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Research Article

Metabolic Factors Limiting Performance in Marathon Runners

  • Benjamin I. Rapoport mail

    brapoport@post.harvard.edu

    Affiliations: M.D.– Ph.D. Program, Harvard Medical School, Boston, Massachusetts, United States of America, Department of Electrical Engineering and Computer Science and Division of Health Sciences and Technology, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States of America

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  • Published: October 21, 2010
  • DOI: 10.1371/journal.pcbi.1000960

Reader Comments (5)

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Assessment of the fuel partitioning model

Posted by G_Henderson on 10 Nov 2010 at 19:47 GMT

The impact of relative exercise intensity upon use of carbohydrate and fat as fuel is well established. The author has attempted to use such knowledge to lead to practical recommendations; this is to be commended as translation to real world application is very important.

Unfortunately, the author has incorrectly modeled the relative contribution of fat and carbohydrate (Figure 1). The model is correct at high-intensity exercise but is incorrect at lower-intensity exercise. Indeed it is true that carbohydrate supplies nearly 100% of fuel at 100% VO2max. However, the y-intercepts (left side of graph) are incorrect. The graph implies that the resting fuel mix is approximately 100% fat (RQ = ~0.7). This is only true during prolonged fasting; this is certainly not the fuel mix of a typical marathon runner on the morning of a race. A typical fuel mix at rest is closer to 50% carbohydrate and 50% fat. Fat contributes a bit more as time since the last meal increases, and carbohydrate use will be higher following meals. Because of the dependence upon the timing and size of recent meals, it is difficult to say exactly what a “normal” resting fuel mix is. However, the resting fuel mix used in the modeling for this paper represents that of an individual who is severely carbohydrate-depleted, not that of a glycogen-loaded runner. Therefore, the model proposed by B.I. Rapoport underestimates the importance of carbohydrate during low-intensity exercise.

No competing interests declared.

RE: Assessment of the fuel partitioning model

crowther replied to G_Henderson on 18 Nov 2010 at 21:52 GMT

I think this may be an issue of fuel use by the whole body vs. fuel use by the muscles. It's true that, at rest, some carbohydrate is normally used by the body, but mostly by nonmuscle tissues like the brain. At rest, muscle use of carbohydrate is extremely low. I think Figure 1 is meant to apply to the skeletal muscles rather than the whole body and is a fairly accurate representation of how muscles' fuel use varies with exercise intensity.

No competing interests declared.

RE: RE: Assessment of the fuel partitioning model

G_Henderson replied to crowther on 21 Nov 2010 at 03:47 GMT

What I stated above about the fuel mix utilized at rest generally applies to both the whole body and the muscle. This has been described in studies in which the gas exchange across the leg was measured in addition to the assessment of energy use at the whole body level. For example:
Bergman et al. Am J Physiol 276:E106-17, 1999.
Friedlander et al. Am J Physiol 292: E107-16, 2007.

No competing interests declared.