Carb Loading Still Works. Most of What You've Heard About It Doesn't.

If you train for endurance events, you've heard the same carb loading advice on repeat for thirty years. Eat a giant bowl of pasta the night before. Cut carbs first to "trick" your body into storing more later. Don't bother for ultras because you can't store enough glycogen anyway. Most of this is either outdated, oversimplified, or just wrong.

Carb loading is one of the most well-studied practices in sports nutrition. It might also be the most misrepresented. The version that actually works in 2026 looks nothing like the carb loading advice your high school cross-country coach handed down.

Here are five myths worth retiring before your next race, with some actual physiology to back up why they need to go.

Is a big pasta dinner the night before a race carb loading?

No. The pre-race pasta party is a cultural ritual, not a science-backed fueling strategy. The original carb loading research dates back to the 1960s, when Scandinavian cyclists figured out that high-carb diets dramatically extended time to exhaustion compared to low-carb ones (Bergström et al., 1967).

Trained athletes can store roughly 400 to 500 grams of muscle glycogen and another 80 to 100 grams of liver glycogen, totaling around 1,800 to 2,400 calories of stored carb energy (Burke et al., 2011). You do not deposit that overnight. Glycogen synthesis is a metabolic process that takes time, and the rate-limiting step is glycogen synthase activity, which gets upregulated over multiple days of high carb intake combined with reduced training volume.

So, a single pasta dinner adds maybe 100 to 200 grams of carbs to your day. That is not nothing, but it is also not "loading." It is just dinner. Let dinner be dinner!

The window for meaningful glycogen loading is roughly 2 to 3 days of carb intake in the range of 7 to 12 grams per kilogram of body weight per day, paired with a taper. Cramming a plate of fettuccine the night before your marathon will mostly leave you bloated, gassy, and contemplating your life choices at the 4 a.m. start line.

Do you need a depletion phase before carb loading?

The original carb loading protocol involved a few days of intense training on a low-carb diet to "deplete" glycogen, followed by a few days of high-carb refeeding. This idea is still floating around the internet like a bad meme. The depletion phase is unnecessary for most athletes and, frankly, counterproductive.

The depletion-then-load model came from research on relatively untrained subjects, where depletion seemed to enhance glycogen supercompensation. But trained endurance athletes already have elevated baseline glycogen synthase activity and higher resting glycogen stores. Sherman and colleagues showed in 1981 that trained athletes could fully saturate their glycogen reserves with three days of high carb intake and a taper, with no depletion phase required (Sherman et al., 1981). More recent work has confirmed and expanded this: a modified protocol of 36 to 48 hours of carb intake at 10 to 12 g/kg/day with reduced training is sufficient for most well-trained athletes to maximize muscle glycogen (Bussau et al., 2002).

The depletion phase adds metabolic stress, increases GI risk, makes your last hard workouts feel terrible, and provides no measurable performance benefit for athletes who already train regularly. Skip it. Eat the carbs.

Does carb loading work for ultramarathons?

This one comes up constantly in ultra circles, and it deserves a careful response. The premise contains a kernel of truth. Liver and muscle glycogen do start getting tapped within the first few hours of hard effort, and at high intensities, you can functionally deplete muscle glycogen in 60 to 90 minutes. But the conclusion that carb loading is therefore pointless for a 15-hour event is dead wrong.

Carb loading does not exist to fuel hour 12 of your race. It exists to maximize your starting reserves so you draw from a fuller tank before you start relying more heavily on fat oxidation and exogenous carbs (which you should obviously be consuming for longer events, longer being over 90 minutes).

Here is the underappreciated piece of physiology: your gut can only absorb so many carbs per hour. Glucose alone tops out around 60 grams per hour due to saturation of the SGLT1 transporter, and you can push that to roughly 90 grams per hour by combining glucose with fructose, which uses a separate transporter (GLUT5) (Jeukendrup, 2014).

That ceiling matters. Even with perfect in-race fueling, your gut cannot keep up with the rate at which your working muscles are pulling fuel. The gap between what you absorb and what you burn has to come from somewhere, and that somewhere is endogenous glycogen. Starting depleted means dipping into that buffer earlier, which accelerates fatigue and degrades fueling tolerance later in the race. Starting topped off means you have more runway before things get hard. For long efforts, that matters most at the start when intensity is naturally a little higher, and it supports your gut's ability to actually absorb and use carbs throughout the race.

