So my primary goal right now is to reduce body fat. I’ve been doing a lot of research on the best way to do this. Cardio? If so, what kind? Weights? Diet? Everything?!

First, I wanted to understand exactly how and when the body breaks down fat.  I’ve came across a few articles, which I was hoping to link to from the end of this entry, but I accidentally X-ed out of my Firefox window while I was trying to move another window…. >_< Anger!

But, if I understand correctly, and I’m not sure I do:

1. The body uses glucose as its primary source of energy, both for cell functions and physical activities.
2. Glucose is derived from the foods you eat, mainly carbs.
3. If the glucose doesn’t get used up soon after your body gets it, it is converted to glycogen which is stored in your liver and muscles.  The liver and muscles can only store a set amount of glycogen, so any leftover glucose that doesn’t get used up for energy (through physical activity), is converted and stored as body fat.
4. During exercise, your body first goes to use any available glucose in your bloodstream.  However, weight lifting, which involves muscle contractions, uses instead the glycogen stored in the muscle.
5. If your body doesn’t have enough glucose in your blood, your liver will start releasing the store glycogen back into your blood as glucose to be used.
6. Since the liver can only store a certain amount of glycogen, at some point (unless you replenish your supplies by eating) your body will no longer have glucose to use as energy.  Unlike the glycogen in your liver, the glycogen stored in your muscles can only be used for muscles and is not “tapped” when the body needs glucose.  Instead, if you use up the glycogen in your muscle, that muscle will feel weak and tired (think of when you struggle to do the last rep on a set), creatine will get involved to try and help (but creatine is for another day, another post).
7. At this point, with no more glucose in your blood, and no glycogen in your liver to depend on, your body starts to use fat stored in your body for energy, and this is when fat loss begins.

So what does this mean?  If I go into the gym and start doing cardio, my body will first use up the glucose in my bloodstream, and if I deplete that, it will use up my glycogen stores, and then it will start burning fat.  Considering it make take some time to deplete those first 2 energy sources, I could be running for a while before I start buring fat!

If I start weight lifting, my body is using both glucose and glycogen.  I will end up depleting my glycogen level in my muscles, so the glucose in my blood will have to go and replenish my muscles, and that means my blood glucose will get low, meaning my liver glycogen will be released to compensate.  It seems that I would be going through my blood glucose and liver glycogen faster by doing weights, and then once I hit the treadmill, my entire cardio session would just be fat burning since there’s nothing else left for my body to use for energy.

Well, that’s my understanding at least.  I need to find all those articles I read again.

Update: Here’s one:

Glycogen - Its Role in BodyBuilding

And another one that seems to explain it clearly:

Weights Before Cardio

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9. Anonymous Says:
   July 20th, 2010 at 12:29 am

   Hi. Good post. I reached the same conclusion as you after reading many of the articles available online. I therefore hope that your advice of doing weights before cardio is correct. However I have a question and i hope you are able to help me. How long does it take for the glycogen stored in the liver or muscles to be transformed into adipose tissue (body fats)? Also assuming the muscles and liver are at their maximum storage capacity of glycogen, what happens to additional glucose present in bloodstream. Is it converted to body fats directly? I need to know the answer so that i can know how long before my workout I should replenish my energy reserves.