Changes

It seems intuitive that excess body fat makes a runner slower. Body fat does not help you run, and adds weight that must be carried. The science seems to support this, as aerobic performance is based on how much oxygen you can use, and it is primarily [[Muscle|muscle ]] that is using that oxygen, not fat. But how much difference does body fat make?

Let's work an example with a hypothetical runner who completes a marathon in 3:10 and weighs 220 Lb. A marathon time of 3:10 is a [[VDOT]] value of 50, which we will assume is also a [[VO2max|V̇O₂max]] of 50. A [[VO2max|V̇O₂max]] of 50 means you consume 50 ml of oxygen per Kg of body weight per minute. If you weigh 220 Lb, which is 100 Kg, and your [[VO2max|V̇O₂max]] is 50, that equates a total Oxygen consumption of 5000 (ml/min). Reversing the equation above, 200 Lb is 91 Kg and 5000 / 91 is a [[VO2max ]] of 55. A [[VDOT]] of 55 equates to a marathon time of 2:56. That's a reduction of 14 minutes in marathon time. The [[VDOT Calculator]] will work out this for you.

One limiting factor in running marathon distance races is our ability to store [[Glycogen]]. Because glycogen [[Glycogen]] is only stored in the muscles and liver, excess body fat reduces the percentage of muscle and therefore percentage of glycogen [[Glycogen]] that can be stored.

You might think that more body fat would help provide fuel on very [[Long Run|long runs]]. However, one pound of fat provides about 3,500 calories, enough to fuel a runner for 35 miles, assuming no other fuel is used (eaten, glycogen[[Glycogen]], etc).