Weight Loss and Performance
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?
1 V̇O2max Changes
1.1 Simplistic assumptions
First assumption is that Jack Daniels Running Formula VDOT is close enough to V̇O2max to be useful. 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.
1.2 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.
1.3 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 V̇O2max of 50. A 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 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.
2 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.
3 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.
4 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/V̇O2max correlation. On the other hand, my personal experience is that losing weight does give a significant boost in performance.
5 The flaws in this idea - too thin to win
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.
6 Body fat as fuel
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).