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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?
=[[VO2max|V̇O<sub>2</sub>max]] Changes=== Wild, optimistic Simplistic assumptions ==
First 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 assumption is that you can lose body fat without losing any muscle.
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. The [[VDOT Calculator]] will work out this for you.
=Body fat and heat dissipation=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.therefore, losing body fat can improve our ability to run at speed in mild or warm conditions. =Body fat and glycogen storage=One limiting factor in running marathon distance races is our ability to store [[Glycogen]]. Because glycogen is only stored in the muscles and liver, excess body fat reduces the percentage of muscle and therefore percentage of glycogen that can be stored. = 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.
If you have 30% body fat, reducing it to 20% body fat, you will get a performance improvement. But if your body fat is already low, reducing it will just impair your health. Reducing your body fat too low is dangerous, as some body fat is essential to life. Concerns with body fat and body image can lead to eating disorders. These problems can come from reducing calorie intake to the point where the diet lacks essential nutrients. However, this can involve pathological dieting - laxative abuse, vomiting, skipping meals, etc. This is very serious. Be mindful of the possibility of eating problems in yourself and those you care about, and seek qualified help if needed.
You might think that more body fat would help provide fuel on very 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, etc).