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Re: Help me calculate an at home "maurten" drink (im bad at math) [BNothling]
BNothling wrote:
Just posting this here after a DM discussion with Dr Alex. I want to preface this by saying that he knows where I'm coming from and I'm not trying to undermine his work. I've been using his mix (dubbed Alacrity Mix) for a few months now and haven't had an issue with it. I've adapted to the 100g CHO/hour and everything is ticking along. I've started seeing a dietician and she asked about my intra-training mix. When I told her, this was her response and after asking Dr Alex for his opinion and insight into her response, we decided it best to post it here so Alex could respond and hopefully enlighten the rest of us.

Fire away Dr Alex!

Hey BNothling, thanks for posting this here, and for your patience for my response. Wanted to do this one justice. Your dietitian is honestly better than average if I were to sample all dietitians for their prescriptive quality and rationale. Better than average even for CSSD's (sport dietitians), too, so I won't blame her for missing the mark in the very small ways that she may have here. Her recommendations are generally considered "best practices" by RD's in general. It's just that the body of ACEND and most RD's are not intimately familiar with endurance sport nutrition and the last 10 years of the most recent literature. So, honestly, kudos to her for being able to make very specific recommendations in the first place.

And, she's lightyears ahead of where I started when I made the plunge into endurance sport nutrition.

Fun anecdote: I once prescribed a male cyclist 75 grams of carbs, TOTAL for a 4-hour bike ride. He told me he was bonking after about 90-120minutes, and so I upped it to 125 grams of carbs. He reported still bonking after ~120-150 minutes, and having a miserable remainder of his 4-hr group ride. Just no power. Needless to say, he, rightly, did not renew his coaching package with me.

(this was circa ~2015, when I didn't have the first clue about endurance sports, and it was that utter coaching dumpster fire that compelled me to get out of weight room nutrient timing & glycogen repletion literature and into actual endurance fueling literature)

To whomever that was, if you ever read this, I'm truly sorry! Happy to credit you with 3 months of free nutrition coaching if ever you stumble into this thread, or lifetime free use of the app I'm writing.

I tell this anecdote to say to anyone learning to write endurance nutrition programs, or implement their own, I promise you already know more than I did when I started, and you're doing a great job.

Okay, let's dive in.

BNothling's dietitian wrote:
The reason I queried this is because of the following;

Glucose (one of the forms of CHO you're taking on, approximately 52% of your current mix) maximally oxidises at 1g/min - which obviously then caps us out at 60g/hour.
Not quite. Depending on who you ask, it's probably somewhere between ....


60-72g/hr for pure glucose (as far as study averages go). FWIW: One poster on ST has reported consistently using 90g/hr of either pure maltodextrin or pure dextrose for all their carbs with no gut issues. The rate of total carb oxidation in the literature is more like 1.5-2.0g/min, IIRC, but with relatively higher incidences of GI distress above 60-70g/hr when consuming pure glucose.

Most importantly, to achieve 60g/hr oxidation of glucose, it requires >60g/hr intake because. If you're capable of oxidizing 100g/hr, total, it may actually take 120-130g/hr intake to cause such an exogenous sugar oxidation rate. This is true for all forms of sugar, independently, and in any combinations. Exogenous glucose oxidation rates exceeding 60g/hr (1g/min) are often found with sugar intakes ranging from 70-120g/hr.

TLDR: You must intake more than will be oxidized. Only way to get those numbers to be identical is to move ever lower towards 0g/hr intake rates.


BNothling's dietitian wrote:
If this is 52% of our mix, this means we are roughly getting 52g/hour.

BNothling's dietitian wrote:
To achieve higher CHO oxidation rates during exercise, we need to use multiple transportable forms of CHO.

BNothling's dietitian wrote:
These limitations are based on intentional absorption of CHO. (Glucose needs the sodium-dependent transporter SGLT1 for absorption across the basolateral membrane of the intestinal lumen).
Also correct.
BNothling's dietitian wrote:
This is achieved with the introduction of fructose (as it uses the GLUT5 transporter) which maximally oxidises at 0.6g/min (36g per hour).
This is probably just an inconsequential misuse of wording here, and I'm only pointing out the semantic error here for the edification of readers.

Trasporters don't oxidize. They transport. Oxidation happens later, in metabolism.

Her cited fructose max transport rate appears to be higher than I've seen reported by an RD before. Cool! Also a bit overly-specific, as is commonly taught in most RD education programs. (My wife is an RD and was remotely educated so I got to witness lots). Usually what is cited is 30g/hr. I'd say that 30-36g/hr is a good rule of thumb for max fructose trasport rate for that one transporter type: GLUT5.

