The ketogenic diet has gained traction among endurance athletes, including runners, as a potential performance enhancer. This article explores the scientific evidence, benefits, and drawbacks of the keto diet for runners, providing a comprehensive overview to help athletes make informed decisions.
Introduction to the Ketogenic Diet
The ketogenic diet (EAKD) is a high-fat, very low-carbohydrate diet designed to shift the body's primary fuel source from glucose to ketones. Traditionally, the ketogenic diet is a high fat (70-80% of calories), low protein (10-20% of calories), and extremely low carbohydrate (5-10% of calories) diet. This metabolic state, known as ketosis, occurs when carbohydrate intake is drastically reduced, typically to less than 50 grams per day. The liver then converts fat into fatty acids and ketone bodies, which the body uses for energy. This approach has been explored for various health benefits, including weight loss, improved insulin sensitivity, and potential advantages for endurance athletes.
The Rationale Behind Keto for Endurance Athletes
The primary rationale for endurance athletes adopting a ketogenic diet stems from the body's limited carbohydrate storage capacity compared to its vast fat reserves. Carbohydrates are stored predominantly as glycogen in muscle (300 g) and liver tissue (90 g), plus glucose in the bloodstream (30 g), totaling approximately 1680 kcal of available energy. Endurance athletes often need to replenish glycogen stores every one to three hours during prolonged activity. This continual consumption can divert nutrients from exercising muscles to the gut to aid digestion, potentially leading to reduced exercise economy and digestive disturbances, compromising performance.
In contrast, fat storage offers a significantly larger energy reserve. One pound of fat yields roughly 3500 kcal, making fat a substantial energy source, even for lean athletes. In theory, athletes who adapt to a ketogenic diet could sustain energy levels for longer periods with less need for refueling, maximizing aerobic benefits during training and competition. Additionally, ketogenic diets have been shown to reduce lactate accumulation after exercise, potentially enhancing recovery.
Scientific Evidence: VO2 Max and Secondary Outcomes
To evaluate the effectiveness of the ketogenic diet for endurance athletes, a review of scientific literature was conducted, focusing on studies that examined the impact of EAKD consumption on maximal oxygen uptake (VO2 max), a key measure of aerobic fitness. The review included studies with endurance athletes (e.g., cyclists, runners, race walkers, triathletes) who followed an EAKD (< 50 g daily carbohydrate) for three or more weeks, achieving ketosis (measured via serum biomarkers).
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Mixed VO2 Max Results
The findings regarding VO2 max outcomes were mixed. Two studies reported significant increases in VO2 max across all diet groups, including the EAKD group. However, four other studies reported no significant VO2 max changes with the EAKD. For instance, Carr et al. reported significant increases in VO2 max from baseline for all diet types (EAKD: 61.1 ± 5.3 vs. 63.4 ± 4.1; HCD: 57.6 ± 4.6 vs. 58.3 ± 4.1; PCHO: 58.1 ± 3.3 vs. 60.2 ± 3.8; p < 0.05), while Burke et al. found a significant increase in VO2 max for all athletes (EAKD: 66.3 vs. 71.1; HCD: 61.6 vs. 66.2; PCHO: 64.9 vs. 67.0; p < 0.001). In contrast, Shaw et al., in a randomized crossover study, found no significant changes in VO2 max from baseline (59.4 ± 5.2) after either 31 days of EAKD or a high-carbohydrate comparison diet (p > 0.05).
Secondary Performance Outcomes
Secondary outcomes, including Time to Exhaustion (TTE), Race Time, Rating of Perceived Exertion (RPE), and Peak Power, also showed varied results.
- Time to Exhaustion (TTE): Of three studies reporting TTE, two found no significant difference based on diet type, while one study reported a significant decrease from baseline for participants consuming the EAKD.
- Race Time: One study reported a significant decrease in race time among high-carbohydrate and periodized carbohydrate groups, while the EAKD group showed a non-significant increase. Specifically, Burke et al. reported a significant decrease in race time among high carbohydrate and periodized carbohydrate groups (HCD: − 190 s; PCHO: − 124 s; p < 0.01), while the EAKD group had a non-significant increase in race time (EAKD: + 23 s; p > 0.01).
