Carbs Per Hour for Runners: Why the Old Fueling Ceiling Moved

Episode: S6E26 | Guests: Jaclyn Sklaver, CNS, LDN and Emma Harvie, CNS, LDN | Aired August 27, 2026 | 54 min

Author: Nick Klastava

Podcast Hosts: Nick Klastava & Amanda Katz

Listen: Spotify | Apple Podcasts | YouTube

How many carbs per hour for runners is the right number? For a long stretch the honest answer was as few as you could get away with. Enough to not fall apart at mile twenty, not so much that your stomach turned on you. That has flipped completely. The people running the fastest times right now are not eating less than everyone else, they are eating far more than anyone thought was physically possible a decade ago, and the science finally caught up to explain why. My honest belief is that the biggest gain available to the average runner over the next five years will not come from shoes or watches. It will come from how much you can fuel around your hardest work. In this episode Coach Amanda and I sat down with Jaclyn Sklaver and Emma Harvie, two board certified nutrition specialists who work with everyone from active gym goers to Olympians, to talk about what actually changed, what is worth your money, and what most runners are still getting wrong.

Key Takeaways

•          The old advice on carbs per hour for runners capped out at 60 grams, and that ceiling was real, but it only applied to glucose. Glucose uses one transporter that saturates at about a gram per minute, and anything past that sits in the gut and causes the bloating and cramping runners blame on carbs.

•          Adding fructose opens a second, separate pathway that does not compete with glucose. That is the entire reason dual source products exist and why 90 to 120 grams per hour became possible.

•          A higher ceiling is not a starting point. Gut training works through faster gastric emptying and repeated exposure, and it needs three to six months, not the two weeks before your race.

•          If you are taking in less than 60 grams per hour, the delivery method barely matters. Dual source, cyclic dextrin, and high carb mixes are solving a problem you do not have yet.

•          Real food counts. One of Emma's clients set a large Boston Marathon personal best on peanut butter and jelly bars. Palatability and consistency beat the perfect product.

•          Carbohydrate needs follow a formula. Sodium losses do not. Individual sweat sodium ranges from a few hundred milligrams to a couple thousand, and even lab testing is affected by what you ate the day before.

•          Bicarbonate research is almost entirely on short sprints and high intensity intervals, and the gains there are measured in a handful of seconds. Thirty more grams of carbs per hour is cheaper and will do more for a marathon.

•          Underfueling is not just a performance problem. Chronic low energy availability drives low testosterone in men, low estrogen in women, iron deficiency, stress fractures, mood changes, and a metabolism that quietly slows down to match what you are giving it.

How Many Carbs Per Hour for Runners Is Actually Right

The first question Jaclyn asks a new athlete is not what they should be doing. It is what they are doing right now, what they have tried before, what has gone badly, and what has gone well. Her reasoning is simple. Most runners are doing it incorrectly, and most of them are underfueling. There is an ideal number of carbs per hour for runners on paper, but the plan only counts if the athlete can actually execute it, so the starting point has to be somewhere in reach of where they already are.

How long they have been training, how experienced they are, how long their runs are, and where they sit in a training block all change the answer. So does hydration, and that piece is more delicate than most people realize. Once you put electrolytes and sugars into the stomach together you are dealing with osmolarity, and the balance between how much fluid, how many electrolytes, and how many carbohydrates you take in, and when, is what separates a plan that works from a plan that sends you looking for a bathroom.

Emma takes the same approach from the other direction. If an athlete arrives with a strategy already in place and it is working, she is not going to break it for the sake of optimizing. What she does correct is the athlete who has no idea what their actual ceiling is, because almost nobody knows how many carbs per hour they can personally absorb.

Why the 60 Gram Ceiling on Carbs Per Hour Moved

Emma explained the mechanism better than I have heard it explained anywhere else. Think of your intestinal wall as having dedicated doors for different types of sugar, and each door can only let so much through per minute. Glucose, the carbohydrate most runners are familiar with, has its own door with a fixed capacity that saturates at roughly one gram per minute. Multiply that out and you get about 60 grams per hour, which is exactly where the old advice on carbs per hour for runners came from.

Once that door is saturated, feeding in more glucose does not help you. The excess just sits in the gut unabsorbed, and that is what actually causes the bloating, the cramping, and the nausea runners have spent years blaming on carbohydrates in general.

Fructose changes the math because it uses a completely different transporter. It is a separate pathway, so it does not compete with glucose for absorption. Combine the two and you are no longer overloading one door, which is how the ceiling climbs to 90 and even 120 grams per hour in very well trained athletes. That is the whole story behind the shift from single source to dual source gels, and behind the high carb drink mixes built on cyclic dextrin that have taken over the sport. It is not a marketing term. It is a second door.

