How to Fuel a 100 Mile Bike Ride Without Bonking (What the Research Actually Says About Carbs Per Hour)

Somewhere around mile 70 of every under-fueled century ride, the same thing happens. Your legs turn to concrete, your mood goes dark, the last 30 miles start to feel like a personal insult, and the gas station burrito at the next stop becomes the most important meal of your life. Cyclists call it bonking. Physiologists call it glycogen depletion. I call it the most preventable disaster in endurance sports.

This summer I spent weeks building and running the crew fueling system for an around the world cycling record attempt by Lael Wilcox, it was planned to be a 78 day effort where the rider needed to eat all day, every day, for eleven weeks straight. When fueling has to work at that scale, you learn very quickly what actually prevents bonking and what is just noise. The good news: the same principles scale down beautifully to your first century. So let’s talk about how to fuel 100 miles, what the research actually supports, and why the number you keep seeing on Instagram (120 grams of carbs an hour) is probably not your number.

What bonking actually is (and why you cannot store your way out of it)

A 100 mile ride at a solid endurance effort costs most riders somewhere between 3,000 and 5,000 calories, and the harder you ride, the bigger the share of that energy that has to come from carbohydrate. The problem is storage. A typical trained athlete carries somewhere between 300 and 700 grams of glycogen in skeletal muscle, plus roughly 80 to 100 grams in the liver (Jensen et al., 2011; Murray and Rosenbloom, 2018). And yes, a big, well trained rider coming off a proper carb load can push muscle stores toward 800 grams, because supercompensation after depletion can roughly double muscle glycogen (Jensen et al., 2011). But even that best case is only about 3,200 calories of stored carbohydrate, part of it locked away in muscles that are not doing the pedaling, and nobody rides down to true zero anyway. Ride at intensity for a few hours without eating and the math stops mathing.

The classic demonstration is nearly 40 years old. Coyle and colleagues had trained cyclists ride at around 70 percent of VO2max to exhaustion. With a placebo drink they lasted about 3 hours. With carbohydrate feedings they held the same power for about 4 hours, a full extra hour, and blood glucose stayed up while the placebo group’s fell (Coyle et al., 1986). Applicability caveat: that was seven trained male cyclists riding indoors at a steady effort, so the exact times will not transfer to your hilly group ride. But the core finding has been replicated for decades and reviewed extensively, including in a systematic review by Trent Stellingwerff, whose work on fueling periodization I lean on constantly: for events over about 2 hours, carbohydrate during the ride reliably improves performance, and longer events benefit more (Stellingwerff and Cox, 2014).

One more thing about the bonk, because I think it is underappreciated: it is not only a fuel gauge problem. Your brain defends your blood sugar. When carbohydrate availability drops, it starts pulling the brake on your muscles well before true empty. That is why a bonk feels emotional, not just physical. The dark mood at mile 75 is neurochemistry, not weakness.

How many carbs per hour? What the dose response research says

For years the standard advice was 30 to 60 grams of carbohydrate per hour. Then researchers actually tested the dose response. In a multi-site trial, cyclists and triathletes rode 2 hours and then did a 20 km time trial, with carb intakes ranging from 0 up to 120 grams per hour. Performance improved as intake went up, peaked at roughly 78 grams per hour, and then declined slightly at higher doses (Smith et al., 2013). Applicability caveat: those were 51 trained cyclists and triathletes, mostly men, doing a 2 to 2.5 hour effort. A 6 or 7 hour century at lower intensity is a different animal, but the shape of that curve is the most useful thing we have: more is better, up to a point, and that point is not infinity.

Why is there a ceiling at all? Absorption. Your gut can only move glucose across the intestinal wall at about 60 grams per hour because the glucose transporter saturates. The workaround, worked out largely in Asker Jeukendrup’s lab, is to add fructose, which uses a different transporter. Mixed glucose and fructose in roughly a 2 to 1 ratio pushes total carbohydrate oxidation toward 90 grams per hour, and in a direct comparison, trained cyclists riding 2 hours then racing a 1 hour time trial went about 8 percent faster with glucose plus fructose than with glucose alone (Currell and Jeukendrup, 2008). Caveat again: eight trained male cyclists, laboratory conditions. But this is why nearly every modern drink mix, gel, and chew lists both glucose (or maltodextrin) and fructose on the label, and why “dual fuel” is not just marketing.

So for a 100 mile ride, the evidence-backed target for most trained riders is 60 to 90 grams of carbohydrate per hour, leaning toward the top of that range if your gut is trained and the ride is hard, and toward the bottom if you are newer to fueling or riding at a gentler pace (Jeukendrup, 2014; Podlogar and Wallis, 2022).

The influencer corner: is 120 grams an hour the new flex?

If you follow cycling or ultrarunning social media, you have watched 120 grams per hour become a status symbol. Pro riders post their carb counts like power files, and the message trickling down to amateurs is that anything under triple digits is leaving performance on the table.

