You're contemplating running a marathon on a ketogenic diet, and the specter of "the Wall" looms large. This dreaded point in a race, often around mile 18 or 21, signifies depleted energy reserves and an acute struggle to continue. Glycogen stores run dry, and the initial runner's high transforms into a debilitating crash. But is hitting the Wall truly inevitable? This article explores how to run a keto marathon by using fat as a primary fuel source, demonstrating that keto endurance is not only achievable but also a valuable asset in any training program. Keto marathon training is gaining recognition as a valuable tool for everyone from record-holding 100-mile runners such as Zach Bitter to novice runners seeking an edge to maintain energy levels during endurance events. It requires consistency, allowing for proper fat adaptation, and discipline, but nothing worthwhile comes easy.
Understanding the Keto Diet for Endurance
A keto diet involves consuming very low carbs and high fat. People eat keto for a variety of health reasons but it has gained in popularity most recently for people working to lose weight. When people eat keto they avoid sugar, grains, starches, root vegetables, tubers, and most fruits. The cornerstone of the ketogenic diet lies in drastically reducing carbohydrate intake while significantly increasing fat consumption. Typically, this translates to a diet comprising 5-10% carbohydrates (20-50 grams), 15-20% protein, and 70-75% fat. This macronutrient ratio compels the body to enter a state of ketosis, where it switches from primarily burning glucose (derived from carbohydrates) to burning fat for fuel. The liver converts fats into ketones, which become the primary energy source for the body and brain.
Benefits of Keto Diet for Marathon Runners
The benefits of keto diet over carb diets are there - it increases fat burning, conserves glycogen, and is good for the brain and the mitochondria (the “power generators” of your cells). The appeal of a keto diet for marathon runners stems from several potential advantages:
- Enhanced Fat Burning: The body becomes highly efficient at utilizing fat stores for energy.
- Glycogen Conservation: By relying on fat, the body spares its limited glycogen reserves, potentially delaying fatigue.
- Stable Energy Levels: Fats and ketones provide a more sustained and stable energy release compared to the rapid spikes and crashes associated with carbohydrate consumption.
- Brain Fuel: Ketones can effectively fuel the brain, potentially preventing mental fatigue during long runs.
Getting Started with Keto Marathon Training
As you begin your keto endurance journey remember this: it takes time. As you know from your marathon training, miracles don’t happen - adaptations happen. And fat adaptations derive from consistency and volume from running on keto. The first rule is to be patient and consistent and don’t look for instant results. Embarking on a keto marathon training program requires a strategic and gradual approach.
Adaptation Takes Time
Becoming fat-adapted takes time. The length of time that people struggle with endurance sports when newly on keto truly does depend on the athlete, but according to nutritionists who study athletes, it can take months. Full fat adaptation can take 8+ weeks. The body needs time to adjust to using fat as fuel, so it’s best to switch to keto thoughtfully and methodically…not just a couple weeks before a race or ultra.
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Pre-Keto Transition
One big part of adopting a keto diet as a runner, or when training for ultramarathons on a keto diet, is understanding that you’ll be starting to buy different foods than you normally would. If you’re seriously considering going keto, I strongly encourage you to take a “test” trip to the supermarket to understand what’s out there. Knowing what you’ll have to work with is a great starting point, and is a great way for you to try out a keto diet before going all-in.
Gradual Carb Reduction
When going keto, you’ll want to cut your consumption of sugar and carbs gradually. The idea here is to do this shift little by little so that you don’t overwhelm your system with a total cut-out. Lowering your carb intake sounds simple, but it’s a bit more tricky in practice than in theory. A good way to start is by switching out one “normal” meal a day for a “keto friendly” meal.
Adjust Training Load
Remember that as you go through this process of transitioning to keto-even if you transition gradually-that you’ll likely notice a decrease in your overall energy level. This is because your body won’t have access to the same level of carbohydrates that it’s used to using as fuel. This has direct implications on your trail running and/or ultramarathon training, because it means you might have to adjust your training load so that you don’t feel overly exhausted after each workout. We really recommend that you do not plan to do any trail races, ultramarathons or even any really hard trainings when you’re first transitioning to a keto diet. The best time to start transitioning to a keto diet, especially as an ultrarunner, is well before your first race goal of the season. Actually, your “off-season” would really be the best time to start making the switch; during your off season, you’re more likely to be doing less volume and intensity in your overall training.
