Not only does the vo2 test determine target heart rate, it also pinpoints the ratios of fats/carbs used for energy in an individual!!
So this is why some guys advocate carbs over fats and vice versa.
I took the test with jason wicker back in march and he is a massive carb burner. I was a 50/50 burner of the two but as my heart rate increased, I used more and more fats!
The thread origonaly posted is a good read
http://www.biohazardforums.com/viewt...t=2064&start=0
As is this one also on bio
http://www.biohazardforums.com/viewt...er=asc&start=0
They are both worth a read