- Rating of Perceived Exertion (RPE): One study reported higher RPE values among the EAKD group post-intervention compared with pre-intervention. For example, Burke et al. reported higher RPE values among the EAKD group post-intervention compared with pre-intervention (p ≤ 0.01).
- Peak Power: One study reported that post-intervention peak power was significantly different between diets, with EAKD athletes improving their peak power and comparison diet athletes decreasing their peak power. McSwinney et al. reported that post-intervention peak power was significantly different between diets, with EAKD athletes improving their peak power and comparison diet athletes decreasing their peak power (EAKD: 8.3 ± 2.2 vs. 9.7 ± 2.3 watts/kilogram; HCD: 9.1 ± 2.6 vs. 8.4 ± 2.2 watts/kilogram; p = 0.047).
Practical Considerations for Runners on a Keto Diet
For runners considering a ketogenic diet, several practical aspects should be taken into account.
Transitioning to Keto
Transitioning to a keto diet requires a gradual reduction in carbohydrate intake to avoid overwhelming the body. It's advisable to start by replacing one "normal" meal with a "keto-friendly" meal. During this transition, runners may experience decreased energy levels as the body adapts to using fat as its primary fuel source. Adjusting training load and intensity is crucial to prevent excessive exhaustion.
Keto-Friendly Foods
A keto diet for runners involves significant changes in food choices. Instead of oatmeal with black coffee for breakfast, a keto-friendly option might include eggs, avocado, and cream in coffee. Post-training meals would shift from pizza or ice cream to steak, salmon, bacon, or cauliflower salad, with dark chocolate and Greek yogurt for dessert. Foods commonly included in a keto diet for runners are nuts, avocados, oils (coconut and olive), eggs, nut butters, and chia and flax seeds, while bread, pasta, rice, and oatmeal are largely excluded.
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Race Nutrition
Race nutrition also needs to be adjusted. While runners on a non-keto diet might consume 60-90 grams of carbs per hour during a race, those on a keto diet may consume just 40-60 grams of carbs per hour. Practicing race nutrition during long runs is essential to understand how the body responds to different fuels.
Potential Risks and Drawbacks
While the ketogenic diet has potential benefits, it also poses several risks and drawbacks for runners.
Reduced Performance
Some research suggests that the keto diet can decrease performance in endurance athletes. Studies have shown declines in running speed and power, with running speed at VO2max declining by 5 percent in some cases. The limited research available indicates that any potential benefits do not outweigh the risks when compared with an evidence-based moderate- to high-carbohydrate diet.
Health Concerns
The keto diet can lead to gastrointestinal distress from increased fat intake, hepatic steatosis, kidney and gallstones, and potential nutrient deficiencies from cutting out major food groups such as fruits, starchy vegetables, whole grains, and legumes. There are also potential risks to restricting carbohydrate such as underfueling related issues like RED-S, which may entail hormone imbalances, eating disorders, and potential increased risk for injury.
Sustainability
Studies on low-carbohydrate and ketogenic diets often exhibit higher dropout and noncompliance rates, suggesting that these dietary interventions may not be sustainable in the long term.
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Exogenous Ketones: A Potential Alternative?
Exogenous ketones are supplements that provide the body with ketones without requiring a strict ketogenic diet. Some elite athletes have started using exogenous ketones during training and races to improve performance in long endurance events.
Limited Research and High Cost
Research on exogenous ketones is limited, but some studies have shown potential benefits, such as improved glycogen synthesis and recovery. One study showed that when carbohydrate intake during and following long efforts was adequate, adding the exogenous ketone product improved glycogen synthesis (IE: the ability to restock glycogen stores) by 50% due to increased insulin levels. However, these benefits come at a high cost, with some products costing upwards of $28 USD per serving.
Potential Benefits
Exogenous ketone supplements may improve recovery and glycogen storage efficiency in male athletes when coupled with adequate carbohydrate consumption. They may also help prevent overtraining symptoms and decrease instances of ketone-related gastrointestinal issues.
The Takeaway
Exogenous ketone supplements may improve recovery and glycogen storage efficiency in male athletes when coupled with adequate carbohydrate consumption. Many endurance athletes are not eating the recommended amount of carbohydrates supported by large bodies of research to benefit from adding ketone supplements right off the bat, so addressing optimizing overall intake should prelude experimenting with ketone supplements, which are a costly addition to an athlete’s ergogenic aid toolkit. More research is needed in this area.
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