Podcast: How to fuel for a marathon (plus show notes)

I recently appeared on Joe Sell's excellent Marathon Running Podcast for a second time to talk about fueling and nutrition for the marathon (see my first appearance here in case you missed it).

The impetus for this podcast was my recent blog post on how to plan fueling for elite marathoners who have "bottle service" at their race, but in the podcast we talk about fueling for all kinds of runners.

We had a great time and went really in-depth on the science behind optimal fueling (both hydration and sports drinks + gels) for marathoning. Check it out here!

Show notes

Here are the references for some of the more technical details we talk about in the podcast:

Caloric cost per mile of running is (almost) the same regardless of speed: Biomechanics of Movement by Uchida and Delp (2021) Figure 3.18 (see slide 19 here)

Typical caloric cost of running per mile: 78% of your body weight in pounds = total calories burned per mile. For a range that covers ~95% of people, use 66-91% of body weight per mile. My unit conversion from averages and 95% range (+/- 2 standard deviations) from Fletcher et al. 1985. I would love a larger/more comprehensive dataset of running economy and cost of transport data; let me know if you know of one!

Proportion of calories per mile that come from carbs is about equal to % of 5k pace (or %VO2max): Rapoport 2010, specifically Figure 1. Note that this "rule" is a simplification, but it works fine for the speeds relevant for casual to elite marathoning (~70-90% VO2max)

Overview of the path that carbohydrates take from ingestion to being burned in muscles: Malone et al. 2021 (Also my reference for "higher blood insulin levels are good because they increase carbohydrate oxidation")

Often Kenyans who are not affiliated with top coaches train without much or any fuel: Renato Canova's comment here

Elite marathon champions often lose 6-10% of their body weight and drink far less than fluid replacement guidelines would recommend: Beis et al 2012

Ad libitum ("just drink when you're thirsty") is equivalent or superior to programmed drinking, even in the heat: Goulet and Hoffman 2019

In a time trial setting (like a marathon) you can lose at least 4% of body weight without performance being inhibited: Goulet 2012

No evidence electrolytes are beneficial: Hoffman et al 2018 (see also Hoffman & Stuempfle, 2015a; Hoffman et al., 2015b in the references)

High sodium intake probably drives high salt content in sweat: McCubbin and Costa 2018 (as opposed to the common theory that some people are "salty sweaters" and thus need to consume a lot of electrolytes)

Electrolyte levels are not associated with cramping: Schwellnus et al 2010

No advantage to hydrogel-like formulations (Maurten gels and others): King et al. 2020

I don't think the UCAN studies are relevant for typical marathon conditions: Davitt et al. 2021

Around 10-20% of runners have GI issues with high carb intake: Pfeiffer et al. 2009

Carbs give you more energy per unit oxygen than fat: Beck et al. 2018

Using multiple sources of carbs is beneficial: Stellingwerff and Cox 2014

Carb absorption is the rate limiting step, because a glucose infusion can lead to much higher exogenous carb oxidation than any consumed supplement: MacLaren et al 1985

It takes about 20 minutes for ingested carbs to be burned for energy: Massicotte et al. 1986, specifically Figure 3 which shows when radiolabeled [13]C starts showing up in exhaled CO2

Low carb / keto diet decrease economy and impairs performance: Burke et al. 2020 and Burke et al. 2017 and Whitfield 2021, though see Beck et al. 2017 for a methodological critique of the economy findings (irrespective of this critique, actual race performance undeniably declined). Note that this is all in race walkers.

Current opinion on keto/low carb for endurance athletes: Burke 2021

Practical tips on how to train in a temporary state of low carb availability: Bartlett et al. 2015

About the Author

John J Davis, PhD

I have been coaching runners and writing about training and injuries for over ten years. I've helped total novices, NXN-qualifying high schoolers, elite-field competitors at major marathons, and runners everywhere in between. I have a Ph.D. in Human Performance, and I do scientific research focused on the biomechanics of overuse injuries in runners. I published my first book, Modern Training and Physiology for Middle and Long-Distance Runners, in 2013.

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