Potential Initial Side Effects
You also need to be aware that you’ll probably feel some not-so-fun things during the first couple of weeks or even months. Some of the things you might feel during the first 30 days of your keto diet are²: being generally low in energy having to bail on some running workouts (both intensity trainings and long runs)some headaches, and overall feeling of achiness in your bodycravings for all the sugary things that you used to eat, to the point that it may feel overwhelmingfeeling especially hungry during the initial switchover.
Maximising Fat Adaptation
Longer and easier-paced aerobic runs push your body to rely on its fat stores for fuel. Athletes who are adapted to a keto diet and to efficiently using fat as a primary fuel source have an advantage - they can efficiently and effectively use two fuel sources for energy: ketones and carbohydrates. To expedite fat adaptation, consider the following:
Read also: Keto Calorie Counting: A Detailed Guide
- Morning Runs on an Empty Stomach: Enhance fat-burning capabilities by performing morning runs in a fasted state.
- Supplementation with Exogenous Ketones: Supplement your diet and keto marathon training with exogenous ketones. Some keto practitioners recommend exogenous ketones; these are supplements that contain lab-produced ketones which can be useful for kick-starting the process.
- Prioritize Healthy Fats: You’ll need a substantial amount of healthy fats in your diet to keep energized. Yes, it feels good to eat all that bacon and cheese, but make sure to eat a lot of vegetables and healthy fats: avocados, nut butters, chia seeds, and the like.
- Electrolyte Balance: When you cut out carbs, you’re often also cutting down on sodium intake. That’s why it’s important to be mindful of your electrolyte levels. Supplement with natural electrolytes to avoid fatigue and cramps during training and races.
- Hydration: That awful foggy, nauseous feeling? Most of the time, it’s dehydration or low electrolytes. Water isn’t enough. Eat more salt than you think you need.
Structuring Your Keto Marathon Training Plan
The marathon is ~97.5% aerobic-most energy you produce to fuel the 26.2 miles will come from aerobic energy systems (glycolysis and oxidative phosphorylation to produce ATP). Training your aerobic capacity should be the main focus of training. High-intensity sprints won’t help you in this race. A well-structured keto marathon training plan should incorporate the following elements:
Base Building
Base or foundation training is a staple in endurance training. This period takes place throughout a major portion of the year before endurance athletes transition into higher level speed training. It also helps that athletes respond better to intense training, perhaps by preventing soreness and fatigue during the transition. Training emphasis in the base-building phase should be on prolonged, easy-to-moderate-intensity workouts. Runs lasting anywhere from 45 - 60 minutes or more at a pace around 65% - 80% of HR max are the staple during foundation training. This intensity corresponds to heart rate zone 2 - 3. As a general rule, mileage during the base-building phase shouldn’t progress more than 10% per week. For example, if you run 40 miles in the first week of this phase, run no more than 44 - 45 the next week.
Speed Work
You won’t need much, but speed work is an important component of a marathon training plan. Race-specific workouts should be introduced following your base-building phase. A great marathon performance requires running for a long time at a high percentage of aerobic capacity (somewhere close to lactate threshold). A higher ability to run fast at lactate threshold is associated with a better marathon performance. The better you can use lactate, the longer you can maintain your goal marathon pace.
Threshold and Tempo Runs
Threshold (tempo) runs and “cruise intervals” are workouts designed to target lactate threshold. These are less intense than high-intensity interval training, occurring at ~85% of HR max. Tempo runs are effective, requiring less recovery than high-intensity interval training. For a variation in threshold training, try cruise intervals. These are tempo-paced running bouts (about a mile in distance) separated by brief recovery periods. Start with a two mile warmup, run 5 x 1 mile “cruise intervals” at your tempo pace, with 60 - 90 seconds of recovery between each.