Training Your Gut to Handle More Carbs Per Hour

The mistake Emma sees constantly is athletes coming to her a couple of weeks out from a race wanting a race day strategy, when the strategy needed to be in place months earlier. There are two separate things that adapt. The first is gastric emptying, meaning how quickly your stomach moves fuel into the small intestine where it can actually be absorbed, and we know that responds to training. The second is straightforward repeated exposure, which appears to improve how well the gut handles a given carbohydrate load independent of how fast the stomach empties.

Her window for raising carbs per hour is three to six months, ideally starting at the beginning of a marathon build. That gives you time to establish a foundational nutrition strategy first and then layer a race plan on top of it, rather than trying to bolt an unfamiliar carbohydrate load onto a gut that has never seen it. Emma put the risk plainly. The research ceiling of 120 grams per hour is real, and someone who has never done it before should absolutely not go try it.

Why Runners Resist Adding Carbs Per Hour

Jaclyn estimates that eight times out of ten, endurance athletes arrive at her practice afraid of carbohydrates. She finds it hard to believe any endurance athlete has ever gained weight from overeating carbs specifically. What actually happens is under eating carbohydrate at the moments it matters, then eating more of everything else later in the day because the brain is still trying to balance the books, which does nothing for training or recovery.

There is a strange collision happening in running culture here. We have been trained our whole lives to believe that more is better, so runners will happily add mileage. We have also been told, in a hundred quiet ways, that smaller is faster, so the same runners will resist adding fuel. The two beliefs work against each other, and the athlete ends up carrying more training load on less energy every year. Jaclyn frames the fix as a choice with only two doors. Either you fuel more or you train less. When the lab work says overtrained and underfueled, one of those has to move.

Emma pointed out the trap underneath all of it. A lot of runners never seek nutrition support because what they are doing seems to be working. Working is not the same as optimal. It does not mean you could not feel better, and it does not mean you are not leaving performance on the table. Low energy availability is rampant in male and female runners alike, and it becomes a loop. You overtrain, you under eat, your hormones stop functioning well, losing weight gets harder, body composition stalls, and you are not eating enough to build or hold muscle in the first place.

Neither of them has ever seen an athlete get worse from eating more. Emma had a client in tears in their second session because she had not known it was possible to have that much stamina in a workout, or that constant low energy was not just the price of training. Jaclyn hears a version of it every time. If you have been living in a bad place long enough, you assume that is what normal feels like.

Recovery Is Where Most Long Runs Fall Apart

A long run has more parts than the run. There is dinner the night before, the carb load if the effort is big enough, breakfast, the fueling during, and then the piece runners skip most often, which is eating afterward. Plenty of people execute the run perfectly and then jump straight into the rest of their Saturday without eating or drinking, and they spend the afternoon wondering why they feel destroyed.

Emma pointed to appetite suppression as the culprit. Adrenaline blunts hunger after a hard effort, and it is easy to read no appetite as no need. It is not. Her correction with every athlete is to refuel after the long effort whether or not hunger shows up, and it does not have to be complicated. Chocolate milk and a banana starts the process.

Jaclyn added the encouraging part. Once you start doing it, the hunger signals wake back up. You cannot be in fight or flight and rest and digest at the same time, but you can nudge your body toward the second one. Start small, and within a few weeks people notice they are actually hungry after training again.

Both of them made the same point about the rest of the week. Fueling and recovery are not race day events. Your training has a rhythm you already understand, and your nutrition needs the same rhythm rather than a scramble the night before a long run. Their functional nutrition background pushes this further than calories. Every system in the body needs specific vitamins and minerals it cannot make on its own, and running generates a lot of oxidative stress, which puts real demand on antioxidants. The carbohydrates you take in during a run provide none of that, which is exactly why the carbohydrates you choose during the other twenty two hours matter.

Hydration and Sodium Are Personal, Not Formulaic

Here is the part that surprised me most. Carbohydrate needs are close to universal across athletes, which Jaclyn still finds remarkable. Sodium is the opposite. One runner might lose 500 milligrams an hour and another 2000, and there is no formula that gets you there.

Jaclyn uses an MX3 analyzer with her athletes and has run enough tests to spot the patterns, including one the manufacturers do not advertise. Diet affects the result. A high sodium meal the night before or an electrolyte drink that morning will move your numbers, so she built her own pre-test rules to keep the data honest. Her conclusion is measured. Testing lets you dial a plan in more precisely, and it is genuinely useful to learn that a heavy sweater is not necessarily a heavy sodium loser. It is also not a prerequisite. She has built successful plans for years without it, and most athletes have no access to it anyway.

