Losing weight affecting running times? (Read 1124 times)

here's a source of this: http://www.runningforfitness.org/calc/weighteffect.php i saw this posted on the runnersworld.com forums. if you click explanation it describes how they arrived upon this formula. apparently they are assuming that your VO2 Max remains constant, but your muscles get more oxygen since there is less body mass requiring oxygen, and also that your overall fitness remains exactly the same. this seems pretty unrealistic to me. i think it would be much more effective to predict speed based on a loss of body fat, and not weight which might be needed muscle. i actually had a question on this topic also: i'm already on the light side of the "healthy" body weight for my height, if i lose >3 pounds i'll be in the "unhealthy" range. right now, running the marathon well is my only concern. at what point does losing weight become detrimental to running performance? i've been trying really hard to eat enough calories, but i'm beginning to wonder if i'm just keeping unnecessary fat on. how can i ensure that i'm losing some extra weight and not strength?

An interesting topic! I think statistics like this are intriguing , but probably not entirely useful (for most people anyways) for many reasons that have already been mentioned. There are too many variables (starting weight, length of run, age of runner, fitness level, etc) for this to be much more than a topic for conversation ... I wouldn't want anyone to get down on themselves because they lost 10 lbs but didn't drop their pace 30 seconds! Losing 10 lbs is a big enough accomplishment by itself! And the pace will take care of itself, eventually... Besides, depending on the conditions (weather, diet, hills, sleep) a runner's mile times WILL fluctuate much more than 3 seconds (assuming equivalent effort).. changes in % of max heart rate would be a more interesting measurment. Teeny: I'm also on the slim side of this range and I have had similar concerns as you, "teeny". In my humble opinion, I think it's probably safest (unless you have 3 nutritionists/trainers weighing your every meal) to overshoot the calories when you're training for marathon distances (undershoot and you may end up like that 15 lb person Wingz speaks of). If you're on the slim side and you're training for marathons, you're probably not going to have any "unnecessary fat"... just the "necessary" kind.. Besides, I think it's a really slipperly slope for a competitive athlete to start measuring calories really precisely and end up with some sort of eating disorder. I saw it happen a lot in high school cross-country and it's really scary (not implying this is you! Just bringing it up as a related topic... ). That said, I think you can probably eat healthy, high calorie foods (less processed, less grease, more carbs) that your body will use most efficiently for your running? But I'm really not an expert on nutrition, so I'm not sure what is best. For me, I try to eat what feels good and what keeps my energy up for my next run.