Fat burning and calorie expenditure during endurance training.

During my long run yesterday, I finally worked out a simple analogy to explain why lower intensity workouts burn a higher percentage of fat than higher intensity workouts. The way that this idea is usually conveyed led to large degree of confusion for a long time for me. I've come to the conclusion that it basically comes down to the general concept being lost in translation. Very often, one is told that lower intensity workouts burn more fat, and thus for weight loss, lower intensity is the better way to go. This is reinforced by seeing a scale on a treadmill of heart rate and training zones, and the lower end of it is labelled with big letters as the 'FAT BURNING ZONE'.

The thing that perplexed me was this - it takes X calories to do an activity, say run 1 mile. Why in hell would it burn more fat if I do it at a slower pace then at a faster pace. Either way, X calories were spent and burned, right? So why does it matter?

Turns out, I was on to something. What is missing from the generalization is that the so-called fat burning zone is operating on a percentage basis of where the energy comes from. A larger percentage of the calories required for the activity comes from fat when doing the exercise at a low intensity as opposed to a higher intensity. However, percentages are not absolutes - and in reality, more calories overall are burned by going at the higher intensity rate.

So the analogy I thought of was this. Imagine you are tending to a small fire, which represents your basic metabolic rate of calories burned, and you have 1 cord of nice, dry, aged firewood available and 3 cords of wet firewood available. The nice, easily burned wood represents the glycogen (energy) in the bloodstream, and the wet firewood represents the energy you have available in fat reserves. The BTU's are there, but not as easily accessible, becaeuse the excess water must be driven off first.

For whatever reason, you have need to stoke the fire and get it burning hotter. This represents the extra energy required by the exercise you are doing. You have a few choices - you can use the dry firewood, you can use the wet firewood, or you can use a little of both types.

If the fire needs to get real hot real quick, which in this analogy means you are sprinting at top speed, the only option available is to dig into your store of nice dry aged firewood. Once hot enough, you can throw in a small amount of the wet firewood so that you conserve a small amount of the ready-to-go firewood, but the vast majority of fuel is the good dry firewood. Lets say in this example that the ratio is 6 pieces of good firewood to 1 piece of the wet firewood. In this scenario, the available fuel of dry firewood is expended relatively quickly, and before too long, there is little to none left. You are out of gas, and can no longer continue at the same speed. You are forced to slow down to whatever meager flame can be attained from burning a fuel mixture comprised mostly of wet firewood.

On the other hand, if you need to get the fire just a little bit warmer than it originally was, which in this analogy means increasing your pace slightly above normal metabolic rate (essentially somewhere in the aerobic zone), then you can add in a little bit of the easily burned firewood, and a little bit of the wet firewood. You may be able to maintain the desired fire with a ratio of more like 2:1, rather then 6:1. As a result, the percentage of wet firewood (fat) to dry firewood (glycogen) is higher, at 33% vs 16%.

However, one needs to keep in mind that all else being equal (time, conditions, etc) - higher intensity ultimately burns more calories. The hotter flame is able to raise the dry firewood to combustion temperatures much more quickly, and thus consume fuel at a faster rate than the smaller flame.

Ok, so maybe this analogy is not quite as ground breaking as I thought it was when I thought of it. If so, I'm blaming it on the fact that I thought of it on mile 7 and during obvious oxygen deficit to the brain :)

Here is a good article I found that discusses aerobic training, anaerobic training, metabolism and all that good stuff.