Emma offered two practical alternatives. The free one is a sweat rate test you can do at home. Weigh yourself before a run, ideally undressed and after emptying your bladder, note how much fluid you drink during a moderate run of three to six miles, then towel dry and weigh yourself again. It is not perfect, but it is a real world number to build from instead of a guess.

The second is asking better questions. Is your sweat noticeably salty? Are your clothes soaked after a run? Do you get headaches after long efforts? Jaclyn adds white salt marks on your hat, your shirt, or your clothes, and whether you cramp. Those answers tell an experienced practitioner most of what a patch would.

On the consumer sweat patches, both were cautious. Emma had a client run one recently and the device overestimated his losses so badly that it contradicted everything his symptoms indicated. Jaclyn noted that among the sports dietitians she knows working with teams, almost nobody is seeking those tests out, which tells you something on its own. Every method here has a margin of error, and the practical takeaway is that with or without data you can build a mostly personalized plan that keeps you from getting dehydrated.

When Data Stops Helping

Jaclyn is watching more athletes feed their numbers into AI tools and build their own algorithms, and she was direct about the limits. Trackers have room for error, spreadsheets full of numbers can drive you into the ground, and a lot of that feedback lands as negative rather than useful. She does love watching HRV climb and sleep improve once an athlete starts fueling properly. She just cannot build a plan out of the data alone, because the person on the other end of it is not a spreadsheet.

Emma framed it in a line worth keeping. Data is helpful, right up until it is not. What the research says is optimal and what is reasonable for a specific human being with a job and a family and a commute are frequently different things, and the actual skill is translating physiology into something a person can execute on a Tuesday.

Bicarb, Ketones, and Why Carbs Per Hour Still Wins

I asked about the exciting stuff, because runners see world records fall and immediately want the thing that got broken out for the occasion. Sodium bicarbonate is the big one right now. Bicarb itself has existed forever. What changed is that a company engineered a delivery system that carries it past stomach acid to the small intestine, where the body can use it as a buffer, without the traditional consequence of spending your race in a bathroom.

Jaclyn's objection is the evidence. Almost all of the research sits on short sprints and high intensity intervals, and there is nothing showing a benefit for endurance. Even in the sprint work the effect is small, on the order of seconds. At roughly seventeen dollars a serving, that is a lot of money for something with no endurance data behind it. She also made a point I had not considered. The sports nutrition world used bicarb years ago and then set it down, because the dose required to get an effect was not tolerable. The only reason it is back in the conversation is that a company invested in encapsulating it and marketing it well.

Emma added a filter I want every runner to steal. She only recommends products with a high potential for benefit and low risk. Bicarb fails on the second half, because the risk of GI distress is genuinely high. Neither of them is dogmatic about it. If an athlete tolerates it, likes it, and does not mind the cost, both would leave it in the plan. It is simply nowhere near the top of the list.

The bigger point is proportion. The athletes breaking records are already taking in 120 grams of carbs an hour and maximizing every other variable, and they are chasing seconds nobody has ever found before. If you are taking in 40 or 60 grams an hour and buying bicarb, another 30 grams of carbohydrate will do far more for you, cost far less, and carry far less GI risk. Emma summarized the whole category perfectly. It is like investing in super shoes while skipping your long runs. Carbs per hour for runners is the lever that moves, and supplements are the decoration on top of it. Supplements sit at the very top of the pyramid of priorities, and the top of a pyramid does not hold anything up.

What Underfueling Actually Does

Low energy availability means your intake does not meet your total daily demand. When that happens, Emma explained, the energy that does come in gets redirected toward the processes required to keep you alive. Basic movement and brain function get funded. Hormones and reproductive function do not.

Run that chronically and systems start failing in a recognizable order. In female athletes you see the triad, meaning stress fractures, menstrual dysfunction, and disordered eating patterns. In male athletes low testosterone shows up, often alongside stubborn abdominal fat driven by cortisol, more frequent illness, and more injuries. Emma also sees iron deficiency constantly in the lab work of female athletes, and she traces low testosterone in male athletes to under eating fat as often as anything else, with intake frequently sitting far below one gram per kilogram of body weight. It is not all carbs. Fat drives hormone production and protein holds the structure together.

Jaclyn and Emma have a name for what walks through the door. They call it the low T diet. Low testosterone in men, low estrogen and low hormones across the board in women, all of it downstream of not eating enough carbohydrate to support the training. Jaclyn pushed into the part nobody wants to say out loud. Sex drive disappears, mood changes, depression shows up, and the way you treat your family and your coworkers changes with it. Low bone mineral density and stress fractures happen in men too, not only in women. And what you are doing at thirty determines a great deal about what your hormones look like at fifty, which is worth sitting with, because the training is supposed to be making you healthier rather than the reverse.

