Physiology Friday #265: Should Carbohydrate Intake During Exercise Be Personalized? A New Study Suggests Yes.
One-size-fits-all nutrition recommendations may be a thing of the past.
Greetings!
Welcome to the Physiology Friday newsletter.
I want to take a brief moment to plug a product and company I’ve been obsessed with—and give you a chance to try them out!
A few weeks ago, I downloaded an app on my iPhone that uses the FaceID camera (the same tech Apple uses to unlock your iPhone or iPad) to scan my feet and make custom running insoles. The company behind the app—FAST Insoles—claims their custom insoles can make you faster and make your running more comfortable.
I’ve been curious about custom insoles for years. FAST isn’t the first company to use an iPhone or iPad to scan your feet for orthotics—it’s actually becoming the standard method doctors and sports medicine clinics use to create a foot cast. FAST is bringing that same technology to the direct-to-consumer space, at about one-quarter the cost of a clinic-made version (think of what Warby Parker did for prescription eyewear).
As a beta tester, I received a complimentary pair. They arrived about a week after I uploaded my scans. The fit was perfect, and the customization was obvious—the topcover was hand-trimmed and sanded flush, and the arch was precisely matched. The 3D-printed portion is flexible and lightweight—not what I expected, since most of the orthotics I’ve seen are bulky and rigid. They slid right into my running shoes and conformed easily to the shoe’s shape.
A few runs in, I started to feel my feet landing differently. I didn’t necessarily feel faster, but with my heel locked in and my foot fully supported, it felt like my form was starting to change for the better. Not to mention my occasional Morton’s neuroma pain was virtually non-existent.
I wasn’t totally sold after the first run. But...I didn’t take them out either. Now, a couple of weeks in, I’m not sure I’ll ever go on a run without these insoles.
In a few weeks, I’m going to dig into the science of FAST insoles with Dr. Jay Spector, a board-certified podiatrist and one of the key thought leaders in this space. Based in Atlanta and a 36-time marathoner himself, he’s fit thousands of runners with products similar to FAST’s insoles.
Right now, you can try FAST insoles out yourself and get $70 off. I’m an affiliate partner, so any purchase you make goes directly toward supporting this newsletter.
Details about the sponsors of this newsletter including FSTFUEL electrolytes, Examine.com, and my book “VO2 Max Essentials” can be found at the end of the post. You can find more products I’m affiliated with on my website.
“More is better" is emerging as the new fueling mantra in endurance sports.
In recent years, athletes have started to push the limits of their palates and gastrointestinal tracts, consuming unprecedentedly high carbohydrate intakes to break records, enhance training intensity, and achieve new performance milestones.
Indeed, numerous records in running and cycling have fallen recently, but it remains debated whether this success is driven primarily by strategic in-race fueling or superior nutrition enabling more rigorous training—likely, it’s some combination of both.
While exceptionally high carbohydrate intake during endurance activities is a recent phenomenon, the concept of carbohydrates as performance enhancers is well-established—substantial scientific evidence continues to reinforce carbohydrates as the optimal fuel for athletic performance (despite what some in the low-carbohydrate diet camp might want you to think).
Current standard recommendations suggest a maximum effective intake of approximately 60 grams of carbohydrates per hour, representing the limit of glucose absorption from external sources.
A one-size-fits-all recommendation like this might seem odd. Wouldn’t someone who’s larger need more carbs during exercise? That’s not what previous data indicate, showing a lack of an association between body size and peak glucose oxidation rate during exercise.
The 60 grams per hour number has been around for quite some time and still represents what most believe to be the highest achievable rate that our body can use glucose during exercise. Exceeding this threshold typically results in undigested carbohydrates lingering in the gastrointestinal tract, potentially leading to discomfort, nausea, and gastrointestinal distress—especially problematic when exercising at intensities approaching 80 to 90 percent of one's VO2 max.
Yet recent insights challenge the rigidity of this guideline, proposing that higher carbohydrate intakes—around 80 to 100 grams or even more per hour—might facilitate greater glucose oxidation rates during exercise. The idea is that supplying the body with higher carbohydrate loads could prompt a corresponding increase in glucose utilization. More in, more out. Factors such as body size, intensity level, and metabolic efficiency may influence the ability to utilize higher carbohydrate doses effectively, as does the type of carbohydrate consumed.
This raises a critical question: Is a uniform carbohydrate intake recommendation truly optimal for all athletes? Physiological variability suggests that the peak rate of glucose oxidation differs significantly among individuals, and personalizing carbohydrate intake could theoretically optimize performance by precisely matching fuel supply to metabolic demand, thereby minimizing gastrointestinal disturbances.
