Effect of weight loss on speed

The effect of a change of weight can be calculated here.

The calculation assumes that everything else is held constant – so that the body’s capacity to turn oxygen into energy is otherwise unaffected. This means that VO2 max would change in direct proportion to the percentage change in weight. The predicted performances are derived by simply calculating the predicted race performance at the new VO2 max.

In real life, however, everything else would not be constant. A reduction in weight might be accompanied by:

  • a higher background level of training;
  • lower muscle mass;
  • reduced immunity;
  • reduced glycogen stores;
  • lower levels of hydration

All but the first of these would be expected to lead to a deterioration in athletic performance. These changes would have to be offset against the direct benefits of the weight loss.

See also

9 Responses to Effect of weight loss on speed

  • RemQuito:

    Hello Owen,
    Thanx from Quito, Ecuador, for your effort to create a very interesting, inspiring and helpful website.
    Also very helpful are your calculators on VO2 max, distance and weight. They have proven me how important/effective weight-loss can be.

    I myself made calculations dividing the speed (min/km) by the bodyweight to calculate and predict my progress, and found in this way also that every lost kg lets me run every km 3-4 seconds faster (which equals your 15-20 sec per 0,5kg on a 10K).
    I think that in this way I don’t just base my calculations on VO2, bodyweight and distance, but also include personal focus/motivation, running style/technique/efficiency, and even terrain/altitude because they are taken from real times on real tracks.

    QUESTION 1: Since I live here in the high Andes on an altitude of 2800m – although acclimatized – I notice a considerable loss of speed comparing to The Netherlands at sea level. Mainly because of the thin air, and especially in little climbs (in flat races from VO2 max I ‘sufered’ decline 45,3 on 10K to 40,8 and less).
    I think that Ethiopia is also quite high, and perhaps you have the same experience. Are there correction factors known in the VO2 max-calculations for altitude and for altitude differences (climbing/descending)?

    QUESTION 2: Do you or others that ran in altitude know how to avoid loosing your breath and recuperate quickly (a small climb, although slowing down, gets me in this altitude minutes in anaerobe circumstances, just long enough for the next climb). Or is it just a matter of patience and training on? It’s quite ironic to have to ask this, as a former mountaineer who has reached 7000m (but running in altitude is quite a different kind of sport!).

    Thank you for your reactions,

  • Courtney's mom:

    I have hunted all over the internet and this is the only calculator like it. It is very helpful, (i found it fun for myself not just my daughter) but could you expand the range?
    my daughter is a sprinter. (freshman in college) Btwn last year and now, she dropped quite a bit of weight. She set PRs of 12.62 and 26.28 in the 100m and 200m at about 122-125pounds (she’s 5’2″) last year. We are curious before the season starts, what sort of impact such a loss of body fat might make on her times once she gets into racing shape. She is now 111 pounds. She feels good, weight lifts a couple times a week in addition to speed running. she did not diet but just doesn’t always have time to eat and doesnt hang out with friends as much where the junk is flowing. LOL

  • […] The best and most cited calculator I found suggests that for every four pounds you lose you improve your marathon speed by 4 minutes. […]

  • […] Thirdly, I wanted to lose 7 pounds of weight. This has proved tricky, I have managed to lose 2 pounds but I’m finding it difficult to lose weight. That’s not really surprising as December is a difficult month for losing weight with all the Christmas celebrations. This website is pretty good at explaining and illustrating why losing a bit of weight would be good. http://www.runningforfitness.org/faq/we […]

  • Angela Richardson:

    According to the calculator, if I weighed 2kg I would be capable of faster than light travel. Contrary to the calculator’s claims that I can always run faster by losing weight, my times are much faster now that I have a BMI of ~20 than they were when my BMI dropped to 17.

    • blah:

      All things being equal.

      So if you weigh a few pounds more, but train more consistently, or other variables have changed, of course its possible to run faster despite putting on some weight.

      Also, these results are based on averages of the people who were studied…meaning that it applies to MOST people, but there can still be exceptions. Your individual results, don’t prove or disprove this.

    • Bob Ranmpton:

      Yeah..you need some strength and flesh to your bones for the calcualtor to work for you. In other words a BMI over 18.

    • chrisgg:

      There is no claim that losing weight can always enable you to run faster. It is made clear that being under a healthy weight could have the opposite effect for a variety of reasons….muscle loss, low carbohydrate stores, reduced hydration, reduced immunity.

  • Russell Walsh:

    Here’s the thing for me. If I am not pretty slim, I am not really a distance runner, don’t really have an easy pace(it’s all relatively hard), I can’t really build mileage to competitive levels (fatigue and injuries). Just 10 pounds takes me out of what I consider the real distance running game. 20 pounds and I am very limited. Just try carrying those kinds of weights in a backpack and run. The demands rob efficiency over distance tremendously. Worse, it kills the potential deep joy of running.

    Aside from PR’s and podiums, it is just so much more of a pleasure to run for an hour or two , or more, as a thin running trained person. I think a lot of people in the new market driven, consumers who run, “running” craze, just haven’t ever had that experience.

    Most comments being made about running and weight these days , miss the point. Good, defend all body types if you want , make a million definitions about what’s “beautiful”, but that’s not really about running at it’s core.

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