No nutrition plan works well for everyone, and no one plan will work for your entire life. It might not even work for an entire week! Nutrition plans need to take many factors into account, both related to the body (e.g., current body mass and composition, target body mass and composition, genetic factors, sleep, stress) and to training (e.g., type of exercise, intensity of effort, proximity to competition). Creating a nutrition plan that balances optimization for long-term goals (like decreased body fat or increased lean muscle mass) with short-term performance (such as key workout sessions or competitions) is tricky. Highly popularized for its ability to help people lose weight quickly, the ketogenic diet is the diet du jour. The keto diet has demonstrated effectiveness for weight and body fat reduction, but its utility for athletes is highly sport-dependent. For ultra-endurance athletes, keto diets can be even more beneficial than other diets. For athletes performing high-intensity or repeated maximal efforts, such as in weightlifting, rugby, or track cycling, keto diets may not be able to replenish energy stores quickly enough to keep up with energy demands. However, keto diets may be an excellent tool for cutting weight without relying on dehydration strategies. This article explains the metabolism of a ketogenic diet and discusses its effects on performance to help you decide if and, more importantly, when to implement it in your training.
 
Ketogenic diets severely restrict carbohydrate consumption in order to switch the body’s metabolic preference from sugar utilization to fat oxidation, a state called ketosis. In ketosis, the body preferentially uses fat stores to synthesize alternate energy molecules called ketone bodies that replace glucose as the body’s primary source of energy when sugars and carbs are unavailable. This process happens naturally when the body is in a fasting state. The brain is able to metabolize ketone bodies for energy, an evolutionary advantage that preserves cognitive function during times of starvation.
 
Getting the body to a state of ketosis is not easy. Carbohydrates must be restricted to 5 percent of total daily calories, or roughly 30-50g/day. The diet is moderate (20 percent) in protein consumption and high (75 percent) in fat consumption in order to maintain total daily calorie requirements. Cells love glucose for its quick and bountiful energy production, so the body is reluctant to switch to utilizing ketones for energy. Inducing a state of ketosis often takes several (two to five) days, and people often report feeling flu-like symptoms during the adaptation stage (often called the “keto-flu”). These symptoms of fatigue, dizziness, and lack of focus subside as the body improves its efficiency with fat oxidation and ketogenesis. The body is medically in a state of ketosis when blood serum ketone levels have increased to 0.5-3mM.
 
When glucose is available as an energy source, it is broken down for energy in a two-part process. First, energy is extracted via the “fast” process of anaerobic glycolysis. Second, even more energy is made through the “slower” process of aerobic glycolysis via the Tricarboxylic Acid Cycle (TCA cycle) and oxidative phosphorylation pathway. When glucose is unavailable as a substrate for the TCA cycle, ketone bodies can be used to build acetyl-CoA, which enters the TCA cycle as if it were derived from glucose directly. In a fasted state, the body relies on the “even slower” process of lipolysis to break down fat stores for energy. The liver contains the enzymes necessary to convert free fatty acids (FFAs) into ketone bodies: acetoacetate and beta-hydroxybutyrate (or, 3-OH-butyrate), and their breakdown product, acetone. Acetoacetate and beta-hydroxybutyrate are water soluble and can move through the bloodstream to supply energy to the heart, skeletal muscle, and the brain. This figure illustrates how ketone bodies made in the liver via lipolysis and ketogenesis can be used in the TCA cycle to create energy in muscle cells when glucose is unavailable.
 
The power of a ketogenic diet on the brain has a rich medical history, where, as early as the 1920s, these diets have been used to control seizures in children with drug-resistant epilepsy. The cognitive benefits of increased blood ketones have also been extended to adults with certain neurological disorders. Research shows that when patients with Alzheimer’s disease supplement with a medium-chain triglyceride (MCT), an FFA precursor to ketone bodies, their cognitive performance increases significantly. This research and similar studies serve as the foundation of the argument in support of MCT supplementation, a regimen followed by Bulletproof dieters with the goal of improved function of both the brain and body.
 
Spending prolonged periods of time (weeks to months) in a state of ketosis can also trigger a huge reduction in weight and body fat. While most of the research in this area has focused on helping people with obesity lose weight quickly, the benefits of ketosis extend to healthy, untrained persons as well as trained athletes. Both groups see decreases in total body mass and body fat percentage, but, critically, also maintain or build lean body mass. In a study on keto ultra-endurance athletes, researchers found an incredible increase in their ability to utilize fat for energy. These runners have mean fat oxidation levels that are 59 percent higher than runners on a high-carbohydrate diet during submaximal exercise, and show similar patterns of muscle glycogen depletion after prolonged, low-intensity exercise. They are able to shunt more fats and, in a state of ketosis, ketone bodies into the TCA cycle to produce energy during their endurance runs. Functionally, these “keto-adapted” ultra-endurance athletes are better at utilizing long-term energy stores, provided that the “slow”/aerobic energy pathways can keep up with the energy demands of their exercise.
 
