Marathon runners train hard and run hard for no less than 16 weeks during their training block, but is there anything else you can do to squeeze out a couple of minutes on race day? We’ve been told that we can gain an additional 2-3 seconds per mile per pound of bodyweight, (2 seconds/km per kg). But does the research show this to be true?
The answer is yes, bodyweight has a role to play, within reason. But the more useful answer requires understanding what the research actually shows, where the popular rules of thumb come from, and how to find a weight that improves your performance without compromising your health or your enjoyment of the sport. This post walks through all of that, drawing on the best science available.
Yes, Bodyweight Matters in Running
Running, unlike cycling or swimming, offers no mechanical advantage and no buoyancy. Every single stride, your leg muscles must generate enough force to lift your entire body weight off the ground, absorb the landing, and propel you forward again. Do that 30,000 to 40,000 times over a marathon and the cost of carrying extra weight really stacks up.
This was calculated by Teunissen, Grabowski and Kram in a 2007 study published in The Journal of Experimental Biology. Their research used a harness system to manipulate how much runners weighed while keeping their actual body mass constant, basically isolating the gravitational component of running cost. What they found was that supporting body weight against gravity accounts for approximately 74% of the total metabolic cost, (the amount of energy used), of running. Nearly three quarters of the energy you burn on a run goes toward simply keeping yourself off the ground.
The practical implication of that finding is straightforward. Any reduction in body weight, provided it comes from fat rather than muscle, reduces the single largest drain on your energy reserves during a run. The question becomes how much of a difference does a given amount of weight loss actually make to your finishing time.
The Calculated Benefits of Weightloss for Marathoners
The most frequently cited study on this question was conducted by Kenneth J. Cureton and co. in 1978, published in Medicine and Science in Sports. Cureton’s team added weighted vests equivalent to 5%, 10%, and 15% of each participant’s body weight and measured the effect on performance across several tests, including a 12-minute run for distance.
The results were clear. Adding just 5% of body weight, which for a 155 pound (~70 kg) runner amounts to 7.7 lbs (3.5 kg), caused participants to cover 292 feet (89 metres) fewer in their 12-minute run. Adding 15% of body weight cut their distance by 909 feet (277 metres), a performance drop of around 8%. When coaches and researchers subsequently converted those findings into pace terms, the figure that emerged was approximately 1.4 seconds per mile per pound of body weight, or roughly 2 seconds per kilometre per kilogram.
Cureton’s team measured distance covered and metabolic cost, not race pace. The seconds-per-mile figure is derived from their raw data by others and has been widely used in coaching ever since, but it was not a number the researchers published. That said, it remains the best figure available from peer-reviewed research.
To translate that into something you can use, losing 11 lbs (roughly 5 kg), would theoretically save you around 3 minutes and 20 seconds over a half marathon and around 6 minutes and 40 seconds over a full marathon, all else being equal. For a runner targeting a four-hour marathon, that is the difference between finishing in 3:53 and 4:00. This is a substantial difference over the length of a marathon course.
There is also a commonly cited coaching rule suggesting that every pound of weight loss saves 2 to 3 seconds per mile. This rule is commonly talked about widely in running communities and coaching literature, but it’s really at the optimistic end of what the research states. The Cureton data points to 1.4 seconds per mile per pound and using the lower, more conservative figure in your expectations is probably more realistic.
My Personal Data
Over a three year period, running the same training program four times, I tracked what happened to my marathon time as my weight changed. At 53, weighing 157 pounds, I ran 3:00:29. That became my baseline. Eight weeks later, still 53, I had dropped to 154 pounds and ran 2:58:33, a reduction of just 3 pounds that took 1 minute 56 seconds off my time, working out to 4.42 seconds per mile faster, or roughly 1.4 seconds per mile per pound lost. A year from that baseline, at 54, I was down to 152 pounds and crossed the line in 2:56:44. That was 5 pounds lighter than where I started and 3 minutes 45 seconds faster, translating to 8.58 seconds per mile and 5.33 seconds per kilometre of improvement. By 55, I had reached 144 pounds, 13 pounds lighter than my baseline, and ran 2:54:23, taking 6 minutes and 6 seconds off that original time. At that point the numbers showed a slight diminishing return in terms of improvement per pound lost, but the direction of travel remained clearly positive. The same training program, four times, with weight as the primary difference between each effort.
The Stillman Formula and Frank Horwill
Before modern sports science had much to say on the subject, runners and coaches relied on a formula attributed to Dr Irwin Maxwell Stillman, a New York physician who published “The Doctor’s Quick Weight Loss Diet” in 1967. Stillman created a height/weight ratio table intended to define a healthy weight for non-active individuals, and this is where his running formula originates.
