Every runner knows that body weight affects running performance. Because your body must overcome the force of gravity with every stride, the heavier you are, the higher is the energy cost of running at any given pace. One study found that every 5 percent of added body weight reduced running performance by 5 percent.
All runners perform best when they are near the bottom of their healthy weight range. There is such a thing as being too light, of course. You won’t run well if you’re undernourished or if you don’t have enough body fat to support basic health. Nor is being at your ideal racing weight a guarantee of successful racing. There’s also a little factor called fitness that plays an important role. But assuming you’re fit, you will generally have your best races when you’re about as light as you can be without compromising your health.
Your ideal racing weight is determined primarily by your body fat level. There’s not much you can do about the other sources of mass in your body: bone, muscle, water, etc. No matter how hard you train or how carefully you eat, all of that weight will stay. It’s excess body fat that accounts for the difference between current weight and ideal racing weight in most runners, and thus it’s fat mass that you must lose to attain your ideal racing weight.
So, what is your ideal racing weight? Given the fact that body fat is the primary determinant of ideal racing weight, the best way to estimate it is to calculate how much you will weigh when you’ve reduced your body fat percentage to the optimal level. Optimal body fat percentage is not the same for everyone. There are many factors that affect how lean an individual runner can become. These include gender, age, genetics, and history of being overweight. However, even runners who have all of these factors working against them can get fairly lean.
The following table presents optimal racing weight body fat percentage ranges for different gender and age groups of runners. Most runners can expect to get their body fat percentage down within the range associated with their gender and age group through proper training and diet.
Age 20-29 = 3-10%
Age 30-39 = 5-12%
Age 40-49 = 6-15%
Age 50+ = 8-17%
Age 20-29 = 10-16%
Age 30-39 = 11-17%
Age 40-49 = 13-20%
Age 50+ = 14-22%
You can expect to reach the lower limit of your ideal range only if you typically lose weight fairly easily, you have never been seriously overweight, and you are willing and able to maintain a high training volume. If your current body fat percentage is well above your optimal range, you should aim only to reach the upper limit of that range initially through increased training and improvements in diet.
Estimating the body fat percentage you can realistically expect to attain at your peak fitness level is not an exact science. Just use common sense and the considerations I named above to make an educated guess for yourself. Also bear in mind that the further you are from your peak fitness level currently and the more room for improvement your diet has, the more you can expect to lower your body fat percentage.
The final step in determining your racing weight is to calculate how much fat weight you will have to lose to get down to your goal body fat percentage. Let’s look at how to do this with the example of a 38-year-old female who currently weighs 140 pounds and has 22 percent body fat and who sets an initial goal of getting down to 17 percent body fat (the upper limit of her ideal range) through improved training and diet.
Step 1: Calculate current body fat mass. Body fat mass = current weight x current body fat percentage expressed in decimal form. In this example: 140 lbs x 0.22 = 30.8 lbs.
Step 2: Calculate current lean body mass. Lean body mass = current weight – fat mass. In this example: 140 lbs – 30.8 lbs = 109.2 lbs.
Step 3: Calculate goal weight. Goal weight = current lean body mass ÷ goal lean body mass percentage. (Note: goal lean body mass percentage is 1.0 – your goal body fat percentage expressed in decimal form.) In this example: 109.2 lbs ÷ 0.83 = 131.5 lbs. Ta-da!
For a more complete description of this method and a step-by-step program to reach your ideal racing weight, check out The Racing Weight Quick Start Guide.