Problems arise when athletes in ketosis attempt to perform anaerobic or maximal efforts. High-intensity exercise requires larger amounts of energy and quicker energy utilization. Under moderate- or high-carbohydrate diets, glucose is available for breakdown into energy via the “fast” process of glycolysis. This figure illustrates the different metabolic energy systems and their power contribution to exercise of different durations: ATP and creatine phosphate (CP) systems power short duration efforts, anaerobic glycolysis powers medium duration/VOmax-type efforts, while aerobic glycolysis powers longer duration exercise when carbohydrates (CHO) are available; lipolysis, or fat breakdown, is utilized for longer-duration/lower-intensity exercise.
 
Under a ketogenic diet, glucose is absent; only aerobic lipolysis and ketogenesis exist for energy production. Keto athletes in certain sports or performing exercises that require greater energy capacity can struggle to perform at these intensities. Wilson et al. tested the anaerobic capacities of trained weightlifters using a Wingate test (a 30-second all-out sprint) and found that athletes on a ketogenic diet had nearly a 100-watt drop in power over the 30-second interval compared to a moderate-carbohydrate group. This loss in power is also observed for even shorter <10-second sprint efforts that quickly deplete the body’s “fastest” access energy (ATP and creatine phosphate) stores. These rapid energy systems are also the ones which are utilized in weight training. Interestingly, keto weightlifters are still able to bench and squat equal 1RM weight as their non-keto counterparts, suggesting that keto diets may not be detrimental to a single, maximal weightlifting effort. Keto athletes will likely need extended recovery between efforts to allow for the slower fat oxidation process to replenish energy stores. This long-duration recovery requirement may make keto diets a poor choice for high-intensity interval training (HIIT) and similar workouts.  Athletes in sports with mixed long endurance and explosive power requirements (e.g., cycling) will need to prioritize fueling for the higher-intensity efforts. A ketogenic diet will fuel long, slow rides but will not be adequate for repeat hill attacks or sprints.
 
Only recently have the effects of a ketogenic diet been investigated on trained weightlifters with an eye on peaking for competition. Wilson et al. designed their study such that athletes followed ketogenic diet (20 percent protein, 5 percent carbohydrate, 75 percent fat) or a higher-carbohydrate diet (20 percent protein, 55 percent carbohydrate, 25 percent fat) for 10 weeks. The keto athletes re-introduced carbohydrates at week 11 during a taper, in hopes of boosting energy stores and performance. The study demonstrated that hypertrophy is not impaired on a ketogenic diet and that keto athletes showed significantly greater body fat loss than the carbohydrate group at week 10. However, keto athlete body mass increased abruptly at week 11 with the reintroduction of carbs. While this increase in weight is most likely due to water retention, it could be problematic for weight-class competition athletes looking to boost performance with additional last-minute energy stores. More research is needed to identify alternate carbohydrate-reintroduction strategies that can help athletes diversify energy stores pre-competition while avoiding associated weight gain.
 
Both the scientific literature and a plethora of self-reported data indicate that ketogenic diets can be powerful tools for decreasing total body mass and body fat while maintaining or even increasing lean body mass. The best ultra-endurance runners can operate very efficiently in ketosis. However, for athletes in high-intensity sports or performing repeated explosive movements, being in a state of ketosis may not be conducive to maximizing performance. An optimal nutrition plan for these athletes might be one where the body is kept in ketosis except for windows of time around competitions. Oscillating between ketogenic and non-ketogenic states is difficult on the body. Firstly, more work is needed to determine how best to re-introduce carbohydrates, possibly for durations of <1 week, in order to avoid the water weight gain response. Secondly, while it only takes a single meal to bounce out of ketosis, reaching a ketogenic state takes several days. Supplement scientists are conducting very preliminary studies on consuming exogenous ketone body supplements to help flip the body’s switch back to ketosis in a matter of hours, or possibly even provide a mid-competition energy boost. One of the major challenges in creating a ketone body supplement is that the active form, a ketone ester (not a ketone salt), is unstable and breaks down quickly. If a manufacturer is able to solve the stability problem, they’re left with a palatability problem: esters often have offensive smell and taste. Once an effective and tolerable ketone ester hits the market (and a few are on their way), it will be exciting to see how athletes—keto and non-keto alike— experiment with them in their training and everyday nutrition.