For men, Stillman allocated 110 lbs (50 kg) for the first five feet of height, adding 5.5 lbs (2.5 kg) for every additional inch. For women, the starting point was 100 lbs (45.4 kg) for the first five feet, with 5 lbs (2.27 kg) added per inch above that.
What many sources don’t mention is that it was Frank Horwill, the highly regarded British running coach and founder of the British Milers’ Club, who appears to have been responsible for bringing Stillman’s table into the running world and applying the event-specific data that runners now associate with the formula. Writing in publications including BMC News and Peak Performance, Horwill proposed that runners should target weights below the Stillman baseline depending on their event: 2.5% lighter for sprinters, 6% for hurdlers, 12% for middle distance runners covering 800m to 10K, and 15% lighter for long distance runners covering 10 miles and beyond.
So a male runner standing five feet ten inches tall would calculate a Stillman base weight of 165 lbs (74.8 kg), then reduce that by 15% for marathon running to arrive at a target of approximately 140 lbs (63.5 kg).
The formula has been widely criticised mostly because it is aimed at professionals and those at the top of the sport worldwide. In addition, It produces a single number rather than a range, ignores individual differences in bone density, muscle mass, and body composition, and the targets it generates are widely reported as too low for most recreational runners to achieve healthily. Defenders of the formula argue that the values reflect what elite athletes actually weigh, which is technically true, but only because the formula appears to have been derived by observing elite runners in the first place rather than through any physiological research.
That makes it descriptive rather than prescriptive. Stillman and Horwill were observing what fast runners weighed, not establishing why those weights produced faster times. If you’d like to see a more updated Stillman calculator, I have added one to my website jmruncoach.com/tools-and-calculators. I do not claim that this is the ideal weight for you. However, it may be a decent starting point for finding your ideal race weight. Finding the right marathon weight more accurately, nothing beats personal experience.
The Difference Between Lighter and Leaner
One of the most important points in this entire conversation is between losing weight and losing fat. The Teunissen study touched on this. When researchers added mass to runners without adding gravitational weight, using a harness to offset the extra load, metabolic cost barely changed. This implies that the penalty in running comes specifically from gravitational load, which is why fat mass is so costly and why losing muscle mass to achieve a lower number on the scale is counterproductive.
Muscle generates force, absorbs impact, and supports the mechanics that keep you injury-free through a heavy marathon training block. A runner who loses 11 lbs (5 kg) of fat will almost certainly run faster. A runner who loses 11 lbs (5 kg) of muscle may well run slower, and will almost certainly get injured more often.
This is why the focus in any weight management strategy for runners should be on body composition rather than total body weight. Two runners of identical weight can perform very differently depending on how much of that weight is active muscle versus fat mass that contributes nothing to propulsion.
Another consideration is the fact that muscle mass directly corresponds with the amount of glycogen a human can store as this runner’s fuel is stored in muscle. The more muscle you have, the more energy you store. The stronger you will feel later in the race, especially when topping up with gels and other carb sources.
Finding Your Own Ideal Racing Weight
No formula can tell you exactly what you should weigh on race day. The body is too individual for that. What the research can do is give you a framework for thinking about the direction of travel and the likely magnitude of the benefit.
A practical approach is to track your training paces, race times, and body weight over a period of several months of gradual change. Look for the range in which your paces improve without a corresponding increase in injuries, fatigue, or difficulty recovering between sessions. That range is closer to your ideal racing weight than any formula will tell you.
The more important point is that weight loss pursued too aggressively tends to undermine the very thing it is supposed to improve. Insufficient calorie intake impairs training quality, delays recovery, disrupts hormonal function, and erodes the muscle mass that makes running possible. Any weight management strategy worth following has to be compatible with your training demands during the entire training cycle.
What This Means In Practice
The research does support that carrying excess body fat makes running harder and slower, and reducing it through sustainable means will likely improve your performance. Where it gets less reliable is the precise prediction of how many seconds per mile a specific weight loss will deliver. Those will vary between individuals depending on their current fitness, running economy, and body composition, age, length of time running, and more.
The one take on this is that bodyweight does affect marathon times. Lighter does not always mean faster. However, if your bodyweight is in the healthy range, and weight loss is done carefully and thoughtfully it will most likely be beneficial. The most important thing to add to this, is that strength work should always be included in a marathon training block and weight should not go below your healthy range. Light and lean is always better than skinny and weak.
Studies referenced in this post: Teunissen LPJ, Grabowski A, Kram R. Effects of independently altering body weight and body mass on the metabolic cost of running. Journal of Experimental Biology, 2007, 210, 4418-4427. Cureton KJ, Sparling PB, Evans BW, Johnson SM, Kong UD, Purvis JW. Effect of experimental alterations in excess weight on aerobic capacity and distance running performance. Medicine and Science in Sports, 1978, 10(3), 194-199.