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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 that is using that oxygen, not fat. But how much difference does body fat make?
== Wild, optimistic assumptions ==
First wild assumption is that [[Jack Daniels Running Formula]] [[VDOT ]] is close enough to [[VO2max |V̇O<sub>2</sub>max]] to be useful. [[VO2max |V̇O<sub>2</sub>max]] is how much oxygen you can use per minute per Kg of body weight. The second wild assumption is that you can lose body fat without losing any muscle.
== Using VDOT for weight/performance changes ==
If [[VDOT/VO2max ]] is how much oxygen you use per Kg body weight, then you can multiply [[VDOT/VO2max ]] by body weight to work out the total max oxygen use. If you lose body fat only, then the total max oxygen should stay the same. You can then divide the total max oxygen use by the new body weight to get a new [[VDOT/VO2max]].
== A worked example ==
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 |V̇O<sub>2</sub>max]] of 50. A [[VO2max |V̇O<sub>2</sub>max]] 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 |V̇O<sub>2</sub>max]] 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. There is an online calculator to make The [[VDOT Calculator]] will work out this simpler at [http://www.runningforfitness.org/calc/weighteffectfor you.php http://www.runningforfitness.org/calc/weighteffect.php]
== The flaws in this idea - losing fat ==
It is very hard to lose weight, and it is even harder to lose weight without losing muscle. I suspect that this is a bigger problem than any issues of [[VDOT]]/[[VO2max |V̇O<sub>2</sub>max]] correlation. On the other hand, my personal experience is that losing weight does give a significant boost in performance.
== The flaws in this idea - too thin to win ==