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?There's a simple answer based on the weight of fat, but the reality might be more complex.

== Simplistic assumptions =The Simple Answer =First The first assumption is that [[Jack Daniels Running Formula]] [[VDOT]] is close enough to [[VO2max|V̇O₂max]] to be useful. [[VO2max|V̇O₂max]] is how much oxygen you can use per minute per Kg of body weight. The second assumption is that you can lose body fat without losing any muscle. == Using VDOT for weight/performance changes ==If [[VDOT]] is how much oxygen you use per Kg body weight, then you can multiply [[VDOT]] 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]].This is worked out in my [[Running Calculator]], and losing 5% of body weight will give a roughly 5% improvement in performance.

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 Running Calculator]] will work out this for you. = The Bigger Impact of Body Fat=Studies have shown that body fat may impact performance more than you'd expect based on simple weight reduction. For instance, a study of 11 elite runners showed that a 5% reduction in body fat resulted in a roughly 15% improvement in running economy<ref name="i754"/>.[[File:Body Fat and RE in Elites.jpeg |center|thumb|300px|Running Economy in Elite Runners]]Another study went further, and found that age and body fat are the strongest predictors of [[VO2max|V̇O₂max]] <ref name="c274"/>. [[File:Body fat and VO2max.png|400px|thumb|center|[[VO2max|V̇O₂max]] and Body Fat]]The study found that their [[VO2max|V̇O₂max]] estimate varied by this formula:[[VO2max|V̇O₂max]] = 129.6−(25.1× Log₁₀(%fat)) − (34.0× Log₁₀(age))Here's a table of how that looks for someone aged 59. As you can see, at high body fat levels, the performance improvement from weight loss is smaller than at low body fat levels. {| class="wikitable" style="margin-left: auto; margin-right: auto; border: none;"! % body fat! age! [[VO2max|V̇O₂max]]! % change |-| 30| 59| 32.3| 100%|-| 29| 59| 32.7| 101%|-| 28| 59| 33.1| 102%|-| 27| 59| 33.5| 104%|-| 26| 59| 33.9| 105%|-| 25| 59| 34.3| 106%|-| 24| 59| 34.7| 108%|-| 23| 59| 35.2| 109%|-| 22| 59| 35.7| 110%|-| 21| 59| 36.2| 112%|-| 20| 59| 36.7| 114%|-| 19| 59| 37.3| 115%|-| 18| 59| 37.9| 117%|-| 17| 59| 38.5| 119%|-| 16| 59| 39.2| 121%|-| 15| 59| 39.9| 123%|-| 14| 59| 40.6| 126%|-| 13| 59| 41.4| 128%|-| 12| 59| 42.3| 131%|-| 11| 59| 43.3| 134%|-| 10| 59| 44.3| 137%|-| 9| 59| 45.4| 141%|-| 8| 59| 46.7| 145%|-| 7| 59| 48.2| 149%|-| 6| 59| 49.9| 154%|-| 5| 59| 51.8| 160%|}

Unless the temperature is quite cold, our maximum running speed is defined by our ability to shed heat. In turn, our ability to shed heat is defined by height and weight and because we cannot change our height, our weight becomes the main determinant.thereforeTherefore, losing body fat can improve our ability to run at speed in mild or warm conditions.