Should You Train on a Low-Carb Diet? Experts Have Strong Opinions
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It all started in 1983. Researcher Stephen D. Phinney and a team of other sports nutrition scientists published a paper detailing a “dramatic physiological adaptation” that resulted in endurance athletes utilizing fat as fuel for exercise, allowing their bodies to spare carbohydrates (in the form of both blood sugar and muscle glycogen) during training.
This groundbreaking paper point-blank declared that four weeks of keto dieting did not compromise endurance training performance; rather, the research suggested that a low-carb diet could unlock a new realm of endurance performance. Indeed, the five athletes studied in the Phinney et al paper exhibited no decline in performance at submaximal training levels (62 to 64 percent of VO2 max) after a month of low-carb dieting.
These findings spiraled into ubiquity and became accepted largely as fact, but proponents of keto diets for endurance athletes often leave out this important bit from the research: “The price paid for the conservation of carbohydrate during exercise appears to be a limitation of the intensity of exercise that can be performed… a throttling of function near VO2 max.”
According to the accumulated body of evidence and modern sports dietitians, “train low, compete high”—a nutrition strategy based largely on the concept of fat adaptation shown in Phinney et al’s study—is dated advice and could be impairing your performance.
Train Low, Compete High: What It Means
The concept is straightforward: Restrict carbohydrates during training, and consume a surplus of them leading up to competition. The “train low” part of the idea posits that by restricting carbohydrate intake for prolonged periods, your body will become “fat-adapted” and use fats for fuel, sparing carbohydrates and saving blood glucose and muscle glycogen for when it matters: during high-intensity spurts of exercise.
The “compete high” part of the idea suggests that increasing carbohydrates after a period of restriction will result in a twofold benefit: Your body will have become efficient at using fat for fuel, and will also benefit from having carbohydrates present. Research doesn’t support the idea that this improves performance, though.
Namrita Brooke, PhD, RDN, endurance coach, sports dietitian, and Bicycling advisory board member explains: Nutrition for endurance performance relies heavily upon having metabolic flexibility—that is, the ability to switch between fuel sources—to maximize usage of both carbohydrates and fats as fuel sources during training or a race.
“During exercise, intensity and duration are the main determinants of fuel source, with carbohydrates being the preferred source as intensity increases,” she says. “However, the storage capacity of carbohydrate in the body is limited, unlike fat.”
Knowing that, curiosity got the best of humans, and scientists have thus been exploring strategies to improve fat utilization in order to spare carbohydrate utilization for decades.
What Science Says About Low-Carb Diets for Endurance Performance
According to the current body of scientific evidence, we have two truths:
Low-carb diets can result in an athlete’s body prioritizing fat for fuel over carbohydrates, also known as becoming fat-adapted, and
Low-carb diets do not result in significant performance improvements among endurance athletes, especially at the elite level.
How can both things be true if fat adaptation is thought to be a good thing? The main reason is that fat adaptation appears to be limited to low-to-moderate exercise intensities.
A leading researcher on the topic, Louise Burke of the Australian Institute for Sport, wrote in a 2023 paper that, in general, adaptation to a keto diet can preserve carbohydrates at intensities of 60 to 70 percent of an athlete’s VO2 max. However, Burke wrote that compromised performance at higher intensities (greater than 80 percent of VO2 max) is of the “greatest relevance.”
The 2023 paper points out four studies that showed “remarkably consistent performance impairments” in sanctioned races, noting losses in exercise economy, and therefore, in race speed.
While, yes, some individual studies do suggest that being in ketosis may improve endurance (like one 2016 study in Cell Metabolism), there’s another one around the corner refuting it (like a 2020 study in the Journal of Physiology).
As with all things, if you dig deep enough into the body of research, you can find a single study to support almost anything you want to believe in. But if you consider the entire body of literature on this topic, you’ll find—as Burke pointedly notes in the 2023 paper—there’s not enough evidence to support keto diets as an all-around improver of endurance performance.
The Benefits of Carbs for Endurance Athletes
Carbohydrates are our body’s main energy source, says Linda Steinhart, RD, CSSD, sports dietitian for the Athlete EDGE program at EDCare.
“If someone limits or restricts carbohydrates, their body has to make an adjustment to become more ‘efficient’ at using fat, but it is helpful to remember that just because our body can make an adjustment, doesn’t mean it is beneficial for health or performance,” Steinhart says.
Here, how carbs can support your rides:
Quick Energy
Carbohydrates provide energy the quickest of all of the macronutrients, Steinhart says. “Dietary fats take longer to produce energy from, while we can make energy from carbohydrates very rapidly,” she says.
Steinhart also points out that research shows that powerful and explosive movements (like sprinting) are compromised if people don’t have enough carbohydrate available, even if they are fat-adapted.
Support for Recovery
Adequate carb intake is critical not just for performance, but for recovery, too. Restoring glycogen post-exercise is considered a fundamental pillar of endurance nutrition. Failing to do so sets you up to begin your next training session with low glycogen availability, which is likely to negatively affect performance, especially if the planned session is supposed to be high-intensity.
Improved Health
Steinhart says that low carbohydrate availability is an independent risk factor for RED-S (relative energy deficiency in sport), which is characterized by bone density issues, impaired iron metabolism, decreased decision making ability and cognitive function, compromised muscle repair, and more.
Strength Building
There are a lot of reasons cyclists and other endurance athletes should participate in strength training, and according to a 2022 systematic review, carbs can help you get the most out of those resistance training sessions.
