Can Losing Weight Make You Run Faster?

Breaking the two-hour marathon has been a hot topic in the running community. Could the key be to simply weigh less? If so, what weight should be lost?

Brock Armstrong
7-minute read
Episode #363

Drawing of one runner that is faster than three others

The authors of a recent study looked at how, among other things, shaving 100g off the weight of the runner’s shoes could contribute to a sub-2. That idea got a lot of runners thinking: if 100 grams is important enough that Nike developed new shoes for the job, how much could losing a few pounds off my actual body weight help me break my own speed records?

When considering this, the first question to ask is: can you lose weight without sacrificing performance? The balance between weight loss and maintaining performance comes with a mixed message. Yes, you can lose weight while training for a race. No, it’s not an easy thing to do. But we are talking about people who run 42.2km (26.2 miles) for fun, so let’s not sell them short.

The Science Behind Weight and Running Times

A new study on body mass reduction and running performance took a fresh look at this idea. During the past 40 years the majority of studies have been centered mainly on the effect of adding external load to the energy cost of walking and running. But in this study, the researchers took eleven trained club level runners (eight male, three female) and had them participate in a series of four maximal trials four to six days apart.

During the first trial, the subjects completed an exhaustive incremental peak VO2 test on the treadmill. On the second visit they completed a three kilometer race time trial on the treadmill running with their normal body mass (BM).

What’s really interesting about this test is that instead of adding weight to the runners (like the previous test), they subtracted five or ten percent of the runners’ body mass using a system of pulleys that lifted the runners up slightly, making them weigh less, in a sense.

In the series of three kilometre race trials, they found that the runners “being lighter” by five or ten percent resulted in improvements of 3.1 and 5.2 percent in their run time. If we do the math, that is a race time improvement of 0.64 percent per pound lost. For the particular subjects of the study, that equalled an improvement of 2.4 seconds per mile per pound. Which we can project would become even more significant over the marathon distance of 26.2 miles. Even a single pound lost could result in slightly more than a minute shaved off their marathon race time.

The study concluded that “the reduction of five and ten percent of inactive body mass may improve significantly 3km performance time … and are supportive of the notion that one way to maximize further running performance is to reduce inactive body mass.” Which means the lighter they made the runner, the faster they could run.

I want to draw attention to the language they used: inactive body mass. In other words, body fat. Not muscle. We’ll talk about that a little more later but for now, I just want you to keep this in mind.

Back in 1978, one study did the opposite of the study we just discussed. They actually added an extra 5, 10, or 15 percent to the body weight of their subjects using a crazy looking harness that was attached around their waist and across their shoulders.

In that study, the runners performed a 12-minute run with no extra weight. Then they ran the same 12 minutes with added weight and found that the distance the runners were able to cover was reduced by an average of 89 meters for each additional five percent of added weight. Again, if we do some math, that equals an extra 1.4 seconds per mile per pound.

This study concluded that “changes in excess body weight can influence VO2 max expressed relative to body weight and distance run performance independent of any change in cardiovascular capacity.” Which means, they ran slower when they weighed more even though their fitness level stayed the same.


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About the Author

Brock Armstrong Get-Fit Guy

Brock Armstrong was the host of the Get-Fit Guy podcast between 2017 and 2021. He is a certified AFLCA Group Fitness Leader with a designation in Portable Equipment, NCCP and CAC Triathlon Coach, and a TnT certified run coach. He is also on the board of advisors for the Primal Health Coach Institute and a guest faculty member of the Human Potential Institute.