Fueling your body is often compared to putting gas in a car. The only problem is, your body is not a car—you don’t just have a full tank or an empty tank—so that overused trope falls flat when it comes to human performance.

The human body is more sophisticated than that. It burns two kinds of essential macronutrients for fuel: fat and carbohydrates. (You can burn protein, a.k.a. your muscle tissue, for fuel through a process called gluconeogenesis, but its far better to fuel appropriately so you don’t force your body to essentially eat itself.) And you can actually train your body to be better at burning one fuel source or the other. How far and fast you go depends on the fuels you put in and how effectively and efficiently you can access and process those fuels.

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Hence athletes’ fixation on carb manipulation in sport; we all have ample stores of fat, but our carb stash is always limited. That’s why we are in a constant quest to nail our carbohydrate intake and utilization, whether it’s by carb-loading, carb cycling, bonk training, carb restriction, or full-on ketogenesis. We want to have all the energy we need on board and to be the best all-around fuel-burners we can be.

Here’s what you need to know about carb manipulation.

First, why do you need carbs?

Your body turns stored carbohydrates and glucose found in your muscles, liver, and brain into glycogen—and that’s what your body burns to fuel hard, higher-intensity efforts. Glycogen also assists in the body’s fat-burning process. If you don’t have enough, your body breaks down muscle tissue and amino acids to make glucose—and as mentioned earlier, this is an undesirable process.

To sum it up in three words: “Glycogen is gold,” says Iñigo San Millán, Ph.D., a researcher at the University of Colorado Sports Medicine and Performance Center (CUSM&PC) in Boulder, Colorado.

Glycogen also regulates muscle calcium function, which you need for muscle contraction. So when your glycogen/glucose levels decline, so does your power output. If you want to ride far and/or fast, you need at least some carbs.

Why not just eat all carbs, all the time?

For a while, people went way overboard on the carbs, forgetting that, again, we’re not cars and we burn more than one type of fuel. It’s a sliding scale, not a flip of the switch: During low-intensity exercise, you burn mostly fat, and few carbs. As you turn up the intensity, your body increasingly uses proportionately more carbs (a.k.a. glycogen) and less fat.

Because your carb stores are limited (you can only store about two hours’ worth) and your fat stores are nearly bottomless, it’s in your best interest to become the best fat-burner you can be. That way, you can spare your precious glycogen stores for when you really need them to go faster and harder for longer. (And you won’t need to stuff as many snacks in your pockets.)

That’s where carb manipulation comes in.

Okay, so what is carb manipulation?

Carb manipulation—an umbrella term that includes carb periodization, carb loading, and carb restriction—is a way to intentionally adjust the amount of carbs you take in, especially in and around exercise, to maximize the fuel you burn.

The premise is that by limiting your carbs sometimes, your body has to rely on fat as the primary fuel source, so you become a better fat-burner. It also helps your body be more sensitive to insulin so you can more effectively and efficiently use carbs when you do put them in the tank.

But there are some important caveats here: You can’t just stop eating all carbs and expect your body to continue to work efficiently, or you can end up undermining your training, which is counterproductive. You also need to train your gut to process what you’ll be eating on race day. If you want it to be able to process a lot of carbs, that needs to be part of your training, too.

Finally, and maybe most importantly, the act of training and becoming more aerobically fit makes you a better fat-burner. Riding your bike long and/or hard already trains your body’s ability to burn any fuel for the energy it needs—that’s part of the training process. Some of these techniques might help fine-tune some of those processes, but the term “marginal gains” applies. That said, if you end up compromising your training, you could end up with more than marginal losses.

A healthy, balanced diet trumps finicky macronutrient manipulation, especially for most recreational athletes who have plenty of room for improvement simply by getting enough sleep and following a structured training plan. So approach any carb manipulation wisely.

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What are the most popular forms of carb manipulation (and do they work)?

Athletes have been manipulating their carb intake for improved performance for decades. Some of the earliest research connecting carbs and endurance performance came in 1967 when a team of Swedish researchers had nine volunteers exercise to exhaustion in a lab while monitoring their glycogen reserves. They concluded, “the capacity for prolonged work is directly correlated to the glycogen store in the working muscles.”

Thus, carb manipulation was born, and we’ve been doing it ever since. Here are the most common forms, starting with the OG: carb loading.

Carb Loading

This oldie is still a goodie, but it’s been modified over the years. In the early days, you would try to ride your glycogen stores dry and then eat a lasagna sandwich chased down with a few dinner rolls (which really was never a good idea).

