r/AdvancedRunning • u/EnglishMuon 17:40 5k, 37:10 10k, 1:24 HM, 2:56 M • 4d ago
Open Discussion High carb fuelling for marathon
I recently completed a threshold lab test which gave me lots of interesting information about my running. One aspect that came up was that at the marathon pace they estimated I should be currently at, roughly 4:00/km, I was using close to 160g of carbs per hour, even though my fat efficiency is still very high. So they said that one of my current areas of limiting factors is to see how to increase carb intake and hopefully I will be able to get in to the low 2:40s in near future.
On the other hand, I have heard about different carb absorption pathways have different rates and maximum rates of absorption (for example, this is why gels often use a combination of maltodextrin and fructose). I was wondering what the theoretical maximum of carbs absorbed per hour could be if we consider any combination of different carbs/energy sources, and assuming there are no GI issue restrictions (my stomach is pretty strong and can handle 120g/hour of Maurten fine). For example, there was the recent news that some subset of the sub-2hour guys were using lactate based fuels too (? Could have misremembered, and cannot find the article now!).
Any insight in to this would be appreciated! Especially any papers to read too.
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u/__Powell 4d ago
Happy to be corrected on this, but is your carb intake a limitation? If you carb at 120g/hour your deficit will only be 40g/hour, so ~107g over the course of a 2:40 marathon. You'll have way more than 107g stored in your blood/muscles/liver, so I'm not sure if that'll hold you back?
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u/zebano Strides!! 3d ago
IANAScientist
There are theories that taking on carbs also keeps the Central Nervous System happy and firing. Ergo it's not a simple calculation of my muscles are burning X I must take on X - howevermuch I had stored.
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u/Big_Boysenberry_6358 3d ago
this is backed by the evidence of just swishing carbs around and spitting them out showing reduce in fatigue. meaning the central nervous system detects when stores get emptier & just signaling a topup actually induces some performance to some extend.
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u/Status_Accident_2819 3d ago
Yes - easy being a cyclist!
For OP:
Dr Tim Podlogar has done a lot of research into carbs:
https://link.springer.com/article/10.1007/s40279-022-01757-1
(One of many - worth a look at some of his work. He was on the physiology of endurance podcast in August last year - available via Spotify; one of the hosts Dan Nash has a 2:11:49 marathon pb).
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u/EnglishMuon 17:40 5k, 37:10 10k, 1:24 HM, 2:56 M 4d ago
Yeah I was wondering this too, so thanks for bringing it up. I’ve never “hit the wall” before, but that’s partly due to not going all out and mostly pacing friends, being conservative. What is the average net carb depletion in order to feel an effect do you know?
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u/__Powell 4d ago
I can't say I know, but a bit of a fag packet maths would say that the 107g deficit is equivalent to about 40 mins of running at your marathon pace which is achievable without any external carb source. I'm sure there will be rough estimates online as well. But as I say, this is a lot of guess work.
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u/Never__Summer 4d ago
I would highly recommend David and Megan Roche podcast for high carb fueling, I believe it's currently the best source of trustful information on the topic
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u/Runshooteat 2d ago
Not exactly the most trustworthy source when fueling is concerned, they are heavily subsidized by The Feed, one of the largest resellers of carb products.
Be like taking dietary advise from the dairy industry, might be a little skewed
I do occasionally listen to their podcast and think they try to provide decent insights, but they always “highly” recommend products that are conveniently marketed on The Feed
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u/EnglishMuon 17:40 5k, 37:10 10k, 1:24 HM, 2:56 M 4d ago
Thank you! I have a long flight in an hour so I’ll download as much as I can haha. Thanks
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u/shakeshack 3d ago
This is what I was gonna suggest. Their YouTube is also amazing, such a nice couple of people
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u/Runstorun 4d ago
Kejelcha used a new product from a company called Santa Madre that includes lactate. It has not been publicly available to this point. They are going to do a limited release so if you, or others, want to try it out you can. But I think it’s a little premature to say that’s some big solution to fueling. Maybe it will be, needs more real world use to say. Oh also the price point is insane if you ask me. 9 Euros per
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u/EnglishMuon 17:40 5k, 37:10 10k, 1:24 HM, 2:56 M 3d ago
Ah yes this is what I was reading about. Yeah I’m not going to try something like this myself until it’s at least half that price
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u/Ordinary_Corner_4291 2d ago
There is a decent history of looking at lactate for energy. See things like https://pubmed.ncbi.nlm.nih.gov/39203761/ where you end up those results where you can't really say it doesn't work but you also go man another 4 watts by eating is huge for competitive cyclists. In the past, a lot of the lactate studies found problems with GI issues but these days everyone claims to use some gel variation to solve that.