A new study published this week directly addresses this question.1
Researchers investigated whether a personalized carbohydrate intake strategy enables athletes to oxidize as much glucose as consuming an extremely high, fixed intake of 90 grams per hour—150% higher than the established upper recommendation, or if such elevated intakes are simply excessive without additional metabolic benefit.
Eleven endurance-trained athletes (six females and five males) participated in two separate cycling trials, each lasting 150 minutes at an intensity just below their lactate threshold (a moderate and sustainable “zone 2” intensity). During the first trial, participants consumed glucose at a fixed high rate of 90 grams per hour. Researchers measured each athlete's peak glucose oxidation rate during this session using a labeled glucose tracer.
For the second trial, researchers calculated a personalized glucose dose for each participant based on their individual peak oxidation rate from the first trial, adjusted for an assumed oxidation efficiency of about 80%. This resulted in a substantially lower average glucose intake of approximately 65 grams per hour (about 28% less than the fixed high dose).
Despite consuming less glucose, the personalized approach achieved an identical rate of glucose utilization to the high fixed-dose. Peak glucose oxidation rates reached 0.91 grams per minute (55 grams per hour) in the personalized carbohydrate condition and 0.90 grams per minute (54 grams per hour) in the high fixed-dose trial.
Importantly, oxidation efficiency was significantly higher in the personalized trial (83 ± 9%) compared to the fixed high-dose trial (58 ± 9%). Less glucose was going to waste.
There were no significant differences observed between trials in terms of total carbohydrate oxidation rates, fat oxidation rates, or endogenous carbohydrate oxidation. Plasma glucose and lactate concentrations showed expected increases over the duration of exercise, but again, no significant differences were observed between the personalized and fixed high-dose trials.
Some minor differences were observed in perceptual responses. The participants reported significantly lower perceptions of stomach fullness and lower ratings of perceived exertion (effort) during the personalized dosing trial compared to the high fixed-dose trial. This suggests that personalizing carbohydrate intake minimizes some gastrointestinal distress, although nausea and abdominal cramping levels weren’t different between the trials (but did increase slightly in both conditions).
Oxygen uptake was slightly lower (though not significantly so) in the personalized trial, whereas carbon dioxide production was significantly higher in the fixed high-dose trial. Additionally, heart rate increased significantly over time and was significantly higher in the fixed high-dose trial compared to the personalized trial, perhaps lending itself to the higher effort level reported above.
The study also investigated whether personal characteristics like body height, mass, and power output could predict peak glucose oxidation rates. While no single variable showed a strong correlation on its own, a combination of body height and power output explained about 68% of the variability in peak glucose oxidation rates. Interestingly, body mass was not a statistically significant predictor. This suggests that while some physical traits contribute to differences in glucose oxidation efficiency, there remains a large degree of individual variability that is not easily explained by body size alone. What explains this variability? Nobody knows (yet).
Although this was a small, proof-of-concept study, I think it points to an interesting direction for endurance sports (and endurance sports research).
Just this week, I was on a phone call with a client I’m coaching who inquired about the necessity of fueling with more than 60 grams of carbs per hour. He’d also got caught up in the high-carb frenzy sweeping the sport and wondered if his performance might improve at a higher intake. The answer to that question is a resounding “maybe.”
On one hand, more aggressive fueling might lend itself to a higher sustainable intensity during a marathon, but at what risk? Fueling too aggressively might leave him with nausea or worse, an unplanned stop at the bathroom. Would a small performance boost be worth the risk?
In my opinion, probably not. This study would (still) suggest that ~60 grams of carbohydrates per hour represents a solid fueling standard to aim for and that higher doses, irrespective of one’s body size, may be excessive.
Unfortunately, you can’t currently quantify your own peak glucose oxidation rate during exercise to personalize your own fueling strategy—but there is the tried-and-true approach of trial-and-error.
The only way to see what works for you is to try out different levels of carb intake, different formulations of carbohydrates, and at different durations and intensities of exercise. It doesn’t take long to figure out what works and what doesn’t, and the promising news is that we can train our guts over time to tolerate more (if that’s your goal).
Personalized nutrition still hasn’t taken over the world of health and performance, but we’re getting closer—at least when it comes to scientifically validated strategies. I suppose that in one way or another, all nutrition is personalized. That’s what makes all of this stuff so fun. Life is an n=1 experiment.
Thanks for reading. See you next Friday.
~Brady~
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This was a great read! Thank you! Thinking of other carbohydrate studies and messages, do you think it's possible for the body to change its glucose oxidation if one were to "gut train" with ~90g/carbs over time? I'd also be curious to see another study like this that looks at muscle damage to see if it gets the same results as this study: https://pubmed.ncbi.nlm.nih.gov/32403259/