Long Runs
To finish a marathon, you need to be good at spending a long time on your feet. This type of staying power comes from running, and running long! This also means you shouldn’t be afraid to run slow. Complete at least one or two 18 - 20 milers before lining up for a marathon. This distance is sufficient to simulate marathon stress, but not long enough to create a high risk of injury or demand extra recovery time. “Never run a marathon to train for a marathon.” This suggestion may seem paradoxical, but is based on sound rationale. The optimal duration for a long run: look to make this workout about ~33% of your total weekly mileage. If you’re running ~45 miles per week, this means a long run of about 15 miles or more.
Read also: Magnesium Supplements for Keto
Practice Runs
Don’t try anything new on race day. Schedule 2 - 3 long runs during your training schedule as “dress rehearsals.” Wake up and start running at the same time as the race, eat and drink the same foods the night before and the day of, and wear the same clothes and running shoes you plan to sport on the big day. While you can’t control the weather, it might also be great to practice running in different conditions to prepare for the race-day climate.
Surge Long Runs
Along with nutrition and gear, you can mix up marathon long runs by practicing race pace during these sessions. An example of this type of workout is called a “surge” long run. These runs train you to run at aerobic threshold (i.e. marathon pace) while fatigued. Run 8 - 10 miles at an easy, comfortable pace. During mile four or five, increase pace to run at ~10k race pace. This small “surge” portion will fatigue your muscle fibers and require a small shift in fuel utilization from fat to more predominantly carbohydrate oxidation.
Training Low
A recent training strategy, termed “train low,” may provide an opportunity to enhance your marathon training if used strategically. “Training low” means commencing some exercise sessions with a low muscle glycogen content-meaning, carbohydrate depleted. Periodize carbohydrate availability for easy days or regular endurance sessions. Achieving this is easy. Strategies include running in the morning after an overnight fast, completing a “two-a-day” workout without adequately refueling after the morning session, and drinking water instead of a sports drink during longer runs. The idea is to force the body to utilize fat and upregulate its ability to do so. Research shows fasted exercise works.
Fueling Strategies
On the flip side of the “train low” paradigm, do your marathon-specific speed sessions, tempos, and long runs with proper fuel availability. For a higher-intensity session lasting 90 minutes or less, eat something easily digestible with adequate energy (mainly carbohydrates) about 1 - 2 hours before. Eating something in the pre-exercise window will “top off” fuel stores. You’ll need it. High-intensity exercise around 90% of V02 max relies primarily on carbohydrate oxidation for energy. Fueling while running can be tricky. The good news is, any workout under about 90 minutes probably doesn’t require intake of anything other than water. The long run may be the only time you to take in some supplemental or exogenous nutrition. Have an 18 - 20 miler planned? You’ll likely be out on the road for around 2 - 3 hours, and performance will benefit from supplemental carbohydrates like a sports drink or energy gel. A 2:1 ratio of glucose:fructose has been shown to be optimal for intestinal absorption-it will maximize availability of carbohydrates and hopefully prevent GI distress!
Post-Exercise Nutrition
Eating post-exercise is important for two reasons: repairing and rebuilding damaged muscle, and replenishing depleted fuel stores. Following a run, consume some form of carbohydrate within one hour-this is the period where the highest rate of muscle glycogen storage / resynthesis occurs. Eating a meal or snack will provide a quick and immediate source of fuel for the muscle to start using for recovery of energy stores. Enhance recovery by consuming carbohydrates along with other macronutrients. The applications of exogenous ketones for endurance performance have also been investigated in this realm.
Keto-Friendly Fueling Options
Many people who distance train on keto choose to drink smoothies before a run. You can mix keto-friendly berries like strawberries, raspberries and blackberries, toss in some avocado, add flax and mix it with unsweetened almond milk. If you find you need to take in fuel while running, there are options for that too. Many runners fuel with small packs of Gu or similar. You can purchase nut butters made and sold in similar packages. If you choose to do carb cycling, you could even fuel yourself before your run with some real food carbs such as a sweet potato or banana. When running ultramarathons on a keto diet, you’ll still need to consume some carbs in the meals prior to race day-plus, fat can also cause stomach distress when running.
