Body composition is important and does affect performance. Let’s first get a caveat out of the way. This topic is a slippery slope! Less may be good to a degree and then it is exceedingly NOT good and in fact life threatening.
The recommended ranges of body fat for the general population are 15-18% for males and 25-30% for females. Of course, as you have certainly read, the general population far exceeds these ranges. So, if you are in that group of the general population and you run, work on getting into the normal range and your times may indeed improve. Most competitive runners are routinely below those levels. Elite male runners are in the single digits – 5-8%. and elite female runners hang in the 10-15% range. Yes, some are below these ranges. These figures come from research on Olympic level runners in the early 90s.
I won’t review all the physiology of fat here. However, fat is essential to life and is critical in the manufacturing of hormones in our bodies. It is why women whose body fat is reduced to those single digit levels often do not menstruate and there are other health risk elements that arise as well (i.e. bone loss later in life).
Most body fat is not readily accessible for energy use. It also is slow to break down into energy compared with carbohydrates. This is why we rely so heavily on carbohydrates as athletes. Excess body fat is certainly not desirable. It would be like wearing a weight vest while running. This is why when someone is on a weight loss program that they invariably get faster as they shed pounds of fat.
Let’s figure you are 165 pounds (75 kg) and have 20% body fat. That is 33 pounds of fat. At 10% you have 16.5 pounds of fat on you. Now, here is an interesting calculation for you. It takes ABOUT one calorie per one kilogram of body weight to move you one kilometer. A marathon is 42.2 kilometers. At 75 kg of body weight it would take 3165 calories (75 x 42.2). There are 9 calories per gram of fat. 3165/9= 352 grams of fat. In fact, research shows that not more than the equivalent of only 200 grams of fat are needed for energy during a marathon for instance. That is about 7 ounces!
By the way, let’s dispel a myth while we’re at it. The pace at which you run does not affect body composition. Though you burn slightly more fat at modest paces, it has been shown not to affect body composition. The modest-pace-fat-burning running advocates are misusing data to earn a buck. They sell “fat burning” workouts to everyone because it sounds so good.
What has been shown effective in losing fat is consistent training and higher speeds of running. Why? You burn more calories per unit of exercise time and increase your basic metabolism post-workout, burning even more! In the end expending more calories – which is the only way to reduce body fat – create a deficit in the calorie intake-output process.
The bottom-line is that though lowering your body fat levels may help you run better it is not a guarantee. If you are over the recommended levels for “average” males or females, it might be healthy to lose some of that fat through a solid exercise program – including weight or resistance training to build muscle. However, running faster, doing the right workouts, and consistency in training are far better predictors of your running success than body fat measurements.
And as a final note, remember we can only be the best “us” we can be. Our body metabolisms and chemistries are unique. So, be careful about jumping to conclusions looking at the person who just finished in front of you with a couple less pounds (or more for that matter!).