I'm going to sign up for a marathon to help me lose weight

This is the intention of many first time marathoners.  The assumption is something like this: Training for a marathon involves a heck of a lot of running; if I sign up for a marathon really get serious about running, I'll lose a heck of a lot of weight in the process.  The reality is far different from this.  A 2010 research study indicated that of novice marathoners completing a 3-month training program (a time period in which they ran literally hundreds of miles and expended tens of thousands of calories), some people lost weight, some stayed the same, and many gained weight.  Here is a breakdown of what happened:

• 78% of runners stayed about the same weight
• 11% gained weight (most of these runners were women)
• 11% lost weight - but every one of them reported making changes to eating habits.

How are these results possible?  How can a person burn literally tens of thousands of additional calories over a 3-month period and lose no weight at all; or in some cases, gain weight?  There are two explanations.  Firstly, a "long" run leads to hunger and the feeling that one "deserves" a special "reward" in the form of food.  A 20-mile training run does in fact expend approximately 2,000 calories.  But as it turns out, its pretty easy to "replace" these calories with a bonus Starbucks muffin and hot beverage post-run along with 2 beers and a half of a pizza in the evening.  The second explanation involves the impact of running or cardio-respiratory exercise on resting metabolic rate (the number of calories expended while we rest).  Although we often say that exercise improves our metabolism, this is only true of strength training or very high intensity exercise.  Following aerobic exercise, our bodies actually find a way to conserve caloric expenditure. Take-home message: Don't use a marathon as a path to fat loss or weight loss.