Weight Loss Impact on Performance
Based on my experience with various athletes and myself, weight loss can have a profound effect on race performance and pace. Most people underestimate the advantage that can be gained by weight loss. This advantage almost always outweighs any potential advantage that can be gained by training alone. Therefore, readjusting body composition should be one of the first areas of improvement sought out by serious athletes.
I have found that one pound of weight loss corresponds approximately to a 0.62% (recently revised based on feedback and more recent data) and 0.42% per pound increase in running and biking pace respectfully. Of course these improvements assume that the individual has fat to lose and that power output on the bike remains the same. Let's look at an example weight loss situation for an Ironman athlete using these figures. An 11:00 Ironman athlete that loses 20 pounds of unwanted fat can expect to reduce his bike time from about 6:00 to 5:30. He can also expect to reduce his marathon time from about 4:00 to 3:30. Therefore, his 20 pound weight loss results in a total reduction of about one hour from his Ironman time!
For most people that have extra fat to lose, a 20 pound weight loss is reasonable given a year's time. For an "already trained" athlete, a reduction of one hour would not be possible with a year's worth of training alone. An approach that includes losing weight and training can have a synergistic effect resulting in even larger improvements in performance.
Obviously, athletes that are at the low end of their acceptable body fat ranges can not, and should not take this approach to performance enhancement. Losing too much body fat can result in detrimental effects on race performance and health.