If zone 2 burns fat, do ultra runners still need carbs?

This is the myth that has spread fastest through endurance running circles in the last few years, and it confuses two different things. Yes, the relative contribution of fat oxidation increases at lower intensities. No, that does not mean you are running on fat alone.

Maximal fat oxidation typically peaks somewhere between 50 and 65 percent of VO2max, depending on the athlete (Achten & Jeukendrup, 2003). Even at that peak, fat oxidation maxes out at roughly 0.5 to 1.0 grams of fat per minute, which translates to around 270 to 540 calories per hour. For most ultra athletes moving at race effort, that is not enough to cover total energy demand. Carbohydrates fill the gap. Even highly fat-adapted athletes still oxidize substantial carbs during prolonged exercise (Volek et al., 2016). The ratio shifts, but the total need does not disappear.

And here is the part that gets ignored: your brain runs almost exclusively on glucose. The brain consumes roughly 120 grams of glucose per day at rest, and that demand goes up under physical and cognitive stress (Mergenthaler et al., 2013). At hour 10 of a 100k, cognitive function shapes every decision you make, including the decision to keep moving. Navigating the course, timing your fueling, deciding whether that ankle pain is a real injury or just regular ultra suffering. All of that is glucose-dependent. Central nervous system fatigue, also known as why your brain feels like soup at mile 50, is partly mediated by reduced glucose availability. A depleted brain makes worse calls than a fueled one.

Is carb loading only for elite athletes?

Back-of-pack runners get told constantly that fueling protocols don't apply to them because they are not fast enough to need optimization. This is nonsense, and you're missing out on valuable performance gains AS WELL AS PIZZA.

Total energy demand for an endurance event is a function of body mass, distance, and time. A 15-hour 100k effort burns more total calories than a 9-hour one because the athlete is on their feet longer. Slower paces do reduce calorie burn per minute, but they do not reduce it proportionally to the time increase. Mid-pack and back-of-pack athletes are out there longer, which means more total glycogen demand, more cumulative GI exposure, and more total cognitive load. They should arguably take pre-race carb loading more seriously, not less.

How to carb load without giving up after one attempt

Here is the pattern we see all the time on the podcast and in Kylee's practice. Someone reads about carb loading. They eat what they think is a lot of carbs for a couple days. They feel different in their body, the scale moves, and they decide carb loading does not work for them. They never actually checked how many grams of carbs they ate, what kind, or whether what they were noticing was the protocol working as designed.

A few things to know up front.

First, on the physiology of how it feels. Glycogen is stored alongside water. Each gram of glycogen in your muscles binds roughly 3 grams of water (Olsson & Saltin, 1970). If you successfully load 400 extra grams of glycogen, you are also storing about 1.2 kilograms of additional water in your muscle tissue. That is not bloat, and it is not fat. It is functional intramuscular hydration that gets released as you burn through glycogen during the race. The fullness, the slight heaviness, the small movement on the scale: those are signals the protocol is doing what it is supposed to do, not signs that something is wrong. If race-week scale numbers are not useful information for you, this is a fine week to step away from weighing yourself entirely. You will not need that data to execute the protocol well.

Second, almost nobody actually eats as many carbs as they think they do. Before assuming carb loading does not work for you, you have to know your baseline. Track your normal intake for three to five days using whatever tool you tolerate (apps, paper, voice memos, whatever you find sustainable). Convert your typical daily carb intake to grams per kilogram of body weight. Most endurance athletes who think they "eat a lot of carbs" land around 4 to 5 g/kg/day in normal training. A real carb load means going to 7 to 9 g/kg/day at minimum, which is a substantial increase. Without that baseline, you are loading blind and likely undershooting the actual target.

Third, do a dress rehearsal. Pick a key long run two to four weeks before your goal race and run a full carb loading practice in the 2 to 3 days leading up to it. Use the same target grams per day, the same food sources, the same timing. This tells you which carb sources sit well in your gut, how much volume you can actually consume, and how your body responds to the protocol in lower-stakes conditions. The first time you experience the full carb loading process should not be the night before a goal event.

