• Peter

    Again, nice gadget! As for the marathon, I think I can make sense of the numbers I get here. But for the half-marathon, the percentage of calories needed from carbs should be higher, relative to the marathon, since the race is shorter, or am I wrong? At least, I see that the carbs stored would never limit my race abilities in the half-marathon (in contrast to the marathon).

    • LukeHansons

      Peter,

      For many, many people, carboydrate depletion is not a concern. I think you have to trust that I put at lease some thought into these calculators and have very good formulas. Why do you feel that the calories from carbohydrate would be more since the race is shorter? I understand that you may be running at a higher intensity, but that doesn’t change how much storage you have?

      • Peter

        My thought was that the amount of energy coming from carbs in a HM is — let’s say — 90%-95% while it is only 80% for the marathon — at some level of fitness. The HM is run closer to Lactate Threshold, and I am pretty sure that not much fat is burned at this intensity, is it?

        I trust that you have very good formulas. As with every such gadget, the question is how to interprete the numbers you get. (E.g., carbohydrate depletion can be a concern in the marathon, but almost never in the half.)

        • LukeHansons

          There is a lot cover here.
          1) where did you get these numbers 80% and 90-95% from? Those seem high to me. 95% may be more representitive of a person running a 5k…
          2) Who says that a half marathon is at lactate threshold? LT is represntitive of how hard a person can run for 50-60 minutes. So unless you are running a half marathon in under an hour, then it is not at LT.
          3) Exactly- If you are running under around an 1:45 half marathon, there really isn’t a worry about running out of carbohydrate. The folks out there running 2:30, 3:00, 3+ hours for the half are at risk of running out of CHO stores. However we represent this because they are going to be less fit and be able to store less- so they’d choose their appropriate fitness level.
          4. Here’s some numbers to think about. A marathon is 99% aerobic. A 10k is still 97% aerobic. So let’s just assume a half marathon splits the difference and is 98% aerobic. So by your numbers above, a 1% difference in anaerobic metablosim is worth 15% more carbohydrate useage?

          Also, think about this, is the diffence in paces between your marathon and half marathon really that much apart? For instance, my marathon pace is 5:08. My 1/2 marathon pace is 4:54. That is about a 5% difference in pace. Does that really equate to a 15% increase in usage?

          My numbers above came from Better Training for Distance Runners by Martin/Coe.

          A final item to think about, even though you are running faster, the total energy amount required to run per mile doesn’t change that much. What I mean is that if you need 100 kcals/mile to run the marathon, it’s still right around 100kcals/mile for the half marathon. That number doesn’t increase to 150 kcals/minute. Why? because even though you are running harder, it takes you less time to cover a mile, so it ultimately balances out.

          • Peter

            Thanks, this is a lot of useful information!

            1) I tried to check if I can find numbers of fat utilisation in a
            marathon and half-marathon, but just wasn’t successful. So, I went back
            and looked up in the gas exchange analysis (spiroergometry) results I
            did several times when I was in race shape. Consistently, I found that I use around 20% energy from fat at marathon pace and around 10% at half-marathon pace, although they are only some 7% different (6:00 versus 6:24 in my case). At 10k pace, this drops pretty much to 0%. (Except once, the first time I followed the Hanson’s method, where I still burned a significant amount of fat at 10k pace, btw.) So, I was maybe too extreme here, and would claim that fat utilisation at HM-pace (at least for my fitness level) is around 10%.

            2) You are right, half-marathon pace is not LT pace, I should say HM-pace is closer to LT than MP.

            4) Well, given the results I describe in 1), I would really say 1% difference in (an)aerobic work can be 10% difference in fat utilisation. Wouldn’t this also make sense? We agree, that — at least for somewhat faster runners — your body stores enough carbs for a half-marathon. Why should the body be then using fat at this pace, given that carbs are easier available?

  • June

    I have a far less technical question. The Humanities major wants to know how you calculate/keep track of your daily intake.

    • http://lukehumphreyrunning.com Luke Humphrey

      Hi June! Not sure what you mean. In the calculator or in general?

      The calculating would be the easy part. Keeping track of is a little tougher. You’d need a journal and a place to put the info. The FDA has a solid free one, although I wouldn’t pay attention to the calorie numbers. It would give you a great breakdown of how much and what you are eating, along with nutrient intake.