- Nuts and Nut Butters: Provide healthy fats and sustained energy.
- Avocado: A versatile source of healthy fats and electrolytes.
- Chia Seeds: Offer fiber, healthy fats, and sustained energy release.
- Coconut Oil and MCT Oil: Easily digestible fats that can boost ketone production.
- Berries (in moderation): Low-glycemic fruits that provide antioxidants.
- Full-Fat Dairy (if tolerated): Greek yogurt, cottage cheese, and cream can provide protein and fat.
Carb Cycling
Many athletes who eat keto choose carb cycling. To stay in ketosis you generally need to stay under 50 net carbs per day. In carb cycling, you do consume a higher number of carbs strategically placed within the workout week. If you place higher carb days around long runs and challenging workouts, you just might get the boost your body needs. Marathon keto eating looks very different from carb keto eating. The keto athlete would focus on fueling with real foods in their most natural state and probably use carb cycling.
Zach now moderates his carb intake based on his training intensity, dipping in and out of ketosis as he needs.
Race Day Nutrition on Keto
Many of us who are ultrarunners suffer from GI issues during races-so the idea of relying less on carbs/sugar (i.e. gels) probably sounds really appealing to a lot of us. That being said, though, it’s important to remember that a keto diet is not a “magic” fix to GI problems on race day. Even when running ultramarathons on a keto diet, you’ll still need to consume some carbs in the meals prior to race day-plus, fat can also cause stomach distress when running. So you could still end up with stomach issues anyhow, even when running an ultra on a keto diet. This is why-regardless of if you eat keto or not-training your race nutrition is absolutely key.
Let’s say that with a non-keto diet, that you aim to consume 60-90 grams of carbs per hour in a race (i.e. 200-300 calories per hour.) On a keto diet, you might consume just 40 to 60 grams of carbs per hour (i.e. 100-200 calories per hour.) Regardless, you’ll still need to put in the calories to keep moving at your ideal pace-so in the end, you’ll really need to practice eating this amount of calories during your trainings and long runs.
The Importance of Monitoring Progress
The best method is via a blood sample tester (here are a few on Amazon) which work by measuring the ketones in a tiny sample of your blood, retrieved by pricking your finger. Another quick and dirty method is to use keto test strips - they’re a simple strip of paper that you pee onto, and they change color based on the % of ketones in your urine. The problem with these strips is that your hydration level seems to affect the result; the more water you drink, the more diluted those ketones are, which will give you a lower reading. Closely monitor your body's response to the keto diet and training regimen. Pay attention to energy levels, performance, and overall well-being.
The Science Behind Keto and Endurance Performance
The ketogenic diet has become popular among endurance athletes as a performance enhancer. In theory, if endurance athletes tolerate the ketogenic diet, they could achieve longer training periods with sustained energy levels and reduced need for refueling, allowing them to maximize the aerobic benefits from training and competing. In fact, there is some evidence that, among highly trained individuals, benefits of the diet include a steady supply of energy for the body and brain during prolonged exercise and accelerated recovery time post-exercise [4].
It has been hypothesized that consuming a ketogenic diet may enhance performance among endurance athletes by promoting a shift in substrate utilization that enhances physiological training benefits [3, 18]. The present review explores this hypothesis by examining associations between EAKD consumption and VO2 max, a biomarker for endurance capacity [11].
Potential Drawbacks and Considerations
It does bear mention that some athletes never seem to fully adapt to fueling for endurance training while eating keto. As with any dietary approach, there are potential drawbacks to consider:
- Initial Performance Decline: You will likely see a decrease in performance. Early keto feels like you’re dragging a sandbag mentally. That’s okay. Nap more, meditate, take extra rest days. Cleveland Clinic says your body needs time to adjust..
- Nutrient Deficiencies: Keto is a pretty restrictive diet, and the best way to ensure compliance is to make it simple for yourself. Try to identify the foods that work for you early on, and eat them regularly.
- Social Challenges: Keto gets tricky when you share a household with family members who aren’t following the same restrictive diet as you.
- Mentally fried?: - Look, keto takes willpower. Sunday long runs? Total punishment.