JC and swimmar-
Fuels sources and when the body chooses them are controversial and somewhat misunderstood topics. It is dependent on exercise intensity and duration as well as the type of exercise and athlete-so we have a lot of variables. But let's use Lance Armstrong since he was mentioned previously and I'll talk about how it works for him. Bear with me-I will use pretty simple concepts and indices
'Low intensity' from an energy system standpoint is APPROXIMATELY an intensity ~50-88% percent of max HR (depending on who you ask-in all actuality HR is just a rough index of workload-to actually determine the workload of Lance in our theoretical model you would have to look at his wattage output, lactate clearance, energy system/metabolic processes and the efficiency of those processes-however HR has been proven as a 'reasonable' index of athletic workload-HR is your body's response to work being done-this could be a whole new thread). This could vary somewhat from athlete to athlete. In this zone a majority of calories burned come from fat, meaning your body is choosing fat as its primary fuel source (60-70% energy comes from fat). As well glucose and protein is used as well. Well-trained aerobic/endurance athletes will have a wider 'zone' for using fat as the primary fuel source.
Your body converts fat down to ATP-same thing happens with glucose and protein. Fat molecules convert to ~ 3 or 4 times as much ATP as a glucose molecule-this is why when conditions allow your body prefers fat. In the "zone" your body still needs glucose as part of the fat breakdon process. Look at available glucose as the igniter of the fat breakdown reaction. Protein gets broken down too, but this is the hardest substance for your body to breakdown to ATP. However some protein breakdown is necessary and part of the energy system/aerobic respiration process.
So Lance is riding along at a HR of 160 which is, say, 80% of max HR (and for our purposes 80% intensity level). His metabolism has been trained very well to use fat for fuel so most of his caloric expenditure comes from fat being burned. After 3hrs, Lance comes to a mountain and accelerates up the climb. His HR increases to 170 which is about 87% percent (close to aerobic/lactic threshold). At this point his body begins to move away from fat a little and demand more glucose. The energy system/aerobic respiration process is speeding up and the body needs ATP ASAP and it is easier to breakdown glucose. Now Lance has been on this mountain for :30min. He is still burning fat but he is burning quite a bit of available glucose too. Lance is near the top and decides to ride all out to the top. He accelerates and his HR goes up to 188 or 95%. Now his body needs energy on demand. The energy system/aerobic respiration process is now making most of its energy from glucose (this could have been released by the liver or taken from food or a sports drink). Most of calories are coming from glucose. At the top Lance relaxes and coasts down the other side. At this point I am kinda fuzzy. I am sure he is gradually burning more fat. When he rests for the day though I believe his energy demands will be low enough they can be met by glucose in the blood or from the liver. At rest I believe the body derives energy from carbs and fat. It isn't fat entirely-otherwise we wouldn't have obese or overweight individuals-everyone would be 15% BF. if their body only burned fat at sedentary workloads (ie get up, go to work, drive home, watch TV, go to mall, movies-do you know what I mean).
So higher intensity does burn more calories and burns much of them from glucose. Low intensity (which you can see really isn't that low-try maintaining 160bpm for 3hrs and then 175bpm for :30min) burns more calories from fat. The higher intensity will raise metabolism for longer post workout. After a lower intensity workout metabolism will return to normal sooner. Metabolism rises in response to the bodies need to refuel/rebuild itself post workout. So it makes sense that after a harder workout your body will take longer to repair.
In well trained endurance athletes they can get into fat burning mode/zone quicker than a poorly trained individual. As well their zone is wider and they may need less fat (converted to ATP) to do the same amount of work as a poorly trained athlete. As well their energy system/aerobic respiration process is much more efficient so they can produce more power from lower amounts of energy (i.e. Lance might only need 500kcal to complete a 30mile race in 1hr; whereas a poorly trained cyclist may need 1500cal to complete the same race in 1:15)
There are a ton of opinions and studies done re: energy system metabolism so you can get somewhat conflicting info here and there.
Currently, BB'ers feel they can burn more calories by doing HIIT, short high intensity interval workouts, rather than longer low intensity workouts. The theory goes like this: after accounting for all calories burned post workout from raised metabolism-they'll burn more calories, thus more fat, from an HIIT workout than from a low intensity workout. During the the low intensity workout they may burn a higher percentage of fat. However since metabolism isn't raised as long post-workout, overall calories burned is less; therefore less overall fat is burned. The flaw with low intensity workouts is often people don't work hard enough-they think 'oh if I just walk at 130bpm for :30min on the treadmill that'll be okay' when really it won't burn much fat at all. They need to get the HR closer to 150 for :45 to really see any fat burning benefit. Either way you burn fat-in general people need to cardio a little harder, a little longer to see results they want (this is a general observation made from posts on the board).
This site had some interesting tidbits (I think ALL their figures are low though-but goes to show the discrepancies in specifics of energy system function).
http://teaching.ucdavis.edu/nut111/carb/carb.html
So there is an energy system primer. Now how ALA affects this...let me regroup and think about that a moment.
FHG
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