Here is what the actual research behind that number looks like. The best known studies come from a Spanish group who tested 120 grams per hour in a mountain marathon: runners taking 120 showed less muscle damage after the race than those taking 60 or 90 (Viribay et al., 2020), and better recovery of neuromuscular function (Urdampilleta et al., 2020). Genuinely interesting findings, and I take them seriously. Now the caveats, because they matter: these were 20 to 26 elite male trail runners who went through weeks of deliberate gut training before race day, and the outcome was muscle damage markers, not finishing time. Nobody has shown that 120 beats 90 for performance in a typical amateur, and the dose response work we just covered actually found performance declining above about 78 grams per hour in trained athletes who were not specifically gut trained for more (Smith et al., 2013). A recent review from Podlogar and Wallis, two researchers I trust on this topic, lands where I do: intakes above 90 grams per hour are a specialized tool with promising but incomplete evidence, not a general recommendation (Podlogar and Wallis, 2022).

On the record attempt crew this summer, we ran the numbers the boring way: a sustainable hourly carb target the rider could actually absorb day after day, built around foods she genuinely wanted to eat. Eleven weeks of fueling does not survive on heroics. Your century does not either. The athlete taking a well practiced 70 grams an hour will drop the athlete choking down an untrained 120 every single time, usually somewhere around the porta potty line.

And the opposite influencer claim deserves a word too, because it refuses to die: ride fasted, become fat adapted, and you will never bonk because you will burn fat forever. Louise Burke’s group tested exactly this in elite race walkers. The low carb, high fat group did become prodigious fat burners. They also became less efficient, their oxygen cost at race pace went up, and while the high carb groups got faster after intensified training, the keto group did not improve at all (Burke et al., 2017). When critics pushed back, the team, including Jamie Whitfield whose fat metabolism work is worth following, ran the whole study again and got the same result (Burke et al., 2020). Even the famous FASTER study of keto adapted ultrarunners found their glycogen use during a 3 hour run was essentially the same as the high carb runners (Volek et al., 2016). Caveat: race walkers and elite ultrarunners are not weekend century riders, and very long, very easy riding does lean more on fat. But fat adaptation does not exempt you from carbohydrate at intensity. Nothing does.

Train your gut like you train your legs

The single biggest reason riders cannot hit 60 to 90 grams an hour is not willpower, it is an untrained gut. The gut adapts to what you ask of it: repeatedly fueling during training increases how much carbohydrate you can absorb and tolerate, which is why Jeukendrup’s review on the topic is literally titled “Training the Gut for Athletes” (Jeukendrup, 2017). In one of the cleaner experimental demonstrations, two weeks of repetitive gut challenge during running reduced GI symptoms and improved how much carbohydrate athletes could handle (Costa et al., 2017). Caveat: that was recreational endurance runners over a short window, and gut training research is still young. But it matches what I see with clients constantly: the rider who practices race fueling for 4 to 6 weeks of long rides can comfortably take in amounts that would have wrecked them in week one.

Practically, gut training looks like this: pick your longest weekly ride, fuel it at your target rate with the exact products and foods you plan to race with, and treat any GI grumbling as training load rather than failure. Start at 40 to 60 grams an hour if you are new to this and add 10 to 15 grams per hour every couple of weeks.

The plan: fueling 100 miles, hour by hour

Here is the framework I use with cyclists, whether the goal is a first century or a gravel race.

• Start full. A century is a greater than 90 minute event, which means the day or two before are real carb loading days, 10 to 12 grams per kilogram of body weight. I wrote a full breakdown of how to do that properly, and why the pasta party does not count, in last week’s post on carb loading.
• Breakfast is part of the ride. Liver glycogen drains overnight, so eat a carb rich breakfast of 1 to 4 grams per kilogram in the 1 to 4 hours before rolling out. Simple, familiar, low fiber.
• Start eating at minute 20, not mile 40. The most common bonk I see is baked in during the first 90 minutes, when riders feel great and forget to eat. By the time you feel hungry or flat, you are chasing a deficit you cannot close, because absorption is capped at roughly 60 to 90 grams an hour no matter how far behind you are.
• Set a number and a timer. 60 to 90 grams of carbohydrate per hour from mixed sources, split into feedings every 15 to 20 minutes. On the record attempt, the system that worked was not sophisticated, it was relentless: fuel showed up on schedule whether the rider felt like it or not. Set an interval alert on your head unit and obey it.
• Use real food early, simple sugars late. Rice cakes, bananas, PB and J, and salted potatoes sit well in the first half when intensity is lower and your stomach is fresh. Shift toward drink mix, gels, and chews as the hours stack up and blood flow to the gut drops. This is also exactly how we structured all-day fueling on the crew: solid and savory while the engine is warm, liquid and simple when it is not.
• Do not forget fluid and sodium. Dehydration slows gastric emptying, which makes hitting your carb number harder. Aim for roughly 500 to 750 ml per hour depending on heat and sweat rate, with sodium in the 300 to 800 mg per liter range, adjusted to how salty a sweater you are.
• Rehearse the whole show. Your longest training rides are dress rehearsals: same breakfast, same products, same hourly targets. Race day is a terrible time for a world premiere.

