The pros and cons of weight loss for runners

Both theory and practice indicate that the energy cost of running is proportional to body weight.  First the theory: the energy cost of running can be subdivided  in to three categories: energy required to do work against gravity; energy required to do work against horizontal ground reaction forces; and energy cost of internal muscle inefficiency. 

The cost of being airborne

We do work against gravity when we become airborne.  The energy required to lift the body is proportional to body weight. Some of this energy is recovered by converting the energy of impact into elastic energy at foot-fall, thereby allowing us to re-use that energy for upwards acceleration at lift-off from stance.  However only a proportion of the energy will be recovered.  Assuming that this proportion is approximately constant (an assumption that depends on not changing running style) the net energy cost of becoming airborne is proportional to body weight.

 The cost of being on stance

Although being airborne has a high energy cost, so does remaining on stance.  While the point of support is ahead of the centre of mass, we experience a braking force (due to the horizontal component of ground reaction) that reduces our momentum.  This braking force at any instant is determined by the angle of our leg and by the force transmitted along the length of the leg, which in turn in proportional to body weight.  Therefore the braking force will be proportional to body weight.  When the point of support is behind the centre of mass, the horizontal component of ground reaction  pushes us forwards.  When running at constant speed, the retarding impulse due to braking must be exactly balanced by the forward accelerating impulse in late stance.   We can capture some of the energy released by the braking force in early stance as elastic energy which helps provide the forwards impulse in the second half of stance, but due to inefficiency we cannot recover 100% of this energy.  Assuming no change in running style, an approximately fixed proportion of the energy will be lost.  So, the energy consumed in opposing horizontal ground reaction forces is also approximately proportional to body weight. 

Unfortunately, it costs energy to be airborne and it also costs energy to spend time on stance.  Both of these costs are proportional to body weight.  Incidentally, as discussed in my posts on efficient running style summarized in the page on the Dance with the Devil, the best way to minimize these costs is to increase cadence, because higher cadence reduces the cost of overcoming gravity  – though there is a limit due to internal inefficiency at very high cadence

Internal muscle inefficiency

The energy costs due to internal muscle inefficiency are less easy to estimate. The process of muscle contraction involves the pumping of various ions, especially calcium, sodium and potassium, across the membranes that separate the different compartments of a muscle fibre.  The molecular pumps are fuelled by the energy molecule ATP, which is regenerated by consumption of glucose. There are also other metabolic processes and frictional processes within the muscles and joints, and as a result the energy consumed by a muscle is greater than the external work done by the muscle.  However, it is probably a fairly good approximation to assume that over the usual range of output, the efficiency of muscles is approximately constant, so the energy wasted internally will be proportional to external work done.  As we have seen the external work done (against gravity and against horizontal ground reaction forces) is proportional to body weight.  Thus the loss due to internal inefficiency will also be approximately proportional to body weight, though variation in the internal regulators of metabolism (such as anabolic and catabolic hormones) might also influence the costs.

Theory compared with practice

Thus, theory predicts that averaged across many individuals, the energy cost of running is approximaltely proportional to body weight, though differences in factors such as efficiency of running style and hormone levels might result in two individuals with the same weight nonetheless having slightly different energy costs.  In fact the energy cost of running averaged across many people, based on actual measurement rather than theory is given by the formula:

Energy cost in  Kcal/min/Kg  = ( 0.0024 * speed2 ) – ( 0.0104 * speed ) + 0.1408

where speed is measured in miles per hour ( http://swingleydev.com/misc/exercise.php ). 

This formula demonstrates that on average, energy cost per Kg is independent of weight (and hence total energy cost is proportional to weight) , but such formulae provide only a rough guide for the costs in each individual.

The conclusion from both theory and from practical evidence is that if you lose weight, you will require less energy per minute to maintain a given speed, so you can maintain a faster speed at any particular fraction of your total aerobic capacity.  Weight loss will lead to improved speed approximately in proportion to the weight loss – all other things being equal.

 

Balancing catabolism and anabolism

The crucial phrase is ‘all other things being equal’.  If the weight loss were to be so extreme that there were no fat reserves for use as fuel, this would result is reduced performance over distances for which fat is a valuable source of energy, such as the marathon and longer distances.  However, because fat is a very efficient fuel (providing lots of energy per gram of fat) it would be necessary to starve almost to death to deplete fat stores below the amount likely to be consumed in a marathon or ultra-marathon.  Nonetheless, even less severe weight loss can trigger hormonal changes (regulated by the hypothalamus) in order to conserve essential body issues, especially the brain, and it is likely that muscle protein would be sacrificed.  In other words, if the weight loss is sufficient to tip the balance from anabolism to catabolism, muscle protein will be broken down and muscle strength decreased.  Hence loss of speed would be expected.  

 

General conclusions

In practice, this means that an out-of-condition runner who has gained weight will almost certainly benefit from losing that weight.  However an athlete who has trained over a substantial period at a training volume just a little less than that required to produce the overtraining syndrome, has probably already reached the optimum balance between anabolism and catabolism and further weight loss would probably be harmful.    

If you want to achieve to your limit, it is probably best to monitor performance regularly while steadily increasing training volume.  When performance shows a tendency to decline despite increasing training volume, it is likely that the balance has shifted too far towards catabolism (break down of tissue), and you should drop back to a slightly lower training volume.  Pushing relentlessly onwards with increased volume despite decreasing performance will almost certainly result in a sustained period of staleness, in which your brain will not allow you to run at your best level.

Personal conclusions

It is also useful simply to monitor weight.  Experience has taught me that when I train regularly my weight stabilizes at around 62-63 Kg, irrespective of exactly how much running I do, at least while I remain below the limit where I feel perpetually tired. Thus I think my brain (and in particular, my hypothalamus) regulates hormonal function so that catabolism equals anabolism when my weight is around 62-63Kg. 

However, while 62-63 Kg appears to be my ‘equilibrium’ weight, is probable that my proportion of muscle to fat is not optimal at present.  Almost 40 years ago when I could run a marathon in 2 hours 25 minutes, my weight was also around 62 Kg.  Since then I have almost certainly lost muscle and increased fat. It is probable that I could improve my running performance by doing more resistance exercise.  Provided it is not excessive, resistance exercise promotes anabolic hormone release and would promote replacement of fat with muscle.  However, a large amount of resistance exercise resulting in bulking of fast twitch muscle fibres and a net gain in weight, without gain in aerobic capacity, would probably decrease my distance running performance on account of the increased energy cost of running with extra weight.

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8 Responses to “The pros and cons of weight loss for runners”

  1. rick Says:

    Thanks once again for the great article, I think the upper body muscle I have put on through 20 years of weight training is holding me back, before weight training I was 10 STONE 8 LB in this years London I hit the scales at 11 1/2 stone [ with very little body fat]!
    Also less weight means less impact, means faster recovery!!!

  2. Ewen Says:

    Thanks Canute – very interesting once again. I’m 3kg over the weight I was in my 30s, and I’m pretty sure that’s extra fat, not muscle!

    I recall one study – not sure how scientific – that said 1kg of fat is worth 30 seconds in a 10k race, so the 3kg for me would be worth 1:30.

    I like the idea of ‘efficiency’ through faster movement of the muscles when at ‘ideal’ racing weight.

  3. lackcy Says:

    It’s tru for me too. As long as I train regularly. I find my weight stabilises, regardless of food intake.

    http://www.best-weightloss-products.com/

  4. Mike Says:

    nice info! already find this.

  5. Aleisha Bevier Says:

    Sorry, aber das bezweifel ich ganz stark…Baer

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