From a comment in my last post, I feel I have to address this once again.
Any coach who says "Lose weight and you'll run faster" should be shunned. They should be banned from the sport. It is not only wrongheaded, it's evil. That idea has killed people.
Fallacy 1: But the elite marathoners are all so skinny - it must be right
The three women and three men who made the US Olympic marathon team in 2012 were
Desiree Davila 5'2" 100 pounds
Shalane Flanagan 5'5" 113 lbs.
Kara Goucher 5'7" 120 lbs.
Ryan Hall 5'10 130 lbs.
Meb Keflezighi 5'7" 127 lbs.
Abdi Abdirahman 5'11" 130 lbs.
They're all quite thin. Would they be faster if they weighed less? No. And neither would you. The type and amount of training that elite marathoners do results in the low weights; it's a by-product and not a goal. 5000 meter runners weigh more than marathoners, but are no less fit. Losing weight in itself has no effect on running performance!
Once again, to be clear: Losing weight doesn't improve performance, but improved performance may result in lost weight. Your body adapts to training and one adaptation, to become more efficient, may be to lose weight. Intentionally trying to lose weight to run faster is completely backward. It doesn't work. It has never worked. There is not one single case where someone has done nothing but lose weight and then become faster.
Fallacy 2: But VO2max is based on weight
VO2max, one predictor of running performance, is measured in milliliters of oxygen consumed per minute per kilogram of body mass. Mathematically, at the same amount of oxygen consumed per minute, the less you weigh, the higher (better) your VO2max will be. Besides being but one predictor, and a questionable one at any distance far from 5000 meters, VO2max's dependence upon body mass is complicated. Losing weight doesn't improve VO2max in a predictable manner; the linear relationship predicted only holds over a very small range and that range becomes vanishingly small in some athletes.
Fallacy 3: But I ran faster when I weighed less
My fastest marathon happened when I weighed 30 pounds less than I do now. If I lost that 30 pounds now, I would not be able to function; I doubt I could even get out of bed without assistance. My weight has gone up and down and my performances varied with that weight - for a while. Even when that happened, however, my weight went up AFTER my performances started to slip and my weight would go down AFTER my race times improved for a brief time. Losing weight didn't make me faster; getting faster caused me to lose weight.
Aid Station: Eugene Curnow
2 hours ago