“The old thought of doing a few days of ‘glycogen depleting’ exercise followed by one to three days of nailing bowls of pasta is gone,” says Andy King, Ph.D., postdoctoral research fellow in the exercise and nutrition research program at Australia Catholic University.

Instead, you just want to eat the same total amount of food, but shift your diet to have a higher percentage of carbs for 24 to 48 hours before your event. “The recommendations are for 10 to 12 grams of carbs per kilogram of body mass (i.e. 750 grams for a 150-pound rider), but in reality, this is a lot of food,” King says.

Indeed. Unless you’re a pro rider heading out for a grand tour, simply prioritizing carbs in your meals will work just fine. You also really only need to carb load if you’re going to be going really hard and/or long (and those levels vary by person). It also won’t work to suddenly stuff yourself full of carbs if you’re typically a low-carb eater. This approach works best for those eating a typically mixed macronutrient diet.

Intermittent Fasting (IF)

Intermittent fasting is when you cycle between periods of eating and not eating, restricting the times you eat into a relatively short window. There are many ways to do it, but cyclists often accomplish it by skipping breakfast and exercising on an empty stomach in a fasted state.

There is some evidence that this might promote better blood sugar control and weight loss, especially in sedentary people. But the research is far from settled and looks far less promising for women. In fact, one study published in Obesity Research found that while IF improved insulin sensitivity in men, women saw no such improvement. Their glucose tolerance actually got worse when they practiced intermittent fasting.

“Fasting has adverse effects on a woman’s endocrine system, so your thyroid function slows down and takes your metabolism with it. Your body is trying to preserve your energy,” says female performance physiologist Stacy Sims, Ph.D., a research associate with AUT University in New Zealand. “This picture gets worse for women who exercise. If you exercise on top of fasting—which is what many women try to do to lose weight—all of these negative effects are exacerbated.”

Though some research suggests that IF may trigger genetic adaptations that make you a better fat-burner overall, that science also gets fuzzy when we start trying to apply it to athletic performance. A 2020 study titled Exercise Training and Fasting: Current Insights concluded that “there is little evidence to support the notion of endurance training and fasting-mediated increases in fat oxidation.”

If you’re still keen to give it a go, the researchers recommend avoiding high-intensity training while fasting, as some studies find that the practice can decrease performance.

“I can’t see it working better than a typical, well-balanced nutritious diet,” King says.

Bonk Training

Bicycling once called this a “non-recommended...dumb way” to get in pro cycling shape fast. It’s a technique that was (and maybe still is) used by some riders in the pro peloton in which you go out one day and do a long, hard, glycogen-draining ride; skip your post-ride recovery, eating just a bit in the hours that follow and then go out the next day and go for a bonk on a hard hour-long ride in a nearly fully depleted state.

Leave this one in the archives. There are just too many opportunities for it to send you backward rather than help you make gains, King says.

“The systemic stress is so high, you risk nullifying your training adaptations. It’s far better to stick to the basics of correct energy intake,” he says.

→ Carb Cycling / Carb Periodization

The idea behind carb cycling is that you periodize your carb intake much as you would (and in concert with) your training. The goal is to train your body to tap into your fat stores and keep cranking when you’re running low on carbs, but also to be able to accommodate all the carbs you plan to use during your actual event. Here’s how this is often done:

  • Start out fueling with plenty of carbs during your long rides at the beginning of a training cycle (like the first few weeks of getting ready for a big event).
  • After a few weeks, scale back on the carbs for your long rides in the middle of the training cycle.
  • Toward the end, do some low-carb long rides.

During this whole process, make sure to include long rides during which you fuel just as you plan to fuel during the event with ample carbs.

This approach can also be done by carb cycling within a week, going low-carb for some sessions and higher for others, rather than strictly periodizing it over a course of several weeks.

From a research standpoint, studies confirm that there are substantial increases in the rates of fat burning after training low on carbs and glycogen. But when you look at performance itself, the manipulation doesn’t always equal gains. Research comparing carb periodization and regular high-carb training among elite race walkers showed that both groups improved their times over a 10K race with the high carb trainers making slightly greater gains.

Still, some athletes find that this type of training helps psychologically. If you’ve done some long rides without carbs, you build confidence knowing you have the metabolic flexibility to make it to the finish line even if you botch your nutrition a bit during a long event.

If you want to give carb cycling a try, the best approach is a common-sense approach: Fuel for the work you need to do. If you’ve got a hard interval day on tap, eat your carbs. If you’re just going out for an hour or hour-and-a-half for a moderate-intensity ride, skipping the pre-ride carbs and training low can help you tap into those fat stores.