GLUT2, however, is pretty neat. See my wife's recent video about monosaccharides, and watch the fructose section for why it matters. Short answer: more fructose intake hourly is possible when combined with glucose intake. The "36" number comes from fructose-only consumption studies.

BNothling's dietitian wrote:
Fructose makes up the other 48% of our mix or 48g/hour.
This puts us at a maximal oxidation possibility of 96g/hour, if 60g was coming from glucose/sucrose and 36g was coming from fructose.
Who knows exactly where it puts max oxidation potential, but I'd posit that if you're not having gut issues, you're transporting all of it peaceably through your GI tract.

BNothling's dietitian wrote:
Things to additionally note:

The calculations I've done are based on the assumption that the "table sugar" i.e. sucrose is 50:50 glucose:fructose as accepted in the literature.
She's a nerd. I love it.

BNothling's dietitian wrote:
I looked up the proportions of glucose:fructose in one container of gatorade and subbed those values in as well, see table at the end of the email.
Super nerd. Fantastic. This is above and beyond service from most sport dietitians. I'm curious what she found for those proportions. I've seen multiple ratios cited.

BNothling's dietitian wrote:
Intensity and time both play roles here too.

BNothling's dietitian wrote:
At high intensity, any exercise <2.5 hours should only maximally need 60g/hour and can be kept to one type of CHO (i.e. glucose alone).
Depends on the person. Some people are going to get hypoglycemic as heck at 60g/hr. She's citing Jeukendrup's work, which has been cited most recently in position stands of the ISSN, which he co-authored. He's excellent, as has been his research. However, the recommendations that he puts forth are very generic, and have led to the under-fueling of probably 30-40% of athletes, to the extent that the believe their fitness is far lower than it actually is.

BNothling's dietitian wrote:
Lower intensities require lower amounts of CHO.
Yes, but intensities have to be VERY low to not merit fueling up to 90g/hr out in the 3-4hr realm, especially for a reasonably well-muscle or high-fitness person. Yes, higher fitness folks are usually capable of burning more fat, both absolutely, and relative to their total fuel oxidation, but they're also simply capable of burning more in general, including carbs. High muscle, high fitness, or both often demand >90g/hr unless we're at recovery intensities, once activity exceeds 2.5 hours.

BNothling's dietitian wrote:
Once we get up past 2.5 hours (and if intensity is still high) that's when we drift up to 90g/hour.
Yes, and sometimes as high as 120-150g/hr is necessary and tolerable for large, muscular, or high-fitness athletes.

BNothling's dietitian wrote:
So, what does all of this mean? Basically, unless you are a freak of nature, you can't possibly be absorbing all of that CHO per hour, which means we can do some tweaking.
Not true.

The AVERAGE of the people studied in all the studies has shown that 90-120g/hr optimizes performance, in the sampled populations. Higher fitness, higher muscularity, or just higher carb burning ability genetically or via training adaptations, can absolutely drive those numbers up 20-50%. I'd posit the 40-50% range (ie. 140-150g/hr) is where you're getting into freak of nature status.

When a study claims to have found the "highest ever exogenous carb oxidation rates" what they mean is that the average of their study subjects produced that record rate, compared to the average of all other study's sample populations. Assuming normal distribution (or where mean ≅ median, roughly) that means literally half the subjects in that study experienced personally higher oxidation rates than the number that is statistically reportable in publication.

Research is especially lacking on larger endurance athletes with very high chronic fuel intake rates because it's never been broadly recommended. If there is anything at all truthful about the "train your gut" movement, I suspect we'll see even higher carb intake, absorption, and oxidation rates in the years to come as wider swaths of athletes start implementing >100g/hr periodically, when it matters.

BNothling's dietitian wrote:
However, I am very aware of athlete's comfort zones, and not wanting to mess around too much outside of what they feel comfortable changing.
@BNothling, I think this is a great opportunity for you to show her that you are indeed using all the carbs. She's allowing you to keep fueling at high rates. Might be fun to report back "hey, so I felt even better when I did 110g/hr." The same thing happened to me when I had an athlete say they'd tried 140g/hr for the first time and felt fantastic on a 5-hr ride. Opened my eyes to the magnitude of human variation that is possible.

IN SEARCH OF APP DEVELOPER. DM me. Equity on table.
Dr. Alex Harrison
PhD Sport Physiology & Performance
Last edited by: DrAlexHarrison: Jun 29, 22 12:20

Edit Log:

  • Post edited by DrAlexHarrison (Dawson Saddle) on Jun 29, 22 12:20: content edit re: glucose oxidation