The study notes that this is especially true if you’re coming off of an eight-hour fast (such as if you’re training first thing in the morning) or training for more than 45 minutes. Carbohydrate is the body’s preferred fuel for high-intensity, short-duration exercise, including high-effort bouts of lifting.
The Connection Between the Intensity of Cycling and Carb Requirements
“Train low, compete high” might work if you performed 100 percent of your training in zone 2 and planned to compete in zone 2, as well. That’s just not the reality for any endurance athlete. Becoming a well-rounded cyclist means training at various intensities, and competing at higher intensities than you train.
Cycling is a dynamic sport that requires frequent bursts of intensity, explains Brooke. This means that potential consequences of low carbohydrate intake include decreased performance capacity, in part explained by reduced glycogen stores and/or reduced capacity for carbohydrate oxidation. (Carbohydrate oxidation is the system by which your body breaks carbohydrates down into usable energy to support exercise.)
Plus, Brooke points out that carbohydrate is still an important energy source during endurance rides.
“Even though you might be riding in or near your ‘fat max’ zone, carbohydrate is still being utilized for fuel, and this adds up the longer the ride goes,” she explains. “If you’re doing these sessions with low carbohydrate availability, you could be setting yourself up for compromised performance and possibly other health consequences associated with low energy availability or low carbohydrate availability.”
As exercise duration increases—that is, the longer you ride—your body begins to rely more heavily on sugars circulating in your blood because stored carbohydrates (glycogen) begin to run low. So, the effect can be twofold:
If your body doesn’t have sufficient stored carbohydrate to begin with, it’ll run through those stores quicker and turn to reliance on blood glucose.
At that point, if you aren’t taking in carbohydrates, you may face the “bonk”—hitting a point of low energy that’s nearly impossible to return from if you continue exercising.
Jena Brown, RD, CSSD, endurance dietitian at Victorem Nutrition, explains that even endurance rides often require metabolic flexibility due to hills, wind, and sometimes traffic, depending on the environment in which you’re training. It’s not uncommon for an overall low-intensity session to include bursts of higher intensities within the session, she says.
“Some amount of carbohydrate availability, either from stored glycogen in the muscles or exogenous sources such as gels or sport hydration products, are likely needed to sustain those intermittent efforts,” Brown says.
If you insist on trying out a “train low” protocol, Brown notes that it would be easier to perform such sessions on an indoor trainer where conditions can be controlled.
Mental Health Considerations When It Comes to Cutting Carbs
Not only can limiting carbohydrate intake negatively affect your athletic performance, it could also have major implications for your psyche.
“Any form of restriction is a huge risk for disordered eating and eating disorders,” says Steinhart. “Going on any form of diet is the number-one risk factor for and predictor of eating disorders. Some research suggests that as many as 1 in 2 female athletes and 1 in 5 male athletes have some form of disordered eating.”
Taken together with the current body of research that shows no significant performance benefit of limiting carbs, Steinhart says she recommends that athletes eat adequate amounts of all macronutrients.
Brown encourages athletes to be aware of potential issues with persistent fatigue and lack of focus, too. It’s possible, she says, that a “train low” approach can lead to energy deficiency and poor recovery.
“If you do not eat properly before and after sessions, your high-intensity efforts and overall training quality will be compromised,” she says. “Not only will your performance decline, but you are also setting yourself up for injury, less mental focus, and low energy availability which leads to many other health concerns.”
A Better Nutrition Approach: Fuel for the Work Required
Brown recommends that athletes adjust carbohydrate intake each day based on the day’s training demands—but also look forward to upcoming sessions if they are wanting to achieve metabolic flexibility and peak performance.
Broadly, longer-duration work and higher-intensity work requires more carbohydrates to sustain performance. Athletes can time carbohydrate intake so it’s in their system before and during training to optimize both intake and performance.
As an example, Brown says that a rest day could look like moderate amounts of carbohydrate at each meal throughout the day, with dinner having more carbohydrates if the next morning’s session includes high-intensity training. (Don’t forget to have breakfast, too, as there’s little evidence supporting fasted exercise for endurance athletes).
According to the National Academy of Sports Medicine and the latest position stand of the Academy of Nutrition and Dietetics (AND), Dietitians of Canada (DC), and the American College of Sports Medicine (ACSM), carbohydrate intakes for athletes should range from:
3 grams of carbohydrates per kilogram of bodyweight per day for low-intensity days up to
12 grams of carbohydrates per kilogram of bodyweight per day for athletes with high training loads of four to five hours
If you’re unsure what ideal carb intake looks like for you, it’s always best to consult a qualified sports dietitian.
Bottom Line: Ditch the Potential Risks and Enjoy Your Carbs
Steinhart encourages athletes to consider the overall body of sports nutrition evidence. “If we take a step back, what science tells us is that adequately fueled, appropriately trained and rested athletes will perform best, feel more motivated for training, and have fewer injuries,” she says.
On top of that, adequately fueled athletes are more likely to “have a balanced and flexible relationship with food that serves them both as athletes and as humans outside of sport,” Steinhart says. “As a sports dietitian, I want people to know that there is always risk when restricting any food group, whether that risk be performance-related, mental health-related, or injury risk-related.”
While experimentation with nutrition and performance on a personal level isn’t inherently bad—athletes should absolutely partake in trial and error to find what works for them—science does not, at this time, support low carbohydrate intake for improved athletic performance in endurance sports.
So, yes, you actually can have your cake and eat it, too: Eat carbohydrates and enjoy your subsequently best performances.
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