A note for the women reading this

Almost everything above comes from studies of trained men: Coyle’s seven cyclists, Smith’s mostly male fifty one, Currell and Jeukendrup’s eight, Viribay’s elite male trail runners. That is not a footnote, it is a real limitation of this literature, and it is a pattern across sports nutrition research. The available evidence suggests hourly carbohydrate targets during riding work similarly across sexes when scaled to the work being done, and nothing in the data says women should fuel less. If anything, the athletes I see underfueling long rides most often are women who have absorbed years of messaging about eating less, and the fix that transforms their riding is simply fueling like the engine they are. But you deserve to know when the confidence interval around advice is wider for you, and here it is.

What I would do this weekend

If you have a century or long gravel day coming up: carb load properly the day or days before, eat a real breakfast, start fueling in the first 20 minutes, and hit a practiced 60 to 90 grams of carbohydrate per hour on a timer, with fluid and sodium alongside. Ignore anyone whose fueling advice is either “just ride fasted, burn fat” or “120 or you are soft.” Both extremes lose to the boring, rehearsed middle.

If you want this dialed to your body, your sweat rate, your gut, and your actual race file, that is what I do all day. My 1 on 1 coaching includes a personalized race fueling plan, gut training progression, and carb targets built off your real training data. More of a DIY athlete? Grab the free fueling guides and cheat sheets on our free resources page, they are the same frameworks we use with clients, including the crew systems thinking from this summer’s record attempt scaled down to one bike and one body.

Fuel like it matters, because it does.

References

Burke LM, Ross ML, Garvican-Lewis LA, et al. (2017). Low carbohydrate, high fat diet impairs exercise economy and negates the performance benefit from intensified training in elite race walkers. Journal of Physiology 595:2785-2807.

Burke LM, Sharma AP, Heikura IA, Ross ML, Whitfield J, et al. (2020). Crisis of confidence averted: impairment of exercise economy and performance in elite race walkers by ketogenic low carbohydrate, high fat diet is reproducible. PLoS ONE 15:e0234027.

Costa RJS, Miall A, Khoo A, Rauch C, Snipe R, Camões-Costa V, Gibson P (2017). Gut-training: the impact of two weeks repetitive gut-challenge during exercise on gastrointestinal status, glucose availability, fuel kinetics, and running performance. Applied Physiology, Nutrition, and Metabolism 42:547-557.

Coyle EF, Coggan AR, Hemmert MK, Ivy JL (1986). Muscle glycogen utilization during prolonged strenuous exercise when fed carbohydrate. Journal of Applied Physiology 61:165-172.

Currell K, Jeukendrup AE (2008). Superior endurance performance with ingestion of multiple transportable carbohydrates. Medicine and Science in Sports and Exercise 40:275-281.

Jensen J, Rustad PI, Kolnes AJ, Lai YC (2011). The role of skeletal muscle glycogen breakdown for regulation of insulin sensitivity by exercise. Frontiers in Physiology 2:112.

Jeukendrup AE (2014). A step towards personalized sports nutrition: carbohydrate intake during exercise. Sports Medicine 44(Suppl 1):S25-S33.

Jeukendrup AE (2017). Training the gut for athletes. Sports Medicine 47(Suppl 1):S101-S110.

Murray B, Rosenbloom C (2018). Fundamentals of glycogen metabolism for coaches and athletes. Nutrition Reviews 76:243-259.

Podlogar T, Wallis GA (2022). New horizons in carbohydrate research and application for endurance athletes. Sports Medicine 52(Suppl 1):5-23.

Smith JW, Pascoe DD, Passe DH, Ruby BC, Stewart LK, Baker LB, Zachwieja JJ (2013). Curvilinear dose-response relationship of carbohydrate (0-120 g/h) and performance. Medicine and Science in Sports and Exercise 45:336-341.

Stellingwerff T, Cox GR (2014). Systematic review: carbohydrate supplementation on exercise performance or capacity of varying durations. Applied Physiology, Nutrition, and Metabolism 39:998-1011.

Urdampilleta A, Arribalzaga S, Viribay A, Castañeda-Babarro A, Seco-Calvo J, Mielgo-Ayuso J (2020). Effects of 120 vs. 60 and 90 g/h carbohydrate intake during a trail marathon on neuromuscular function and high intensity run capacity recovery. Nutrients 12:2094.

Viribay A, Arribalzaga S, Mielgo-Ayuso J, Castañeda-Babarro A, Seco-Calvo J, Urdampilleta A (2020). Effects of 120 g/h of carbohydrates intake during a mountain marathon on exercise-induced muscle damage in elite runners. Nutrients 12:1367.

Volek JS, Freidenreich DJ, Saenz C, et al. (2016). Metabolic characteristics of keto-adapted ultra-endurance runners. Metabolism 65:100-110.