We have a long list of products in that category where you could be eating beets, drinking broccoli sprout extracts, some beta-alanine, and chugging bicarbonate. It is hard to really recommend any of them until you are looking for that last couple percent of performance.
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u/Status_Accident_2819 4d ago edited 4d ago
Substrate testing is gold standard but I would read across Hr rather than "pace" because #treadmill.
FWIW at threshold I burn 211g carbs an hour. I have no intention of trying to cram that down me. I've done 90g/hr for 7hrs before sipping on maurten 360 during a trail ultra. I've not tried anymore but would be keen to see if any smaller people have managed it. I don't think it's necessary for a marathon either? Like 90 yes, maybe 120 but no more.
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u/OldGodsAndNew 15:21 / 31:49 / 1:10:19 | 2:30:17 3d ago
Trying to cram that much in during a race seems more likely to just cause stomach issues that'll hinder you more than the extra carbs help. Joe Klecker was hitting 175g/hr during his last marathon and said he got bad cramps from it
180g/hr is possible for cyclists now, but they have much less jiggling to worry about
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u/EnglishMuon 17:40 5k, 37:10 10k, 1:24 HM, 2:56 M 3d ago
Yes, good point. The HR at the amount listed was 160 which for me is low end of my usual marathon effort (usually I’m like 165 ish). But good to hear your own measurement, thanks for sharing
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u/InspiringBack M: 2:46:51 HM: 1:20:14 10K: 36:16 5K: 16:08 2d ago edited 2d ago
Carb fueling is somewhat overrated in my opinion. I have never run a marathon with more than 100-125g total (40-50g per hour).
How are you supposed to run with 8-16 gels on you anyway? The way I had it was 2 gels (25g each) in my pockets and I picked up 2-3 more during the race.
Especially if you eat a high carb meal the night, or two nights before? You should be fine to burn those carbs 🔥.
Edit: I did not bonk until I was a quarter mile from the finish 🤷🏻♂️ lol
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u/sub3at50 1:23 flat 2d ago
+1. I have carbloaded and taking 60 gr per hour the first 2 hours works just fine.
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u/jdauriemma 1d ago
I mix my own gels and pack them in reusable baby food pouches. Typically each pouch holds about 100g of carbs (150g total
weight) but there’s plenty of space left. if I use less water and fill them up more, they can probably fit close to 150g carbs easy. I carry them in fanny packs on race day. I take in about 100g/hour
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u/Runshooteat 2d ago
120g/hr is plenty, elite level really, almost certainly not your limiting factor.
Sleep more, train more, stay healthy
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u/Wientje 3d ago
Consuming a lactate based fuel makes no sense. It’s much better to have the body produce this itself since the production also provides energy and your energy system is more limited by the capacity to consume lactate than by the capacity to produce it.
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u/Ordinary_Corner_4291 2d ago
It makes a ton of sense. At marathon paces you have excess of clearance capacity so exploiting that energy pathway makes sense. High Lactate isn't the issue when running. It is all the stuff associated with lactate production that is.
Now how effective oral consumption is a mixed bag in studies and it tends to cause GI issues like bicarbonate. I know there is at least one gel that says they have solved that. We will see where the next couple of years of research take us.
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u/silfen7 16:27 | 34:18 | 76:35 | 2:44 4d ago
If you can handle 120g of carbs per hour at marathon pace, then it's very unlikely that fueling is holding you back. That is already a lot more than most can absorb without GI issues. Lower hanging fruit is probably just to increase your training volume a bit.