→ Train High, Sleep Low

Recognizing that you need carbs to nail your high-intensity workouts, some have opted for the “train high, sleep low” approach. This strategy involves performing a high-intensity training (HIIT) session in the late afternoon with plenty of glycogen on board. Afterward, you avoid restocking your stores with carbs, so you can maintain that glycogen depletion overnight. You cap this off with a low- to moderate-intensity session in the morning after your overnight fast.

The idea is to get the best of both worlds, getting the training benefits of HIIT sessions supported by high-carb availability while triggering the metabolic adaptations of having low glycogen availability. So it’s similar to intermittent fasting (and carries the same caveats), but with a high-carb training twist.

A 2016 study on 11 highly-trained male cyclists performing either the sleep-low strategy or who ate carbs normally throughout their training for one week found that the sleep-low group improved performance by about 3 percent over the continuous carb consumers in a 20 km time trial. Interestingly, the sleep-low group didn’t show any significant changes in their fuel metabolism (a.k.a. they didn’t seem to become better fat-burners). But they did crank out higher power for the same perceived exertion in the second half of the TT. The researchers hypothesize that the benefit may come from improvements in glycogen storage capabilities among those who had manipulated their carb intake during training.

The obvious caveat here is that this is a pretty convoluted approach to training and this research was done on a very small group of elite-level male cyclists. Chances are, most of us could make equal or greater gains just by eating well, sleeping well, and adhering to a structured training plan.

→ Delayed Recovery

The newest study (published in February 2021) to the carb manipulation mix examined a concept called “delayed recovery.” The idea here is that research shows training in a low-carb state can trigger genetic adaptations that boost your endurance fitness. But not having adequate carbs compromises your training sessions. One potential solution: Train hard with all the carbs you need, but then delay your recovery for a few hours to force your body into that carb-depleted state for a longer period of time.

For this study, the researchers had eight recreationally active men perform two cycling sessions four weeks apart. Each session consisted of 60 minutes of steady-state riding followed by six 30-second all-out sprints. The riders then received either a carb-rich recovery drink or a fake carb-free recovery drink, and the researchers measured their muscle glycogen levels and genetic activity among key genes related to mitochondria production and oxygen metabolism (both of which improve aerobic fitness performance) three hours after they were done.

The results were a wash. Delaying recovery didn’t trigger any additional molecular genetic response. All it did was slow down the recovery process.

Of course, this is only one small study, so there’s no hard and fast conclusion, King says. “I’m happy to reserve judgment, but it appears not to work.” And like the other manipulations, it has the potential to actually set you back by delaying the recovery process.

→ Ketogenic / Low-Carb, High-Fat Diet

Of course, there are those who manipulate their carbs by paring them down to the bare minimum, à la ketogenesis or eating a low-carb, high-fat (LCHF) diet.

There is good scientific evidence that athletes following a LCHF diet see substantial increases in their ability to burn fat for fuel, maybe in as little as five to 10 days. The problem is you end up compromising your ability to use carbs during exercise, which ultimately reduces your capacity to perform high-intensity exercise.

Research shows athletes following a LCHF diet can perform well during low-intensity exercise tests, but have a dramatic decrease in performance compared to high-carb consumers once the intensity ramps up to over 80 percent of VO2 max or during high-intensity sprints.

“There’s a small portion of athletes who can train well on it, particularly those doing slow ultra-type events, but personally I’m not a fan,” King says.

The Bottom Line on Carb Manipulation

Though a little carbohydrate manipulation, such as adding a few easy to moderate low-carb sessions into your training, can have some benefits, it’s easy to send your progress backward or hinder your training when you attempt too much manipulation.

Sticking to a well-balanced diet that fuels your workouts, regulates your mood, and helps you sleep well instead of obsessing over carb-counting is far better for recreational, competitive, and (research is showing) maybe even most pro athletes. That way, you can take all that energy you would spend tracking food and channel it into quality training instead.

“I think for many people, the message is simple: get the basics of your nutrition and training dialed in without looking for a supplemental trick or silver bullet,” King says.

Headshot of Selene Yeager
Selene Yeager
“The Fit Chick”
Selene Yeager is a top-selling professional health and fitness writer who lives what she writes as a NASM certified personal trainer, USA Cycling certified coach, Pn1 certified nutrition coach, pro licensed off road racer, and All-American Ironman triathlete.