Quote:
Originally Posted by JayZeeJay
I did an experiment (accidentally) on this very thing about 3 years ago. I was running marathons infrequently and decided to train for a 50 mile race. Prior to that, I was running a consistent, medium number of miles each week and was maintaining my weight around 145 lbs at the time. I tracked everything very precisely for a month and determined that I maintained that weight on 1400 NET calories per day, subtracting out the calorie burn from running (using an estimate of 90 calories/mile; I was running entirely on flat land at the time and at a consistent pace, so there was little variation).
I created a spreadsheet for my training that listed the run mileage for each day and I calculated the approximate increase in calories burned running the additional distances. My goal was to complete 4 months of training and to lose 5 pounds in the process, to give my knees some relief, so I calculated a net calorie intake to achieve that weight loss. I was quite religious (obsessive, in retrospect) about matching my exact calorie goals for each week, but at the time I lived alone and it was pretty easy.
Three months in, I was at my peak mileage running about 75 miles/week. According to my spreadsheet, I should have been down 4-5 pounds. Instead, I had gained 3 and it was a consistent, permanent gain (not glycogen/water fluctuation). The most logical conclusion was that my metabolic efficiency during running had increased with training and I was no longer burning nearly the same number of calories per mile as before. It seems obvious now, but I think about this every time this subject comes up on the forum. How many of us constantly change our exercise routines to keep from becoming too efficient at them and burning fewer calories over time? I am guessing it's not a high percentage of people - routine makes life easy. This could become a significant weight loss factor in the long haul (and it's ALL about the long haul).
I have an alternate explanation:
You lost lean mass, so you were no longer burning as many calories at rest. Don't get me wrong; cardio is awesome - it tones and firms and improves stamina and burns off calories. But it also reduces lean mass.
Addition: This apparently applies mainly to longer cardio sessions. Also, switching up types of cardio can be helpful, and doing interval training instead of longer stretches of repeated exercise, like running.
Also I hear it helps to make sure you eat before cardio so you can burn off food instead of lean mass, I guess. Your body doesn't necessarily target the fat first when trying to get fast fuel. There are multiple factors at play.
Here's one take on the subject