That's when I got thinking about a question that Runner's World (sort of) answers in one of its online columns this month: "Is there an ideal running weight?"
It's commonly said that every pound lost makes you 2 seconds faster per mile. (See here, here, here, and here.) That might not seem like much, but a 5K race is 3.1 miles, so every pound lost would supposedly make you 6 seconds faster in such a race. If you were to lose 5 pounds, that would be 30 seconds (well, technically 31 seconds), which is not insignificant in such a short race.
That kind of thinking, however, may explain why eating disorders are not unheard of among distance runners:
The pressure of sports performance and competition involved with long distance running can be a breeding ground for eating disorders. If other factors are involved, such as social pressures or low body image/self-esteem, the risk for developing an eating disorder can be increased.Obviously, it can't be indefinitely true that every additional pound lost will make you 2 seconds/mile faster, because you aren't going to be infinitely fast at 0 pounds. (You're going to be dead!) Even well before that absolute limit, there will come a point where additional pounds lost are coming from muscle/skeletal mass, not fat, so we wouldn't expect the conventional wisdom to be true on a linear scale.
Anyway, here's RW's response:
As long as you enjoy the journey from the start to the finish, body type does not seem to make much difference. However, when you add speed into the mix, lighter is probably better if lighter means reducing total fat mass; it does not apply if “lighter” means reducing lean body mass or cutting into essential body energy stores.If you really want to maximize your performance-to-weight ratio, I guess you'd need to experiment a bit to find the perfect point at which any additional weight loss will be coming from lean body mass. I'm guessing that if I really worked at it, including cleaning up my lunch diet, I could probably target a few more pounds of fat - after all, Galen Rupp is an inch taller than I am and several pounds lighter - but then again, I'm far from an elite runner, and in the real world, I'm already pretty lean these days. That means dropping my 5K time is going to have to come from improved performance, not weight loss.