the liver) are burned preferentially. As i recall this is because the glycogen is most quickly and easily burned. unforunately, the body is pretty limited in its ability to store glycogen, and it runs out after 30-45 minutes of tough exercise - the notorious Bonk.

at intensities around 60-80% of max HR, the percentage of energy coming from fat is greatest compared. sorry, i don't know what the actual percentage is, although i think it is still well unfer half of total energy expenditure. consequently, this range has been called the "fat burn zone" in recent years. however, recent research indicates that interval training may actually be more effective at burning fat. the link below is to an article related to this idea (and VO2, which we are all interested in anyway). not directly related to your question, but it might help nonetheless.
It's complicatedKerry
Aug 16, 2002 4:34 PM
But it is really all about equilibrium and the level of activity. If you go to bed hungry (limited glycogen reserves) then your body will tend to burn more fat for its calories. If you go to bed on a stomach full of pasta, of course the body will use that as fuel first. Of course, you don't burn that much at night (60-80 calories per hour). Your body makes "choices" because the metabolic pathways are controlled by enzymes and other agents that "sense" what fuels are available and route the energy stores accordingly. It's called the Crebs cycle. You will always be burning some mix of the three, and the ratio will depend on exercise intensity and availability of a given source. Further, a trained athlete will have a different mix than a sedentary person. During prolonged exercise, you will burn between 200 and 300 calories an hour from fat (about 200) and protein (about 100). The rest MUST come from sugar, which starts as carbs. In order to fuel bodily processes, everything must be converted to sugar or close to it, so fats and proteins get broken down. Your statement is not true, though at high intensities, the vast majority of the fuel does come from carbs. At very low intensities, you body will do some "glycogen sparing" and get much of its power from fat. This means riding about 12 mph or so! Once you have bonked, that's all the faster you can go, give or take.
That's the we we were taught 25 years ago ...Humma Hah
Aug 16, 2002 4:37 PM
... in the last millenium, but evidently its not so cut and dried as was thought at the time.

Back then, biochemists understood the Krebs Cycle and the Emerhoen-Myerhoff pathway (I'm sure I've mis-spelled that last) down to a gnat's eyelash. Those biochemical pathways are how carbs are metabolized. Carbs include simple sugars, double sugars (like table sugar), starches, and glycogen.

Those same textbooks had very little to say about fat metabolism.

I've picked up a smattering since. Evidently there is a lot more metabolism of fat during moderate exercise than was previously realized. There are reports that a gentle warmup helps trigger fat metabolism. Fat metabolism is generally considered limited to about 200 calories per hour, for an average adult (females may be able to burn it faster than men).

Carbs may have a similar mechanism that kicks in a metabolic afterburner, triggered by an intense interval. I believe I've experienced this by engaging in a "rabbit chase" early in a ride. The effect, a boisterous release of energy resulting in uncharacteristically high average speed, lasts about 50 miles. A couple of times, it has ended in a near-bonk.

Either phenomenon will burn fat in the long run if you don't pig out after a ride. I personally prefer the carb-afterburner approach for shorter rides, as it burns a lot of calories fast and it really is fun at the same time.
Thanks but one more questionPODIUMBOUNDdotCA
Aug 16, 2002 5:09 PM
How do all these results correlate to just sedetary activity while not training?

All excess calories turn to fat ...Humma Hah
Aug 17, 2002 9:05 AM
... and a weight-loss diet causes the body to shed muscle to conserve energy, to stop the loss of fat.

That's why diet AND exercise are important -- to force the body to not lose muscle, and preferably put some on.

Actually, what happens to excess calories is not cut and dried. Depending on genetic makeup, certain health factors, and diet, some people will turn every excess calorie into fat, while other people, those annoying people who can eat anything without getting fat, just turn it into heat and burn it away.
isn't age a factor in what and how one metabolizes..Djudd
Aug 17, 2002 11:42 AM
fat? Asking as a soon-to-be forty year-old.
Yes and noKerry
Aug 17, 2002 3:05 PM
Yes because older people usually are losing muscle and therefore have a smaller engine to burn the fat. Maybe because your levels of hormones change. No because the basic metabolic pathways don't change.
I'm lighter and stronger at 49 than I was at 39 ...Humma Hah
Aug 18, 2002 7:40 AM
... thanks to getting back into cycling in a serious way.

My endurance is better than it was at 19. I'm sure of that because I'm riding the same bicycle, and can ride if further and faster than my peak performance when younger, at age 24.
"Fuel Up" - buy the book270bullet
Aug 18, 2002 1:15 PM
"Fuel Up - Using the Principles of Sports Nutrition to Perfom Like a Pro".

This book by Dr. Eric Sternlicht will answer all your questions. He is an Exercise Physiologist and a cyclist. He's won the CA State Time Trial Championships 4 times and is a masters National Champion.