The ketogenic (keto) diet, marked by a high-fat, very low-carbohydrate intake, has become a popular topic among endurance athletes. It aims to shift the body's primary fuel source from carbohydrates to fat, with proponents suggesting benefits for performance and fat loss. However, the effectiveness and suitability of this diet for endurance athletes remain a subject of debate.
Understanding the Keto Diet
A keto diet typically consists of 5-10% carbohydrates, with fat making up the majority of macronutrient intake. The goal is to induce ketosis, where the body produces ketones from fat to use as an alternative fuel source.
Fuel Utilization During Exercise
The body uses different fuel sources depending on the intensity and duration of the activity:
- High-intensity, short-duration exercise: Primarily utilizes creatine phosphate for the first few seconds, followed by muscle glycogen for up to sixty seconds.
- Endurance exercise (lasting over two minutes): Initially uses muscle/liver glycogen, transitioning to fat metabolism, and eventually amino acids.
- Low-intensity exercise: Primarily burns fat.
At higher intensities common in endurance training and competition, carbohydrates are typically the primary energy source.
Comparing Keto to a Typical Athletic Diet
A typical athletic diet emphasizes carbohydrate intake to fuel high-intensity exercise and replenish glycogen stores. In contrast, the ketogenic diet drastically reduces carbohydrate intake, forcing the body to rely on fat for fuel.
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Potential Drawbacks of the Keto Diet for Endurance Athletes
While the keto diet may offer some benefits, it also has potential drawbacks for endurance athletes:
- Ketoadaptation: It can take up to four weeks for the body to efficiently use ketones as fuel.
- Carbohydrate Dependence for High-Intensity Exercise: Endurance events lasting up to three hours often require carbohydrate-based foods, and fat-based foods can downregulate carbohydrate metabolism.
- Potential Impairment of Performance: Despite increased fat utilization, the keto diet may impair performance during high-intensity exercise.
- Muscle Mass: Research suggests that while the keto diet can decrease fat mass, it may not increase muscle mass with resistance training.
Scientific Evidence on Keto and Endurance Performance
Several studies have investigated the effects of the keto diet on endurance performance. These studies have examined various outcomes, including maximal oxygen consumption (VO2 max), time to exhaustion (TTE), race time, rating of perceived exertion (RPE), and peak power.
VO2 Max
VO2 max, a measure of aerobic fitness, has shown mixed results in studies on keto diets. Some studies reported significant increases in VO2 max across all diets, while others found no significant changes. One study found a smaller increase in VO2 max in the keto group compared to a comparison diet group, with the increase potentially inflated by weight loss.
Secondary Outcomes
Secondary outcomes have also yielded mixed results. Some studies reported no significant difference in TTE between keto and high-carbohydrate diets, while one study found a significant decrease in TTE for athletes on a keto diet. Race times have shown significant decreases in high-carbohydrate groups but non-significant increases in keto groups. RPE has been reported as higher in keto groups post-intervention. Peak power has shown mixed results, with one study reporting improved peak power in keto athletes and decreased peak power in comparison diet athletes, while another study found a mean decrease in peak power in keto athletes.
Limitations and Heterogeneity
The limited and heterogeneous findings from existing studies make it difficult to draw definitive conclusions about the efficacy of the keto diet for endurance performance. More randomized trials are needed to better understand the potential effects of keto diets on endurance performance.
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The Keto Diet and Fat Oxidation
The ability of ketogenic low-carbohydrate high-fat (K-LCHF) diets to enhance muscle fat oxidation has led to claims that it is the 'future of elite endurance sport'. There is robust evidence that substantial increases in fat oxidation occur, even in elite athletes, within 3-4 weeks and possibly 5-10 days of adherence to a K-LCHF diet. Retooling of the muscle can double exercise fat use to ∼1.5 g min-1 , with the intensity of maximal rates of oxidation shifting from ∼45% to ∼70% of maximal aerobic capacity. Reciprocal reductions in CHO oxidation during exercise are clear, but current evidence to support the hypothesis of the normalization of muscle glycogen content with longer-term adaptation is weak.
Importantly, keto-adaptation may impair the muscle's ability to use glycogen for oxidative fates, compromising the use of a more economical energy source when the oxygen supply becomes limiting and, thus, the performance of higher-intensity exercise (>80% maximal aerobic capacity). Even with moderate intensity exercise, individual responsiveness to K-LCHF is varied, with extremes at both ends of the performance spectrum. Periodisation of K-LCHF with high CHO availability might offer opportunities to restore capacity for higher-intensity exercise, but investigations of various models have failed to find a benefit over dietary approaches based on current sports nutrition guidelines.
The Importance of Individual Needs
No diet is universally suitable for every athlete. Individual and sport-specific needs should guide dietary choices, particularly concerning the type and duration of activity. Ingesting carbohydrates during exercise may delay fatigue and improve oxygen-delivery rates and performance.
Ketosis and Gastric Distress
Athletes in ketosis may perform well at a steady endurance pace and for extended periods while consuming fewer calories than carbohydrate-dependent competitors. This may be a solution for athletes who struggle with gastric distress during ultradistance events. During exercise lasting 9-24+ hours, changes in blood volume, heat stress, and hydration status can slow or halt gut motility.
Disruptions to Training
If carbohydrate is available it is the go-to fuel for muscles and the brain. Only in carbohydrate’s absence will the body transition to producing and using more ketone bodies for energy. When sugar from plants was available to our ancestors they could gorge on it, use some for energy and store the rest as fat. During times when there were no plants to eat, their carbohydrate stores ran out and they transitioned to ketosis to fuel themselves from their stored fat. To achieve ketosis voluntarily - instead of through inadequate insulin production - you have to essentially eliminate carbohydrate from your diet. Initially, you will have neither enough carbohydrate nor ketones to fuel your brain. While you are always producing ketones, it takes time (up to 2-3 weeks) for your body to increase production to the point you are relying on them as a primary energy source. During the first week people often experience the “keto flu”, which is not an infection but rather a set of symptoms reminiscent of the flu: Headache, foggy brain, fatigue, irritability, nausea, difficulty sleeping, and constipation. Training performance will definitely suffer (and lifestyle performance may suffer as well). Your power output will be lower than normal. Your running pace will be slower than normal. Perceived exertion will go up, at all intensity levels. Recovery from training sessions will be hindered. Once you are adapted to fueling yourself primarily on ketones for day-to-day living, you still need to adapt to performing optimally as an athlete fueled by ketones. This can take months, during which time your only progress will be in fat adaptation, not aerobic development, the ability to produce power, or the ability to achieve faster paces. If you are going to try ketosis as an athlete, the best time to experiment would be a period of general aerobic endurance training. For summertime athletes in the Northern Hemisphere, this typically means fall or winter. It would be a mistake to try making this transition during a period of important, race-specific, high-intensity training.
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