The metabolic adaptation piece is the sneakiest. Under eat long enough and your metabolism slows to match, and you start conserving energy in ways you never notice. Parking closer to the store. Feeling flat on easy runs. Nutrient insufficiencies creeping into the lab work. Muscle mass you cannot hold onto.

The upside is that bodies respond fast when you give them what they are asking for. Emma worked with an Olympic track and field athlete who came to her with low energy availability. Over four to six months they doubled her calorie intake. Her muscle mass increased, her body fat decreased, and she went on to win every race of her season.

What They Wish Runners Understood

At the end of the episode both of them landed in the same place. Jaclyn wishes people understood that nutrition is not chicken and broccoli. Every metabolic process in your body is affected by what you put into it, and a professional can do far more than tell you to hit 30 grams of protein and 40 grams of carbs. That is surface level. Emma agreed and added the counterweight. It can be complex, but it does not have to be. If you eat enough, eat enough plants, get your protein and fiber in, and stay consistent, you have the pattern that produces long term results, and the right carbs per hour for runners falls out of that foundation rather than replacing it.

The other thing they both hear is fear. People avoid working with a nutritionist because they assume their favorite foods are about to be taken away. Neither of them has any interest in that. The entire point of personalized nutrition is a pattern you enjoy and can maintain, and if the plan is not sustainable it was never going to work anyway. As Jaclyn put it, the thing to be scared of is staying exactly where you are.

Guest Bios

Jaclyn Sklaver, MS, CNS, LDN, is a board-certified integrative sports nutritionist and founder of Athleats Nutrition, where she works with active gym-goers, competitive athletes, and everyday clients seeking sustainable performance and body composition results. Drawing on a functional medicine approach, Jaclyn goes beyond calories and macros to address the root causes behind plateaus, fatigue, and stalled progress,  from gut health and hormone balance to inflammation and recovery. Her personalized nutrition programming has supported some of the world's top athletes, including professionals in the NBA and NFL, alongside a wide range of committed gym-goers looking to train harder, recover faster, and feel better in their bodies. With over a decade of clinical experience, Jaclyn pioneered the private-practice model in sports dietetics and continues to bridge performance nutrition with whole-body, functional care. 

Emma Harvie, MS, CNS, LN, is a Licensed Nutritionist and Board Certified Nutrition Specialist specializing in functional medicine and sports performance nutrition. She brings a lifelong history of athletics to her clinical work, including as a former professional bodybuilder and current avid CrossFit athlete. Born and raised in Duluth, Minnesota, Emma has run Grandma's Half Marathon every year since she was 15 years old, now an 18 time finisher and one of her favorite hometown traditions. Drawing on her firsthand understanding of what the body demands under training and competition, she works with clients using a practical, realistic, and root cause approach, building highly personalized nutrition strategies that recognize the unique needs and lifestyle of each individual patient. Emma has had the privilege of working with NBA players, celebrities, and elite, collegiate, and Olympic athletes to reach their health and performance goals. She loves helping clients overcome barriers to health to achieve meaningful, sustainable, and lifelong improvements in well-being.

Frequently Asked Questions

How many carbs per hour should a runner take in?

The old guidance of 60 grams per hour reflected the absorption limit of glucose alone, which saturates at roughly one gram per minute. Combining glucose and fructose uses two separate transporters and raises the ceiling to 90 and even 120 grams per hour in well trained athletes. Most runners are not close to 60 grams per hour yet, so the practical answer is to work upward from wherever you are now rather than jumping to the research maximum.

What are dual source carbs and do I need them?

Dual source products combine glucose and fructose so they are absorbed through two different pathways instead of competing for one. They matter once you are trying to exceed roughly 60 grams of carbohydrate per hour. If you are taking in less than that, the delivery method makes very little difference and your energy is better spent on total intake and post run recovery.

How do I train my gut to handle more carbs?

Gut training works through two adaptations, faster gastric emptying and improved tolerance from repeated exposure. Both take time. Start at the beginning of a training block and give yourself three to six months of progressive practice during long runs rather than testing a new carbohydrate load in the final weeks before a race.

Is sodium bicarbonate worth it for endurance runners?

The available research on bicarbonate is concentrated on short sprints and high intensity intervals, where the measured gains are very small, and there is little evidence supporting a benefit for endurance events. It also carries a meaningful risk of GI distress and costs around seventeen dollars a serving. Increasing your carbohydrate intake by another 30 grams per hour is cheaper and far more likely to change your race.

What are the signs I am underfueling as a runner?

Common signals include runs that feel consistently flat, poor recovery, frequent illness or injury, stress fractures, menstrual cycle changes in women, low testosterone symptoms in men including low sex drive and mood changes, iron deficiency on lab work, and difficulty building or maintaining muscle. A slowing metabolism and unconscious energy conservation often accompany the pattern.