I just finished the book Racing Weight by Matt Fitzgerald. It was a good read and made a strong case that weight is an important variable requiring attention if we are ever want to continue to get better in endurance sports. From experience, I know that the lighter I am, my running improves. Even a few pounds can make a significant difference. Last year my weight was in the low 140s, but this year my weight was in the upper 140s. Carrying this extra weight was probably a contributor to running my marathons about 8 to 10 minutes slower this year. Studies show that a runner who is 160 lb runner expends 6.5% more energy than a 150 lb runner at the same pace. This may not seem like much, but over a marathon distance, this extra effort may add several minutes to your time.
But it is just not weight, but also body fat. The combination of weight and body fat percentage have been shown to be more strongly correlated with finish times than any training variables such as average weekly training time. Of all endurance sports, body fat affects running performance the most because runners have to overcome gravity – more so than sports such as cycling and swimming. The average body fat percentage among elite male marathon runners is just 7.3%, which is lower than every sport except body building.
Most of us will never get to the elite body fat level, but even if we have a goal of getting a little faster, losing some weight and body fat is a strong indication you are on the path to success. The book states everyone has an optimal performance body composition associated with your highest athletic performance. Unfortunately, the author doesn’t provide any simple formulas, and it will take time through trial and error to try to find that sweet spot of optimal body composition since everyone is different.
My best marathon times have been when my weight has been in the low 140s and my body fat around 8%. The lightest I’ve ever been was about 142 lbs in early 2012 and not sure I will ever get to that weight again for a couple reasons. Currently I’m around 147 lbs with body fat a little over 9%. I’ve been incorporating more strength training to reduce the running injuries I’ve been plagued with this year. The downside is this strength training will mean I’m putting on a little more muscle, so inevitably I’m probably going to weigh more with the same training/nutrition effort. But I’m hoping the extra muscle I put on offsets the extra weight.
I’ll be monitoring my body composition closely over the next few months to try and find that elusive optimal weight/body fat percentage. My short-term goal is to get close to 145 lbs and 8% body fat by the AFC Half Marathon in mid-August.
Running Inspired Blog 
I really like this book, especially the focus on the quality of foods you eat and how they are metabolized as opposed to the common and error-filled calories in -calories out model. It’s also great to see running/athlete specific books out there that you actually can sit and read and use for reference much like all the fad-diet books.
Hi Annabelle! I agree, the book had some interesting studies and tips. What I like most was the advice on eating a balanced diet, nothing extreme, just good clean food and using our common sense to manage our appetites. I’ve experimented with a few diets, and for me anyways, I’m coming to the conclusion that higher carbs, with plenty of fruits and vegetables and a little bit of dairy seems to work better for me.
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Thank you. Awesome info!