For sources, lean on familiar, easily digestible carbs. White rice, white bread, regular pasta, potatoes, bananas, pretzels, bagels, low-fiber cereals, hydration mixes, and sports drinks are the workhorses. Pull back on high-fiber and high-fat options for those few days, because reducing fiber temporarily gives your gut less to process and lowers the odds of a race morning bathroom emergency. Spread carbs across all your meals and snacks rather than back-loading into a single dinner. Eight smaller carb-rich snacks are easier to absorb than three giant ones.

On race morning, eat a carb-forward meal 2 to 3 hours before the start, lower in fat and fiber. The goal is topped off, not stuffed. And then the part most people still get wrong: keep eating during the race. Consistent intake of 60 to 90 grams of carbs per hour during long endurance events is what protects the work you did pre-race. Carb loading is the foundation. In-race fueling is the actual building.

The bottom line: does carb loading still work?

Yes. It is not the giant pasta dinner of 1985, and it is not irrelevant for ultras. The current evidence supports a 2 to 3 day window of increased carb intake at 7 to 12 g/kg/day, paired with a taper, with practical adjustments to fiber and fat. It applies to every endurance athlete regardless of pace, and it is most effective when paired with consistent in-race fueling. Track your baseline, expect water weight, practice once before race week, and stop trusting Reddit's metabolic takes.

[Link a carb loading or fueling episode by name here if one exists. If not: "We're going deep on this one on the podcast. Subscribe so you don't miss it."]

If you want to run your own numbers past an actual dietitian, the weekly Nutrition Question Thread on Patreon is where Kylee answers exactly this kind of thing, and where "wait, how many grams is that in bagels" gets asked more than you'd think.

Sources

Achten, J., & Jeukendrup, A. E. (2003). Maximal fat oxidation during exercise in trained men. International Journal of Sports Medicine, 24(8), 603–608. https://doi.org/10.1055/s-2003-43265

Bergström, J., Hermansen, L., Hultman, E., & Saltin, B. (1967). Diet, muscle glycogen and physical performance. Acta Physiologica Scandinavica, 71(2–3), 140–150. https://doi.org/10.1111/j.1748-1716.1967.tb03720.x

Burke, L. M., Hawley, J. A., Wong, S. H. S., & Jeukendrup, A. E. (2011). Carbohydrates for training and competition. Journal of Sports Sciences, 29(sup1), S17–S27. https://doi.org/10.1080/02640414.2011.585473

Bussau, V. A., Fairchild, T. J., Rao, A., Steele, P., & Fournier, P. A. (2002). Carbohydrate loading in human muscle: An improved 1 day protocol. European Journal of Applied Physiology, 87(3), 290–295. https://doi.org/10.1007/s00421-002-0621-5

Jeukendrup, A. (2014). A step towards personalized sports nutrition: Carbohydrate intake during exercise. Sports Medicine, 44(Suppl 1), S25–S33. https://doi.org/10.1007/s40279-014-0148-z

Mergenthaler, P., Lindauer, U., Dienel, G. A., & Meisel, A. (2013). Sugar for the brain: The role of glucose in physiological and pathological brain function. Trends in Neurosciences, 36(10), 587–597. https://doi.org/10.1016/j.tins.2013.07.001

Olsson, K. E., & Saltin, B. (1970). Variation in total body water with muscle glycogen changes in man. Acta Physiologica Scandinavica, 80(1), 11–18. https://doi.org/10.1111/j.1748-1716.1970.tb04764.x

Sherman, W. M., Costill, D. L., Fink, W. J., & Miller, J. M. (1981). Effect of exercise-diet manipulation on muscle glycogen and its subsequent utilization during performance. International Journal of Sports Medicine, 2(2), 114–118. https://doi.org/10.1055/s-2008-1034594

Volek, J. S., Freidenreich, D. J., Saenz, C., Kunces, L. J., Creighton, B. C., Bartley, J. M., Davitt, P. M., Munoz, C. X., Anderson, J. M., Maresh, C. M., Lee, E. C., Schuenke, M. D., Aerni, G., Kraemer, W. J., & Phinney, S. D. (2016). Metabolic characteristics of keto-adapted ultra-endurance runners. Metabolism, 65(3), 100–110. https://doi.org/10.1016/j.metabol.2015.10.028