Nutrition guidelines consistently evolve, and athletic dietary recommendations are no exception. While traditional sports nutrition has historically stressed high-carbohydrate meals (e.g. pasta, bread, and sports drinks), experts are now considering low-carbohydrate diets and their potential to enhance athletic performance. The ketogenic diet, perhaps the most extreme low-carbohydrate eating pattern, has grown increasingly popular—but is it safe and effective when combined with exercise?
Conventional Sports Nutrition
Gatorade’s successful debut among University of Florida football players in the 1960s prompted researchers to begin investigating connections between athletic performance and food. The notion that both carbohydrates, in the form of simple sugars, combined with electrolytes, in particular sodium, would successfully power athletic pursuits, notwithstanding type or intensity, became widely accepted.
This rationale makes sense, as the human body’s default energy source is carbohydrate. Through processes called glycolysis, the Krebs Cycle, and the electron transport chain, humans (and most other organisms) use simple carbohydrates and oxygen to create the molecule ATP, essentially the body’s “energy currency.” The body extracts energy from carbohydrates relatively easily and rapidly during aerobic (requiring oxygen) activity, but it can also anaerobically (without oxygen) ferment carbohydrates to produce ATP during high intensity physical activity when oxygen is absent. So why advocate fat as fuel instead?
Advocating Fat as Fuel: What the Science Says
The ketogenic diet is a high fat, moderate protein, and very low carbohydrate diet (~5% of daily calories)—defying traditional sports nutrition recommendations. After an adaptation period, ketogenic dieters preferentially burn fat, not carbohydrates, as their primary subsistence. This initial adaptation period’s duration varies between people, but generally requires between one and four weeks.
Employing fat as fuel works differently than carbohydrates. Rather than glycolysis, the Krebs Cycle, and the electron transport chain, the body performs ketogenesis. This aerobic process ultimately converts fat molecules from food into smaller molecules known as ketones. Ketones then power ATP production in lieu of carbohydrates, ultimately driving the body’s energy needs, including supporting exercise.
Research investigating the ketogenic diet fueling and possibly enhancing sport performance has skyrocketed over the last decade. The rationale is in the diet’s heart: fat. One gram of fat versus one gram of carbohydrate or protein provides nine calories versus four, respectively. Therefore, harnessing the body’s fat stores (approximately 140,000 calories worth for a healthy male) can create, theoretically, “endless energy.” Accordingly, rather than needing consistent refueling breaks, a fat-adapted, ketogenic athlete or exercise enthusiast could sustain physical activity longer than his/her carbohydrate-adapted peer.
However, efficiency and effectiveness are the crux of the matter. Consider casual exercisers who do not need abundant energy reserves, is a keto diet the best option or is a carbohydrate-based eating pattern preferable? And power athletes require fast busts of energy. Since insufficient oxygen is available to facilitate ketogenesis during extremely high intensity workouts, does the keto diet adequately support fitness fanatics and high-power athletes? Examining the science offers some clarity.
Keto for Untrained Individuals or Those New-to-Exercise
One study published in Obesity fed two 30-person groups of obese participants the same number of calories. Aiming to facilitate weight loss, all participants consumed 30% fewer calories than their daily energy expenditure, but one group ate a high-carbohydrate diet (46% carbohydrate) and the other a ketogenic diet (4% carbohydrate). At the study’s start and finish, researchers measured participants’ weight, time to exhaustion on a treadmill walk, and hand and knee strength. After eight weeks, the high-carbohydrate and ketogenic groups lost the same amount of weight, neither group experienced a decrease in strength, and both groups treadmill walked further before exhaustion (suggesting increased fitness). The only difference between groups was no surprise: laboratory testing proved that the ketogenic group burned fat, while their high-carbohydrate counterparts utilized carbohydrates.
Further studies echo these findings. When following a ketogenic diet, overweight subjects can successfully engage in regular, moderate-intensity exercise, lose weight, and increase treadmill-walking time to exhaustion. Additionally, a small study involving obese women who began a resistance training program showed that a keto diet helped reduce body fat without decreasing muscle mass—but no more so than a regular diet did.
Evidently, current research indicates that the ketogenic diet does not significantly affect fitness gains and performance among untrained individuals but does adequately support exercise.
Keto for Endurance Exercisers
A review in the Journal of the International Society of Sports Nutrition examined several studies, but could not offer a definitive conclusion regarding how the ketogenic diet impacts endurance exercise. The researchers failed to discover any differences between athletes following normal and ketogenic diets, measured via VO2 max (maximum oxygen consumption), time to exhaustion, and maximum power output tests.
The journal Nutrition and Metabolism published research that compared two studies among trained, long-distance cyclists. During the first two weeks of the ketogenic diet, the cyclists experienced decreased performance levels. Six weeks later though, performance and VO2 max levels were restored. Researchers attributed this initial performance decrease and subsequent restoration to a necessary ketogenic diet adaption period. After adaptation, the ketogenic diet nourished the cyclists as well as their regular, higher carbohydrate diets.
Similar to the studies involving untrained individuals, this research demonstrates that the ketogenic diet may support, but does not necessarily improve endurance exercise performance.
Keto for Power-Based Athletes and Exercise Aficionados
Mobilizing fat to create ketones and thus energy requires oxygen and time. During power exercises, such as sprinting, lifting heavy weights, or gymnastics, oxygen is limited and energy is needed immediately, raising questions about the ketogenic diet’s efficacy.
Nutrients featured a study examining ketogenic diet interventions among CrossFit, gymnastics, and power-lifting athletes and failed to reveal any significant, keto-attributed athletic performance or power improvements. Additionally, the authors stated that the ketogenic diet may even impair new muscle mass formation despite power training.
Another pilot study including twelve CrossFit athletes, published in Clinical and Sports Nutrition, discovered that after twelve weeks adhering to a ketogenic diet, participants lost body fat but did not experience any performance or strength changes.
Clearly, while the ketogenic diet successfully fuels casual as well as long, endurance-based efforts as those aforementioned, it may hamper power-based performance.
Still, perhaps the most notable finding is that ketogenic diet adaptation may impair future carbohydrate usage. This means that if a keto-adapted athlete decides to boost his/her performance for an extremely high-power exercise or competition via carbohydrate consumption, the athlete’s body may not even efficiently process it. The result?—no performance benefits, and possibly even undigested carbohydrates provoking gastric distress.
The Bottom Line
Researchers studying the ketogenic diet and how it impacts exercise face numerous challenges, including difficulties controlling participants’ diets and exercise regimens, which consequently encourages large study dropout rates, error margins, and small sample sizes. Additionally, since ketogenic diet adaptation normally requires as long as four weeks, further questions emerge regarding the validity of shorter studies.
With these obstacles in mind, the science generally fails to provide proof that the ketogenic diet definitively enhances exercise performance but does indicate that it effectively supports exercise, though possibly excluding power-based activities. For many active individuals, this diet is potentially worth trying. No universal eating pattern exists, however. Rather, consuming whole foods and limiting or omitting grains and legumes, dairy, and added sugars provides a healthy nutritional foundation, whether or not one pursues a ketogenic diet.